Evolution by natural selection is just one of its driving forces. Genetic changes occur over time through evolution by mutation, migration evolution and genetic drift. Natural selection plays a role in determining which traits are favored for survival and reproduction; for instance, as it relates to moth coloration, if darker colored moths are able to blend with their environments better and have greater chance of survival vs lighter colored moths, there will be a greater number of darker colored moths represented in the population. The challenge remains for evolutionary biologists to identify the driving factors behind evolutionary changes.
When most people think about evolution, they think of Charles Darwin and the survival of the fittest. Darwin is well known for coming up with the mechanism of evolution that explains the diversity of life: natural selection.
However, there are more forces that drive evolution. Natural selection is just one of them.
The simplest definition of evolution is change over time. To be more precise, it is genetic changes over time. If I dye my hair pink, that is a change over time, but my offspring will not be born with pink hair. In evolutionary biology, when we talk about changes over time, we mean changes from one generation to the next.
So what drives these changes? Well, let's start with natural selection. The way that natural selection works is that not everyone is able to contribute equally to the next generation. That is simply impossible. It would mean each species would grow exponentially and we would quickly run out of food and space. So, there is competition for survival and for reproduction, and only the genes of those who are able to make it through this fierce competition for survival and reproduction are represented in the next generation. So, if an organism has a trait that would increase its survival chances, for instance a moth [FIG 1] with a slightly darker color that would help it blend into its dark environment, it is more likely to survive predation. The moths who have this darker color are more likely to survive and reproduce than the lighter colored moths, so the gene for darker color is then expected to be slightly more represented in the next generation than the gene for lighter colors.
However, as I said before, there are other mechanisms that lead to evolutionary change, too. Since evolution is simply genetic changes over time, this could be driven by naturally occurring mutations that lead to new variants in a population. It could also be driven by migration. For instance when we have new black moths fly into a lighter colored moth population and mate with those lighter colored moths, we have evolution happening due to migration. Evolution can also be driven by genetic drift. The easiest way to define genetic drift is ‘evolution by chance’. If darker moths just happen to reproduce more one generation than lighter moths, but it had nothing to do with being more adapted, it was just pure luck, we see more darker moths the next generation not because of natural selection but because of genetic drift.
One of key challenges for evolutionary biologists is to figure out whether evolution is happening, and if so, what was the driving factor behind it.