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Why are hybrids infertile?

Why are hybrids infertile?


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Let's take a quote from Wikipedia about zebroids.

Donkeys are closely related to zebras and both animals belong to the horse family. These zebra donkey hybrids are very rare. In South Africa, they occur where zebras and donkeys are found in proximity to each other. Like mules, however, they are generally genetically unable to breed, due to an odd number of chromosomes disrupting meiosis.

First, if I understand meiosis, the resulting cells don't actually end up with half the number of chromosomes, but closer to a full set of halves of chromosomes. How is the meiotic process disrupted?

Then,

A donkey has 62 chromosomes; the zebra has between 32 and 46 chromosomes.

Apparently this difference doesn't obstruct producing (infertile) offspring. How comes the process of recombination of such vastly different number of chromosomes in gametes is viable? What happens to chromosomes that don't find their 'pair'?

And then,

Horses have 64 chromosomes, while most zebroids end up with 54 chromosomes.

54 is an even number. How comes zebroids can't just normally produce fertile offspring with other zebroids of the same number of chromosomes?


A critical step in meiosis is the formation of tetrads. In diploid organisms like donkeys, they have a paternal version and a maternal version of each chromosome. Two chromosomes 2, for instance. During prophase 1, these two "matching" or homologous chromosomes form a tetrad and cross over, swapping alelles. There is no recombination and no tetrad formation in mitosis, which is used for growth and daily living.

How is the meiotic process disrupted?

In a hybrid, there are no matching maternal and paternal chromosomes. In the zebroid testes or ovary, a lonely donkey chromosome 2 is wandering around, looking for another donkey chromosome 2, and there isn't a homologous chromosome. This mucks up Prophase 1, and the first division pulls crazy numbers of chromosomes to each daughter cell, and the gametes from this division are likely full of extra chromosomes and missing some critical ones.

Apparently this difference doesn't obstruct producing (infertile) offspring. How comes the process of recombination of such vastly different number of chromosomes in gametes is viable?

An embryo needs a solid set of genes to grow up, but chromosomes don't have to be homologous for mitosis, and organisms use mitosis for growth and living, not meiosis. As long as the crazy mis-matched chromosomes in a zebroid have everything needed for a healthy organism, the hybrid will be healthy. It isn't until the zebroid starts to make gametes via meiosis that a problem occurs, as the homologues ONLY form tetrads in making eggs and sperm. The inability to recombine only occurs in the making of gametes.

54 is an even number. How comes zebroids can't just normally produce fertile offspring with other zebroids of the same number of chromosomes?

Because even chromosome number doesn't help if they are not each homologous with another chromosome.


I am a little bit out of practice but i researched the genetics of the horse tribe during the 1990s at the start of the Internet. Male hybrids of the diverse equine species are all infertile. However, a fairly large number of fertile female mules have been observed for over a century and much genetic research has been done to them, both in the West as in China. A few fertile female hinnies have also been proven. With zebroids only one fertile mare, a zebra-horse mix, is known and sadly her remains as that of her foal have not been subjected to serious research. That donkeys are closer related to zebras as horses is not generally accepted. There exist three species of zebras -and possibly one recently extinct species, the Quagga- but all three species have interbred and at least a few of the female hybrids have produced foals, in the wild even!


If you think about the complexity of successful reproduction -- sperm recognizing egg, entering egg, the intricate choreography of cell division and differentiation -- it's staggering that any two individuals of the same species ever successfully interbreed, and it's obvious that enormous selective pressure is needed to maintain this perfect compatibility. It's much less surprising that genetic drift would randomly lose one or more of the thousands of adaptations that are needed to guarantee compatibility.

So random drift is probably the reason for most genetic incompatibility, but in cases where closely-related species come into contact (whether through sympatric speciation or through changes in the range of one or both), there's also positive selection that will actively prevent cross-species interfertility. Hybrids are almost certainly going to be less well adapted to either parental niche than are purebreds, and so investing energy into developing hybrids is likely to be much less evolutionarily successful than putting the same energy into purebreds.


Why are hybrids infertile? - Biology

I occasionally hear people claim that different species can't produce hybrids. That's clearly absurd but it probably comes from a misrepresentation of another false claim that is much more widely held to be fact. That is that different species can't produce fertile hybrids, and if they do then that indicates they aren't really different species at all and have been classified incorrectly. That's also wrong, but I've heard it so often that I suspect it's sometimes taught in schools and textbooks.


Fertility in bird hybrids

It is demonstrably the case that some hybrids can be fertile. There are hundreds of examples I could give you - let's say Western Gull x Glaucous-winged Gull, two species with an extensive overlap in their ranges and where in the centre of the overlap zone most birds are hybrids and freely and successfully breeding with one another.

It is less easy to prove that some hybrids are necessarily infertile. However if a hybrid is very common but backcrossed hybrids are unknown, then it seems reasonable to conclude that they are. One example of a hybrid that is apparently always infertile is Muscovy Duck x Mallard. Domestic examples of these have been bred together, deliberately and accidentally, and such a hybrid has never (I believe - correct me if I'm wrong) produced any offspring of its own.

Another good example is Greylag Goose x Canada Goose. Such hybrids are very common but I have never seen or heard of any evidence to suggest that they can produce offspring.

The fact that most Muscovy Duck x Mallard hybrids or Greylag Goose x Canada Goose hybrids don't produce offspring doesn't prove that they are necessarily infertile. It doesn't mean that one day we might find one that does produce offspring. But with common hybrids like this there is sufficient evidence at least to say that these hybrids are either much less likely to be fertile, or much less fertile.

And here's a point. It may not be a black and white question of fertile or infertile, but a shades of grey question of how fertile. Western Gull x Glaucous-winged Gull hybrids are very fertile and Muscovy Duck x Mallard hybrid are very infertile. Are there other hybrids that are a bit fertile, but not very fertile? Do you know of any examples of hybrids that have shown they can be fertile but appear to have reduced fertility compared to their parent species

In an earlier draft of this article I speculated that Mallard x Northern Pintail hybrids might provide such an example. They turn up fairly often, especially in NW USA/SW Canada it seems, and they're pretty consistent in appearance most of the time.

If Mallard x Northern Pintail hybrids were as fertile as their parents, I figured we should expect to see a few backcrossed birds now and then, looking much more like one or other parent than the typical F1 (first-generation) hybrid? I was aware of one or two records that we've wondered about and that do seem quite likely to have been backcrossed hybrids, but not often and not many. I concluded tentatively that perhaps Mallard x Northern Pintail hybrids are fertile, but not very fertile, or perhaps not all of them are fertile. Well, Joern Lehmhus has brought to our attention studies of captive birds that have shown that Mallard x Northern Pintail hybrids are in fact very fertile. This seems to be true of many other ducks in the genus Anas (and also in Aythya).

Most hybrids of very closely related taxa - like where the two taxa are treated as conspecific by some authorities and distinct species by others - seem to be very fertile. Several records of vagrant Black Brants in Norfolk (UK)'s Dark-bellied Brent Goose flocks have remained with their carrier species to breed and now Dark-bellied Brent Goose x Black Brant hybrids are rather frequent. Many of these have bred with Dark-bellied Brent Geese and produced young, although once the family ties are broken they are nigh-on impossible to identify. They are evidently very fertile.

