Population Evolution and Island Biogeography

In most areas of empirical science, including biology, we take an experimental approach for granted. However, in evolutionary and related studies, large-scale natural or field experiments are rare because the large spatial and temporal scales of evolutionary and biogeographical processes render experimentation problematic. Classically, biogeography is about large-scale pattern: How many species are there on an island? Is the number related to extent of isolation, island size, or complexity of vegetation? For example, why are there so many tree lizard (anole) species on each of the Greater Antilles but only one or two on each of the Lesser Antilles? Is it simply island size that is accountable, even though the small islands are environmentally heterogenous and complex? This is not readily subject to direct experimentation. However, these large-scale biogeographic patterns are mediated by small-scale population-level processes. These include ecological processes such as competition between species and habitat usage, and evolutionary processes such as adaptation by natural selection, ancestor-descendant relationships, and speciation (splitting into distinct species, which do not interbreed). Exceptionally, it may be possible to manipulate these fundamental population processes experimentally to gain insight into biogeographic patterns, although even population-level experimentation is difficult. For example, experimental introduction of small Caribbean anoles onto islands, and experimental translocation between large enclosures within islands, have revealed much about the evolutionary and ecological population processes underlying their biogeography. Large-scale translocation of the small tree lizard Anolis oculatus (Dominica, Lesser Antilles) has demonstrated the rapid effect of natural selection on a wide range of genetically controlled traits in response to wet or dry habitats, thus explaining the nature and cause of the geographic variation. In addition, experimental introduction of Anolis sagrei onto Bahamanian islands has shown how predators can alter the behavior, niche usage, and their selection for prey. These studies also suggested how introduced predators may render their prey more vulnerable to extinction by catastrophes such as hurricanes.