Cichlids are an example of what Mary Jane West-Eberhard calls a “multidirectional radiation” – that is an adaptive radiation that produces a large amount of diversity and specialization of related forms. There are at least 1,500 species of haplochromine cichlids. Within Lake Victoria there are at least 120 species that display a wide variety of behavioral and morphological specializations.
Cichlids are well know for their variable jaw morphology and accompanying dietary specializations. For example, there are scale eaters, scrapers, diggers, choppers, piscivores, and snail eaters, to name a few. Some of the variability in jaw morphology is actually driven by dietary factors. In aquarium reared Astatoreochromis alluaudi (a mollusk specialist) fed on a soft diet the jaw is much more weakly developed, there are fewer enlarged pharyngeal teeth, and there is less calcium content in the bones. This mirrors the morphology of Astatoreochromis alluaudi in other lakes where snails are rare.
Cichlasoma minckleyi is a New World species of cichlid. It comes in two morphs, which at one time where thought to be different species. One morph seems specialized towards plant food, while the other seems to be specialized towards snails. What makes this example interesting is that in times of food abundance both morphs feed on much the same food. In times of scarcity the diets diverge towards the specializations one can see in their anatomy.
This can be seen even more clearly in Darwin’s finches. As is well known, A large number of Darwin’s finches are seed eaters. In times of plenty the diet is pretty generalized, but in times of scarcity diet is determined largely by bill size. The best example of this is Geospiza magnirostris. During times of plenty it eats a pretty generalized seed diet, but in times of scarcity – such as the droughts induced by El Niño – it falls back on mericarps.
In each case, the specializations are driven by selection. As West-Eberhard (pg 577) puts it:
This observation of increased specialization under food scarcity fits a pattern seen in other rapid trophic radiations, namely, that directional change is driven in episodes of strong selection, when food limitation and trophic competition force dietary specialization, including those influenced by morphological predispositions…
As Robinson and Wilson (1998) point out, however, this difference between phenotypic specialization and diet soon led some to question the role of competition as an evolutionary force in the diversification of species. This discrepancy between phenotypic and ecological specialization is what has come to be known as Liem’s Paradox.
Which brings us to Paranthropus boisei and the reconstruction of paleodiets. There are a number of ways one can get at paleodiets among hominins. One can look at tools, for example bone tools used for termite fishing have been found at Swartkrans – some have argued that they were made by P. robustus. One can look at the gross morphology of teeth. Whether the cusps are high shearing cusps or low grinding cusps can tell you something. One can also look at microwear and reach some conclusions based on pitting and scratching. Finally, one can look at the chemical composition of the teeth in the form of stable isotope analysis. The recent paper by Unger et al looked at dental microwear in P. boisei along with tufted capuchin monkey, the grey-cheeked mangabey, the mantled howler monkey, the silvered leaf monkey, and Australopithecus africanus and P. robustus. As Brian and Kambiz both point out P. boisei was considered to be a hard food eater based on its cranial and dental morphology. Unger et al come to a different conclusion:
Comparisons with the South African hominins suggest that while Paranthropus boisei may have consumed foods with similar ranges of toughness as those eaten by Australopithecus africanus, the eastern African “robust” hominin did not eat harder and brittler foods than the South African “gracile” form. Further, the patterns for P. boisei and P. robustus are very different. Paranthropus robustus likely ate foods that were on average much harder and less tough than P. boisei. The differences in both central tendencies and ranges of variation suggest different feeding strategies, and by implication, that the two species of Paranthropus probably had markedly different diets or foraging strategies.
Consequently, they argue that this is an example of Liem’s Paradox. As I point out above, though, Liem’s Paradox can readily be interpreted in terms of strong directional selection in times of food scarcity and really isn’t a paradox.
Robinson, B. W. and Wilson, D. S. (1998) Optimal Foraging, Specialization, and a Solution to Liem’s Paradox. The American Naturalist 151(3): 223-235
Ungar PS, Grine FE, Teaford MF (2008) Dental Microwear and Diet of the Plio-Pleistocene Hominin Paranthropus boisei. PLoS ONE 3(4): e2044. doi:10.1371/journal.pone.0002044
West-Eberhard, M. J. (2003) Developmental Plasticity and Evolution. New York: Oxford University Press.