In natural selection, disruptive selection selects against the average individual in a population. Typically, this type of population would have phenotypes (individuals with groups of traits) at both extremes, but very few in the middle. As the rarest of the three types of natural selection, disruptive selection can cause species lines to diverge.
Basically, it comes down to the individuals in the group who get to mate-who survive. These are the ones who exhibit traits on the extreme end of the spectrum. Individuals with just middle-of-the-road characteristics have less chance of surviving and/or reproducing to pass on “average” genes.
When intermediate individuals are most populous, population functions in stabilizing selection mode. When conditions change, such as habitat changes or resource availability changes, disruptive selection occurs.
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Disruptive Selection and Speciation
When disruptive selection is present, the bell curve does not have a typical shape. There are almost two bell curves in this graph. There are peaks at both extremes and a deep valley in the middle, where the average individual is represented. Speciation can result from disruptive selection, which can result in the extinction of middle-of-the-road individuals. Due to this, it’s also called “diversifying selection.”
As individuals compete with each other for food to survive and/or partners to pass on their lineage, disruption selection occurs in large populations with lots of pressure to find advantages or niches.
Similarly to directional selection, disruptive selection can be influenced by human interaction. Disruptive selection can drive animals to choose different colors for survival as a result of environmental pollution.
Disruptive Selection Examples: Color
In many different species, color can serve as a useful example of camouflage because the individuals that can hide from predators the most effectively will live the longest. Those who cannot blend into either extreme of an environment, whether they are moths, oysters, toads, birds, or anything else, will be eaten quickly.
London’s peppered moths are one of the most studied examples of disruptive selection. The peppered moths were almost exclusively very light in color in rural areas. These same moths, however, were very dark in color in industrial areas.
There were very few medium-colored moths at either location. Darker-colored moths survived in industrial areas by blending in with polluted surroundings. The lighter moths were easily seen by predators in industrial areas and were eaten. Rural areas experienced the opposite. Due to disruptive selection, there were very few medium-colored moths left in both locations.
The light- and dark-colored oysters could also have a camouflage advantage over their medium-colored relatives. Oysters with light colors would blend into the rocks in the shallows, while oysters with dark colors would blend into the shadows.
Intermediate oysters would show up against either backdrop, giving them no advantage and making them easier prey. Therefore, with fewer medium individuals surviving to reproduce, the population eventually has more oysters with extreme colors.
Disruptive Selection Examples: Feeding Ability
It isn’t all a straight line when it comes to evolution and speciation. A group of individuals may be under multiple pressures, such as drought pressure, for example, that is just temporary, so the intermediate individuals do not disappear completely.
There is a long timeframe involved in evolution. When there are enough resources for all types of diverging species, they can coexist. A population might specialize in food sources in fits and starts, only when food supplies are under pressure.
Mexican spadefoot toad tadpoles:
There are higher populations of spadefoot tadpoles at the extremes of their shape, with each type having a dominant feeding pattern. In general, omnivorous individuals have round bodies, while carnivorous individuals have narrow bodies.
Body shapes and eating habits of intermediate types are smaller (less well-fed) than those at either extreme. There was an additional, alternate food source available to those at the extremes as opposed to those at the intermediates, according to a study.
Those that were more omnivorous fed more effectively on pond detritus, and those that were more carnivorous fared better on shrimp. The intermediate types competed for food, resulting in individuals on the extremes being able to eat more and grow faster.
In the Galapagos Islands, fifteen species evolved from a common ancestor 2 million years ago. They differ in terms of beak style, body size, feeding behavior, and song. Over time, different types of beaks have evolved to adapt to different food resources.
On Santa Cruz Island, ground finches eat more seeds and some arthropods, tree finches eat more fruits and arthropods, vegetarian finches eat leaves and fruit, and warblers eat mostly arthropods. There is an overlap between what they eat when food is abundant. Even when it isn’t, this specialization, the ability to eat a certain type of food better than others, helps them survive.