In this thread I want to consider the biology of hybrids. I want to do this to increase my understanding of the underlying biology of reproduction and the definition of "species".
Why are hybrids such as mules and ligras infertile? Why are hybrids even biologically possible from an evolutionary point of view? What interesting examples of hybrids are known to exist or be possible? How closely related do species have to be in order to successfully breed a hybrid? What do we actually mean by "closely related"? What does this mean in terms of DNA, genetics, chromosomes etc. etc.? 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? What does the existence of hybrids tell us about evolution as a theory?
Ignoring the moral questions how feasible is a chimp-human hybrid? From a purely biological perspective how feasible is this as a physical possibility?
I would prefer that this thread not be cluttered up with anti-evolutionists. Nor do I want the moral question of human hybrids to become the focus. I want to understand the pure biological science of these questions.
Female ligers and tigons are fertile. Apparently, the homogametic sex in hybrids (XX females in the case of mammals) are more likely to be fertile than the heterogametic sex. I don't know why this is, but the following explanations are offered by wikipedia:
Homogametes are protected from any deleterious recessive genes on the sex chromosones, whereas the heterogametes aren't.
In mammals, sexual selection drives faster evolution in male development patterns, so the Y chromosone is more divergent in the different species and less able to make a viable animal (though you'd think this would have the opposite effect in birds, where males still tend to be more heavily influenced by secual selection but where they are the homogametic sex).
Plus another couple that I don't think I've quite grasped from their brief overview. It's here in the article on Haldane's Rule.
ABE: I just had a quick read, and apparently female mules can produce offspring with either horse or donkey males.
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.
As far as I know, most hybrids are pretty much forced, either in a lab by humans, or by extreme environmental factors, so that in the wild, in most cases, these animals are reproductively isolated from one another, and as such, we can only assume more changes will develop unless we artificially hold them in their current forms. If and when these changes overpower the similarities, hybridization will no longer be possible.
But again, I'm so much a layperson on this that I'm downright recumbent.
If my tigon and liger-interested friends were right, I believe a liger is almost always bigger than either parent, and I believe he said that a tigon is almost always smaller...
I'm not sure about fertility, but I do know it does happen. Relatively rare it might be, but it does happen.
There are also documented occurences of half-cow-half-moose births, although I don't know how long they live and if they're fertile either.
To be honest, I would expect that the existence of hybrids amongst relatively similar life-forms would point to the ToE being vindicated - if they were entirely different, there should be NO hybrids at all.
If we've all come from common ancestors, sometimes perhaps (through the magic of the genetic code which we're far from fully unravelling) hybrid offspring are possible even when we think it unlikely...
I believe a liger is almost always bigger than either parent, and I believe he said that a tigon is almost always smaller
I'm sure this phenomenon is caused purely by the interacting differences involved. Similar to how two closely related people can end up letting all sorts of genetic errors into their child because they don't have an infusion of new genes to compensate, having regulatory genes that are different can either increase production of an inhibitor or decrease it, both altering the size of certain features or even the whole shebang.
There's also the possiblity of having two different proteins or hormones that might duplicate the effects of ones from the other parent, making either additive or counteracting effects.
I am trying to recall the details from a devo class I took in grad school, but iirc the hybridization issue is even more complex on the fertilization level. That is, fertilization to multiple cell stages can occur between even widely diverse taxa (even though they do not progress further). The distance between taxa can be extreme if the protein coat of the sperm is removed or altered. I will search for the papers from the class (more likely similar topics as it has been about 8 years!) to ref, but I think I am remembering the concept accurately, just not the specifics. Neat stuff and (imho) a lot more important to speciation than is widely known.Doctor Bashir: "Of all the stories you told me, which were true and which weren't?" Elim Garak: "My dear Doctor, they're all true" Doctor Bashir: "Even the lies?" Elim Garak: "Especially the lies"
Was this a natural occurrance or (as I am assuming) bred in a lab?
