DEPARTMENT OF BIOLOGY
The Basics of Evolution
Mutating in Order to Survive
We often develop the idea that evolution occurs because a species of plants or animals mutates "in order to survive." It seems as if this must have happened, because the plants and animals that are now alive are well adapted to their environments, and appear to be much better adapted than their ancestors that are recorded in the fossil record. Indeed, the ancestral species died out, which seems to indicate that they were not "good enough" to make it, while the ones that could make themselves "better" are with us today.
This notion is also satisfying from a human-centered view of the world. We are here now, so we must be the "best" species ever to evolve. Ancient species were much more primitive, so evolution would seem to work as some kind of drive to become better -- and eventually become human.
But does this make sense? It is even possible?
If we think about the evolution of plants, in which they acquire narrow leaves in dry climates, or broad leaves in wet climates, we must wonder how they can do this. As far as we know, plants can't think. As far as we know, they cannot predict what the world will be like in a few hundred generations. And, as we know for certain (see above), mutations occur randomly. There is no way to prevent mutations from happening, and no way to cause them to occur in specific genes.
If it is not possible to make mutations happen on cue, it simply cannot possible to mutate in order to adapt to new conditions.
And yet, so many species seem so well-suited to their environments. How can this have happened if they didn't know what they were doing? The examples above should help resolve this conundrum. Random mutation is sufficient to account for the observation, if there is enough genetic diversity in the population that some individuals have characteristics that are advantageous, and enable them to survive changing environmental conditions. As long as environmental conditions don't change too fast, the continued occurrence of mutations is capable of producing genetic variations -- some of which may happen to be useful. Fortunately, the rate of mutation is high enough that useful mutations have occurred sufficiently often to accommodate many of the environmental changes that have occurred.But not always. If useful mutations don't happen to occur, or if the environment changes too fast (as might occur if a giant asteroid strikes the earth), then species die out. They become extinct. It is instructive to consider the fact that most of the earth's species have gone extinct. Those of us alive now are the lucky remnants of various genetic lineages in which advantageous mutations did occur.
An excellent example of how selection operates on individuals of a population, and how the individuals do not "plan" their mutations is offered by the following story, which is taken from Shadows of Forgotten Ancestors by Sagan and Druyan.
Instincts are inborn behaviors that animals display without necessarily thinking logically about it. We recognize a great many instincts in different breeds of dogs--such as chasing and retrieving sticks (retrievers), digging (beagles, selected for hunting rabbits), and pulling (huskies, selected to work in dogsled teams). We don't usually think of our own instincts, but we have them.
One human instinct that is variable, but present in a significant fraction of the population is fear of the dark. Many of us recall being afraid of the dark, or of "monsters" when we were children. Most of us recall our parents telling us "there's nothing to be afraid of; go back to sleep." If Mom and Dad were telling us not to be afraid, then where did we get our fear of the dark? It cannot be a learned behavior, but must be an instinct.
There are good reasons to have an instinctive fear of the dark. In our history, before civilization, the world was a scary place. There were many predators that hunted at night. In a very real sense, there were monsters out there. The world in which our ancestors lived was perilous.
So, picture the following: Our ancestors are sitting around the fire at night--or maybe they are perched on the limbs of trees.* Most of them have a vague unease, and are afraid to venture out into the night. But, this genetically-coded uneasiness, like any genetically-coded trait, is variable. Some individuals are more afraid, some are less afraid.
So here is everyone sitting around being afraid, and one guy says, "You guys are wimps. I'm going for a walk." He goes out into the night, and is eaten by lions.
Whose genes did we inherit?
This simple scenario offers several important insights into the nature of evolution and natural selection. They are:
1. Selection operates on individuals. Individuals can be eaten, or they can live until morning. The entire tribe did not get eaten at once, nor did the entire tribe live until morning.
2. "Survival of the fittest" really is a poor phrase to describe this. We don't think of "the wimps" as being "fit," but they are the ones who passed on their genes. In evolutionary terms, "fitness" refers to the production of offspring, and not to anything else. In this case, being fearful increased an individual's fitness in this particular lion-filled environment.
3. Selection operates on traits that already exist. The individuals who were afraid survived; the individuals who were not afraid were likely to be eaten. No one was eaten by lions, and then developed the fear-of-the-dark trait.
4. There is no planning in evolution, no "mutating in order to survive." Our ancestors did not sit around the fire thinking, "I bet that in a few hundred thousand years, our descendents will be better off if they are afraid of the dark--so I'm going to mutate, and become afraid." Instead, each individual lived his or her life normally. The ones who happened to have genetically-coded traits that were advantageous in that environment passed on their genes to more offspring.
last updated:Jan. 15, 2009