Biologists agree that the huge diversity of species seen on earth have evolved from a common ancestor through the process of natural selection. However the details of how this occurs have been hotly debated. How do populations of organisms adapt to their local environment? How are new species formed? One theory called gradualism (which was the mechanism initially proposed by Darwin) suggests that adaptive changes occur in incremental steps. So for example, if a bird had a short beak which was well adapted to environment A and some individuals from that population suddenly found themselves in environment B (lets just say that it is and island and there no subsequent contact between the two populations) and environment B favors individuals with long beaks then there will be natural variation of beak length within the population and those individuals with longer beaks will survive better and produce more offspring. Over many generations mean beak length will become longer and longer until eventually population B have longer beaks than population A. Such differences between populations could be modulated by relatively small changes in a large number of genes. One question is whether adaptive changes can also occur on a more rapid timescale. Examination of the fossil record indicates that in many cases a species changes very little over time and speciation events are relatively rapid (on a geological time scale) leading to an idea called punctuated equilibrium. One question is whether traits instead of shifting gradually through intermediate stages can be changed directly from one state to another. Using the previous example immediately after the birds from environment A colonized environment B the vast majority of birds would have relatively short beaks but a few individuals would have substantially longer beaks creating a bimodal distribution of beak size. The longer beaked individuals would survive better and produce more offspring and over time the majority of individuals in population B would have long beaks. In this case beak length would likely be controlled by a small number genes such that changing the regulation of one or a few genes would result in large changes in the size of the beak.
Stickleback fish have provided a great natural experiment for studying evolutionary mechanisms. The marine form of sticklebacks have invaded many North American lakes in separate colonization events. Once a stickleback population becomes established in the lake environment several adaptions occur including a loss of armor plates and reduction of the pelvis. Comparing marine and freshwater populations has allowed evolutionary biologists to observe how these fish adapt to their new environment and in some cases discover the underlying genetic mechanisms.
Tomorrow: What do sticklebacks have to tell us about the mechanisms of adaptive change?
