Natural Selection

Adaptation & Descent with Modification as consequences of Natural Selection

    Consider a species of snails that display a range of variation in shell color, from very light to very dark. As in other snail species, shell phenotypes are largely due to genotype (heritability).  Snails occur in a variety of environments, including light sandy soils to dark forest loams. Such environments vary over short distances (which may be long distances for snails), for example between two backyards or along a stream course.

The degree of match between shell color and soil type confer an advantage from avian visual predators. On lighter soils [left], light snails are more cryptic and dark snails are preferentially eaten by birds. On darker soils [right], the opposite is the case and darker snails are more likely to survive.

    The surviving individuals reproduce, in such a way that population size remains constant. Each population now shows better
adaptation [is better adapted] to its local circumstances, in that the character distribution of its members is such that they are are on average more likely to survive and reproduce than those in the preceding generation. Because offspring resemble their parents, the next generation will have a different character distribution than the preceding. Descent with modification has occurred, such that each population has greater fitness [is more fit]. Further, the populations have diverged from each other: if continued indefinitely, the populations may over time evolve into distinct species.

Figure modified from the original; text material © 2021 by Steven M. Carr