  • The more closely related two taxa are, the more likely their hybrid offspring are to be fertile (and the more distantly related they are the more likely their hybrid offspring are to be infertile)
  • The more closely realted two are, the more fertile their hybrid offspring are likely to be (and the more distantly related they are the less fertile their hybrid offspring are likely to be)

Of course in nature things aren't always simple, so it's likely that hybrids of some closely related species won't be fertile and it might be that some hybrids of more distantly related species are fertile. It would be good to hear from you if you know of cases that fall outside of expectations.


Finding evidence of hybrid fertility

In captivity it might be possible to do more, but with observations of birds in the wild we can only rely on collecting data from hybrid pairings where the hybrids are clearly different from either parent species. If you see a lot of hybrids that look completely different from their parent species but consistent with one another then they're probably all first-generation hybrids. If you see most hybrids are consistent but a minority look much closer to one or other parent species, it may be reasonable to conclude these are second-generation backcrossed hybrids.

For more variable hybrids, or hybrids that are not very dissimilar from one or both parent species it can be much harder to detect backcrosses and thereby prove that the hybrid is fertile. Sometimes, as with the Dark-bellied Brent Goose x Black Brant hybrid's family shown above, their behaviour might give it away even when the birds' appearance does not.

Joern Lehmhus has also pointed out another factor that may influence our perception of fertillity. He points out that the display of most Anas species has a series of several elements which are varied between the species. Several species may even lack some elements. These behavioral traits are inherited (this is shown in a study of Mallard x Northern Pintail hybrids in Behaviour 27: 259-272 (Sharpe & Johnsgard, 1966). Therefore a hybrid does often not show the same combination as either of the parent species. If a male F1 hybrid is displaying to a female of one of the parents (or a male of one parent species displays to an F1 hybrid female) some elements will be 'wrong' and may reduce the success of reproduction in a hybrid, even if the bird is as healthy and viable as the parent species.

Viability in hybrids (the ability for a hybrid to maintain life) is much harder to gather information about. At least we can say that those hybrids that have been recorded are, or can be, viable. But we cannot say that those hybrids we do not observe are not viable - it may simply be that they are rare.

  • In his "Handbook of Avian Hybrids of the World" (Oxford University Press 2006), Eugene M McCarthy's small section on viability/inviability includes a reference to a brood of European Greenfinch x Yellowhammer eggs. Three eggs were sterile, one died in its shell and the fifth survived to maturity. McCarthy admits that such data is of limited value for predicting viability but its inclusion presumably infers some relevance. I would take this as anecdotal evidence that supports the idea that Greenfinch x Yellowhammer hybrids may have low viability, but of course much more data is required before conclusions can be drawn. There's also a report of a European Goldfinch x Yellowhammer pairing - hybrids reportedly started developing in the eggs but died before hatching. Both of these examples were published in Cage Birds journal in the early 50s - we're not quite sure how reliable they are.
  • Also in McCarthy's book there is a comment about Mandarin hybrids. Many breeders and researchers have tried and failed to breed Mandarins with other species without any success. It has been suggested that because Mandarins have a different chromosome count to other ducks they cannot hybridise (the implication being that those few Mandarin hybrids that have been reported are erroneously identified) but McCarthy points to frequent hybrids between different species of Muntjac deer which have different chromosome counts and suggests that a better explanation might be that Mandarin does hybridise but only rarely. I wonder if it might even be that they do hybridise but have poor viability. McCarthy cites a case of two captive Mandarin x Laysan Duck hybrids - both lacked eyes on hatching with one surviving at first but dying in juvenile plumage.
  • When a Swoose (swan sp. x goose sp. hybrid) was discovered by the Radipole Ringing Group their research found that such hybrids do not normally survive past fledging (Radipole Ringing Group Report 2007). They didn't cite any references so I'm not sure of the source of that information, but if it was based on good evidence then that would suggest that swan x goose hybrids are not normally very viable. Exceptions exist though, as the bird they were reporting on was discovered as a cygnet/gosling in 2002 and was still surviving when I saw it in 2010 and thereafter until at least December 2011.

I would be willing to speculate that hybrids of distantly related species are less likely to be viable than hybrids of closely related species.

Let us know if you have any information or experience that might help us build a clearer picture.


Species mentioned:
Mute Swan Cygnus olor
domestic goose - in this case Anser cygnoides x Anser anser
Greylag Goose Anser anser
Canada Goose Branta canadensis
Dark-bellied Brent Goose Branta (bernicla) bernicla
Black Brant Branta (bernicla) orientalis (formerly nigricans) or Branta (nigricans) orientalis
Muscovy Duck Cairina moschata
Mandarin Aix galericulata
Mallard Anas platyrhynchos
Laysan Duck Anas laysanensis
Northern Pintail Anas acuta
Western Gull Larus occidentalis
Glaucous-winged Gull Larus glaucescens
Yellowhammer Emberiza citrinella
European Greenfinch Chloris chloris (formerly Carduelis chloris)
European Goldfinch Carduelis carduelis


Species and the Ability to Reproduce

A species is a group of individual organisms that interbreed and produce fertile, viable offspring. According to this definition, one species is distinguished from another when, in nature, it is not possible for matings between individuals from each species to produce fertile offspring.

Members of the same species share both external and internal characteristics, which develop from their DNA. The closer relationship two organisms share, the more DNA they have in common, just like people and their families. People&rsquos DNA is likely to be more like their father or mother&rsquos DNA than their cousin or grandparent&rsquos DNA. Organisms of the same species have the highest level of DNA alignment and therefore share characteristics and behaviors that lead to successful reproduction.

Species&rsquo appearance can be misleading in suggesting an ability or inability to mate. For example, even though domestic dogs (Canis lupus familiaris) display phenotypic differences, such as size, build, and coat, most dogs can interbreed and produce viable puppies that can mature and sexually reproduce (Figure (PageIndex<1>)).

Figure (PageIndex<1>): The (a) poodle and (b) cocker spaniel can reproduce to produce a breed known as (c) the cockapoo. (credit a: modification of work by Sally Eller, Tom Reese credit b: modification of work by Jeremy McWilliams credit c: modification of work by Kathleen Conklin)

In other cases, individuals may appear similar although they are not members of the same species. For example, even though bald eagles (Haliaeetus leucocephalus) and African fish eagles (Haliaeetus vocifer) are both birds and eagles, each belongs to a separate species group (Figure (PageIndex<2>)). If humans were to artificially intervene and fertilize the egg of a bald eagle with the sperm of an African fish eagle and a chick did hatch, that offspring, called a hybrid (a cross between two species), would probably be infertile&mdashunable to successfully reproduce after it reached maturity. Different species may have different genes that are active in development therefore, it may not be possible to develop a viable offspring with two different sets of directions. Thus, even though hybridization may take place, the two species still remain separate.

