Diversifying selection, also known as disruptive selection, is a pattern of natural selection where individuals with traits at both extremes of a spectrum are favored over those with intermediate traits. This process promotes the existence of distinct forms within a population and increases the variety of observable characteristics within a group of organisms.
How Diversifying Selection Works
Diversifying selection occurs when environmental pressures make extreme phenotypes more advantageous for survival and reproduction than intermediate ones. This leads to a bimodal distribution of traits within a population, where two distinct groups emerge. For example, if a habitat contains both very light and very dark surfaces, individuals that blend into either extreme will have a better chance of surviving predation than those with a medium coloration. This type of selection maintains or increases genetic variation within a population by favoring multiple alleles that correspond to these extreme traits. Over time, individuals with intermediate phenotypes are selected against, reducing the frequency of their associated genes while increasing those linked to extreme traits. The environmental conditions that drive diversifying selection are often heterogeneous, presenting distinct niches or challenges best met by specialized forms.
Examples in Nature
One example of diversifying selection involves populations of mice living on a beach environment. If the beach has patches of light-colored sand and dark, tall grass, light-colored mice would blend into the sand, and dark-colored mice could hide in the grass. Medium-colored mice, however, would stand out against both backgrounds, making them more susceptible to predators. This scenario leads to the survival and reproduction of both light and dark mice, while intermediate colored mice are less successful.
Another illustration can be found in animal populations that exhibit multiple male mating strategies, such as lobsters. Large, dominant males secure mates through aggressive encounters. Small males employ a “sneaking” strategy to obtain furtive copulations. Medium-sized males are often too large to effectively sneak and too small to compete successfully with the dominant males, making their mating strategy less effective. This results in the selection for both large and small male phenotypes, while intermediate sizes are disadvantaged.
Its Role in Evolution and Speciation
Diversifying selection has long-term evolutionary consequences, often leading to the formation of distinct sub-populations or morphs within a species. This divergence can create genetic differences between the extreme groups, even within the same geographic area.
Over time, if these distinct sub-populations become reproductively isolated, diversifying selection can drive speciation. Reproductive isolation means that individuals from different groups are unable to produce viable offspring or avoid mating with each other. This can occur through various mechanisms, such as differences in mating behaviors, habitat preferences, or physical characteristics that prevent successful interbreeding. When reproductive isolation occurs within the same geographic area, it is known as sympatric speciation, and diversifying selection is considered a significant factor in this process.
Diversifying Selection Versus Other Natural Selection Types
Diversifying selection differs from other forms of natural selection in how it shapes the distribution of traits within a population. Directional selection favors one extreme phenotype, causing the population’s average trait value to shift in that direction over generations. For instance, if an environment consistently favors larger individuals, the average size of the population will increase over time. This reduces variation at one end of the phenotypic range.
In contrast, stabilizing selection favors intermediate phenotypes, leading to a reduction in the frequency of extreme traits and a narrowing of the trait distribution around the average. An example of this is human birth weight, where both very low and very high birth weights are associated with lower survival rates, favoring an average weight. This type of selection decreases genetic variation within a population.
Diversifying selection, distinct from both directional and stabilizing selection, promotes polymorphism by favoring two or more extreme phenotypes. It leads to a bimodal distribution of traits, where the population effectively splits into distinct forms, increasing overall genetic variance. This process maintains diversity, unlike stabilizing selection which reduces it, and it drives divergence within a population, unlike directional selection which shifts the entire population towards a single extreme.