Lecture #16—Selection

This topic is the key part of Darwin and Wallace’s hypothesis of evolution; it is the driving force: Some organisms survive and others don’t; the survivors are the ones who leave their offspring to populate the next generation.

Darwin said that nature worked just like the Artificial Selection that was practiced by farmers. Humans choose particular animals or plants with desirable traits for breeding over several generations. Thus there is a gradual change in the farmer’s stock and crops.

Darwin coined the term Natural Selection to mean that there is a differential survival and reproduction of individuals due to differences in their phenotype. Recall there are 4 requirements which need to occur for natural selection to work:

1. Overproduction of offspring—more are produced that can survive.

2. Genetic variation in the population—individuals differ.

3. Struggle for existence—some individuals are better able to survive than others.

4. Successful reproduction—the survivors pass on their genes to the next generation.

Artificial selection can produce rapid changes in animals and plants (silver fox experiments in Russia and many other examples given. You should know several). It is now clear that natural selection can also be rapid.

Small selective differences can lead to major changes in a few generations (e.g. silver fox). The speed depends upon if the alleles involved are dominant or recessive. If the alleles are dominant and thus expressed so they are seen by the farmer or by the environment, then they can be rapidly eliminated (negative selection) or chosen (positive selection). If they are recessive, their elimination or preservation will be much slower because the traits will only show up in individuals who are double recessive. Individuals who are heterozygotes will be carrying the alleles and not be identified.

Sometimes the best adapted individuals are heterozygous; neither the dominant or recessive are as evolutionarily fit (=likely to survive) as the heterozygotes (Africans with sickle-cell anemia). The environment makes a difference as to who is best fit; if malaria is absent as it is in the USA, the heterozygotes are not favored.

Since most traits are polygenic, the effectiveness of selection is seen in a gradual shift in the proportions of individuals with or without a trait. There are 3 different types of selection: directional, stabilizing, and disruptive.

Terms/Concepts to Define:

Artificial selection

Natural selection

mtDNA

Pleiotropy

Positive selection

Relative fitness

Achondoplastic dwarfism

Cystic fibrosis

Polygene traits (polygenic)

Directional selection

Stabilizing selection

Disruptive selection

Can you answer these questions?

  1. How can we distinguish between artificial selection and natural selection?
  2. Horses have been selected for their speed, but in recent years the average speed for race horses hasn’t changed very much. Why is that?
  3. Discuss how silver foxes have changed over the time of their domestication and what seems to have transpired with their genetics.
  4. Using the major steps of Darwin’s model of natural selection, explain how insect resistance to a pesticide develops.
  5. Using the major steps of Darwin’s model of natural selection, explain how a parasite might influence the evolution of a host species.
  6. Suppose we have a rabbit population with brown and white varieties. The brown variety produces 5 offspring each breeding season and the white variety produces 2 offspring. What is the fitness coefficient of each variety assuming everything else is equal between the two?
  7. Why do we say that when selection favors the heterozygous condition, that complete dominance must not be in effect in the genetic system?
  8. Explain why directional and disruptive selection can lead to species formation but stabilizing selection can’t.