I recently had the funny thought that the evolution of classical cases of ‘adaptive radiations’, such as the Darwin’s finches and the cichlid fishes, are very similar to the evolution of domesticated animals, such as dogs and cats. With the only difference that we talk about different ‘breeds’ when it comes to cats and dogs, and different ‘species’ (most of the time) in case of finches and cichlids. Nevertheless, both categories show the spectacular morphological, phenotypic evolution typical for adaptive radiation.
Typical for adaptive radiation in the classical sense is that the group shows a very fast rate of evolution, resulting in many different species, supposedly because of strong selection pressure by the environment. In case of the finches this has resulted in species specialized on different food sources, and consequently showing a spectacular change in beak morphology, where ground finches have large beaks for cracking seeds, and warbler finches have more elongated small beaks for spearing insects, for example. The cichlids – those occurring in the African Great Lakes- seem to adapt to different lake features and show disparity in body sizes and colors. This last feature contributes to strong sexual selection, where females of one species are more attracted to the color pattern of the male of the same species, than to the color pattern of another species’ male. Clearly, if we stick to mating with the same species, this will accelerate the speciation process, because ‘gene flow’ between non-species will inhibit the evolution of new species.
They recently discovered the underlying genetics of Darwin finches and the genes coding for their classical beak morphologies. Interestingly, although these finches all look very different, there was “extensive sharing of genetic variation among populations […], particularly among ground and tree finches, with almost no fixed differences between species in each group” (Lamichhaney et al. 2015). This suggests that, although there is selection on the genes coding for beak morphology, there are not much genetic differences between species. This raises the question: are they really different species? Or are they in the process of becoming different species?
What about the parallel to our cats and dogs? In this example we, humans, are the selective environment, selecting for genes coding for long fur / short fur, pointy ears / floppy ears, long tail / short tail, big dog / small dog, etcetera, which has resulted in the spectacular morphological diversity of domesticated dogs and cats. Funnily, these breeds are still the same species or subspecies (Canis lupus familiaris for the dog, and Felis catus for cats). Why don’t we call these different forms different species, resulting from adaptive (human) radiations? Or, perhaps better, are Darwin’s finches and cichlid fish, without much genetic differentiation, really different species?
Sangeet Lamichhaney, Jonas Berglund, Markus Sällman Almén, Khurram Maqbool, Manfred Grabherr, Alvaro Martinez-Barrio, Marta Promerová, Carl-Johan Rubin, Chao Wang, Neda Zamani, B. Rosemary Grant, Peter R. Grant, Matthew T. Webster, Leif Andersson. Evolution of Darwin’s finches and their beaks revealed by genome sequencing. Nature, 2015; DOI: 10.1038/nature14181