Mutational Meltdowm

John Conery and Michael Lynch

Several factors can lead to the extinction of a species. Environmental influences, such as severe climatic changes or loss of habitat, are major causes of extinction. Demographics, or population dynamics, also plays an important role. For example, some species are territorial in nature, while others migrate over a wide range of territories. The complex interaction of these demographic variables with environmental changes can make it very difficult to assess the probability of extinction or to devise population recovery plans. A third category of factors that has not been studied until recently is genetics. It has long been known that random mutations occur quite frequently, and that the majority of mutations are harmful, but biologists have not agreed on the extent to which mutations or other genetic factors might contribute to extinction. For asexual species, in which offspring are genetically identical to their parent, mutations build up over succeeding generations and eventually will cause extinction, but for species that reproduce sexually it was generally thought that recombination would tend to flush out harmful mutations.

However, our computer simulations have shown that under the right circumstances the accumulation of muta tions alone will cause sexually reproducing species to go extinct. The simulations show that mutations build up slowly at first, but eventually the mutations begin to affect the size of the population. With a smaller population new mutations are even more likely to be passed to the next generation, which leads to an even smaller popula tion. Eventually the population reaches a critical point after which extinction occurs within a few generations. This snowballing process is known as mutational meltdown.

This work is made possible by the extensive use of high performance computer systems. For simple systems, such as the buildup of mutations in asexual populations, it is possible to develop analytical models in the form of math ematical equations that predict the number of generations to extinction as a function of the mutation rate, the aver age effect of each mutation, and other genetic variables. However, models of mutation accumulation in sexual species are too complex to predict with mathematical models. Realistic models of a particular ecosystem, for exam ple salmon in Northwest river systems, must take into account all three categories of variables - environmental, demographic, and genetic - and will be even more complex. Understanding the interaction between these effects will depend heavily on the use of high performance computer systems and will require cooperation between teams of biologists, mathematicians, and computer scientists.

Lynch, M., Conery, J.S., and B|rger, R. Mutation accumulation and the extinction of small populations. To appear in American Naturalist.

Lynch, M., Conery, J.S., and B|rger, R. Mutational meltdowns in sexual populations. To appear in Evolution.

Conery, J.S., Lynch, M., and Hovland, T. Optimizing irregular computations on SIMD machines: A case study. In Proc. Fifth Symposium on Frontiers of Massively Parallel Computation, (MacLean, VA, Feb. 6-9), 1995, pp. 222- 230.