Monster Seminar Jam - Chance & Necessity in Highly Fecund Marine Life
Dr. Dennis Hedgecock, Biological Sciences, University of Southern California
Most evolutionary biologists consider Natural Selection, particularly balancing selection, as the primary force behind the maintenance of biodiversity. In the 1980s, positive correlation between fitness-related traits, such as growth and survival, and allozyme heterozygosity in bivalve mollusc populations was taken by some as evidence that balancing selection maintained these protein polymorphisms. However, in the late 1990s, experimental work on the Pacific oyster revealed selection against recessive mutations as the more plausible alternative explanation for this correlation. Evidently, this oyster and, by inference, other highly fecund marine fish and shellfish generate a large load of deleterious recessive mutations, simply as a by-product of having to produce tens of millions of eggs (Necessity). Selection against homozygotes for these mutations likely causes the correlation of fitness with allozyme heterozygosity in natural populations. Why doesn't selection eliminate these mutations or drive them to low levels? A large part of the answer is Chance, in the form of sweepstakes reproductive success, which is made possible, again, by high fecundity and which reduces effective population size (Ne ), increasing genetic drift and inbreeding in marine populations. Genetic drift may allow even highly deleterious mutations to persist longer than expected in marine populations; these mutations may also be introduced at frequencies greater than 1/2N, through a process known as clustered mutation. Inbreeding depression, resulting from mutational load at the genomic level, coupled with sweepstakes reproductive success, may thus account for much of the standing variation in fitness (growth & survival) in marine fish and shellfish. Since many deleterious mutations are expressed in early life stages, these findings have implications for understanding the recruitment problem in fisheries.
Date and Time:
January 31, 2008,
11:00 am - 12:30 pm