Wednesday, November 25, 2009

Extinction is evolution

I often enjoy Olivia Judson's science blog on She has an actual science background, and gets the science badly wrong far less often than most other reporters of science.

Her most recent post is more in line with what I expect from science journalism. Without recapping her whole argument, she writes, "But if [evolution] has that much potential — how come organisms keep going extinct in nature? In other words, why does evolution keep failing?"

She then goes on to make a big deal about how evolution fails each time a population fails to evolve. But in so doing she fails to admit the distinction between evolution on the one hand and adaptation on the other.

Evolution is genetic change in lineages with time. Adaptation is the process by which the genetic makeup of a population is altered in a way that helps individuals of that population be more successful in their environment. Adaptation is one mechanism, and result of, evolution. It happens to be the best known mechanism, and the one that in the popular imagination defines evolution. But evolution has lots of other mechanisms. Genetic drift for example. Imagine you have a small population of squirrels on an island, some gray, some black. A disease comes through and kills a significant portion of the squirrels, and even though they are all equally susceptible to the disease, through the randomness of epidemiology more grays than black catch it and die. After the disease, the population has a higher proportion of black alleles than before. The population has evolved, because the proportional genetic makeup of the population has changed. But this is not an example of adaptation, because the genetic change was not caused by one version of the color gene conferring a selective advantage over the other. The allele frequencies "drifted" without being pushed by selective considerations. The next time the disease comes though more black squirrels could happen to catch it, and the allele frequencies could drift back.

So adaptation is not the be all and end all of evolution. Much of the evolution that happens is non-adaptive. And another important mechanisms of evolution is extinction. Imagine now that the disease killed not just some, but all of the squirrels on the island. That population is extinct. They are no longer represented in the genetic diversity of squirrels more generally, of rodents more generally, of mammals more generally. The remaining diversity, through the removal of that population, has changed. The extinct population has ceased evolving, but through their removal the larger group is left genetically less diverse. The genetic makeup of the larger lineage has changed, so evolution has occurred. It is also possible that the residual populations are more fit than the pre-epidemic average, because they are resistant to that particular disease. Whether or not this qualifies as adaptation depends on the fine details of the definition of adaptation you prefer. There is no standardly accepted definition, or rather different groups of biologists prefer different definitions. But all evolutionary biologists, save those who have crossed over into journalism, agree that extinction is an example of, not a failure of, evolution.

No comments: