That's Allopatric speciation, Folks!
May 18, 2014 11:11 AM   Subscribe

How quickly does genetic drift lead to speciation in higher life forms? More specifically, how long would it take for two isolated human populations, exposed to similar but not identical selective pressures, to become incapable of producing viable offspring due to genetic drift?

It appears from my clumsy, amateur research that the size of the initial populations matters, so assume several hundred individuals are in each founding group. I'm curious if there is a rate at which isolated groups of vertebrates have been observed to "drift" or if the variables are simply too complex, and the numbers of observed instances too few, to calculate.

This is for reasons of fiction, not actual human experimentation.
posted by itstheclamsname to Science & Nature (10 answers total) 5 users marked this as a favorite
Considering how far back the split between Australian aborigines go with the rest of the human genetic tree (1 2), it could be over 75,000 years. They also think that we might have previously mistaken normal variation within a species as different species for early hominids. The concepts of adaptive radiation and punctuated equilibrium might be useful to you, however, though I'm sure you bumped into them on the Wikipedia article on Allopatric speciation.
posted by foxfirefey at 11:54 AM on May 18, 2014

It would depend on the particular mutation(s) that arose. Rapid environmental changes that force adaptive responses or a novel mutation that gives an organism purchase on a new resource could speed things up rapidly. For example, if something like this happened, it wouldn't take many generations for the new mutants to overtake those lacking the variant.
posted by Crotalus at 11:55 AM on May 18, 2014

There's no standard answer to that, because there is so much randomness and conditionality involved.

For instance, if they are in challenging environments (say, 75% of each generation dies before breeding) then the process will be somewhat faster. If it's a comfortable environment, it's going to take a lot longer. Evolution is quicker when there's a very high death rate.

It also depends on how different each environment is. If they're about the same (say, two different islands in the South Pacific) then the evolutionary pressure will be in the same direction, so differentiation won't be so quick. You'd get faster divergence if one population lived in Hawaii and the other in northern Siberia.

But you're probably talking half a million generations, if not more. There's recent work that suggests that Cro Magnons from Africa bred with Neanderthals in Europe before the Neanderthals died out. And Neanderthals branched off something like 400,000 years ago, and that from a line of hominids who left Africa maybe a million years ago.

Speciation is not a very fast process.
posted by Chocolate Pickle at 12:39 PM on May 18, 2014 [1 favorite]

The other answers here are good - speciation is a very random and poorly understood process. It's also highly dependent on the type of organism your discussing, as the relevant parameters are not typically time, but number of generations, amount of gene flow, and population size. Nevertheless, the general thinking has been that speciation should take on the order of 100,000 years - probably not less than 10,000, probably not more than 1,000,000.
posted by Buckt at 1:50 PM on May 18, 2014

To help your intuition, I should add that in evolutionary time, 1,000,000 years is actually a very small number, and 10,000 is so small as to be considered virtually instantaneous. Most of the large scale changes that you think about (different types of animals, for example) happen on the order of 100 million years.
posted by Buckt at 1:52 PM on May 18, 2014

By the way, I think it important to point out that two groups of animals are considered different species if they don't naturally interbreed when in close proximity. It doesn't mean they cannot interbreed.

That can happen if fertility displays change, for instance.

Once that happens, then they are reproductively isolated even if not geographically isolated, and natural processes will cause them to diverge ever more widely as time goes on.
posted by Chocolate Pickle at 2:35 PM on May 18, 2014 [1 favorite]

The rule of thumb is that one migrant per generation will keep subpopulations from differentiating.
posted by SandiBeech at 7:58 PM on May 18, 2014

Reproductive isolation is a surprisingly blurry line. It can range from "could theoretically mate, but choose not to" to "outbreeding depression" where two groups can and do interbreed, but the hybrids are less fit than the "purebreds", thereby leading to reduced gene flow, all the way out to separate species whose crosses are always sterile--except when they're not. A surprising number of what we typically consider to be separate species do hybridize in the wild. Oaks are notorious for this.
Because humans can be quite, um, open-minded in their sexual partners (Captain Kirk, anyone?), and because we have the ability to protect and care for individuals with a disability that might result from outbreeding depression, we might have a very high threshold for genetic distance before we become mutually incompatible.
The Island Fox is estimated to have had only 10,000-16,000 years to speciate. It seems likely some populations of humans have had much longer than that, as foxfirefey mentioned. The island fox is considered a separate species, but if somehow the Channel Islands were to crash back into mainland California, it might well begin to hybridize with the gray fox.
I guess the take-away here is that you could probably end up with green-skinned aliens that everyone agreed were a separate species, but that could still be reproductively compatible with humans.
It's notable that many species are genetically compatible with other species, but maintain reproductive isolation by preventing intercourse through physical or behavioral means. Some species even have a "lock and key" arrangement with the female and male genitalia to prevent cross-species mating. Again because humans are so resourceful, you might have to draw an arbitrary line on where speciation has occurred. If two pseudo-human "species" can mate, but only do so when really drunk, have they speciated? What if they require in-vitro fertilization? What if two pseudo-human species are incompatible without extensive genetic manipulation of the embryos, but they do in fact engage in that manipulation regularly?
I think the answer to your question could be anywhere from hours ("let's make a pact to never fuck redheads") to billions of years ("let's insert that bacterial gene into corn!") depending on what your definition is.
posted by agentofselection at 8:58 PM on May 18, 2014 [1 favorite]

