What determines rates of ecological speciation?

Speciation models are the most beautiful thing in evolutionary biology. This is widely known, and those who disagree are the crazies. Other models have their place, and empirical evidence for speciation, and insights from there into how speciation takes place are crucial for progress. But real understanding of this question of questions in evolutionary biology only comes once a model is constructed and validated. It is the ultimate goal of scientific work to condense knowledge in terms we can share and peruse, and this we do in models - which you may call theory, but I tend to regard this distinction with distrust.

Case in point is a model of ecological speciation in a population of sexually reproducing diploids adapting to different resources scattered heterogeneously in the environment: Patterns of Species Ranges, Speciation, and Extinction.

Specific questions asked and answered include: What is the effect is limited vs. wide dispersal, i.e., how is the rate of speciation affected by individuals occupying a small geographical area? How does the distribution of resources (food, a place in the sun, etc.) affect the rate of speciation?

And the answers are: The more widely the individuals roam, and the more mixed resources are in the environment, the lower the rate of speciation is. So when individuals only get their resources from a small geographical area, and when there are few types of resources within that area, the more speciation happens.


Examples of environments with many types of resources within a small area (A) and resources distributed such that individuals will mostly find only one type within the area they roam.

With four different types of resources, the simulations are started with only 20 identical individuals in the corner of an area like the ones above. By reproduction and mutation the population grows and diversifies. Whether resource specialists or generalists emerge depends primarily on the two factors already mentioned: range and resource distribution. Specialists, which are very good at utilizing a single resource, can coexist with other specialists when they use different resources. You eat the bananas, and I'll the the leaves, and then we can get along. However, different generalists, in this model defined as those who use more than one resource, may not get along so well, because they overlap in resource use, and so one will tend to outcompete the other. We thus have that the emergence of specialists equates to higher rates of speciation.

Here are some results:



A and B: highly mixed resource environments. C and D: fragmented resources. A and C: low dispersal range. B and D: large dispersal range.

In A there are three specialists using only one resource (red), five generalists using two resources (blue), and two three-resource generalists (green) coexisting.

In B there is only a single generalist (no speciation).

In C there are four specialists coexisting.

In D there is one specialist and two two-resource generalists.

And Birand et al. did lots of simulation runs to get a good handle on the importance of the different parameters governing speciation rates. In addition to the two already mentioned (dispersal and resource distribution), another important one is the strength of trade-offs. It can be set as more or less difficult for individuals to be good at utilizing more than one resource. As an example of trade-offs, think of lemurs: Some lemurs eat leaves, and in order to digest the hard-to-digest cellulose, they have really long intestines. Other lemurs eat fruit containing lots of sugar, which is easy to digest. They don't need to have long intestines for that, so they have short ones, and thus can't digest leaves very well. At the same time, lemurs with long intestines can't eat fruits, because they tend to rot while making it all the way to rectum, which makes the lemurs sick. Thus, there is a trade-off between which foods the lemurs can eat, which makes the two species able to coexist (at least in theory - I don't know if they have overlapping ranges). Competitive exclusion (wiki) prevents lemurs that use the same resources from coexisting in the same area, while two species that occupy different niches won't have a problem with each other.

But, while trade-off are important to have for speciation to occur, varying the strength of it had little effect compared to the effects of limited dispersal and resource distribution.

In fact, geography turns out to be the most important feature in this model:

Indeed, when resources were distributed randomly, which resulted in highly fragmented landscapes (figs. S1, S11), there was always only one species regardless of the dispersal range D and trade-off coefficient b (of the 90 simulations initiated with all the parameter combinations of b and D, only one simulation resulted in two species).

That is to say, if it wasn't because resources are distributed in a patchy way, then speciation just doesn't occur at all. When individuals can find all four types of resources within their dispersal range, then generalists are always of higher fitness, and specialization is not favored. This also means that this model cannot account for (strict) sympatric speciation, where there is no geographical structure, and where individuals are not limited to mate with those close by, or use resources in their neighborhood. Not surprisingly, parapatric and allopatric speciation (where geography is a factor) is much easier than sympatric speciation (where geography is not). For that something else is needed. Stay tuned.

Reference:
Birand A, Vose A, & Gavrilets S (2012). Patterns of species ranges, speciation, and extinction. The American Naturalist, 179 (1), 1-21 PMID: 22173457