Blackwell Publishing

Natural selection and variation - Summary

• Organisms produce many more offspring than can survive, which results in a 'struggle for existence', or competition to survive.

• Natural selection will operate among any entities that reproduce, show inheritance of their characteristics from one generation to the next, and vary in 'fitness' (i.e., the relative number of offspring they produce) according to the characteristics they possess.

• The increase in frequency of the melanic, relative to the light colored, form of the peppered moth Biston betularia clearly illustrates how natural selection causes both evolutionary change and the evolution of adaptation.

• Selection may be directional, stabilizing, or disruptive.

• The members of natural populations vary with respect to characteristics at all levels. They differ in their morphology, their microscopic structure, their chromosomes, the amino acid sequences of their proteins, and in the DNA sequences.

• The members of natural populations vary in their reproductive success: some individuals leave no offspring, others leave many more than average.

• In Darwin's theory, the direction of evolution, particularly of adaptive evolution, is uncoupled from the direction of variation. The new variation that is created by recombination and mutation is accidental, and adaptively random in direction.

• Two reasons suggest that neither recombination nor mutation can alone change a population in the direction of improved adaptation: there is no evidence that mutations occur particularly in the direction of novel adaptive requirements, and it is theoretically difficult to see how any genetic mechanism could have the foresight to direct mutations in this way.

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