Figure (PageIndex<2>): The (a) African fish eagle is similar in appearance to the (b) bald eagle, but the two birds are members of different species. (credit a: modification of work by Nigel Wedge credit b: modification of work by U.S. Fish and Wildlife Service)

Populations of species share a gene pool: a collection of all the variants of genes in the species. Again, the basis to any changes in a group or population of organisms must be genetic for this is the only way to share and pass on traits. When variations occur within a species, they can only be passed to the next generation along two main pathways: asexual reproduction or sexual reproduction. The change will be passed on asexually simply if the reproducing cell possesses the changed trait. For the changed trait to be passed on by sexual reproduction, a gamete, such as a sperm or egg cell, must possess the changed trait. In other words, sexually-reproducing organisms can experience several genetic changes in their body cells, but if these changes do not occur in a sperm or egg cell, the changed trait will never reach the next generation. Only heritable traits can evolve. Therefore, reproduction plays a paramount role for genetic change to take root in a population or species. In short, organisms must be able to reproduce with each other to pass new traits to offspring.


HYBRIDS: If lions and tigers were intelligently designed, why are offspring of crosses between them infertile?

The original question was:
If the tiger and lion were intelligently designed, why are they able to interbreed, but create only sterile offspring? The same is true with horse and donkey, they produce sterile offspring. Why?

Answer by Diane Eager

We need to start by asking just why we think these animals should be able to mate and have fertile offspring anyway. The bottom line is that our classification system has put them in the same genus, so therefore we conclude they should be able to interbreed. The humbling factor is that whatever we label them does not necessarily make them part of the same original kind. Our classification system is simply a way of organising our knowledge of living things based on out observations of their structure, function and genetics today. We were not there to see the original kinds.

We do know that some creatures that are classified in the same genus, but labelled different species, can combine to produce fertile offspring, which is one indicator they were all part of and original kind. The best example of this is the Galapagos finches. See our report Finch Gene Flow here.

Furthermore, the world has degenerated since the original created kinds were made. It is possible some that some similar, but mismatched, species can mate and reproduce for one generation only because there has been loss of whatever mechanisms (genetic, cellular, behavioural, etc.) originally kept them separate.

Let’s look at the two examples given in the question.

Horses and Donkeys

The sterile offspring of the horse/donkey combination, i.e. a mule, can be explained due to a mismatch in the number of chromosomes. Horses have 64 chromosomes and donkeys have 62. A mule ends up with 63. To understand why this results in sterility we need to look at what happens to chromosomes when living things reproduce, and the importance of even numbers.

Chromosomes normally come in pairs – one member of the pair inherited from the mother and one from the father. Therefore, horses have 32 pairs, and donkeys 31 pairs. This is important when it comes to sexual reproduction, which involves the union of two cells, i.e. a sperm and an egg, to form a new individual. If the sperm or egg (sex cells) had the same number of chromosomes as any other cell in the body there would be a doubling of chromosome numbers with each generation. That wouldn’t work. Within a few generations there wouldn’t be room for them all. Therefore, during the formation of sex cells the number of chromosomes is halved by splitting up each of the pairs and giving one of each pair to the sex cells. Therefore, a sex cell from a horse will have 32 chromosomes and a donkey sex cell will have 31. When these are combined the total is 63 made of 31 pairs and 1 odd one. Because horse and donkey chromosomes are similar enough, the new combined cell is able to live and grow into a new individual, a mule. Growth involves making many more cells with the same number of chromosomes. To do this all the chromosomes are copied, and each new cell gets one whole set.

However, when the adult mule tries to makes sex cells, there is a problem. There pairs have to be split up. The normal process of making sex involves getting all the chromosomes to line up neatly in pairs in order to split off one of each pair to give to the sex cells. But for an animal with an odd number of chromosomes this can’t happen. There will always be an odd one out, so the process tends to stop there.

This problem will occur with all hybrids resulting from parents with mismatched numbers of chromosomes, not just horses and donkeys.

Lions and Tigers

Lions and tigers have the same number of chromosomes, 38, or 17 pairs. Therefore, an odd one out caused by unequal chromosome numbers is not the problem. In spite of this, hybrids resulting from a lion and tiger combination are usually sterile, so there must be a different reason.

Lions and tigers are both classified as felines, but even if they were part of an original Cat Kind, they have been separated for many generations of degeneration since their ancestors left the ark. Since then mutations have occurred in all animal populations, including lions and tigers, but not all mutations are the same in every population. This means that lions and tigers of today have accumulated different mutations. Not all mutations are lethal, they can simply result in variation in gene activity, or they can result in genes being moved out of their original place on the chromosome, which is how another problem can occur during the formation of sex cells. To see where this can happen we need to look at the process of copying the chromosomes during sex cell formation.

We will look at what happens to one pair of chromosomes, but it will happen with each pair. Remember each pair consists of one chromosome from the mother and one from the father. In the diagrams below they are coloured differently, but they are a matched pair.

The first step is to copy each chromosome of the pair. The copies remain attached to one another until the last step in the process, when they are separated and each new chromosome goes into the mix with the other new chromosomes to be incorporated into sex cells. The diagrams to the right and below show the process.

The double pairs then line up, and during this process, sections of chromosome are exchanged between the pairs, as shown in the diagram below. This process is known as “crossing over”, or recombination, and results in some come chromosomes containing a mix of genes from the two different parents. After recombination the attached copies, some with recombined sections, split up into separate chromosomes.

During this process it is important to get the chromosomes of each pair lined up very precisely, so that the pieces that are exchanged contain exactly the same genes, including the sections that regulate the activity of those genes, i.e. turn them on and off at appropriate times. This precise alignment may not be able to happen in the offspring of similar parents from two separate populations if mutations have resulted in displacement of genes or alteration in gene activity. The newly made composite chromosomes may be missing some genetic information, or it may have a combination of gene regulators that do not work well together. Genes do not act alone, they need to function in a coordinated way with other genes. Therefore, gene regulators, i.e. switches that turn genes on and off are just as important as genes that code for the structure and function of the body.

This recombining process happens for every chromosome pair. Therefore, it is almost inevitable to each newly formed sex cell will have at least one of the new re-combinations in its chromosome mix.

Lions, tigers, horses and donkeys were intelligently designed in the beginning, with no dysfunctional mutations. Since then all living things have been affected by the degeneration that came into the world as a result of man’s sin and God’s judgement. Therefore, it is possible that some kinds have lost some mechanisms that kept them separate to other kinds, and they can now reproduce, but give rise to dysfunctional offspring or some original kinds have been split into subpopulations that have been subject to different mutations that stop them from reproducing fertile offspring when they meet up again. Whatever the cause it is the result of degeneration in living things, not because there was anything wrong with the original designed Kinds.

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Why do some hybrids (Such as africanised honeybeees) produce fertile offspring, while most (like Ligers and mules) produce infertile offspring?

I recall one of my earliest understandings on the definition of a species is a distinct individual who can breed with members of its species to produce fertile offspring, but whose offspring with another species will either terminate or be infertile.

I've heard the term "Subspecies" before, but I'm not sure what it actually means.

Similarly, cross pollination seems common in plants - is that also within a single species?

