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Darwin saw the ecological relationships between species being the source, or one of the sources, of selection pressure. That’s where the struggle for existence happens; it happens between species and between individuals in an ecological context. So a 20^th Century biologist described the situation as there being an evolutionary play inside an ecological theatre - and there are lots and lots of examples.

A favourite one of mine would be the evolution of the banded snail. This is eaten by thrushes. Thrushes search for these things and, when they find them, they typically take the snails to a particular stone within their territory, called an anvil, and they beat the hell out of the snail and the shell breaks off and then they eat the snail inside. You can look at what the thrushes have been eating by looking at the remains around these thrush anvils.

And, what’s nice about this banded snail is, actually, it varies a lot in its shell colours and patterns, and so experiments have been done using the remains found around the thrush anvils and this sort of thing, which demonstrate that the patterns of these banded snails are adaptive, and the snails that are more cryptic, that are more difficult to find, appear, are eaten less often, turn up at anvils dead less often than the ones that are more conspicuous.

And this is, basically, a selection process going on, natural selection, where the agent of natural selection, the thing doing the selection if you like, is the thrush in its predation.

Why are there so many species? Well we know these species evolve through natural selection, but it’s not obvious how natural selection alone would produce these species. And the reason for this is the following: that natural selection favours those individuals that breed the most often. Different species occupy different niches, that is to say different roles in what Darwin described as “the economy of nature”, or what we’d describe today as ecology. They have different ecological roles.

So these could be different positions in the food web, so there are plants and there are things that eat plants, there are things that eat the things that eat plants, and so on. And there are many different ways of being a plant and being a herbivore, and there are different ways of capturing your food if you’re a carnivore, and these are the different niches, they’re like the roles in an economy, hence Darwin’s term “the economy of nature”. And that is the answer to why there are so many species when you combine it with natural selection, so that’s another way in which ecology and evolution are counterparts and have to be seen as different aspects of the same thing.