Natural Selection and Adaptation

Lecture Part 2


II. Darwin's Theory

Natural selection is the differential success in reproduction, and its product is adaptation of organisms to their environment.

A. Darwin was aware of and used the following to support his ideas

  1. Improvements of breeding stock by artificial selection
  2. Malthus's essay on population which contained the following points

 

  1. all species have enormous reproductive potential and increase geometrically.
  2. resources increase arithmetically (in a linear fashion).
  3. populations are kept in check by starvation, disease, etc. so very few young survive.
  4. consequently only the strongest are able to survive.

 

B. During Darwin's voyage on the Beagle with Captain Fitz Roy, he began to put together "his theory". The Theory of Natural Selection is based on the following 4 observations:

  1. Variation is found in individuals of natural populations
  2. Most variations are heritable
  3. Populations quickly become too large for the local environment
  4. Individuals with the best combination of traits for inhabiting their environment tend to leave more surviving offspring.

 

C. Darwin spent the rest of his life searching for evidence to support his theory and found many more examples including:

  1. Species can disperse - especially important for biological communities found on Islands. Darwin discovered that a variety of species can survive in sea water for long periods of time.

     

  2. Domesticated plants and animals are extremely variable although descended from wild ancestors which usually exhibited less variation.

     

  3. Variation is heritable - more examples of artificial selection - but Darwin's ideas about inheritance were not entirely correct.

     

  4. Malthus' idea of a "struggle for existence" could be seen everywhere in nature.

 


There are some Subtleties of Natural Selection that require clarification.

One is the importance of populations in evolution.

For now, we will define a population as:

a group of interbreeding individuals belonging to a particular species and sharing a common geographic area.

A population is the smallest unit that can evolve.

Natural selection involves interactions between individual organisms and their environment, but individuals do not evolve.

Evolution can be measured only as change in relative proportions of variations in a population over a succession of generations. Furthermore, natural selection can amplify or diminish only those variations that are heritable.

Natural selection edits populations - favoring the survival and reproduction of some individuals over others. Natural selection is utilitarian, picking traits that work best for the present situation.

 

Researchers have published more than 100 accounts of natural selection in the wild - the most famous of which is the account of Biston betularia, the peppered moth. A more recent example of natural selection in action is the evolution of antibiotic resistance in bacteria.