Main points Sexually-reproducing organisms are organized into populations of interbreeding individuals. Although there are difficulties with the various species concepts available, organizing populations of individuals into species allows us to understand the diversity of organisms that we find in nature, and gives us a unit for tracking changes in populations of individuals over time. The recognition of the importance of variation was a milestone in the development of biological thought.
Variation is the raw material which allows populations to change over time.
Hereditary mechanisms over the course of species life cycles provides a means to maintain constancy of phenotypic characteristics between parent and offspring. Sexual reproduction by itself does not lead to changes in characteristics of species over time. Describe the biological species concept. Describe how phenotypic variation differs between polymorphism and continuous variation. Polymorphisms occur when the trait involved is determined by a one or a few genes, each of which has a large effect, with little environmental variation.