Hybridization is statistically more successful at producing fertile offspring when species are closely related. If species are closely related, they share more genes, and have fewer incompatible genes. These incompatible genes can result in hybrid infertility. There can be various genetic mechanisms in which genes do not work well together and result in either infertility or inviability (inability to survive).

You are referring to the biological species concept, in which it is stated that a species is a population of potentially interbreeding individuals that produce fertile offspring. This is an attempt to categorize and explain biological phenomenon, but is not necessarily a strict rule that is always followed due to the complexity of nature. Thus, we have exceptions and nuances that include hybridization.

Subspecies describes a subpopulation within a species that has some unique feature that sets them apart from other population within the species. The article I linked above also has some information about subspecies. Subspecies still retain the ability to produce viable offspring with other populations of their overall species.

Cross pollination occurs between species, resulting in hybridization. In plants, hybridization is much, much more common than in animals.


Contents

Hybrid incompatibility occurs when the offspring of two closely related species are not viable or suffer from infertility. Charles Darwin posited that hybrid incompatibility is not a product of natural selection, stating that the phenomenon is an outcome of the hybridizing species diverging, rather than something that is directly acted upon by selective pressures. [4] The underlying causes of the incompatibility can be varied: earlier research focused on things like changes in ploidy in plants. More recent research has taken advantage of improved molecular techniques and has focused on the effects of genes and alleles in the hybrid and its parents.

Dobzhansky-Muller model Edit

The first major breakthrough in the genetic basis of hybrid incompatibility is the Dobzhansky-Muller model, a combination of findings by Theodosius Dobzhansky and Joseph Muller between 1937 and 1942. The model provides an explanation as to why a negative fitness effect like hybrid incompatibility is not selected against. By hypothesizing that the incompatibility arose from alterations at two or more loci, rather than one, the incompatible alleles are in one hybrid individual for the first time rather than throughout the population - thus, hybrids that are infertile can develop while the parent populations remain viable. The negative fitness effects of infertility are not present in the original population. [5] [6] In this way, hybrid infertility contributes in some part to speciation by ensuring that gene flow between diverging species remains limited. Further analysis of the issue has supported this model, although it does not include conspecific genic interactions, a potential factor that more recent research has begun to look in to. [4]

Gene identification Edit

Decades after the research of Dobzhansky and Muller, the specifics of hybrid incompatibility were explored by Jerry Coyne and H. Allen Orr. Using introgression techniques to analyze the fertility in Drosophila hybrid and non-hybrid offspring, specific genes that contribute to sterility were identified a study by Chung-I Wu which expanded on Coyne and Orr's work found that the hybrids of two Drosophila species were made sterile by the interaction of around 100 genes. [7] These studies widened the scope of the Dobzhansky-Muller model, who thought it likely that more than two genes would be responsible. [5] [6] The ubiquity of Drosophila as a model organism has allowed many of the sterility genes to be sequenced in the years since Wu's study.

With modern molecular techniques, researchers have been able to more accurately identify the underlying genetic causes of hybrid incompatibility. This has led to both the development of expansions to the Dobzhansky-Muller model. Recent research has also explored the possibility of external influences on sterility as well.

The "snowball effect" Edit

An extension of the Dobzhansky-Muller model is the "snowball effect" an accumulation of incompatible loci due to increased species divergence. Since the model posits that sterility is due to negative allelic interaction between the hybridizing species, as species become more diverged it follows that more negative interactions should develop. The snowball effect states that the number of these incompatibilities will increase exponentially over the time of divergence, particularly when more than two loci contribute to the incompatibility. This concept has been exhibited in tests with the flowering plant genus Solanum, with the findings supporting the genetic underpinnings of Dobzhansky-Muller:

"Overall, our results indicate that the accumulation of sterility loci follows a different trajectory from the accumulation of loci for other quantitative species differences, consistent with the unique genetic basis expected to underpin species reproductive isolating barriers. . In doing so, we uncover direct empirical support for the Dobzhansky-Muller model of hybrid incompatibility, and the snowball prediction in particular." [8]

Environmental influences Edit

Though the primary causes of hybrid incompatibility appear to be genetic, external factors may play a role as well. Studies focused primarily on model plants have found that the viability of hybrids can be dependent on environmental influence. Several studies on rice and Arabidopsis species identify temperature as an important factor in hybrid viability generally, low temperatures seem to cause negative hybrid symptoms to be expressed while high temperatures suppress them, although one rice study found the opposite to be true. [9] [10] [11] There has also been evidence in an Arabidopsis species that in poor environmental conditions (in this case, high temperatures), hybrids did not express negative symptoms and are viable with other populations. When environmental conditions return to normal, however, the negative symptoms are expressed and the hybrids are once again incompatible with other populations. [12]

Lynch-Force model Edit

Though a multitude of evidence supports the Dobzhansky-Muller model of hybrid sterility and speciation, this does not rule out the possibility that other situations besides the inviable combination of benign genes can lead to hybrid incompatibility. One such situation is incompatibility by way of gene duplication, or the Lynch and Force model (put forth by Michael Lynch and Allan Force in 2000). When gene duplication occurs, there is a possibility that a redundant gene can be rendered non-functional over time by mutations. From Lynch and Force's paper:

"The divergent resolution of genomic redundancies, such that one population loses function from one copy while the second population loses function from a second copy at a different chromosomal location, leads to chromosomal repatterning such that gametes produced by hybrid individuals can be completely lacking in functional genes for a duplicate pair." [12]

This hypothesis is relatively recent compared to Dobzhansky-Muller, but has support as well.

Epigenetic influences Edit

A possible contributor to hybrid incompatibility that fits with the Lynch and Force model better than the Dobzhansky-Muller model is epigenetic inheritance. Epigenetics broadly refers to heritable elements that affect offspring phenotype without adjusting the DNA sequence of the offspring. When a particular allele has been epigenetically modified, it is referred to as an epiallele A study found that an Arabidopsis gene is not expressed because it is a silent epiallele, and when this epiallele is inherited by hybrids in combination with a mutant gene at the same locus, the hybrid is inviable. [1] This fits with the Lynch and Force model because the heritable epiallele, ordinarily not an issue in non-hybrid populations with non-epiallele copies of the gene, becomes problematic when it is the only copy of the gene in the hybrid population. [1]


Varying Rates of Speciation

Two patterns are currently observed in the rates of speciation: gradual speciation and punctuated equilibrium.

Learning Objectives

Explain how the interaction of an organism’s population size in association with environmental changes can lead to different rates of speciation

Key Takeaways

Key Points

  • In the gradual speciation model, species diverge slowly over time in small steps while in the punctuated equilibrium model, a new species diverges rapidly from the parent species.
  • The two key influencing factors on the change in speciation rate are the environmental conditions and the population size.
  • Gradual speciation is most likely to occur in large populations that live in a stable environment, while the punctuation equilibrium model is more likely to occur in a small population with rapid environmental change.

