One common misconception about evolution is that the processes that produce phenotypic change between species (macroevolution) are different from those that produce phenotypic variation within species (microevolution). For example, creationists might accept microevolution, since this can easily be observed in a lab, but would argue that since macroevolution is difficult to observe over the span of one human lifetime, there is no evidence for it and that large-scale differences must be the result of some designer. In reality, however, there is no real distinction between microevolutionary and macroevolutionary changes: the same microevolutionary processes that result in variation within species give rise to macroevolutionary differences between species. And this afternoon, evolutionary biologists from the University of Maryland and Syracuse University published a study in the journal Proceedings of the National Academy of Sciences that nicely illustrates this link between microevolution and macroevolution. Their study, titled “Craniofacial divergence and ongoing adaptation via the hedgehog pathway“, highlights the genetic basis of within and between species variation in jaw shape in African cichlid fishes.
Different species and genera of cichlids are defined in part by the shape of their jaws and mouth, a complex structure that helps alternate species feed on different food stuffs. Cichlids from the genera Labeotropheus and Metriaclima differ tremendously in the shape of their mouth and jaw. Labeotropheus have a large jaw and flat mouth adapted for biting algae from rocks, whereas Metriaclima have a small jaw and mouth adapted for sucking plankton from the water column. The two shapes are unmistakably different and clearly represent a macroevolutionary difference between species and genera (see Figure 1).
Using an ingenious combination of genetic and association mapping in Labeotropheus and Metriaclima, the authors identified a genetic variant near the gene patched1 (ptch1) that explains some of this variation in jaw shape. Ptch1 is a component of the famous hedgehog signaling pathway that, among other things, promotes bone development and growth. The authors found a genetic variant that explained ~ 8.6% of the variation in the length of the retroarticular process (RA), a part of the jawbone that acts as an inlever for jaw opening. Lengthening the RA, as seen in Labeotropheus, slows the speed of jaw opening (called the mechanical advantage of opening, or MAo), whereas shortening it as in Metriaclima increases the MAo (Figure 1). The authors found no difference in the sequence of the ptch1 gene between these species, but they did find that ptch1 expression is greater in the developing RA of Labeotropheus which, compared to Metriaclima, have thicker jaws and longer RAs. Finally, by chemically knocking down the activity of the hedgehog pathway in developing Labeotropheus, the authors produced Labeotropheus individuals with Metriaclima-like jaws (Figure 2). Thus, their results clearly demonstrated that a regulatory mutation near the ptch1 gene is responsible for macroevolutionary differences in jaw shape between these species.
However — and perhaps more interestingly — the authors also examined the link between jaw shape and the ptch1 regulatory mutation in other cichlids, including multiple populations of the genus Tropheops. Unlike Labeotropheus or Metriaclima, Tropheops individuals exhibit considerable microevolutionary variation in jaw shape. Despite being members of the same species, Tropheops individuals collected from shallow locations feed on algae-covered rocks and have Labeotropheus-like jaws, while individuals collected from deep locations suck plankton and other invertebrates and have Metriaclima-like jaws. By comparing MAo and alleles of ptch1 across Labeotropheus, Metriaclima, and Tropheops, the authors found that the same alleles linked to macroevolutionary divergence in jaw shape between Labeotropheus and Metriaclima are linked to microevolutionary divergence within Tropheops (Figure 3). The authors expanded their analysis to even more species and found the same thing: obligate biters have the long-RA (Labeotropheus-like) ptch1 alleles, while suction feeders have the short-RA (Metriaclima-like) alleles. Thus, the authors deftly demonstrated that the same genes responsible for macroevolutionary divergence in jaw shape between Labeotropheus and Metriaclima also cause microevolutionary divergence in jaw shape within Tropheops (Figure 3).
Some questions remain unanswered, including whether ptch1 mutants contribute to adaptive variation in jaw shape among other vertebrates, and how exactly hedgehog signaling changes bone development. But one thing is clear: the only difference between the evolution of macroevolutionary and microevolutionary traits is time. Macroevolutionary changes are the result of microevolutionary changes over long periods of time that become fixed or nearly so, where intermediate forms are missing either due fixation or extinction. However, by examining closely-related and rapidly evolving organisms like cichlids, it is possible to investigate both macro- and microevolution at once. The macroevolutionary divergence in jaw shape between Labeotropheus and Metriaclima evolved over the last 0.5 to 1 million years, and intermediate forms are mostly absent since the causative alleles are nearly fixed in both species. However, the same process is currently ongoing in Tropheops, whose populations are likely less than 100,000 years divergent. Thus, by examining ongoing evolution in the mouths of these remarkable fish, it is possible to clearly demonstrate that the processes of microevolution and macroevolution are the same. Macroevolution is microevolution.
As a final note, some of the authors of this study work close by — literally next door — and could be coerced into answering readers’ questions. Feel free to ask away in the comments below. Don’t be shy.Roberts, R.B., Hu, Y., Albertson, R.C., Kocher, T.D. (2011) Craniofacial divergence and ongoing adaptation via the hedgehog pathway. Proceedings of the National Academy of Sciences, epub ahead of print. DOI: 10.1073/pnas.1018456108.