Saturday, January 23, 2010

Speciation in one country

How does sympatric speciation occur? New models clarify how species can diverge in place.

Despite Darwin's great work, the origin of species still remains something of a mystery, since beyond the depth of circumstantial evidence and the empirical demonstration of the details and mechanisms of evolution, speciation itself takes time- time that scientists don't have, as a rule, to stand around and watch. Traditional theories of speciation (Mayr and Dobzansky) demanded the geographic separation of two populations, giving them time to diverge by random processes without contaminating each other by interbreeding. But this is wholly insufficient to account for the facts of evolution. The Amazon is a hotbed of speciation, (or was, at any rate), and one can't possibly claim countless geographic barriers for so many speciation events. Sympatric speciation has to happen.

(Note that the terms "sympatric"- occurring in the same territory or "fatherland", and "allopatric"- occuring in different territories, were clearly devised in the patriarchial days of yore, possibly by German scientists!)

Allopatric speciation is clearly applicable to islands- the countless endemic species related to their mainland ancestors are clear evidence of such divergence. But how could 15 species of Darwin's finches diverge in place on the ~ten or so Galapagos islands? And how did the thousands of butterflies in the Amazon come about? They are mobile and can interbreed during their speciation.

The answer has got to be sympatric speciation. But evolutionary theory has had a hard time modelling that process, since any genetic divergence between two nascent species- two sub-populations of an existing species- is going to be swamped by interbreeding, exchanging genes that need to be kept separate if divergence is going to take place.

A recent paper by van Doorn et al. in Science takes a large step to resolving this dilemma by improving models of speciation to take sexual selection into account, finding that under realistic conditions, sexual selection synergizes with ecological selection to allow sympatric speciation. The situation they give themselves is an ecological setting where two modes of getting a living work well, such as a mix of large seeds and small seeds, (leading to Darwin's finches), or two differently structured plant flowers (leading to differentiated butterflies).

In this setting, organisms are favored which specialize on one of the two conditions, and disfavored if they express the average condition. Incidentally, this is one of many different evolutionary scenarios. Often a population benefits by the retention of diversity, such as in the case of human personalities and temperaments, such that all are better off when a variety of skills and attitudes are kept in the flock, as it were. But if ecological space presents a reason to diversify, then the question is whether organisms can follow suit to the point of speciation, even if they occupy the same physical territory.

The key to this new work is the realization that the occurrence of male ornaments that function both as marks of fitness and as female attractants allows females to select those males that do well in one of the two conditions. The marks do not have to be differentiated between the proto-species to start with, and nor do the female have to know which males are which, at least at first. Male ornamentation, like the colors of many birds or the dramatic designs of many butterflies, often acts as a sign of fitness- if the male is doing well, the colors are brilliant. If not, then not so brilliant, or perhaps in tatters. If females choose carefully, then they will reinforce the natural selection of males well-adapted to one or the other condition, even in the same territory.

After time, this process generates two sub-species that functionally specialize, even if they look identical, even to each other. When hybrids occur between well-adapted males of one specialization and females of the other, their offspring are less well-adapted, and especially in the case of males, less likely to propagate. I can't vouch for the math involved or all the assumptions, but the general idea makes sense. It would be quite difficult to put numbers on the various parameters, so the authors give ranges in some of their graphs:

Left- the relation between tendency to speciate (colors) vs migration rate between the proto-populations (Y-axis) and ecological specialization pressure (X-axis). Green represents the traditional modeling approach, where sympatric populations (migration of 1) only speciate with extreme selective pressures for specialization.

Yellow represents the addition of the theory of this paper, which adds female choosiness and male fitness signalling to the mix, allowing specialization to be amplified by sexual selection. On the right, relations are graphed between each of the above variables and time to speciation based on arbitrary modeled values and starting from a completely homogeneous population:

(B assumes migration quotient of 0.3, while C assumes a sigma/selection for specialization of 0.75. I'd note that these are rather permissive conditions for the theory presented, since I was really interested in fully sympatric speciation. On the other hand, there are other possible mechanisms at work that further contribute to speciation, like the ability of females to recognize one or the other male specialist, which is not part of the base theory presented here.)

What does sigma mean in this data? The authors state that it represents the (inverse of) intensity of "stabilizing selection within habitats". Which is to say- how strongly the ecological situation penalizes in-between hybrids versus pure-plays of either specialization. The left graph shows in proper fashion (lower right corner) that even if there is no selection of this kind, allopatric (island) populations will eventually speciate anyhow. On the other hand, sympatric species require some kind of push from their ecological setting to differentiate and speciate. In its absence, there is no reason to do so.

Biological traits involved in these models are:
x- the ecologically selected variation, such as bill size, which responds to the bimodal ecological issue at work.
t- investment in the male ornament, which is not differentiated with respect to x, but affects mating success.
p- female choosiness, which is what makes t useful.

The models also assumed a rate of mutation and evolution: mutations occur 1e-5 per allele per generation, and have effects on x, t, and p of 0.1, 0.1, and 0.05 respectively, in either direction at random. This is realistic for such issues as bill size, which are as likely to vary in one direction as the other.

So, in the end, this work provides one rationale whereby evolutionary theory can be fitted more closely to evolutionary reality, for speciation among organisms that make use of sexual selection (ornaments, female choice, etc.). This encompasses a large number of complex organisms (notably birds and mammals), and constitutes one theoretical explanation, among several others, for their particularly rapid speciation in the face of relatively low population numbers and long life-spans (relative to, say, bacteria).

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