Key Terms

  • punctuated equilibrium: a theory of evolution holding that evolutionary change tends to be characterized by long periods of stability, with infrequent episodes of very fast development
  • gradualism: in evolutionary biology, belief that evolution proceeds at a steady pace, without the sudden development of new species or biological features from one generation to the next

Varying Rates of Speciation

Scientists around the world study speciation, documenting observations both of living organisms and those found in the fossil record. As their ideas take shape and as research reveals new details about how life evolves, they develop models to help explain rates of speciation. In terms of how quickly speciation occurs, two patterns are currently observed: the gradual speciation model and the punctuated equilibrium model.

In the gradual speciation model, species diverge gradually over time in small steps. In the punctuated equilibrium model, a new species changes quickly from the parent species and then remains largely unchanged for long periods of time afterward. This early change model is called punctuated equilibrium, because it begins with a punctuated or periodic change and then remains in balance afterward. While punctuated equilibrium suggests a faster tempo, it does not necessarily exclude gradualism.

Graduated Speciation vs Punctuated Equilibrium: In (a) gradual speciation, species diverge at a slow, steady pace as traits change incrementally. In (b) punctuated equilibrium, species diverge quickly and then remain unchanged for long periods of time.

The primary influencing factor on changes in speciation rate is environmental conditions. Under some conditions, selection occurs quickly or radically. Consider a species of snails that had been living with the same basic form for many thousands of years. Layers of their fossils would appear similar for a long time. When a change in the environment takes place, such as a drop in the water level, a small number of organisms are separated from the rest in a brief period of time, essentially forming one large and one tiny population. The tiny population faces new environmental conditions. Because its gene pool quickly became so small, any variation that surfaces and that aids in surviving the new conditions becomes the predominant form.


Variety Within a Kind

Creation scientists use the word baramin to refer to created kinds (Hebrew: bara = created, min = kind). Because none of the original ancestors survive today, creationists have been trying to figure out what descendants belong to each baramin in their varied forms. Baramin is commonly believed to be at the level of family and possibly order for some plants/animals (according to the common classification scheme of kingdom, phylum, class, order, family, genus, species). On rare occasions a kind may be equivalent to the genus or species levels.

Baraminology is a field of study which attempts to classify fossil and living organisms into baramins. This is done based on many criteria, such as physical characteristics and DNA sequences. For living organisms, hybridization is a key criterion. If two animals can produce a hybrid, then they are considered to be of the same kind.1 However, the inability to produce offspring does not necessarily rule out that the animals are of the same kind, since this may be the result of mutations (since the Fall).

Zonkeys (from a male zebra bred with a female donkey), zorses (male zebra and female horse), and hebras (male horse and female zebra) are all examples of hybrid animals. Hybrid animals are the result of the mating of two animals of the same “kind.” Perhaps one of the most popular hybrids of the past has been the mule, the mating of a horse and donkey. So, seeing something like a zorse or zonkey shouldn’t really surprise anyone, since donkeys, zebras, and horses all belong to the horse kind.

The concept of kind is important for understanding how Noah fit all the animals on the Ark. If kind is at the level of family/order, there would have been plenty of room on the Ark to take two of every kind and seven of some. For example, even though many different dinosaurs have been identified, creation scientists think there are only about 50 “kinds” of dinosaurs . Even though breeding studies are impossible with dinosaurs, by studying fossils one can ascertain that there was likely one Ceratopsian kind with variation in that kind and so on.

After the Flood, the animals were told to “be fruitful and multiply on the earth” ( Genesis 8:17 ). As they did this, natural selection, mutations, and other mechanisms allowed speciation within the kinds to occur. Speciation was necessary for the animals to survive in a very different post-Flood world. This is especially well illustrated in the dog kind in which current members (e.g., coyotes, dingoes, and domestic dogs) are confirmed to be descended from an ancestral type of wolf.2

Hybrid animals are usually the result of parent animals that have similar chromosome numbers. Many times the hybrids are infertile due to an uneven chromosome number that affects the production of eggs and sperm. However, this is not always the case, as even some mules (horse + donkey) have been known to reproduce. Consider some of these amazing animal hybrids:

Zonkey, Zorses, and Mules

These hybrids are the result of mating within the family Equidae. As we’ve said before, zonkeys are the result of mating a male zebra and a female donkey zorses are the result of mating a male zebra and a female horse and mules are the result of mating a male donkey and a female horse. But reverse matings (such as hinnies produced from a male horse and female donkey) are rare, although still possible. All are considered “infertile” due to uneven chromosome numbers, but fertility has been observed in some cases. Zonkeys and zorses have a mixture of their parents’ traits, including the beautiful striping patterns of the zebra parents.

Ligers, Tigons, and Other Cats

These hybrids are the result of mating within the family Felidae. Ligers are the result of mating a male lion and a female tiger. Ligers are the largest cats in the world, weighing in at over 1000 lbs (450 kg). Tigons are the result of mating a female lion and a male tiger. These matings only occur in captivity, since lions live in Africa, tigers live in Asia, and the two are enemies in the wild. Female hybrids are typically fertile while male hybrids are not.

Other hybrids in this family include bobcats that mate with domestic cats and bobcats with lynx (Blynx and Lynxcat). There have been mixes of the cougar and the ocelot, as well as many others. This shows that large, midsize, and small cats can ultimately interbreed, and therefore, suggests that there is only one cat kind.

Wolphin

Turning to the ocean, this hybrid is the result of mating within the family Delphinidae. The wolphin is the result of mating a false killer whale (genus Pseudorca) and bottlenose dolphin (genus Tursiops). Such a mating occurred in captivity at Hawaii’s Sea Life Park in 1985.3 The wolphin is fertile. This hybrid shows the difficulty of determining the species designation, since a major criterion is the ability to interbreed and produce fertile offspring. Even though the whale and dolphin are considered separate genera, they may, in fact, belong to the same species. This shows how difficult it is to define the term species. Of course from a biblical perspective it is easy to say they are both the same kind!


Why can’t hybrid animals breed?

You may have heard of the tales of chimera, a fire-breathing lion with a tail ending with a snake’s head and the head of a goat emerging from its back. Even though this monstrous hybrid creature only exists in ancient Greek myths, hybrid animals do exist in our real world and some of them play an important role in our lives.

Chimera. Photo credit: Alexandra Korey via Flickr

Hybrid animals are the results of interspecies mating. To name a few, liger (the offspring of a male lion and a female tiger), beefalo (the offspring of a domestic cattle and an American bison) and mule (the offspring of a male donkey and a female horse). However, animals from different species rarely mate unless there is environmental stress or human interventions.

While many hybrid animals are kept in zoos, some of them especially mules have played an important role in mankind history. Historically, mules were widely used for transportation, agriculture and even fought alongside British soldiers during the Second Anglo-Afghan War. In modern days, although not as widely as in the past, they are still used in Central Asia and South America for agricultural purposes.

A mule transporting luggage. Photo credit: Jeroen Mirck via Flickr

All of those contributions of mules to mankind history are made possible due to their inherited patience, strength and long life-span from donkeys, and intelligence and speed from horses.

Although hybrid animals might look very cool and have many advantages over their parents, it is extremely rare for them to have babies.

Why can’t hybrid animals have babies?

