What is needed to demonstrate that some evolutionary change and the resulting characters were produced by a process of natural selection? The short answer (from the previous post): It’s hard work to establish evidence for natural selection. Longer answer: It requires demonstrating both a functional and a temporal correlation between the character and the differential reproductive success, i.e. analysis of the character’s effect and its consistent origin in time with respect to the environmental circumstances/ stress/ challenge.

In practice, most evolutionary biologists or writers about evolution don’t undertake that work.  They seek out or imagine special conditions that increase the chances of natural selection serving as an explanation of the historical change in the frequency of one character:

• a) Single function (or co-ordinated composite function) for the character (so you can discount other effects in the functional correlation).
• b) Effective independence of the character and its effects from those of others (again so you can discount other effects in the functional correlation); e.g heavy metal tolerance in plants on mine tailings.
• c) Character reproduced over time (generations) (so you don’t have to deal with changing characters and their functional correlations); e.g. single locus phenotypes.
• d) Consistency of the organism-environment relationship over time (again so you don’t have to deal with changing functional correlations).
• e) Limited time span (again so you don’t have to deal with changing functional correlations).

From our knowledge of biology, these special conditions are likely to be rare. The next post addresses the consequence of their rarity.