Natural Selection, Part II

Natural Selection, Part II.

Over long enough periods of time, natural selection can cause populations of a species to differentiate into two species, creating a branching pattern of diversification.

daphkick. An Example of Natural Selection: Daphnia Adapts to Toxic Cyanobacteria (Blue-green Algae).

401446aa. Phosphorous pollution in Lake Constance, Germany: increased cyanobacteria in the 1970s; later the lake was cleaned up and cyanobacteria densities fell (mostly).

401446ab. Daphnia eggs deposited after the increase in cyanobacteria density showed greater resistance to the toxin.

Note Darwin’s three postulates in action:. Variation (even before the cyanobacteria became a major problem, there were some resistant individual Daphnia ) Heritability (clones from different periods differed genetically) “Struggle for existence” (toxin-resistant strains outreproduced the less resistant strains) in the face of “hostile forces of nature”.

Clarifications and Corrections of Common Misunderstandings of Natural Selection.

1. Adaptations generally benefit individuals ’ survival and reproduction, even at the expense of the species’ well-being.

...yet... 2. What changes in evolution is the composition of a population , not the characteristics of individuals (there’s no “personal evolution”).

Example: deeper beaks better for larger seeds. 3. Natural selection is not a circular argument (“the traits that persist are the ones that persist”): in fact, the traits that persist are those that an engineer would regard as better “designed”.

4. Chance plays a role in evolution, but natural selection is not a “random” process.

Finch Example: Small beaked birds can’ t find enough food Large beaked birds have higher juvenile mortality Selection favors intermediate beak size At equilibrium, selection will maintain stasis (no change).

6. Natural selection can build complex adaptations, step by step.