Math Grad Seminar Oct 11, 2006

How Can One Tell In What Direction Evolution is Going?

Stanley Sawyer

Abstract:
In the long run, most important changes in biological organisms are due to the replacements of genes by new genes that do a better job for the organism. However, in large, established populations, many biologists believe that most evolutionary change is, instead, due to the replacement of genes by slightly deleterious variants.

The reason for this is that most mutations are harmful rather than helpful, and that mildly harmful mutations can replace a better, established gene by the chance effects of who mates with whom and who happens to survive. This would take a long time for a large population, but most large populations have been around for a very long time. We can study this question from the distribution of DNA in populations in the present. The distribution of a new gene is different if it is advantageous, deleterious, or selectively the same as the old gene. We can obtain more information from the differences between two related species. Current data suggests that most recent evolutionary changes in fruit flies were advantageous, but that most recent evolutionary changes in a common weed were deleterious. Investigating this process in detail leads to difficult problems in probability theory and statistics, as well as to the use of a technique called Markov Chain Monte Carlo.