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Author | Topic: Hybrids and Evolution | |||||||||||||||||||||||||||||||||||||||
Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 8.4
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Why are hybrids such as mules and ligras infertile? 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.
Why are hybrids even biologically possible from an evolutionary point of view? 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.
What interesting examples of hybrids are known to exist or be possible? 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 gynogenesis I'll return to the other questions in another post.
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RAZD Member (Idle past 1426 days) Posts: 20714 From: the other end of the sidewalk Joined:
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Hi Bluejay.
Given all the subjectivity involved in biological definitions, I’m not sure we can actually say that hybridization really means all that much. Hybrid is, itself, a rather vague term, defined as the fusion of two other things that were previously classified as distinct from one another based on equally vague and subjective terms. I'm surprised that nobody has mentioned hybrids between varieties within a species, as this is well known and often used in breeding. Heterosis - Wikipedia
quote: Dogs are probably the best example of hybridization between varieties. The other place where we see hybridization being important is in hybrid zones between varieties of a species, where gene flow occurs between what would otherwise be isolated populations in a small area where the populations overlap. You could have a linear distribution of population centers and multiple hybrid zones between them to maintain gene flow, however the rate of flow is unlikely to keep the ends of the distribution from diverging. When we talk about ring species we see situations where the linear distribution folds back on itself and ends up with varieties that overlap but don't hybridize.
Perdition, Message 7: As a complete layman, I've always assumed hybrids are a result of two recently diverged lineages. If they're both still near the "fork in the river" so to speak, they have enough similarities to overcome the differences that are building up. Or diverging populations, where speciation has not occurred yet but is immanent. If the isolation of the main populations increases and the hybrid zones become smaller so gene flow slows, the interfertility between the populations decreases as they accumulate mutations that are increasingly incompatible. Thus hybridization can be seen as attempts to reunite populations that have become estranged, possibly resulting in some varieties showing hybrid vigor that can also increase and either draw the populations back together or meld with one or the other population. Lots of possibilities here. Enjoy. by our ability to understand Rebel American Zen Deist ... to learn ... to think ... to live ... to laugh ... to share. • • • Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click) • • •
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Coragyps Member (Idle past 756 days) Posts: 5553 From: Snyder, Texas, USA Joined: |
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. But, as noted upthread, domestic and Przewalski's horses are interfertile, and they have differing chromosome numbers - diploid #'s are 64 and 66, IIRC. So a hybrid has 65, but is still interfertile. I'm guessing that's because the 2n=66 is from a recent fission in a 2n=64, and the resulting broken chromosome hasn't accumulated many mutations. But I'm not a biologist.....
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 8.4 |
But, as noted upthread, domestic and Przewalski's horses are interfertile, and they have differing chromosome numbers - diploid #'s are 64 and 66, IIRC. So a hybrid has 65, but is still interfertile. I'm guessing that's because the 2n=66 is from a recent fission in a 2n=64, and the resulting broken chromosome hasn't accumulated many mutations. 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... I'm not a biologist either... yet.
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 8.4 |
How closely related do species have to be in order to successfully breed a hybrid? 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.
What do we actually mean by "closely related"? What does this mean in terms of DNA, genetics, chromosomes etc. etc.? 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.
Where is the line that divides the ability to reproduce fertile offspring, infertile offspring and the inability to breed at all? Are there definite dividing lines or is it a graduated scale? 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.
What does the existence of hybrids tell us about evolution as a theory? I can't see that it tells us much at all.
Ignoring the moral questions how feasible is a chimp-human hybrid? 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.
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caffeine Member (Idle past 1045 days) Posts: 1800 From: Prague, Czech Republic Joined: |
Thinking about it, isn't it essential that at least some combinations of different numbers of chromosones are capable of producing viable offspring? Otherwise, it wouldn't be possible for secually reproducing organisms to evolve different numbers of chromosones in the first place. The first mutant with a duplicated or deleted chromosone would be necessarily infertile except in the extremely unlikely case of bumping into a member of the opposite sex with exactly the same mutation.
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 8.4 |
That would follow in animals, yes.
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derwood Member (Idle past 1897 days) Posts: 1457 Joined: |
There are a number of species that maintain karyotypic polymorphism in their populations. A nice example of the ability of species with differing karyotypes being able to interbreed and produce viable offspring are the domestic horse and Przewalski's horse.
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Godless Dave Junior Member (Idle past 5261 days) Posts: 1 Joined: |
The post I'm replying to is a month old but...
quote: Half-cow half-moose? Did you mean to say bison instead of moose? Because I'm pretty sure cattle and moose can't interbreed!
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Iblis Member (Idle past 3917 days) Posts: 663 Joined: |
Hi Dave, welcome to the great monkey-or-not poop toss.
Yeah, the moose-cow is a tabloid myth. Ranchers in Minnesota occasionally blame deformed calves on the horny bull moose; the two examples that got some press turn out to be endocrine problems, probably caused by a combination of recessive genes.http://jhered.oxfordjournals.org/cgi/pdf_extract/30/10/457 Beefalo, on the other hand, are delicious, and have moved on from problematic attempts to a succesful breed of cattle. They have totally sabotaged "pure" bison conservation, though, as they fill the niche better than their endangered kinfolk.Beefalo - Wikipedia Remember that old Star Trek where they talk about the buffalo being extinct? It's the one with the sexy salt-sucker in it who puts the make on everyone in the ship, McCoy's ex-girlfriend, tells Uhura he is a "Swahili" etc. Anyway I won a huge bet on that one time, I had a buddy who really believed they were gone! He had faith in Mr. Spock ...
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greyseal Member (Idle past 3883 days) Posts: 464 Joined: |
Yeah, the moose-cow is a tabloid myth. Ranchers in Minnesota occasionally blame deformed calves on the horny bull moose; the two examples that got some press turn out to be endocrine problems, probably caused by a combination of recessive genes. http://jhered.oxfordjournals.org/cgi/pdf_extract/30/10/457 Well I'll have to ask my farmer-friend who keeps cows for some evidence then. It came up in a discussion where we were talking about calves since it was that time of the year up here - they'd just had one that was born "inside out" and he'd had to put it down. There are lots of moose where we live and the cows here are let out to roam around so there's more than enough chance for it to happen. I don't think he was deliberately lying to me and feel it more than likely that he believed what he was saying...
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