DEPARTMENT OF BIOLOGY
The Basics of Evolution
Natural Selection – If Mutation is Random, Why Does Evolution Occur at All?
Every time that scientists have examined the process of mutation, seeking to learn if there are recognizable patterns, the answer seems to be that mutation is essentially random. If we think of the causes of mutation, such as chemical mistakes in DNA replication or repair, or physical damage due to cosmic rays or other radiation, we see that there is no reason to expect mutation to be anything except random. And yet, evolution has produced highly-complex life forms, with a great many specialized adaptations that make them appear as if they were specifically designed to live where and how they do. How can an apparently random process result in apparently directed evolution?
This question is one of the "logical" problems that many people have with evolution. It is simply counter-intuitive that a random process can give rise to highly-ordered structures, and to adaptation to specific environments. Perhaps the best way to address this issue is to provide some examples in which we model the process.
Example 1: using colors to represent individuals
In this example, we consider two populations of 10 individuals. Each individual can reproduce, but because of ecological constraints (food supply, for example), the environment maintains the population at a maximum of 10 individuals. Using random choice (throwing dice, for example), we have assigned different colors to the 10 individuals--and we use the same colors for each population. The two populations look like this:
The left-hand population is in a cool environment, then individuals colored blue, green, and purple have a competitive advantage over the other colors, with purple being most successful. The right-hand population is in a warm environment, with red, orange, and yellow having a competitive advantage. Red is most successful. In the next generation, we have this:
There isn't really much difference between this generation and the previous generation. Most of the different colors (genetic variants) are present in each population. Some of the variants have increased in frequency, while others have decreased in frequency. Compared to the prior generation, each one looks pretty much like the parental generation, except that some of the genetic variations are a bit more common.
In the next generation, we have this:
And then this:
And then this:
Each population changes slowly with time, from generation to generation, as some individuals have more offspring than others. The two populations began with identical genetic diversity, based on random "mutation." However, the environmental conditions were different, so selection was different. The cool environment selected for the cool colors, and against the warm colors. The warm environment selected for the warm colors, and against the cool colors. Mutation was random, but selection provided a direction to the evolution.
Example 2: leaf shape
In this example, a species of shrub has spread across a valley, and up into the mountains on either side of the valley. As the climate warms, the shrubs lower down (in the valley) die out, and the shrubs higher up (in the mountains) survive. But, on the north side of the valley, the shrubs are on mountain slopes that receive full sun; rain water dries rapidly. On the south side of the valley, the shrubs are on mountain slopes that receive little sun; rain water dries slowly. Thus, one population is in a dry environment, and the other is in a wet environment.
Both populations start with identical genetic diversity, resulting from random mutations that affect leaf shape. Some leaves are wide, some are narrow, some are in-between. They look like this:
Wet environment Dry Environment
Of course, the shape of a leaf is not a trivial matter, if you are a plant. A broad leaf can capture more sunlight, perform more photosynthesis, and thus provide more food for the whole plant. A narrow leaf is much less effective. However, the more broad a leaf is, the more stomata it has--openings that allow CO2 to enter and O2 to escape. The more stomata a leaf has, the more water it loses by evaporation. Therefore, a plant with broad leaves requires more water than a plant with narrow leaves; it is much more likely to wilt on a hot, dry day. From these considerations, we can see that in a wet environment, where water loss is not a serious problem, wide leaves would be advantageous. However, in a dry environment, where water loss is a problem, wide leaves would be a liability.
After a few generations, the distributions of leaves in the two populations look like this:
After a few more generations, they look like this:
And, after a few more generations, they look like this:
Wider leaves lose more water during the day, so in the dry environment, wide leaves are selected against. Narrow-leaved plants produce more seeds than wide-leaved plants. However, wide leaves can carry out more photosynthesis than narrow leaves can, so in the wet environment, wide leaves are selected for . Wide-leaved plants produce more seeds than narrow-leaved plants.
The genetic variation in leaf shape was determined by random mutation. However, the environmental conditions determined which variations were more successful, and provided a kind of direction to evolution.
From these examples, it should be evident that the random nature of mutation does not cause evolution to be random. Random mutation simply provides an array of genetic variants for selection to choose among. If there are genetic variants that are successful in the particular environment, then those genetic variants prosper. They produce more offspring. Eventually, they become the norm.
What if there are no genetic variants that are particularly successful? What if the environment changes rapidly, and there just don't happen to be any mutations in the population that enable any individuals to do well? Then, as has happened over and over during the history of life on earth, the population will die out. The species may go extinct.
last updated:Jan. 15, 2009