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Natural Selection

Evolution

Evolution refers a change in the gene frequency of a population. For example, suppose that in a certain human population in 1990, 65% of the eye color genes were for blue eyes and 35% were for brown eyes. In 2000, the number of blue eye genes was 67%. This small evolutionary change may not be noticeable, but over time, small differences accumulate to produce larger differences. A number of natural phenomena can act to change gene frequencies. Organisms moving into or out of a population (migration) can cause gene frequencies to change. Random fluctuations can also cause changes, particularly in small populations. Natural selection (described below) is particularly important in causing changes in gene frequencies.

Adaptation

Adaptations are structures or behaviors that allow efficient use of the environment. For example, the webbed foot of a duck enables it to swim better than a foot that is not webbed.

Adaptations are due to genes, that is, they are inherited.

Natural Selection

As was noted in the introductory paragraph above, natural selection is one of several different mechanisms that cause evolutionary change in populations. Natural selection produces changes in the genetic composition of a population from one generation to the next. As a result, organisms become better adapted to their environment.

Natural selection occurs because

  1. Individuals within a population vary; they are not all identical.
  2. Some variants are "better" than others. As a result, they have more reproductive success.
  3. The traits that vary are heritable.
If the above three items are true, then the "better" individuals will have more success reproducing and will have more offspring. In successive generations, more offspring will have the better traits; the population will change. These items are discussed below.

Variation

For many traits that occur in a population, individuals are often not all identical.  For example, if running speed were measured, some individuals would likely be able to run faster than others but most individuals would probably be intermediate.  

If number of individuals is plotted against the trait in question (running speed for example), a graph like the one shown is often produced.

We would get a similar bell-shaped curve if we plotted height, weight, performance on exams, etc.
Note the variation in coloration seen in these ladybird beetles (Coccinellidae).
Click image to enlarge.

Some Variants are Better

Some individuals are bound to be better than others. Perhaps their body structure allows them to escape predators better or to find food faster or to better provide for their young. For example, suppose that the faster-running animals diagrammed below are better able to escape predators than the slower ones. You would expect that more of the faster ones would survive and reproduce than the slower ones.

The slower rabbits will not reproduce as much because predators kill them more than they kill the faster rabbits.

Traits Are Heritable

Those individuals that reproduce more will pass their superior genes to the next generation. Individuals that reproduce less as a result of "poorer genes" will not pass those genes to the next generation in high numbers. As a result,  the population will change from one generation to the next. The frequency of individuals with better genes will increase. This process is called natural selection.

Fitness

We often hear natural selection described as "survival of the fittest." The word "fitness" used in a biological context means "reproductive." It does not have anything to do with physical fitness or strength. In the example above, it is the fastest rabbits that reproduce the most, not the strongest.

Natural Selection Produces Evolutionary Change

If the conditions discussed above are met, the genetic composition of the population will change from one generation to the next. This process is called natural selection.

The word "evolution" refers to a change in the genetic composition of a population. Natural selection produces evolutionary change because it changes the genetic composition of populations.

A variety of other mechanisms can also produce evolutionary change. For example, suppose that 65% of the eye-color genes in a population were for individuals with blue eyes and 35% of the genes were for brown eyes. If most of the immigrants entering the population carried the blue gene, the overall composition might change from 65% blue to 70% blue.

Although natural selection affects individuals, it is important to note that multicellular organisms cannot change their genes. Changes in the genetic composition of a population occur as a result of changes in reproduction or survival of individuals.

Example of Natural Selection: The Peppered Moth

There are two forms of the peppered moth (Biston betularia) in England- a dark-colored form (carbonaria) and a light form (typica).

In the early 1800's, most moths were the light form. The first dark form was reported in 1848. The dark form increased in frequency during the last half of the 1800s. By 1895, 98% of the Moths in Manchester were the dark form.

The increase in the dark (carbonaria) form of the moth occurred at a time of rapid industrialization in England- the industrial revolution. During this time, an increase in the amount of coal-burning factories caused widespread pollution. The pollution killed light-colored lichens, causing the trees to be darker. The trees in polluted areas were also covered with dark soot.

In 1896, J. W. Tutt proposed that bird predation was responsible for the increase in abundance of the dark form of the moth.  He reasoned that birds had difficulty seeing the dark form on the dark trees; the moths were camouflaged and survived better.  In clean areas, the trees were covered with lichens, making the pale form more difficult for birds to see.  In the early 1800s, the trees were light and the light form were more difficult to see. 

To test the bird predation hypothesis, H.B.D. Kettlewell  released moths of each type and then measured the number that were later recaptured. The experiment was performed in a polluted area in Birmingham, England and in a clean area in Dorset. The moths were marked with a dot of paint so that he could identify them after they were released and then recaptured. In the unpolluted area, he recaptured 13.7% light, 4.7% dark indicating that the light form survived better.  In the polluted area, he recaptured 13% light and 27.5% dark suggesting that the dark form survived better.

These results support the hypothesis that color change was due to bird predation.  Birds ate moths that were easiest to find.

Sexual Reproduction and Evolutionary Change

Variation

Later in this course, we will discuss how sexual reproduction acts to increase variation in populations by shuffling genes. Offspring have some genes from each of two different parents and therefore are not identical clones of their parents. The increased variation due to sexual reproduction allows natural selection (and thus evolution) to produce changes in populations as described above.

Fluctuating environments

Evolutionary change due to natural selection would not be necessary if the environment never changed and the organisms within the environment were optimally adapted to the environment. For example, imagine a plant that is adapted to an environment that has an average annual  rainfall of 100 cm. If the climate were to change so that the amount of rainfall decreased, individuals that could tolerate less rain would survive and reproduce better, thus establishing their drought-tolerant genes in subsequent generations. If there was no variation in the plant population, there would not be any drought-tolerant individuals and the species would likely go extinct in areas of decreased rainfall.

Sexual reproduction therefore, enables species to survive in fluctuating or changing environments because it promotes variation, which in turn allows natural selection.