To understand the infertility in hybrid animals, we must go deep and look into the world of chromosome. Chromosomes are thread-like structures consist of DNA and protein. They carry the genetic information that determines the body plan of animals. Chromosomes are arranged in pairs, one set from father, and one set from mother.

In order to have babies, animals need to produce sex cells. The production of sex cells requires paired chromosomes to exchange genetic information, so that the chromosomes from the father will carry some of the genetic information from the mother, and vice versa. For example, a blonde hair gene from the father exchanges for a black hair gene from the mother. This exchange process is called genetic recombination.

Genetic recombination. Image credit: yourgenome via Flickr

Genetic recombination is the process that goes wrong in hybrid animals and causes their infertility. In normal animals, because their father and mother are from the same species, the genetic information that are exchanged from their mother/father to their father/mother can still be processed. However, in hybrid animals, things could get really nasty. Because hybrid animals have parents from different species, the exchange of genetic information can cause many malfunctions in the chromosomes. This can result in the production of infertile sex cells and infertility.

There is an exception to every rule!

In ancient Rome, when “impossible” things happened, people often describe them with the saying “when a mule foals”. However, “impossible” things sometimes do happen.

For some unknown reasons, there are incidences that some mules have skipped the process of genetic recombination and produced fertile sex cells. Although we cannot explain those incidences now, we can definitely say to the ancient Romans that sometimes mules do have babies.


Why are hybrids infertile? - Biology

WHY BREED HYBRIDS?
HOW NEW SPECIES ARE CREATED
COMPLEX HYBRIDS

Hybrid big cats are artificial creations. They are unlikely to occur in the wild except in unnatural situations e.g. in very isolated populations where there is no mate of the appropriate species available. Because of the fertility issues, valuable genes may be lost by breeding dissimilar species together. Most conservationists condemn deliberate hybridization as wasteful in terms of genes and in terms of money. So why are they bred?

Many are bred out of curiosity. Exotic animals, especially ligers (the largest big cats on the planet), are great crowd-pullers. Pony-sized striped big cats and leopard-patterned lions are undeniably magnificent creatures. Others occur by accident where two animals are housed together from an early age in the belief that they won't mate with each other. The mating instinct is strong enough that a puma allowed herself to be mated by an ocelot one third of her size! This occurs where there is limited accommodation e.g. private collections, travelling circuses etc. Even experienced zoos have accidentally bred hybrids this way e.g. the servical. Believing that hybrids are always sterile, some keepers have housed a hybrid big cat with pure-bred big cats only to discover that hybrid females are fertile.

Private menageries also breed hybrids, sometimes as exotic pets. Some are bred to bypass restrictions on ownership of purebred big cats. Loopholes in some legal systems means that hybrids are not subject to the same legal restrictions on ownership or transportation as pure-bred tigers or lions! Many privately-owned curiosities end up at rescue centres when they grow too large, become to expensive to keep or prove to be temperamental. There is also an element of salacious - as well as genuine scientific - interest in the act of inter-species copulation.

There is a limited amount of hybridization for scientific reasons. This may be for research into how physical or behavioural traits are inherited or to discover how closely two species are related. The ability of pumas to produce offspring with ocelots (South American cat) and also with leopards (African cat) helps scientists to work out the taxonomy of pumas i.e. how closely they are related to other cat species. More worryingly, big cats have been hybridized in an attempt to create domestic big cats.

Hybrids do not generally give rise to new species. Because hybrid males are mostly infertile, female hybrids are mated back to pure-bred animals. In only a few generations, the "alien genes" are absorbed into the gene pool of the species she is bred back to. Theoretically, a new sub-species may arise if the population is isolated, but they will only have subtle differences such as lions retaining spots into adulthood as a result of a few lurking leopard genes in the gene pool from a leopon many generations back.

Speciation (one species evolving into two) is usually an excruciatingly slow process. Different species usually cannot mate and reproduce (reproductive isolation). If the species are closely related, such as certain cat species, they can produce hybrids, but those hybrids have reduced fertility. The more easily two species form hybrids, the more closely they are related in evolutionary terms. One way reproductive isolation occurs is genetic mutation. One group of animals might be geographically isolated from others of the same species. Each group accumulates slightly different mutations over many generations - some genes affect appearance, others affect behaviour. Many generations later, the two groups have diverged and are different enough that even if they can mate, they can't produce fully fertile offspring.

Sometimes, one species splits into two through behavioural isolation. Some individuals develop behaviour patterns which limit their choice of mates e.g. they might be attracted to certain colours or might be active at different times of day. Though they are fully capable of interbreeding with the other group, their different behaviours keep them apart. If their habitat changed, behavioural barriers might break down and allow interbreeding the hybrids might become new species.

Another way reproductive isolation occurs is when fragments of DNA accidentally jump from one chromosome to another in an individual (chromosomal translocation) The mutant individuals can only reproduce with other mutant individuals - not much good unless the individual has mutant siblings to mate with! There are also "master genes" which govern general body plan (Hox genes) and those which switch other genes on and off. A small mutation to a master gene can mean a sudden big change to the individuals that inherit that mutation. Sometimes, those radical mutations can "undo" generations of divergent evolution so that two unrelated species can mate with each other and produce fertile young (so far, this has only been seen in micro-organisms).

Hybridisation is frequently a dead end because the hybrids are not fully fertile. If the hybrids are fertile, they are usually absorbed back into the population of one or other parent species and most of the alien genes are bred out. More rarely, hybrids can become new species or new sub-species. In the hands of breeders, some domestic/wildcat hybrids can become breeds these are not new species because the wildcat genes are largely bred out by crossing with domestic cats, until only the wildcat pattern remains.

Although big cat species rarely, if ever, form hybrids under natural conditions, in other species, hybridisation might possibly play a larger role in evolutionary biology than previously believed. Most hybrids face handicaps as a result of genetic incompatibility, but the fittest survive, regardless of species boundaries. Life may be a genetic continuum rather than a series of self-contained species.

In wild sunflowers, hybridisation causes an explosion of genetic variation some hybrids become new species capable of exploiting new ecological niches. In this case, hybridisation may be more important than genetic mutations in causing rapid, widespread evolutionary transitions because hybridisation creates variations in many genes or gene combinations simultaneously. Laboratory hybrids of annual sunflowers were back-crossed over one or more generations to one of the parent species. Enough "alien" genes were retained in later generations to allow them to thrive in conditions where neither parent species could live. Computer simulations suggest that the successful hybrids could evolve into new species within 50 to 60 generations. Similarly, genes from GM crops will inevitably leak into the wild gene pool.

In Heliconius butterflies genes have leaked from one species into another through hybridisation. Heliconius hybrids are relatively common and are a long way from the biology textbook stereotype of a sterile and deformed hybrid. These hybrids can successfully breed with either parental species or with other hybrids. However, there is natural selection against hybrids. Pure-bred Heliconius butterflies have warning colouration recognised by predators. The hybrids, equally unpalatable, have an intermediate pattern which is not recognised - the predators have not yet adapted and so the hybrids are disadvantaged.

