Figure 7.13: Natural selection determines the survival of groups of organisms. Flight as
shown in these geese is an evolutionary step that probably aided in the survival of many
birds. ( 15 )
types of selection.Directional selectionshifts the frequency curve away from the average
by favoring individuals with an extreme form of the variation. The curve would still be bell-
shaped, but it would have shifted to the left or right, in the direction of the lighter or darker
alleles.Stabilizing selectionselects for a group of phenotypically average individuals, with
individuals with either extreme phenotype selected against. Disruptive selectionselects
for groups of individuals with extreme phenotypes, selecting against individuals with the
average phenotype.
Lesson Summary
- Evolution is change in species over multiple generations.
- Natural selection is how evolution occurs.
- Adaptations are the result of natural selection.
- Charles Darwin is credited with developing the Theory of Evolution by Natural Selec-
tion - Darwin collected much of his evidence on a five year voyage around the world, with
much of his data collected on the Galápagos Islands. - The work of many others contributed to Darwin’s theory.
Review Questions
- What is biological evolution?