In a cave, a bear gives birth to two cubs one long dark night. In the morning, the weak winter light reveals something strange: the cubs’ fur is white, in stark contrast to the dark fur of their mother. They are freaks… or are they?
What is evolution? Easy, you might think: it’s the way living organisms change over time, driven by natural selection. Well, that’s not wrong, but it’s not really how evolutionary biologists think of it.
Picture those bear cubs. Here we see a dramatic physical change, but it isn’t evolution. Among black and brown bears, white bear cubs are not that uncommon. But white bears don’t have more cubs than other bears, so the gene variants for white fur remain rare.
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Among one group of brown bears living in the Arctic, though, white fur was an advantage, helping them sneak up on prey. There white bears thrived and had more offspring – their “fitness” increased – so the proportion of white bears rose until the entire population was white. This is definitely evolution. It happened as polar bears evolved from brown bears a few million years ago.
So although we tend to think about evolution in terms of the end results – physical changes in existing species or the emergence of new ones – the key concept is the spread of genetic variants within a population.
This results of this process can appear purposeful. Indeed, it is convenient to talk as if they are: “polar bears evolved white fur for camouflage”. But it all comes down to cold numbers: a random mutation that boosts fitness spreading in a population.
What’s more surprising is that even mutations that don’t increase fitness can spread through a population as a result of random genetic drift. And most mutations have little, if any, effect on fitness. They may not affect an animal’s body or behaviour at all, or do so in an insignificant way such as slightly altering the shape of the face. In fact, the vast majority of genetic changes in populations – and perhaps many of the physical ones, too – may be due to drift rather than natural selection. “Do not assume that something is an adaptation until you have evidence,” says biologist Larry Moran at the University of Toronto, Canada.
So it is wrong to think of evolution only in terms of natural selection; change due to genetic drift counts too. Moran’s minimal definition does not specify any particular cause: “Evolution is a process that results in heritable changes in a population spread over many generations.”
It does not even have to involve many generations, says Michael Kinnison of the University of Maine in Orono, who studies how living species are evolving. Evolution occurs almost continuously, he says. It usually takes time for populations to change significantly, but sometimes it happens very fast, for instance when only individuals of a particular genetic type survive some catastrophe, or when only tumour cells with a particular mutation are not killed by a cancer drug.
In these cases, there is no need to wait for the survivors to reproduce to determine that the population has changed. “I would say evolution occurs whenever some process changes the distribution of heritable traits in a population, regardless of the time scale,” Kinnison says. “While evolutionary biologists like to treat evolution as a generation-to-generation process, that is often more a matter of convenience than reality.”
Suppose those white bear cubs somehow reached an island and founded a new bear population. The interbreeding of white bears always produces white offspring, and thus being white would be normal there. So we can boil down the concept of evolution to just six words: Evolution is what makes freaks normal.
Read more: “Get your head around the 13 boldest ideas in science”
This article appeared in print under the headline “How to think about evolution”
