Sidebar. Population bottlenecks, human and other

Species clearly go extinct. They can also go “almost extinct” if they reach a very small population size or a “bottleneck.” Cheetahs are a classic example of a species that has gone through a bottleneck. They may even be doomed apart from poaching, loss of habitat, and other human interferences. It’s hard to recover from a bottleneck because the population has low diversity of genes while having many different genetic variants may be needed for recovery as the environment continues to change. The primary genetic variants are different alleles, which are different forms of the same gene such as for coat color of an animal, drought tolerance of a plant, or metabolic thriftiness of a human.

The best guarantee of sufficient genetic diversity is a large population size. It’s not pure numbers that matter but the effective population size, Ne. It’s basically a measure of genetic mixing and sorting. This is lower than the full population size or census. Various factors reduce Ne. These include past fluctuations in population size, a ratio of males to females skewed in either direction, and the distribution of, well, litter sizes. Various books and websites discuss the concept.

Humans look rather diverse phenotypically, that is, in outward appearance and performance. Still, for our huge current population, we are not very diverse genetically. (This also bears on the unreality of defining races.) Our effective population size is 10,000 or even much less, while chimpanzees and gorillas with their tiny populations may have Ne twice as big.

What causes population bottlenecks, and what evidence is there for the causes of the bottleneck in the human species? Many things can go wrong for a species biologically and geologically. For us, there are diseases, famines, and wars, and in the past, perhaps effective predators or competitors. We have records of some of these, increasingly very fragmentary going deeper in time. Even direct population counts are decisive, in view of the factors cited above. For pure population numbers over time, we can’t go digging up human remains all over to get their numbers, then get their ages with, say, carbon dating, and put together a plot of actual population size over our lifetime as anatomically modern humans, 200,000 years. Instead, we can look into the detailed genetic structure of the whole human population. With modern genetic sequencing that’s possible. That’s what gave us various estimates of our effective population size… but not of the dates of our near crashes. Geological events are suspects in the “crime.” The massive eruption of the Toba volcano 70,000 years ago has been proposed as a possible cause. The evidence is sketchy for a worldwide effect. Climate changes from causes other than volcanoes are other possibilities. Particularly for pre-agricultural times it would be difficult to estimate how badly such changes affected wild food sources or stressed human temperature tolerances.

On the basis of direct biological function rather than genetics, ecologists have developed the concept of a minimal viable population size. They look at means and variability in reproductive success of individuals, the same in death rates, and more. The idea is that random variations generate trends akin to the gambler’s ruin. Random variations plus and minus can have a run of negatives that lead to the bankruptcy of a gambler (with any size of initial stake) or the extinction of a species. The link to the genetic concept of effective population size is still marginal.

What disposes a species to be able to recover from a population bottleneck is just what Thomas Malthus pointed out – the ability to repopulate quickly with a high rate of reproduction. This lends itself to exhaustion of resources that then limits the population. That can cause a sequence of subsequent crashes and recoveries, and, of course, can cause much misery. In any event, a species that does not have the capacity for overpopulation doesn’t have the capacity for recovery, which means that such as species doesn’t exist. Species that have some capacity may still go extinct.

The lesson for evaluating habitability is that species have their ups and downs, and that some of the downs are terminal. In fact, the downs are eventually terminal for all species. The Earth has been overall habitable for life of any form for over 3 billion years but every species does go extinct. For large vertebrates the average lifetime is about 2 million years, with wide variations among species. At what stage are we? What may we expect for life on other habitable planets? For one thing, we don’t expect “progress,” with longer overall habitability yielding “bigger, better, smarter” life forms. Expect to see evidence of major changes, including many truncated branches in the tree of life as well as mass extinctions. Of course, mapping the course of life on an exoplanet is beyond possibility. We have to use other tools. We may look for evidence of chemical disequilibrium. An example is the presence of methane in an oxygenic atmosphere, which is very likely to be from life forms generating methane.

Bottlenecks, extinction, adaptation to new conditions, habitability – they’re all tied up together in ways that we are just appreciating on Earth. That should inform us of what to expect on exoplanets.