- There was something about the environment they were in? For instance, an ancestral species happens upon new habitat with unfilled niches. (Speciation driven by external causes.)
- They have some special feature? Ancestral species has some key innovation innovation, which is so adaptable that it allow it to diversify faster. (Speciation driven by internal causes.)
A paper by Wagner and colleagues tries to weight these two possibilities against each other using African cichlids. Several rift lakes in Africa are famous for having a lot of cichlid species in them, like Lake Victoria and Lake Tanganyika. What is less well known is that there are lots of examples of African rift lakes (created recently, so lots of unfilled niches) that were invaded by cichlids (which have some interesting features in the ways their jaws work, so lots of evolvability)... but that were not followed by a burst of evolutionary innovation for which cichlids have been so well studied (Seehausen, 2006).
This means there are a lot of natural experiments that allow you to figure out why there are somethings veritable explosions of diversity, and sometimes... matches fizzling out.
They found three factors that were most correlated with adaptive radiations: two external, and one internal.
First, the depth of the lake. This makes sense to me: the deeper the lake, the more potential niches there are for fish to inhabit. Cichlids are often very visual animals, and the changes in light quality as you descend could be an important factor in creating new niches. It’s not just size of the lake, as surface are doesn’t come out as a strong predictor. (I wish they estimated volume, though.)
Second, solar radiation. Now, I would have thought that most of these being more or less in equatorial regions that there wouldn’t be much difference in the amount of sunlight from lake to lake, but this is why you do the experiment and not count on intuition. (Additional: I was thinking of day length, but it occurred to me that cloud cover and climate would affect this, too.) They authors suggest that the greater “energy” in the system facilitates speciation.
Third, the difference in appearance of the males and female of each species is a predictor of speciation (sexual dichromatism). Why should this matter? The authors matter that it probably doesn’t, but that the difference in appearance between the sexes indicates that there is a lot of choosiness who gets some sweet lovin’ sexy time in the rift lakes of Africa. In other words, big differences in appearance means more sexual selection is going on in the population.
This may also be correlated with lake depth, since earlier studies showed that visual signals were critical to sexual selection in many of these species. In fact, there are cases when waters started to get cloudier, different cichlid species started to hybridize again.
This and other research is suggesting more and more that some evolutionary events are predictable, at least in broad strokes.
Seehausen O. 2006. African cichlid fish: a model system in adaptive radiation research. Proceedings of the Royal Society B: Biological Sciences 273: 1987-1998. DOI: 10.1098/rspb.2006.3539
Wagner CE, Harmon LJ, Seehausen O. 2012. Ecological opportunity and sexual selection together predict adaptive radiation. Nature: in press. DOI: 10.1038/nature11144
Photo by RevisionTwo on Flickr; used under a Creative Commons license.
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