Natural hybrids are found among butterflies, birds and fish. Blue whales will hybridise with fin whales. Interspecies matings have been witnessed in dolphins. Wolves, coyotes and dogs all produce fertile hybrids, so much so that some wild canids are becoming increasingly mongrelised. Usually, where there are two closely related species living in the same area, less than 1 in 1000 individuals will be hybrids because animals rarely choose a mate from a different species (unless mates from their own species are in short supply, the reason some endangered species are further threatened by hybridisation).

So why don't genetic leaks cause species boundaries to break down altogether? One, seen in butterflies, is because predators may not recognise the hybrids as inedible. Another is because hybrids cannot compete against the parent species for resources. Healthy hybrids between Darwin's Galapagos finches are relatively common, but their beaks are intermediate in shape and less efficient feeding tools than the beaks of the parental species so they lose out in the competition for food. In a 1983 storm, changes to the local habitat meant new types of plant began to flourish and some hybrids had an advantage over the birds with specialised beaks.

The hybridisation of the native European white-headed duck and the introduced American ruddy duck means that pure white-headed ducks are being hybridised into extinction. While humans want to protect the white-headed duck evolution wants to utilise the ruddy duck genes. Once a species is introduced into a new habitat and the process starts, any Endangered Species legislation is trying to work against the inexorable and far more ancient forces of nature.

In 2011, it was reported that hybridisation between different species of mice resulted in strains of mice resistant to rodenticide poisons. German and Spanish mice have rapidly evolved the poison-resistant trait by breeding with an Algerian species. The European and Algerian mice separated as species between 1.5 and 3 million years ago. Fast-track evolution through hybridisation is uncommon in mammals because many of the hybrid offspring are sterile. Mice resistant to Warfarin (which causes fatal bleeding) have evolved naturally - and slowly - in parts of the world, but the German and Spanish mice developed resistance rapidly by cross-breeding to the Warfarin-resistant Algerian mice.

While male mammalian hybrids are generally sterile, the females may be fertile and able to breed with either parental species. Some of those fertile female hybrid mice evidently bred with local European mice, passing the Warfarin-resistance gene into the gene pool. As result, most of the mice in Spain are now Warfarin-resistant and the number of resistant mice in germany is growing. It is due to human travel that the Algerian mice came into contact with the European species. Isolated from their own species, but with a biological urge to mate, they would have bred with the European mice instead. Thanks to their genetic advantage over Warfarin-susceptible mice, the strain resistant mice are likely to spread throughout Europe.

Also in 2011, it was shown that hybridisation between the House Sparrow and the Spanish sparrow had given rise to a new and separate species known as the Italian Sparrow. DNA studies have found that the species originated as a hybrid, but it no longer breeds with either of the parental species with which it shares habitat, therefore meeting one definition of "species". Italian Sparrows and Spanish Sparros live side by side, but appear to have a reproductive barrier that prevented the hybrids being absorbed into the Spanish Sparrow population. Scientists believe this shows that the crossing of two species to form a new species might be more common in nature than previously realised.

If a fertile female tigon or liger offspring were mated back to the lion, the percentage of lions genes in the offspring increases and the percentage of tiger genes decreases. Assuming that the offspring at each generation were fertile females, the tiger genes would eventually be swamped by continually back-crossing to a lion. The end result (after several generations) would show no visible signs of tiger ancestry. The same would happen if the tigon or liger was backcrossed to a tiger and the fertile female offspring of each generation were backcrossed to a tiger etc. After several generations of backcrossing to tigers, the percentage of lion in the offspring would decrease to such a point that the offspring would appear to be wholly tiger. After several generations of backcrossing to one parental species, the offspring become indistinguishable from that species as the effect of the alien genes is swamped out. This applies to the nuclear DNA - the DNA which transmits physical and physiological traits.

There is a second type of DNA which is found only in the mitochondria and which is inherited only from the mother. In simple terms, mitochondria are the energy factories inside cells. They are present in the egg and sperm cells, but when fertilization occurs the sperm mitochondria are left outside the fertilized egg. If a female liger and her female offspring were repeatedly backcrossed to a lion (as detailed above), the nuclear DNA becomes almost all of lion origin. However, the mitochondrial DNA come from the original tiger mother. In later generations, the male hybrids become genetically close enough to being all lion to be fertile and can breed with lionesses. Only at that point is the original tiger mitochondrial DNA lost. The same happens when tigons are backcrossed to tigers over many generations. The mitochondrial DNA is lion DNA until such time as fertile males are produced and breed with tigresses who pass on tiger mitochondrial DNA .

Where there is suspicion of hybridisation in the past, it is possible to test the mitochondrial DNA. Unless something prevented all of the later generation female hybrids breeding (so that they couldn't pass on their mitochondrial DNA), there will be traces of the other species mitochondria DNA in what appear to be pure-bred lions or pure-bred tigers.

EXAMPLE: BACKCROSSING LION TO TIGER

A female liger is 50% lion and 50% tiger. This is backcrossed to a purebred male tiger. At each generation, the female offspring is backcrossed to a purebred male tiger. The percentage of tiger genes goes up in each generation until they reach 99% at which point it could be considered purebred. It will never quite reach a round total of 100%.

The lion and tiger genes won't be inherited in neat 50/50 splits as the genes are shuffled about (though over several individuals it averages at 50%), so breeders talk of "pure-bloodedness" instead. How close is each generation to being a pureblooded tiger? The arithmetic here is rounded up to one or two decimal places.

F1 cross: 50%
F2 backcross: 75%
F3 backcross: 87.5%
F4 backcross 93.75%.

Once the hybrid is 90% one species or the other, the male hybrids are likely to be fertile (based on information from Bengal and Savannah cat breeders).

Each successive backcross after that gives:

In captivity, breeders are able to control the relative percentages of genes and maintain them at a stable level. In wild/domestic cat hybrids this is by selecting for looks, but the degree of wild genes is limited because temperament is also a factor and breeders are breeding domestic pets, not recreating a wild species. In the wild, the introduced genes will be swamped out by genes from the species present in the greatest numbers unless the hybrids slectively mated only among themselves (assuming perfect fertility), which is unlikely. The infertility of the first several generations means the alien genes are already diluted since the cats mate indiscriminately it would take human intervention and intense selection to create cats closer to the wild type than to the domestic type.

Question "Can you get really complex hybrids? I mean lion x tiger x leopard x panther? What would it look like?"

Because the female hybrids are often fertile, it is theoretically possible to create a very complex hybrid. It would depend on the females of each generation being fertile - conceivably, there could be a point where there are so many different or incompatible genes in the mix that the offspring are no longer fertile.

The most complex hybrid so far was a lijagulep (li-jagleop). First a jaguar and a leopard were crossed. The female offspring, a jagulep (jagleop) was crossed to a lion to produce the lijagulep. This was 50% lion and (roughly speaking because the genes might have been passed on unevenly) 25% leopard + 25% jaguar. It therefore looked more like a lion than like a leopard or a jaguar.

If the lijagulep had been female it might have been fertile. If so, it be crossed to a tiger. Lion x tiger matings produce offspring, but tiger x leopard matings have been unsuccessful in captivity. If a female lijagulep was fertile and produced offspring when mated to a tiger, it would result in a ti-lijagulep. A ti-lijagulep would be 50% tiger, 25% lion, 12.5% leopard and 12.5% jaguar. No-one has ever bred one, but based on the percentages of genes it would probably look like a ti-tigon since it contains more tiger and lion genes than leopard or jaguar genes and it has twice as many tiger genes as lion genes.

