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Natural selection is often associated with the phrase “survival of the fittest”•The fittest individuals are those that not only survive, but are able to leave the most offspring behind •The selection on phenotypes affects genotypes present in the population–If environmental conditions favored tall pea plants, then tall pea plants would leave more offspring–These offspring would carry tall alleles•Individuals are not born with invariable levels of fitness; instead, fitness can change in accordance with changes in the surrounding environment.
Three Modes of Natural SelectionStabilizing selectionDirectional selectionDisruptive selectionRange of a particular characteristic(in this instance, lightness or darkness of coloration)In stabilizing selection,individuals that possessextreme values of acharacteristic—here, both the lightest and the darkest colors—are selected against and die or fail to reproduce. Over succeeding generations,an increasing proportion of the population becomes average in coloration.In directional selection, one of the extremes of a characteristic is better suited to the environment, meaning that individuals at the other extreme are selected against. Over succeeding generations, the coloration of the population moves in a direction—in this case toward darker coloration.In disruptive selection,individuals with averagecoloration are selectedagainst and die. Oversucceeding generations, part of the population becomes lighter, whilepart becomes darkermeaning the range of color variation in the population has increased.Time (many generations)
Stabilizing Selection•Stabilizing selectionwhen environmental conditions are relatively constant•Phenotypic variability declines–Intermediate phenotype is favored–Both extreme phenotypes are selected against•e.g. body size in Aristelligerlizards–Smallest lizards have a difficulty defending territory–Largest lizards more likely to be eaten by owls
Stabilizing SelectionStabilizing selection moves a given character in a population toward intermediate forms and hence tends to preserve the status quo. infantbirthsInfant mortality is lowest among infants of averagebirth weight.infantdeaths302010062345Birth weight in pounds40506070789101150101520Percent of infant deathsPercent of births in population
Directional Selection•Directional selectionoccurs when environmental conditions change in a consistent direction•Average phenotype shifts in a consistent direction–One of the extreme phenotypes is favored–Both intermediate and opposite extreme phenotypes are selected against –e.g. pesticide resistance, antibiotic resistance
Directional SelectionDirectional selection moves a given character toward one of its extreme forms.