(part III) Principles of Evolution (Chapters 22-25 in Campbell et al.)

Natural Selection – the mechanism of evolution as told by Wallace and Darwin

**Natural Selection - differential reproductive success that results from the interaction of organisms with their environment.**

By way of example, we illustrated the process of natural selection with the peppered moth story.

I. Some Background

Remember three important subtle points (see also p. 437 “Some Subtleties of Natural Selection”):

1) Natural selection can amplify or diminish only heritable variations. Only heritable variations are affected; acquired characteristics play no role in evolution.

2) Populations evolve, not individuals.

3) Natural selection is situational (the direction it takes depends on the environment, i.e., the selective forces).

To illustrate these subtle points using the case of the peppered moth… 1) moth color is inherited perhaps under control of a single gene; 2) the changes in nature over time involve whole populations, that is, in pre-industrial times the dark moth form was quite rare and perhaps in the distant past the dark mutation had not yet appeared. With time and natural selection at work, the dark moth became the most common form within the population. 3) the 3^rd subtle point is quite important, for what is advantageous in one environment may not be in another. In the saga of the peppered moth, under polluted tree bark conditions following the industrial revolution the dark from became most common, yet a return to a predominance of light colored forms follows restoration of healthy, non-polluted tree bark. (The story of the peppered moth has additional complexities but we will ignore them).

A. About heritable variations

Heritable variations are governed by genes, and as we have seen, genes are mere chemicals containing coded information in the form of DNA. DNA’s ability to mutate is the fundamental basis for evolutionary change. While this primary source of variation is the result of random, chemical events (i.e. gene mutations), other aspects of the process of evolution (e.g. natural selection) are very nonrandom, as we will see.

B. About Populations

Natural Selection (evolution) is a population level phenomenon, i.e. populations evolve, not individuals, and furthermore, the variations upon which natural selection operates exist within populations (variations exist only in the context of populations).

Population - a) a group of individuals of the same species, b) live together in same area at same time,

c) exchange genes (reproduce sexually)

Sources of Variation w/in populations:

1) Mutation - creates new alleles/new traits/new phenotypes- The fundamental basis of change

2) Sexual reproduction - recombines alleles (recall Punnett-squares)

Benefit of sexual reproduction: genetic variation in offspring allows some to survive in changing environment (e.g. clonal algae turn sexual during unfavorable conditions)

3) Gene Flow - gain or loss of alleles (genes) from a population

gain alleles - immigration (arrival of new individuals) lose alleles - emigration or death

C. About Situations or Circumstances

The type of change natural selection can bring about depends on the situation or circumstances of the environment. For example, for a population of domesticated, pedigree dogs the continued existence of each breed is dependent on an environment in which owners continue to breed pure lines for the survival of the breed. Perhaps of more interest is the situation (environment) in which breeders desire new combination of characters and experiment with hybridization between breeds. Breeders hope to develop new lines of pure breeding dogs in this way. Each decade, new breeds of dogs, cats, rabbits, etc. are created in this process (=artificial selection) that is not unlike the process of natural selection.

Expounding on the situational aspect in which evolution operates, Stephen Jay Gould, Harvard paleontologist and prolific writer popularizing evolutionary concepts, puts fourth an interesting idea that he calls the “contingency of evolutionary history” in his 1989 book, Wonderful Life: The Burgess Shale and the Nature of History. The title was inspired by the 1946 movie It’s a Wonderful Life in which George Bailey learns how his own life was a contingency for many other events. One such contingency that became a prerequisite for human evolution was the extinction of the dinosaurs that made way for mammal diversification. Apparently, dinosaur extinction was contingent on an asteroid that impacted Earth 65 million years ago. Had these events not occurred, humans may never have evolved. (The question of why humans evolved (to what purpose) is not a question science can address; the question of how humans evolved is explored through the process of science. See pages 709-715 if you are interested in human evolution.)

II. Types and examples of Natural Selection (see p. 458--459)

Given that genetic variation within a population produces a diversity of phenotypes, there is a genetic basis for the inheritance of variant phenotypes that may become adaptations (depending upon the situation/environment of course). Adaptations result from natural selection. The following examples illustrate natural selection.

1. Stabilizing selection - favors intermediate phenotype

- does not lead to change

- extremes selected against

ex. Birth weight in humans and gall fly on golden rod

2. Directional selection - important when environmental change occurs

ex. resistance to pesticides/antibiotics

- favors an extreme phenotype, leads to changes in populations & eventually (if you accept theory of uniformity, uniformitarianism) to new species formation.

3. Disruptive (Diversifying) Selection - the intermediate phenotype selected against.

ex. predator favors medium sized prey

In all 3 modes: Survival of the Fittest where fitness=reproductive success (not strength)

4. Sexual selection - Selection for mate attracting traits

ex. Male’s bright plumage in birds, males greater strength, larger antlers

Sexual selection provides an explanation for sexual dimorphism - male & female differ beyond gonad and genitalia. Recall the extreme case of the dwarf male angler fish. Based on a morphological species concept alone, male angler fish may have been named as a separate species when originally captured from the depths of the ocean. Only after the realization that the small angler fish were simply the male’s which spawned with the much larger female angler fish would a single species rather than two be recognized. The knowledge of the capacity for interbreeding is of course the basis for the biological species concept. (see p. 465)

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