Evolution's Downward Ratchet
When a species runs out of time and space
Something we don’t talk about very often is the fact that a species doesn’t usually die out because it is incapable of adapting. It dies out because it runs out of time.
Now, this for sure sounds like a paradox because evolution is supposed to be all about adaptation, and about how organisms become better suited to their environment. The phrase “survival of the fittest” is typically at the forefront when people talk about evolution.
But if you look at the fossil records, it tells an entirely different story. Most species that have ever lived are extinct.
You could even argue that the ones that disappeared the fastest, the ones that left behind the tiniest fossil record, the ones that appeared and disappeared over a few million years, often weren’t out-competed by rivals, nor were they destroyed by some kind of natural catastrophe. They simply degraded.
The mechanism has to do with the long-term effective population size, which can be much smaller than it looks.
When a population is small for a long time, it loses genetic variation, and this is a very well-known issue. What matters even more is the kind of variation that it loses. Random loss of DNA sequences can happen at a steady rate, and some of those losses are good sequences, some are bad sequences, and the majority are neutral. The bad mutations that disappear might have been selected against anyway. In each generation, a handful of novel mutations appear. Most of these are either slightly harmful or they have no effect, and they’re effectively neutral with respect to the fitness of the organism. In a large population, the slightly harmful ones should be weeded out by natural selection. But in a small population, they have a chance, because of random chance, of becoming present in every individual, and becoming a permanent resident in the species.
So now you can see where the difficulty lies. A small population doesn’t collapse in a single generation, but over dozens or perhaps hundreds of generations, the ratchet clicks. Metabolic efficiency might decline, or developmental stability might erode, or the immune function weakens, or fertility might drop. On their own, none of these changes is catastrophic. However, they compound over time.
The end result is that you have a population that is fragile and vulnerable to change. Or, to put it another way, when change comes, it has no genetic reserves to draw on, to deal with that change. Along comes a new pathogen, or the climate shifts, or a competitor arrives. If the population was large, standing genetic variation would probably mean that some individuals are resistant or better adapted or more flexible. But in a population with a long-term history of being of small size, you simply don’t have that ability.
Once the process starts, it feeds itself.
Populations with a long-term small size degrade quickly. Degraded populations are more vulnerable to disturbance, which in turn shrinks the ecological range of the species, and the smaller range means an even smaller population.
The details differ from species to species, but this is roughly the path taken.
Evolution in a small, stable population is dominated by random genetic drift - the random sampling of the next generation from the former. Individuals with “good” genetics can be randomly lost, while those with “bad” genetics can reproduce and spread these suboptimal genetic types. Populations that are small over the long term risk the gradual accumulation of damage. Most of this damage is invisible. Some of it is deleterious. None of it is catastrophic on its own.
Random genetic drift doesn’t get to dominate the scene in much larger populations. Random sampling of the generation does, of course, take place in every generation, but in large populations, selection is strong enough to see even the tiniest of deleterious mutations, and over time, it manages to eliminate them.
In the long run, in small populations, drift will win out, and the ratchet almost always clicks downward.
It would be wrong to think this is a statement about evolution being, in some way, wasteful or inefficient. It is not. This is a statement about the asymmetry of forces. Selection can only do so much work when the population is small. In particular, when the population is small, natural selection only gets to see the mutations that have enormous effects. Mutations of small effect, even if they are deleterious, can go under the radar and natural selection never picks them up. They accumulate.
The tragedy for any given species with a small effective population size isn’t that they cannot adapt. It is that they run out of time before they have the chance to. The population probably doesn’t feel itself degrading and from the inside, every generation looks pretty much like the last. In fact, we would only see the fragility in the rear-view mirror, when the disturbance arrives and the species simply has no reserves to call upon to deal with it.
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