Much change is due to random genetic drift rather than positive selection. It could be called the survival of the luckiest.
Take a look in the mirror. The face you see is rather different to that of a Neanderthal. Why? The unflattering answer could be for no other reason than random genetic drift. With features that can vary somewhat in form without greatly affecting function, such as the shape of the skull, chance might play abigger role in their evolution than natural selection.
The DNA in all organisms is under constant attack from highly reactive chemicals and radiation, and errors are often made when it is copied. As a result, there are at least 100 new mutations in each human embryo, possibly far more. Some are harmful and are likely to be eliminated by natural selection – by death of the embryo, for instance. Most make no difference to our bodies, because most of our DNA is useless junk anyway. A few cause minor changes that are neither particularly harmful nor beneficial.
You might think that largely neutral mutations would remain restricted to a few individuals. In fact, while the vast majority of neutral mutations die out, a few spread throughout a population and thus become “fixed”. It is pure chance – some just happen to be passed on to more and more individuals in each generation.
Although the likelihood of any neutral mutation spreading by chance is tiny, the enormous number of mutations in each generation makes genetic drift asignificant force. It’s a little like a lottery: the chance of winning is minuscule but because millions buy a ticket every week there is usually a winner.
As a result, most changes in the DNA of complex organisms over time are due to drift rather than selection, which is why biologists focus on sequences that are similar, or conserved, when they compare genomes. Natural selection will preserve sequences with vital functions, but the rest of the genome will change because of drift.
Drifting through bottlenecks
Genetic drift can even counteract natural selection. Many slightly beneficial mutations can be lost by chance, while mildly deleterious ones can spread and become fixed in a population. The smaller a population, the greater the role of genetic drift.
Population bottlenecks can have the same effect. Imagine an island where most mice are plain but a few have stripes. If a volcanic eruption wipes out all of the plain mice, the island will be repopulated by striped mice. It’s a case of survival not of the fittest, but of the luckiest.
Random genetic drift has certainly played a big role in human evolution. Human populations were tiny until around 10,000 years ago, and went through a major bottleneck around 2 million years ago. Other bottlenecks occurredwhen a few individuals migrated out of Africa around 60,000 years ago and colonised other regions.
There is no doubt that most of the genetic differences between humans and other apes – and between different human populations – are due to genetic drift. However, most of these mutations are in the nine-tenths of our genome that is junk, so they make no difference. The interesting question is which mutations affecting our bodies or behaviour have spread because of drift rather than selection, but this is far from clear.