It is common for similar anatomical structures and physiological processes to evolve independently in different species of animals and plants. This phenomenon (known as convergent evolution) reflects the fact that similar ecological pressures impose similar functional requirements; in other words, each task calls for a specific tool. What is not clear is whether convergently evolved traits share the same genetic and molecular basis. It is possible that different molecular pathways can be co-opted in different evolutionary lineages to produce superficially similar structures and adaptations. This hypothesis will be tested by identifying the genes responsible for evolutionary changes in color patterns in several species of fruit flies (Drosophila). Drosophila has been chosen as a model for this research due to the ease and low cost of genetic and molecular experimentation in this insect. If different genes are found to produce similar color patterns in different species, it will indicate that natural selection can utilize different sources of variation to achieve the same functional outcome. An opposite finding will suggest that the action of natural selection is constrained by the inherent structure of genetic pathways that control animal development. Given the increasing importance of understanding adaptive processes in human pathogens, agricultural pests, and other biological species that affect human society and economy, resolving this interplay between chance and necessity in evolution will help in the development of biological and chemical control strategies. An additional goal of this project is to train young scientists prepared to take the lead in analyzing molecular variation on genome-wide scales, and relating it to anatomical and physiological traits and environmental factors. Such training is essential for further advances in human medicine, agriculture, forestry, and biotechnology. This project will provide an opportunity for students to acquire first-hand research experience and expand their career options by complementing theoretical education with a practical application of the latest concepts and techniques.
What are the relative roles of chance and constraint in evolution? S. J. Gould has famously proposed that "if we could rewind the tape of time and play it again, everything would come out differently". Short of finding life on other planets, convergent evolution (independent evolution of similar traits in different organisms) is our only chance of re-running the evolutionary tape. Elucidating the mechanistic basis of convergent evolution can help us evaluate the extent to which the random forces of natural selection are tempered by developmental and functional constraints. Convergent evolution is a pervasive phenomenon that affects all levels of biological organization, from molecules to morphology. A fundamental question raised by the widespread occurrence of convergent traits is to what extent trait convergence reflects an underlying similarity of molecular mechanisms that control individual development and metabolism. Are convergent traits generated by the same molecular mechanism in each case, or can different mechanisms produce a similar evolutionary outcome? The evolution of Drosophila color patterns offers an excellent model for addressing these questions. The development of pigmentation is simple compared to most other traits, and we have considerable knowledge of its genetic basis. The color and spatial pattern of pigmentation vary dramatically within and among Drosophila species, and present numerous instances of phenotypic convergence. Finally, Drosophila is an experimental organism par excellence, and is amenable to a variety of molecular and genetic approaches. In this project, we examined the genetic control of identical color patterns that evolved independently in different species of Drosophila. We used recently developed genomic methods to map the genes responsible for color pattern variation in each of several interspecific crosses. We found that convergently evolved color patterns are controlled by distinct though overlapping sets of genes, and we are now close to identifying some of these genes at the molecular level. This work suggests a "toolkit model" of convergent evolution. We suggest that, for any given trait, only a limited set of genes (the toolkit) is capable of evolving in a way that generates viable phenotypic variation. Each evolving lineage samples genetic variants in toolkit genes in a stochastic manner dependent on mutation, demography, and natural selection. In this way, a combination of chance and constraint leads to similar but not identical genetic underpinnings of trait variation in different evolutionary lineages.