Tigers and lions don't really coexist in nature: there is only a small population of Asian lions left, and I don't think any tigers live in the same vicinity.
So, this was definitely a zoo/circus stunt.
How do we genetically account for a hybrid animal of this type that is twice the size of either of it's parent species?
I'm not sure that this is actually known, but I'll share with you the version I've been told.
As social animals, lions benefit from being larger, because larger animals become dominant in prides, particularly larger males. So, there are some growth factors the promote large body size in lions. But, large cubs tend to maim mother lions, so mother lions have the ability to suppress the growth factors.
So, when you mate a male lion to a female tiger, you get the growth factors without the inhibiting factors (which only come from female lions). So, ligers grow very large.
A male tiger does not produce the growth factors that a male lion produces, so, if you mate a male tiger to a female lion, you get the inhibitors without the growth factors, resulting in a small, frail animal (a tigon).
As far as I know, most hybrids are pretty much forced, either in a lab by humans, or by extreme environmental factors...
It's actually really common among birds.
Mallards will mate with many other kinds of duck. Indigo and Lazuli buntings interbreed where their ranges overlap. Baltimore and Bullock's Orioles used to be thought to be one species (the Northern Oriole), because they interbreed readily.
Hybrids usually have poor survival in relation to purebreds, so interbreeding can actually be a major problem if one species is much more common than another. If the less-common species readily breeds with the more-common species, but the hybrids are less fit, the less-common species will experience rapid population decline.
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.
I was really hoping for some serious and detailed bio tech answers to these questions. Bluejay and others have given some brief answers but surely the EvC bio contingent can provide better answers to the questions posed than have been forthcoming so far?
How closely related do species have to be in order to successfully breed a hybrid? What do we actually mean by "closely related"? What does this mean in terms of DNA, genetics, chromosomes etc. etc.?
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 quickly learning that this is one of the central themes of biology: we like things to live by rules, so we can define them and categorize them and organize our information about them... but, life simply isn’t bound by any of the rules we think apply to it.
We can design theories that effectively explain patterns of behavior across populations of organisms, but we can’t use these theories to predict, with any useful degree of accuracy, the behavior of any particular individual organism.
The Theory of Evolution has provided us with at least two frameworks for evaluating the likelihood of the occurrence of certain behaviors: one called optimization theory and one based on game theory. Optimization obviously refers to the tendency of natural selection to amplify the segment of a population whose behaviors are most successful in their environment. Thus, under this criterion, we expect that nature will be largely full of species that are “optimized,” or well-adapted, to their environment. Game theory refers to the interplay between different organisms all competing for the same resource or “payoff.”
But, evolution works on a scale to which individual organisms are largely insensitive. So, evolution does not drive the behavioral patterns of organisms, because it can only function ex post facto, and its only means of feedback is through differentialy success of phenotypes. Thus, the reason an organism performs some certain behavior (e.g. sex) has nothing to do with its evolutionary benefits: nobody has sex for an evolutionary advantage: they do it because it tickles their pleasure centers. A pleasure center is nothing but a way to attach a short-term reward to a successful long-term strategy.
So, that some organisms end up being attracted to the “wrong” kind of potential mate, evolutionarily speaking, doesn’t really mean much, except that there is lots of wild, random variation within populations. From an evolutionary standpoint, hybridization, homosexuality, and other things that have long been thought of as “sexual perversities” are dead-ends. But, because the behaviors of individual organisms do not respond to evolutionary pressures, we shouldn’t expect all behaviors to be adaptationist.
As far as genetics go... biological molecules are also insensitive to evolution. If a hybrid is viable, all it means from the standpoint of molecules is that a massive network of chemical reactions is proceeding in a stable, cyclical fashion. The more divergence you have between the interacting parts of the chemical network, the more chances you have to throw the reaction off balance.
But, context defines everything... I think it’s going to be very difficult to discuss the genetics behind hybridization without some serious equivocation on the “rules” that define the process.