You could cross the ti-lijagulep back to a leopard to get 56.25% leopard, 25% tiger, 12.5% lion, 6.25% jaguar (56.25% = 50% leopard + 6.25% leopard from the earlier mating). In fact, as long as the female offspring are fertile, you could go on like this forever. There are dozens of permutations, but as a rule as the percentage of genes from one species decreases, the offspring looks less and less like that species and will most closely resemble the parent whose genes make up the greatest percentage. Other genes will be more and more dilute - too dilute to show up visually.

WHAT IS A SPECIES? HOW IS "SPECIES" DEFINED?

There are 7 main concepts of "species", and over 20 variations based on combinations of those 7 concepts. If you put n (any number) of biologists in a room and ask them to define "what is a species?", you'll end up with a minimum of n+1 definitions and quite a few frayed tempers. The main 7 are:

  • Agamospecies (microspecies) - isolation maintained through reproducing asexually (no gene mixing) so each generation is identical to its parent
  • Biospecies (reproductively isolated sexual species) - species that are incompatible enough they cannot interbreed
  • Ecospecies (ecological niche occupiers) - highly specialised for their own niche
  • Evolutionary species (evospecies, Darwinian species) - evolving lineages
  • Genetic species - defined by sharing a common gene pool or a common genetic fingerprint with only small variations e.g. colour/pattern variations
  • Morphospecies - species that are defined by their form, but may turn out to comprise two or more identical-looking sister species
  • Taxonomic species - whatever a taxonomist calls a species depending on whether he's a "lumper" or a "splitter"

Mixes of these definitions have given biologists lots more definitions:

  • Cohesion species - a mix of mate-recognition and isolation mechanisms maintain the population as distinct
  • Cryptic species - a "hidden species" which is visually indistinguishable, but reproductively isolated from sister species
  • Demographic species - based on range, behaviour and appearance
  • Isolation species - similar to biospecies, maintained as distinct through some form of isolation
  • Mate-recognition species - based on individuals recognising each other as potential mates
  • Phylogenetic (cladistic) species - based on appearance, similarity/relatedness to other species and having common ancestors
  • Phenotypic (phenetic) species - based on having same appearance
  • Polyphaisc - looks at several lines of evidence to determine whether a population is a species
  • Recognition species - based on physical and behavioural compatibility i.e. the individuals recognise each other as same-species
  • Reproductive species - same as biospecies
  • Ring species - neighbouring populations can interbreed along a geographical corridor, but the populations at each end of the ring do not interbreed even if they overlap each other in a ring)
  • Typological species - based on fixed properties including appearance and developmental stages
  • Wildlife species (Evolutionarily Significant Unit) - the conservationist definition based on a mix of species definitions to decide what is significant enough to be conserved

Several concepts of species are based on physical appearance. but what if there is variation within a species? Dogs are all the same species, but vary dramatically in appearance. The Great Dane and Chihuahua are anatomically unsuited to mating so should they be considered separate species? On the other hand, some bird populations visually appear to belong to one species, but are genetically two or more species (cryptic, or hidden, species) that don't interbreed.

The genetic and biospecies concepts rely on the idea that species cannot interbreed, or that if they do interbreed (usually in artificial conditions) any offspring will be non-viable or infertile. Grizzly Bears and Polar Bears are considered different species, but they interbreed in the wild and the hybrid offspring are fertile.

An evolutionary species doesn't so much define what a species is, but how a species arises e.g. through some form of isolation so that certain traits/mutations become fixed in a population. Since traits are associated with genes, this means a common gene pool based on whatever the foundation stock had.

The phylogenetic species concept (the Linnaean system giving us the 2 part species name e.g. Panthera leo) is a mix of species concepts such as morphospecies, biospecies or evospecies - it looks at the traits of a creature and tries to work out what it is related to. If it looks like a Lion and lives in Asia instead of Africa, let's call it the Asiatic Lion and assume it's related to the African Lion. It doesn't always work, because convergent evolution can produce very similar-looking animals from different lineages and then it's up to genetics to help us out.

Agamospecies do not reproduce through sex. Many are likely to be ecological niche adaptations, though some might be random genetic mutation. They are rare - even bacteria sometimes exchange genes with each other and some parthenogenic creatures switch between sexual and asexual reproduction depending on environmental conditions. True sexual reproduction requires the organisms to have compatible anatomy and genetic compatibility. A lion and a leopard have compatible anatomy, but limited genetic compatibility resulting in their hybrid male offspring being sterile. Other species may be genetically compatible, but their anatomy prevents them exchanging genes with each other. The more closely related the 2 species are, the more likely they can successfully interbreed.

Lions and tigers separated from each other around 3.7 million years ago, but can interbreed giving fertile female hybrids and infertile male hybrids. Both lions and tigers co-exist in India's Gir forest region, but they don't interbreed. Tigers are solitary hunters that live in woodland or forest. Lions are co-operative, familial hunters that occupy grassland and open scrub. The only place they might meet is scrubland, but the females of each species have different oestrus cycles and different receptiveness signals that prevent interbreeding. There may be some anatomical differences in genitalia, but not enough to be a barrier otherwise zoos would not have ligers or tigons.

Some supposedly distinct species have hybrid zones where 2 closely related species interbreed and where their fertile hybrids form a distinct population that can be recognised as a species in its own right! For example Gyrfalcons and Saker Falcons interbreed and the hybrids are recognised as Altai Falcons. Populations of Altai Falcons breed among themselves and are recognised as a separate species.

Along a geographical corridor there may be a connected series of populations which hybridise with their closest neighbours, but the populations at each end of the corridor are too different, or too distantly related, to interbreed with each other. Even if the corridor forms a ring so that the end points overlap, they remain distinct from each other. The populations are like links on a chain - are they all one species or are they all separate species? How far apart do the populations have to be on the ring to be unable to interbreed? The classic example is that of Herring Gulls and Black-backed Gulls. These exist as distinct species in the British Isles, but are also end points on a ring of connected gull populations around the North Pole (ring species may have branches in other directions as well - more like a network than a single ring).

The bottom line seems to be that species are fluid rather than fixed, something abhorrent to humans who want to fit everything into neat pigeonholes. Evolution doesn't simply create branches that grow away from each other, it also merges branches or intertwines branches. In plants, hybridisation allows offspring to rapidly colonise new habitats that neither parent can thrive in - a ecological niche species is born. In grizzly and polar bears it's likely that they diverge when there are two distinct habitats, but converge into one species when the ice habitat is lost. Darwin's finches, cited as classic examples of speciation, show a pattern of divergence and convergence as their habitat goes through cyclical changes they diverge into various extremes when there are numerous distinct food sources and converge to a smaller number of more generalised forms when the habitat is less rich (but they retain the ability to diverge again later on).



Comments:

  1. Bodwyn

    I would like to talk to you, to me is what to tell.



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