BIOS Instant Notes in Genetics by Fletcher Hugh & Hickey Ivor

Author:Fletcher, Hugh & Hickey, Ivor [Fletcher, Hugh]
Language: eng
Format: epub
ISBN: 978-1-136-16033-2
Publisher: Taylor and Francis
Published: 2012-11-26T16:00:00+00:00

Figure 1. Modes of selection. The shaded region is selected against. Arrows show changes in phenotype frequency expected over time. Vertical solid lines show means, dashed lines allow comparisons.

Directional selection

Directional selection (Figure 1b ) favors individuals with characteristics at one end of the range found in the population. An example which changes direction with the climate relates to Darwin’s finches, Geospiza fortis . During drought in the Galapagos Islands, grasses and small plants do not grow well, but bigger deep-rooted plants with larger seeds still produce fruit. There is then selection which favors individual finches with larger beaks capable of eating large seeds, and selection against those with smaller beaks best-suited for eating grass seeds. This is reversed after wet years when small seeds are plentiful. A common form of directional selection causes character displacement when two species compete. If there are two similar species of finch on one island (sympatry), one species will specialize in large seeds and evolve a large beak, the other will specialize in small seeds or insects and evolve a suitably small beak. Either species alone (allopatry) has an intermediate size beak. This effect is very common among related species.

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