Evolution and Natural Selection

I have called this principle, by which each slight variation, if useful, is preserved, by the term Natural Selection. - Charles Darwin, The Origin of Species

In this reading, we wish to ask:

Use the links below to quickly jump to a section

Darwin's Theory    Natural Selection    Natural Selection Requires    

Evidence of Natural Selection    Local Adaptation    Stabalizing Selection    Summary

1. Darwin's Theory

Darwin's theory of evolution has four main parts:

  1. Organisms have changed over time, and the ones living today are different from those that lived in the past. Furthermore, many organisms that once lived are now extinct. The world is not constant, but changing. The fossil record provided ample evidence for this view.
  2. All organisms are derived from common ancestors by a process of branching. Over time, populations split into different species, which are related because they are descended from a common ancestor. Thus, if one goes far enough back in time, any pair of organisms has a common ancestor. This explained the similarities of organisms that were classified together -- they were similar because of shared traits inherited from their common ancestor. It also explained why similar species tended to occur in the same geographic region.
  3. Change is gradual and slow, taking place over a long time. This was supported by the fossil record, and was consistent with the fact that no naturalist had observed the sudden appearance of a new species. [This is now contested by a view of episodes of rapid change and long periods of stasis, known as punctuated equilibrium].
  4. The mechanism of evolutionary change was natural selection. This was the most important and revolutionary part of Darwin's theory, and it deserves to be considered in greater detail.

2. The Process of Natural Selection

Natural selection is a process that occurs over successive generations. The following is a summary of Darwin's line of reasoning for how it works (see Figure 2).

  1. If all the offspring that organisms can produce were to survive and reproduce, they would soon overrun the earth. Darwin illustrated this point by a calculation using elephants. He wrote:

    "The elephant is reckoned the slowest breeder of all known animals, and I have taken some pains to estimate its probable minimum rate of natural increase; it will be safest to assume that it begins breeding when 30 years old and goes on breeding until 90 years old; if this be so, after a period from 740 to 750 years there would be nearly 19 million elephants descended from this first pair."

    This unbounded population growth resembles a simple geometric series (2-4-8-16-32-64..) and quickly reaches infinity.

  2. As a consequence, there is a "struggle" (metaphorically) to survive and reproduce, in which only a few individuals succeed in leaving progeny.
  3. Organisms show variation in characters that influence their success in this struggle for existence. Individuals within a population vary from one another in many traits. (Animal behavioralists making long-term studies of chimps or elephants soon recognize every individual by its size, coloration, and distinctive markings.)
  4. Offspring tend to resemble parents, including in characters that influence success in the struggle to survive and reproduce.
  5. Parents possessing certain traits that enable them to survive and reproduce will contribute disproportionately to the offspring that make up the next generation.
  6. To the extent that offspring resemble their parents, the population in the next generation will consist of a higher proportion of individuals that possess whatever adaptation enabled their parents to survive and reproduce.
Process of Natural Selection
Figure 2: The Process of Natural Selection

The well-known example of camouflage coloration in an insect makes for a very powerful, logical argument for adaptation by natural selection. Development of such coloration, which differs according to the insectıs environment, requires variation. The variation must influence survival and reproduction (fitness), and it must be inherited.

Later, when we have merged genetics with evolution, we will define natural selection this way:

Natural Selection is the differential reproduction of genotypes.

3. Natural selection requires...

For natural selection to occur, two requirements are essential:

  1. There must be heritable variation for some trait. Examples: beak size, color pattern, thickness of skin, fleetness.
  2. There must be differential survival and reproduction associated with the possession of that trait.
          Unless both these requirements are met, adaptation by natural selection cannot occur.

Some examples:

In addition, natural selection can only choose among existing varieties in a population. It might be very useful for polar bears to have white noses, and then they wouldn't have to cover their noses with their paws when they stalk their prey. The panda could have a much nicer thumb than the clumsy device that it does have.

Later, when we incorporate genetics into our story, it will be more obvious why the generation of new variations is a chance process. Variants do not arise because they are needed. They arise by random processes governed by the laws of genetics. For today, the central point is the chance occurrence of variation, some of which is adaptive, and the weeding out by natural selection of the best adapted varieties.

4. Evidence of natural selection

Let's look at an example to help make natural selection clear.

