Hybrids and Evolution

You want bizarre hybridisation? Get a load of the fishes. Gynogenesis and hybridogenesis are wonderfully wacky.There are two reasons for inability to hybridise, one is boring: stuff what is different don't work together none well. The second is rather cool: it is adaptive if you have overlapping populations with differing environmental circumstances (and thus selective pressures) to be unable and unwilling to mate. Fantastic examples of this come from the Cichlid fishes.

The exact reason why a particular hybrid is infertile varies, but is commonly down to failure of meiosis to function correctly. During one of the early stages of meiosis, all chromosomes need to be paired up so they can then be divided into the haploid daughter cells.Where there are a differing number of chromosomes in the two parents species the chromosomes obviously can't pair up; this means that the gametes produced do not contain a correct number of chromosomes and are thus not viable. This can also occur simply because the chromosomes present are sufficiently different. Hybridisation can occur because of the commonalities in mating systems, and because there is no selective pressure to prevent them except in cases of sympatric evolution. Related species from the same location are more likely than related species from different locations to be unable to hybridise and to behaviourally avoid hybridisation. Poeciliopsis monacha-lucida and Poecilia formosa are wonderfully interesting examples. P. monacha-lucida is a female only species that is produced by the crossing of a female P. monacha and a male P. lucida. It can mate with P. lucida to produce offspring, however the genetic material passed on by P. monacha-lucida only comes from P. monacha, the entire P. lucida half of the genome is simply discarded. This is called hybridogenesis, it has been observed in other species, such as some frogs (that link also has a helpful diagram). Poecilia formosa is also all female; it is formed from the hydridisation of P. latippina and P. mexicanuum. It reproduces by gynogenesis; it must mate with a male P. mexicanuum in order to reproduce but the offspring produced is a genetic clone. The sperm from the father has its genetic material entirely discarded. This is called gynogenesisI'll return to the other questions in another post.

I don't know the details of this for certain, but I will speculate a little.While the hybrids are fertile, they're less fertile than either normal domestic or Przewalski's horses. I would suggest this is probably because a significant number of the gametes produced by meiosis are not viable. Probably what's happening is that matching up can occur correctly with most of the chromosomes (because they are sufficently similar), and then providing that either the combined domestic chromosome and not the extra Przewalski, or both Prezwalski chromosomes and not the combined chromosome end up in one cell - and, of course, crossing over doesn't mess it up - then the resulting gamete is viable. This would happen in roughly 1 in 4 gametes, explaining the reduction in fertility.I suspect this has probably been investigated at some point though I'm not a biologist either... yet.

There is no clear criteria of the basis of relatedness for this. Especially once you leave the large mammals. Some very closely related species can't hybridise; some remarkably remote species can. There isn't a specific definition. It could mean several different things. The most meaningful from an evolutionary point of view is probably the number of generations seperating them, via the last common ancestor population (usually more easily - but less accurately - expressed as a time, often the time to the seperation), but you could also look at genetic similarity, phenotypic similarity, and so on. Common usage is pretty casual on what exactly you're talking about. It's very graduated, and isn't even a scale (see gynogenesis and hybridogenesis as in my earlier post). Quite often, even "fertile" hybrids are actually less fertile than pure bred examples from either parent strain. I can't see that it tells us much at all. I do not believe a humanzee is possible. Certainly there appears to be no evidence of one ever having actually occurred; despite experiments apparently being performed and documented cases of female chimps being used as prostitutes. From a biological point of view, it would seem unlikely that a chimp/human hybrid would be viable because of the huge differences in development timings and physical morphology; it seems quite probable that an embryo, even if created, would not reach term.

That would follow in animals, yes.