Zoology · Ch 6 — Evolution
Natural Selection — Types of Selection
Natural Selection — Types of Selection
Natural selection occurs whenever a particular allele, or combination of alleles, makes its bearer more or less likely to survive and reproduce in a given environment; alleles that reduce fitness tend to decline in frequency across generations, while those that raise it tend to increase. Selection can act on a population's phenotypic distribution in three characteristic patterns (Fig. 6.7). Stabilising selection (centripetal selection) operates in a stable environment, favouring average phenotypes and eliminating both extremes, so the population's phenotypic spread narrows over time without any new species forming — measurements of sparrows that survived a severe storm clustering tightly around the population mean, while the sparrows that died were disproportionately from the extremes, is the textbook example. Directional selection operates when the environment itself is gradually changing, and shifts the population's phenotype distribution progressively toward one end of the range — differences in body weight between male and female sparrows, where females show a directional shift relative to males, illustrate this pattern. Disruptive selection (centrifugal selection) operates when a once-uniform environment becomes heterogeneous, favouring both phenotypic extremes while eliminating the intermediate phenotype; this is a comparatively rare pattern but can split a single population into two or more diverging sub-populations, effectively becoming a route to adaptive radiation — Darwin's finches, where beak size track …
What this figure shows. The figure illustrates natural selection using a beetle population shown across two generations in genotypes AA, Aa and aa against a dark rock background. A caption reads 'light grey beetles are spotted and eaten by birds more often than dark ones' and 'If X is eaten by birds then only survivors reproduce', with beetle icons scattered over the dark background to show light-coloured individuals standing out and being picked off. The 'Next generation' panel below shows a new array of AA/Aa/aa genotype icons in different relative numbers than the parent generation, visually demonstrating that differential predation on the conspicuous light phenotype shi …
What this figure shows. Three bell-shaped graphs, each plotting 'Number of individuals with phenotype' on the vertical axis against a phenotype range on the horizontal axis, are shown side by side and labelled (a), (b) and (c). Graph (a) 'Stabilising' shows the peak of the curve becoming higher and narrower over time, with the caption 'Peak gets higher and narrower', meaning average phenotypes are favoured and both extremes are removed. Graph (b) 'Directional' shows the whole peak sliding toward one end of the phenotype axis, captioned 'Peak shifts in one direction', meaning one extreme phenotype is increasingly favoured as the environment changes. Graph (c) 'Disruptive' shows the single original peak splitting into two separate peaks at opposite ends of the axis, captioned 'Two peaks form' and 'Medium-sized individuals are favoured' pointing to the trough between them, meaning both extremes …