investigating life
If speciation typically takes thousands to millions of years, how do biologists conduct experiments to study this process?
Although speciation usually takes thousands or millions of years, and although it is typically studied in natural settings such as Lake Malawi or in the field experiments described in Investigating Life: Flower Color Reinforces a Reproductive Barrier in Phlox, some aspects of speciation can be studied and observed in controlled laboratory experiments. Most such experiments use organisms with short generation times, in which evolution is expected to be relatively rapid.
William Rice and George Salt conducted an experiment in which fruit flies (Drosophila melanogaster) were allowed to choose food sources in different habitats. The habitats—where mating also took place—were vials in different parts of an experimental cage (Figure 22.18). The vials differed in three environmental factors: (1) light; (2) the direction (up or down) in which the fruit flies had to move to reach food; and (3) the concentrations of two aromatic chemicals, ethanol and acetaldehyde. In just 35 generations, the two groups of flies that chose the most divergent habitats had become reproductively isolated from each other, having evolved distinct preferences for the different habitats.
Figure 22.18 Evolution in the Laboratory For their experiments on the evolution of prezygotic isolating mechanisms in Drosophila melanogaster, Rice and Salt built an elaborate system of varying habitats contained within vials inside a large fly enclosure. Some groups of flies developed preferences for widely divergent habitats and became reproductively isolated within 35 generations.
The experiment by Rice and Salt (see American Naturalist 131: 911–917, 1988) demonstrated an example of habitat isolation as a prezygotic isolating mechanism. Even though the different habitats were in the same cage, and individual fruit flies were capable of flying from one habitat to the other, habitat preferences were inherited by offspring from their parents, and populations from the two divergent habitats did not interbreed. Similar habitat isolation is thought to have resulted in the early split between cichlids that preferred the rocky versus the sandy shores of Lake Malawi. In controlled experiments like this one, biologists can observe many aspects of the process of speciation directly.
Biologists can now sequence entire genomes of organisms, which allows them to compare divergent genes in pairs of recently divergent species, and to identify the genes responsible for prezygotic and postzygotic isolation. These same techniques also allow researchers to detect any level of hybridization between species. Thus we are now able to determine the genetic basis of reproductive isolation, and measure the rate at which diverging species become isolated from one another. With these advances, biologists can now appreciate the details and genetic basis for the origin of species.