Richard Dawkins at his Substack newsletter, The Poetry of Reality:
Natural Selection is the pressure that drives evolution up the slopes of Mount Improbable. Pressure really is rather a good metaphor. We speak of ‘selection pressure’, and you can almost feel it pushing a species to evolve, shoving it up the gradients of the mountain. Predators, we say, provided the selection pressure that drove antelopes to evolve their fast running legs. Even as we speak, though, we remember what this really means: genes for short legs are more likely to end up in predators’ bellies and therefore the world becomes less full of them. ‘Pressure’ from choosy females drove the evolution of male pheasants’ sumptuous feathers. What this means is that a gene for a beautiful feather is especially likely to find itself riding a sperm into a female’s body. But we think of it as a ‘pressure’ driving males towards greater beauty. No doubt predators provided a selection pressure in the opposite direction, towards duller plumage, since bright males would presumably attract predator, as well as female, eyes. Without the pressure from predators the cocks would be even brighter and more extravagant under pressure from females. Selection pressures, then, can push in opposite directions, or in the same direction or even (mathematicians can find ways of visualizing this) at any other ‘angle’ relative to one another, Selection pressures, moreover, can be ‘strong’ or ‘weak’, and the ordinary language meanings of these words fit well. The particular path up Mount Improbable that a lineage takes will be influenced by lots of different selection pressures, pushing and tugging in different directions and with different strengths, sometimes cooperating with each other, sometimes opposing.
But pressure isn’t the end of the story.
More here.