Saturday, June 03, 2006

An Example of Speciation: The Cichlid

Muddled in the festering maw of the evolution debate has been the concept of speciation. Creationists blithely dismiss that evolution is even possible by zeroing in on knowledge gaps on specitation. Although the record is not complete, scientists have gained ground. Ernst Mayr has said that evolution is no longer a theory it is fact (Mayr 2001), and today we have many examples of speciation.

By studying the relationship between genetic variation and differences in outward characteristics (shape, behavior, ecology, physiology) among species that are closely related scientists can document evidence of speciation. So rather than drawing on correlation between a wide variety of species (which may or may not be related), the next generation of evolutionary studies will attempt to tie population genetics with molecular genetics of closely-related species.

However, one problem with evolutionary biology is the arcane terminology, and speciation is no exception. With terms like allopatric, sympatric, and parapatric speciation it’s no wonder that people–even many biologists–get tongue tied and confused. The classic concept of speciation has focused on how new species arise out of geographic separation. This concept has helped explain some aspects of evolution, but not all. There is also evidence of speciation when the populations are not separated, but they diverge in adjacent areas. Nevertheless for decades many biologists dismissed the idea that speciation could occur in overlapping populations. But more recently, acceptance of new species arising within the same geographic region has been studied and documented. How can this be? Ecological and sexual selection, and reproductive isolation (subtle changes in the timing or behavior of reproductive processes) are examples of how species can differentiate within an overlapping area.

So what are some living examples of speciation: everyone is familiar with the Galapagos finches, which diverged from a few species into many. Other examples include the Arctic char (a trout- like fish from northern latitudes), anole lizards (like the chameleons you can get at the pet store), and the honeycreeper from Hawaii (long-billed birds that have different bill shape based on feeding strategies). Cichlid fish are another spectacular example, and there has been extensive research on these fish in recent decades. Let’s spend some time on cichlids.

The Cichlid Model
Kocher (2004) provides an excellent overview, and the text below is a synopsis of his paper.Cichlid fish inhabit tropical and subtropical freshwater lakes and rivers in three continents (Africa, southern Asia, and the Americas) and Madagascar. Their diversity is greatest in East Africa, where some 2,000 species exist. The speciation of cichlids has been relatively recent (geologically speaking), over the past 10 million years. What’s particularly interesting is that the “radiation” of species (i.e., speciation) has not necessarily occurred through geographic separation. They have diverged through other means: habitat separation, changes in shape, and variation in color pattern and behavior. The fossil record has documented the radiation of these fish, and scientists have used genetic analysis has been able to chronologically document the relationship among species.

For example, scientists have shown a phased evolution of cichlids. In the first stage, these fish diverged by habitat separation–some became rock dwellers and others became dependent on sandy habitats. The next phase, driven by natural selection, resulted in more species that became different by exploiting different food sources–thus their feeding style, and anatomy, changed (diverged) over time. Species that evolved most recently developed color variation, primarily based on sexual selection. Sexual selection is through mate choice, where the female selects
mates based on color, pattern or behavioral traits. The recent rapid radiation of many varieties of cichlids may be largely due to sexual selection. This phased evolution of species has also been documented in the char, anole lizards and honeycreepers.

The divergence within lakes is incredible. For example, in Lake Tanganyika alone there are 1,000 species of cichlids. What has happened over the past 2 million years is that sub populations splinter off, very close geographically to the parent population but they stay within their subpopulation without much intermixing (known as gene flow). There is enough difference in habitat type, feeding opportunity, and predator pressure that over time and through natural selection they become a slightly different fish. And then, through sexual selection (typically by females selecting their mates) results in a greater species diversity in color patterns and behavior.

What makes the cichlids particularly useful for study is that with new genetic techniques biologists can develop analytical links between theory and observation. They can posit a hypothesis, study populations of closely related cichlids (e.g., habitat use, feeding strategies, coloration and mating strategies) and link that information with genetics. It is through this type of science that we will learn much more about speciation and how it works. What is being learned about cichlids is also being applied to other species such pocket mice and salmon.

What makes the modern study of evolution so robust is that a number of independent sub-disciplines, such as genetics, anatomy, paleontology, geology, ecology, and animal behavior have come together and synthesized information that corroborates and clarifies much of what has been theorized since Darwin’s time.

Kocher, T.D. 2004. Adaptive Evolution and Explosive Speciation: The Cichlid Fish Model. April 2004, Volume 5.

Mayr. E. 2001. What Evolution Is. Basic Books.


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