When does a dramatic physical difference become evolution? Steven Kazlowski/Science Faction/Corbis
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.
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 …
