Disruptive Selection: Definition, Examples, Significances

Disruptive selection, also referred to as diversifying selection, is a type of natural selection that favors individuals with the smallest and largest trait values.

Natural selection, proposed by Charles Darwin and Alfred Russel Wallace in the 19th century, is a fundamental mechanism explaining the process of evolution.

Natural selection is the process through which inherited traits that improve the survival and reproductive success of organisms become more common in a population over generations. It explains the diversity and adaptation of life on Earth. Differences among individuals drive natural selection, influencing their likelihood of survival and reproduction.

There are three main types of natural selection:

1. Directional selection favors individuals at one end of the trait distribution, causing a gradual shift in the average phenotype as time progresses. Example: Long beaks in birds becoming more prevalent in a population due to changes in food availability.
2. Stabilizing selection favors individuals with intermediate values of a trait, reducing the overall trait variation in the population. Example: Intermediate birth weights in humans have a higher survival rate than extremely low or high birth weights.
3. Disruptive selection favors individuals at both extremes of the trait distribution, leading to a bimodal distribution with fewer intermediate phenotypes. Example: Fish populations where both small and large individuals have advantages over medium-sized individuals.

Understanding natural selection and its types helps us to understand how species adapt to their environments and how biodiversity is generated over time. 

What is Disruptive Selection?

Disruptive selection occurs when individuals with more extreme characteristics, whether in terms of appearance (phenotypes) or genetic makeup (genotypes), have a fitness advantage compared to individuals with intermediate traits.

• Among the three types of natural selection, disruptive selection is the least common and can result in the deviation of a species line.
• This type of selection occurs during periods of change, such as changes in habitat or resource availability. 
• Individuals with traits at the extremes of the spectrum enjoy better chances of survival and reproduction, leading to a population with few individuals in the middle. 
• Individuals with intermediate characteristics face a fitness disadvantage, hindering their ability to pass on genes. They are not as successful at survival or breeding.
• In simpler terms, nature favors the extremes rather than the middle ground, leading to a population where individuals with highly distinct traits have better chances of survival and reproduction.
• Disruptive selection explains the diversity and adaptation of life on Earth by favoring heritable traits that enhance the survival and reproductive success of organisms, making them more prevalent in successive generations. 

Speciation and Disruptive Selection

• Disruptive selection has the potential to result in speciation, forming two or more distinct species.
• If disruptive selection keeps happening, it might slowly split the group into two parts. These parts become so different that they don’t share traits anymore. 
• As long as there are some individuals with traits from both groups, we call it disruptive selection. If, at some point, no hybrids are left, and the split is complete, we call it divergent directional selection.

Sympatric Speciation and Disruptive Selection

• Sympatric speciation is believed to occur through disruptive selection. It is a controversial concept in evolutionary biology. 
• Sympatric speciation is an uncommon process that stands out from other types of speciation.
• Sympatric speciation translates to “same place” in Greek and in this process an ancestral species undergoes a split into two or more reproductively isolated groups without geographic separation. 
• According to sympatric speciation, the formation of a new species can occur without any physical barriers preventing members of a species from mating with one another, and all members are in close proximity. 
• In sympatric speciation, a new species emerges based on a different food source or characteristic, without the need for geographic isolation. Over time, these differences in environmental dependency can lead to reproductive isolation. 
• The crucial element of sympatric speciation is that it occurs when emerging species are in physical contact with each other, allowing them to interbreed and exchange genes. 
• This concept challenges traditional ideas about the necessity of geographic isolation for speciation to occur.

Examples of Disruptive Selection

Peppered moths in industrial and rural areas

One of the popular examples of disruptive selection is ​London’s peppered moths. In industrial areas with pollution, darker-colored moths were more likely to survive predation because they blended in with the polluted surroundings. In rural areas, lighter-colored moths had a survival advantage. There were very few moths with medium color in both places. This led to a bimodal distribution in wing coloration, representing disruptive selection.

Darwin’s finches on the Galapagos Islands

Different species of finches have evolved with distinct beak shapes and sizes, adapted to different food sources. For example, ground finches have beaks suited for eating seeds, while tree finches have beaks adapted for feeding on fruits and arthropods.

Mexican spadefoot toad tadpoles

Tadpoles exhibit a resource polymorphism with two extreme phenotypes – omnivores and carnivores. Omnivorous individuals have round bodies, and the carnivorous tadpoles have narrow bodies. The intermediate ones are smaller than those at either extreme of body shape and eating habits. Intermediate phenotypes experience increased competition and reduced fitness, leading to disruptive selection.

Lazuli Buntings

In habitats with limited nesting sites, both the dullest and brightest yearling males have higher fitness, obtaining high-quality territories and attracting females. Yearling males with intermediate feathers face aggression from adults, resulting in disruptive selection. 

Disruptive Selection Video Animation

Significances of Disruptive Selection

• Disruptive selection has the potential to generate significant variation within a population. This variation contributes to the evolution of new species through the process of speciation.
• Disruptive selection can lead to ecological sexual dimorphism, where different traits are favored in males and females based on ecological factors.
• The process of disruptive selection contributes to the alteration of a population’s genetic makeup over an extended period.
• Disruptive selection is considered a fundamental aspect of evolution, playing a crucial role in shaping the diversity of life on Earth. Darwin’s study of finch populations in the Galapagos is an example showcasing the role of disruptive selection in evolution. 
• Disruptive selection is often connected with other selection mechanisms, such as directional stabilizing selection.

References

1. 19.3B: Stabilizing, Directional, and Diversifying Selection – Biology LibreTexts
2. Disruptive Selection – Definition and Examples – Biology Online Dictionary
3. Hill, W. G. (2013). Disruptive Selection. Brenner’s Encyclopedia of Genetics, 333–334. doi:10.1016/b978-0-12-374984-0.00411-3
4. Holsinger, K. E. (2001). Natural Selection. Brenner’s Encyclopedia of Genetics, 8–13. doi:10.1016/b978-0-12-374984-0.01020-2
5. https://www.cell.com/current-biology/pdf/s0960-9822(07)01625-9.pdf
6. Migration: 3.3 Disruptive selection – OpenLearn – Open University
7. Scoville, Heather. “Types of Natural Selection: Disruptive Selection.” ThoughtCo, Aug. 28, 2020, thoughtco.com/what-is-disruptive-selection-1224582.
8. Speciation (nationalgeographic.org)
9. Thoday, J. M. (1972). Review Lecture: Disruptive Selection. Proceedings of the Royal Society B: Biological Sciences, 182(1067), 109–143. doi:10.1098/rspb.1972.0070