Selection Pressures

Natural selection needs something doing the selecting. A drought, a predator, a pesticide, a fussy female — anything that makes some individuals more likely to reproduce than others is a selection pressure. The last one on that list is the interesting one, because it can push a species in a direction that makes survival harder.

What counts as a selection pressure

A selection pressure is anything in the environment that makes survival and reproduction more likely for some individuals than for others. It is the thing that decides which variations pay off.

The word “environment” here is broad. It includes the weather, the soil, the predators, the diseases, the neighbours competing for the same food — and the members of the opposite sex who decide who gets to breed.

Abiotic selection pressures

In biology, abiotic means non-living. An abiotic factor is a non-living factor within an ecosystem, and these affect which individuals in a population survive — which is why population size fluctuates from year to year.

Many abiotic factors are density-independent: a cold snap kills roughly the same proportion of a population whether that population is crowded or sparse. Compare that with a disease, which spreads far more easily when individuals are packed together.

Sexual selection

Sexual selection is a form of selection that occurs because one sex prefers certain characteristics in individuals of the other sex. It affects the evolution of animal species, and like every selection pressure it needs variation to work on.

Within the male cohort of a population there is variation in physical and behavioural traits, and those traits are visible to females (and the reverse also happens). Females often treat these differences as an indicator of a male’s overall fitness — a peacock with vibrant, healthy tail feathers is likely to be carrying a lower disease burden than one with dull, sparse feathers. So the trait affects who succeeds in attracting a mate, and over generations the alleles behind it become more common.

Sexual dimorphism

Sexual dimorphism is a distinct difference in size or appearance between the sexes of an animal species. Birds of paradise are the classic example: males are brightly coloured and perform elaborate courtship displays, while females are grey and brown.

The logic runs like this: a physical or behavioural trait in the male phenotype is used by females as an indicator of fitness → females are more likely to mate with males showing that trait → the alleles for it are inherited by the next generation → the trait becomes more exaggerated over time.

Reproductive isolation

Reproductive isolation occurs when changes in the alleles and phenotypes of some individuals prevent them from breeding successfully with other individuals of the species. For example, an allele might change a male’s courtship behaviour so that females no longer find him attractive. Those females stop choosing to mate with those males, and the two groups become reproductively separated even though they still live in the same place.

The peacock problem: when the two selections disagree

This is the part examiners like, because it breaks the simple story. Natural selection tends to produce populations that are well adapted to their environment. Sexual selection does not.

Peacock tail feathers are iridescent, long and covered in eye-spots, and they are used heavily in courtship. Over generations sexual selection has produced longer tails and more elaborate patterns. But a long tail reduces agility, makes flying harder, and makes the bird far easier for a predator to spot. In other words, the trait that improves mating success actively reduces the chance of survival.

What settles the argument is a trade-off. The trait grows more showy only up to the point where the extra mating success is cancelled out by the extra risk of being eaten.

Natural selection vs sexual selection

Worked examples