Competition in nature is relentless. Predation, famine, disease, and disaster all threaten to prevent individuals from reproducing. In this competitive environment, mutations that make survival more likely are more likely to appear in future generations, and mutations that make survival less likely probably won’t stick around in the population for long. This tendency for beneficial mutations to fix (meaning everyone in the population eventually gets it) and for deleterious mutations to be lost is called natural selection, and it’s what most of us go to when we think about evolution.
However, natural selection really only plays a partial (though critical) role in the diversity of life we see across our planet. Though the phrase “survival of the fittest” has become almost synonymous with evolution, the actual process of evolution includes much randomness. In fact, the majority of genetic changes separating one species from another have absolutely no impact on fitness. It seems that who actually gets eaten and who lives a full life with many grandchildren really comes down mostly to luck, though “survival of the luckiest” does seem less inspiring.
Selective Effects of Mutations
Oh no! Your DNA sequencing results came back, and you have a *gasp* mutation! In fact, you probably have a set of about 100-200 new mutations never before seen in the human population. How bad is this? Well, probably not very. Most of these mutations occur in intergenic regions (sections of DNA that do not occur in genes), and though the jury is still out on how important those intergenic regions are in macroevolution, most mutations there don’t seem to have any appreciable effects on individuals. For the mutations that do occur in genes, about a third of them are “silent,” meaning that they do not change the amino acid sequence of the proteins the gene encodes. Again, though that pesky jury still can’t agree how important those silent mutations are for translation efficiency and mRNA stability, it seems that silent mutations, in the majority of cases, don’t really seem to do much of anything either.
That leaves the non-silent mutations that will actually change the protein sequences, which has got to have some effect on fitness, right? Well, sometimes. Other times, the amino acid could be so chemically similar to another amino acid that the change doesn’t really matter. Or, the mutation could be in a region of the protein that is not that important to its function. Of course, when the mutation is in a region of the protein that is important, it could have a pretty big effect on fitness. Mostly, this will be negative and cause the protein to malfunction, but sometimes it could cause the protein to function better than it did before. But again, even if the mutation has a visible effect on an organism’s phenotype, it could still be overall neutral in the eyes of evolution, like a mutation which causes a butterfly’s wings to have one arbitrary (but beautiful!) pattern instead of another.
We could then bin all mutations into three categories. Neutral mutations have a selective effect of around zero, meaning that they make it neither more nor less likely that you survive and reproduce. Deleterious mutations have a negative selective effect, meaning that they will make it less likely that you survive and reproduce. Beneficial mutations have a positive selective effect and will make survival and reproduction more likely. Still, having a deleterious mutation does not make survival impossible and having a beneficial mutation does not make survival inevitable. Unless the mutation is so bad that it kills you, all these mutations can do is modify the weights of the dice evolution casts. Thus, good mutations can be lost from a population and bad mutations can fix for no good reason.
Let’s drive this home with an example. Why do we have five fingers on our hands instead of six? Some might say that it's because our ancestors who survived and reproduced had five fingers. Of course, this must be true in a literal sense because all traits that we inherited had to have come from our ancestors. But is it true to say that our ancestors survived and reproduced because they had five fingers? Well, not necessarily. If we had an evolutionary six-fingered uncle who got eaten by a saber-toothed tiger, it’s a bit of a stretch to claim that he would have been just fine if he had only five fingers instead of six. And there are populations of humans today with six fingers who, despite some trouble with counting, seem to do just fine. Thus, having exactly 5 fingers, like having 32 teeth or 12 ribs, may have nothing to do with evolutionary fitness and everything to do with random chance.
Evolution isn’t an omniscient process with goals or direction. It’s a mostly random process that is driven by many forces, only one of which being the tendency to favor traits which make replication more likely. Though it may be unsatisfying, the final answer to why we have some of the traits we have could just be “it happened to work out that way.” But in the end, I’m still glad we came out with five fingers. I think six would be a handful!