Evolution of Sex and Recombination, Mutation Accumulation, Evolution of cooperation

Prof. Howard is interested in a wide range of evolutionary and ecological questions. A major focus of his research is directed toward understanding the adaptive significance of sexual reproduction and the associated event of genetic recombination. Accounting for the widespread distribution of sex in nature is an important and enduring problem in biology; all else equal, organisms that reproduce asexually are expected to increase at twice the rate as their sexual counterparts. The cartoon below illustrates this point. The asexual female (on the left) gives rise to four grandchildren compared to two for the sexual female (on the right). This "cost of sex" should result in the very rapid replacement of sexual populations by clones. The challenge is to explain why we are not living in a world dominated by asexual species.

One intriguing possibility is that sex is adaptive as a genetic defense against parasites. This idea, known as the Red Queen hypothesis, predicts that sexual populations suffer reduced levels of parasitism due to their ability to recombine genes. Asexual lineages, on the other hand, lack recombination and instead make exact genetic copies of themselves.

The fact that clonal offspring are genetically identical to their parents should make them highly susceptible to infection by coevolved pathogens. The Red Queen hypothesis predicts that clones are prevented from replacing sexual populations because infection by debilitating parasites erodes their intrinsic twofold reproductive advantage. Recent empirical work suggests that parasites may indeed play a role in the maintenance of sex, but it is unclear as to whether or not they are the sole selective agent.

A competing hypothesis maintains that sex is adaptive because it purges the genome of harmful recurrent mutations. According to this idea, sexual lineages persist because they are more efficient than asexuals at clearing their genomes of such mutations. Given enough time, asexual lineages are expected to be eliminated due to the accumulation of a large mutation load. A potential shortcoming of this idea is that mutation rates must be high (>> 1.0 per genome per generation), and mutation number must map onto fitness in a very specific way.

Prof. Curt Lively (professor of biology at Indiana University) and Prof. Howard have made extensive use of individual-based computer simulation models to examine the interaction between parasitism and mutation accumulation in generating an advantage to sex. The results from these studies suggest that when both mechanisms operate in the same population, sexual populations may be rendered evolutionarily stable against invasion by clones ( the data - slide provided by Curt).

Another area of current research involves mathematical modelling of the rates of mutation accumulation in non-recombining lineages. Prof. Howard is involved in an ongoing collaboration with Professor J.F. Fontari and his colleague Alexandre Colato (mathematical physicists) at the Institute of Physics at Sao Carlos, Brazil. They recently co-authored a paper on mutation accumulation in growing asexual populations, and are now working to understand the process in populations which undergo dynamic flucations in size.

Professor Howard is also interested in various other aspects of population biology and evolutionary theory, especially as they relate to the evolutionary consequences of infectious disease. His graduate students have worked on a variety of problems, including sexual selection in birds, habitat partitioning in shrews, mortality trends in migratory songbirds, and the maintenance of color polymorphisms in insect populations. Please feel free to email him with questions or to express interest in a research project.