D.4.1 Natural Selection

natural selection

process where organisms better adapted to their environment survive and produce more offspring than competitors; the mechanism through which evolution occurs

1. Organisms produce a far greater number of progeny than ever give rise to mature individuals

2. The number of individuals in species remains more or less constant.

3. Therefore, there must be a high mortality rate.

4. The individuals in a species are not identical, but show variations in their characteristics.

5. Therefore, some variants will succeed better than others in the competition for survival. So the parents for the next generation will be selected from those members of the species better adapted to the conditions of the environment.

6. Hereditary resemblance between parents and offspring is a fact.

7. Therefore, subsequent generations will maintain and improve on the degree of adaptation of their parents, by gradual change.

fitness

an organism’s ability to pass its genetic material to its offspring; its reproductive success

infraspecific competition

competition between individuals of the same species; compete for the same resources and have the same biotic and abiotic interactions that influence the growth, survival and reproduction of the species

interspecific competition

competition between individuals of different species

sexual dimorphism

differences in appearance between males and females of the same species, such as in colour, shape, size, and structure

sexual selection

selection arising through preference by one sex for certain characteristics in individuals of the other sex; driving force for evolution as an increase in breeding allows more genetic variation

gene pool

all the genes (and their alleles) present in a breeding population

stabilizing selection

occurs when there is an increase in the frequency of an average trait; it maintains a favorable characteristic, eliminating variance or abnormalities that are useless or harmful

directional selection

occurs when one extreme phenotype is favored over the other extreme and intermediate phenotypes

disruptive selection

occurs when environmental conditions favor the extremes of the phenotypic range over intermediate phenotypes

artificial selection

choosing individuals for breeding that have desirable traits