Industrial melanism is a phenomenon that affected over 70 species of moths in England. It has been best studied in the peppered moth, Biston betularia. Prior to 1800, the typical moth of the species had a light pattern (see Figure 3). Dark colored or melanic moths were rare and were therefore collectors' items.

Image of Peppered Moth
Figure 3

During the Industrial Revolution, soot and other industrial wastes darkened tree trunks and killed off lichens. The light-colored morph of the moth became rare and the dark morph became abundant. In 1819, the first melanic morph was seen; by 1886, it was far more common -- illustrating rapid evolutionary change.

Eventually light morphs were common in only a few locales, far from industrial areas. The cause of this change was thought to be selective predation by birds, which favored camouflage coloration in the moth.

In the 1950's, the biologist Kettlewell did release-recapture experiments using both morphs. A brief summary of his results are shown below. By observing bird predation from blinds, he could confirm that conspicuousness of moth greatly influenced the chance it would be eaten.

Recapture Success
light moth dark moth
non-industrial woods 14.6 % 4.7 %
industrial woods 13 % 27.5 %

5. Local adaptation - more examples

So far in today's lecture we have emphasized that natural selection is the cornerstone of evolutionary theory. It provides the mechanism for adaptive change. Any change in the environment (such as a change in the background color of the tree trunk that you roost on) is likely to lead to local adaptation. Any widespread population is likely to experience different environmental conditions in different parts of its range. As a consequence it will soon consist of a number of sub-populations that differ slightly, or even considerably.

The following are examples that illustrate the adaptation of populations to local conditions.

Image of Rat Snakes
Figure 4: Subspecies of the rat snake Elaphe obsoleta, which interbreed where their ranges meet.

Image of Finches
Figure 5

6. Stabilizing, directional, and diversifying selection

Finally, we will look at a statistical way of thinking about selection. Suppose that each population can be portrayed as a frequency distribution for some trait -- beak size, for instance. Note again that variation in a trait is the critical raw material for evolution to occur.

What will the frequency distribution look like in the next generation?

First, the proportion of individuals with each value of the trait (size of beak, or body weight) might be exactly the same. Second, there may be directional change in just one direction. Third (and with such rarity that its existence is debatable), there might be simultaneous change in both directions (e.g. both larger and smaller beaks are favored, at the expense of those of intermediate size). Figures 5a-c capture these three major categories of natural selection.

Categories of Natural Selection
Figures 6a-c

Under stabilizing selection, extreme varieties from both ends of the frequency distribution are eliminated. The frequency distribution looks exactly as it did in the generation before (see Figure 6a). Probably this is the most common form of natural selection, and we often mistake it for no selection. A real-life example is that of birth weight of human babies (see Figure 7).

Figure 7

Under directional selection, individuals at one end of the distribution of beak sizes do especially well, and so the frequency distribution of the trait in the subsequent generation is shifted from where it was in the parental generation (see Figure 6b). This is what we usually think of as natural selection. Industrial melanism was such an example.

The fossil lineage of the horse provides a remarkable demonstration of directional succession. The full lineage is quite complicated and is not just a simple line from the tiny dawn horse Hyracotherium of the early Eocene, to today's familiar Equus. Overall, though, the horse has evolved from a small-bodied ancestor built for moving through woodlands and thickets to its long- legged descendent built for speed on the open grassland. This evolution has involved well- documented changes in teeth, leg length, and toe structure (see Figure 8).
Figure 8

Under diversifying (disruptive) selection, both extremes are favored at the expense of intermediate varieties (see Figure 6c). This is uncommon, but of theoretical interest because it suggests a mechanism for species formation without geographic isolation (see the previous lecture on speciation).

7. Summary

Darwin's theory of evolution fundamentally changed the direction of future scientific thought, though it was built on a growing body of thought that began to question prior ideas about the natural world.

The core of Darwin's theory is natural selection, a process that occurs over successive generations and is defined as the differential reproduction of genotypes.

Natural selection requires heritable variation in a given trait, and differential survival and reproduction associated with possession of that trait.

Examples of natural selection are well-documented, both by observation and through the fossil record.

Selection acts on the frequency of traits, and can take the form of stabilizing, directional, or diversifying selection.