I've read— and unfortunately I can't find a cite, so take this with a large grain of salt— that aboriginal Australian people and Indonesian/Asian/African/European people already have detectably lower interfertility. Obviously we're still the same species, but I suppose this could be a starting point for estimating how much longer Australia would have had to be isolated for speciation to occur. (Assuming it's real and not just some misremembered crackpot footnote.)

The rule of thumb is that one migrant per generation will keep subpopulations from differentiating

Out of curiosity, how isolated is Australia thought to have been, between its early settlement(s) and modern contact? I would think that people would find their way between Austrailia and New Guinea or maybe even Timor every now and then, clinging to a raft or fishing boat after a storm.
posted by hattifattener at 1:24 AM on May 19, 2014

I'm an evolutionary biologist specializing in the evolution of sex; I do *not* specialize in speciation, and I personally find it a little brain-meltingly complicated.

You've gotten some good and well-informed answers already, and I'll try not to just repeat what's been said before.

Think of it this way: *Don't* interbreed comes before *Won't* interbreed comes before *Can't* interbreed.

"Don't" interbreed comes from physical or temporal barriers to speciation -- the populations are separated with no or very low (less than one migrant per generation, as above) gene flow. Such barriers can rise effectively instantaneously.

"Won't" interbreed comes from changes in mating cues. Changes in mating cues happen due to drift combined with sexual selection, and can happen *very fast*, because sexual selection can drive/be driven by antagonistic coevolution between the sexes, and so there's a very tight connection -- "no, I want someone with a BIGGER tail" "I have a bigger tail!" "But I want a BIGGER BIGGER tail" etc. And then they're brought back into sympatry with the other population where they're going, "OH GROSS THOSE TAILS GAG ME". The "gag me" response is going to be reinforced if there's hybrid incompatibility due to other evolutionary forces (drift or natural selection). Such barriers can arise in thousands of generations.

"Can't" evolves as a consequence of "Won't" or "Don't". "Can't" can mean, "my Tab A and your Tab B are not compatible", or "Your sperm can't penetrate my ova", or "Chromosomes wrong shape AAAAAAAAA" or any other mechanisms by which viable fertilization is prevented rather than just a bad idea.

The fastest speciation rates in animals are either in abalone or in beetles, depending on what you really consider a species. Because of broadcast spawning, abalone need really really really specific lock-and-key stuff on the eggs and sperm level to make sure that they're the same species and are going to create viable offspring. But because the pickiest eggs get the most compatible sperm, there's a constant evolution going on that can cause rapid speciation once allopatry is established.

Beetles (inordinate fondness, and yes, Haldane really said that, a lot, see Gould for details) are pretty famous for being speciation maniacs. One possible super awesome reason: there's a negative correlation between horn size and copulatory apparatus size, so intra-sexual competition driving horn size drives reproductive changes, which drive speciation. Google, um, Armin Moczek and Harald Parzer for more.

"A few hundred individuals" is a VERY SMALL population with respect to genetic viability. If your populations have basic genetic theory, they can mitigate this somewhat (careful breeding management can increase effective population size ABOVE census population size, for instance), and if they have the ability to do real genetic testing and maybe IVF selection, they can do even better, especially if they're looking to preserve MHC diversity and avoid known deleterious recessives.

Consider reading Carl Zimmer's abridgement of Darwin's _The Descent of Man, and Selection in Relation to Sex_. There's a reason Darwin thought those topics were inextricable -- he thought that sexual selection accounted for the physical differences between the putative human races. (This was an alternative idea to the then-current idea that the physical differences were due to some people having been separately created by God to be the slaves of other people.)

Anyway, if I were writing your story, I'd give your populations at least 1,000,000 of your Earth years to reach "largely incompatible, but offspring are possible, just a bad idea", and 5,000,000 for "nope nope nope nope nope". I'm basing that largely on the relationship between tigers and lions, because I think their semi-inter-fertility is neat (look up genomic imprinting and ligers.)
posted by endless_forms at 10:31 AM on May 19, 2014 [3 favorites]

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