Disruptive Selection Examples: See How Evolution Works!

Evolutionary biology, the science underpinning the study of adaptation, provides a framework for understanding disruptive selection example. The rock pocket mouse offers a compelling case study often cited to illustrate the power of natural selection. Charles Darwin’s pioneering work on the origin of species laid the groundwork for comprehending these complex processes. Specifically, disruptive selection example, is a process where extreme values for a trait are favored over intermediate values, leading to the divergence of populations over time.

Disruptive Selection Examples: A Detailed Look

Disruptive selection, also known as diversifying selection, is a mode of natural selection that favors extreme values for a trait over intermediate values. It plays a crucial role in driving evolutionary change. The central concept revolves around individuals with traits at either end of the spectrum being more successful than those in the middle. Understanding this requires examining specific disruptive selection example scenarios.

What is Disruptive Selection?

Disruptive selection leads to increased genetic variance within a population by selecting against the average phenotypes. This often results in a bimodal distribution of traits, where two distinct groups become more prevalent. The original population, which may have displayed a normal distribution for a particular trait, is essentially pulled apart towards the extremes.

How it Differs from Other Types of Selection

It’s useful to differentiate disruptive selection from other modes of selection:

• Directional Selection: Favors one extreme of the trait range. The bell curve shifts towards one side. Example: Dark-colored moths becoming more common in polluted environments.

• Stabilizing Selection: Favors the intermediate phenotype, reducing variance. The bell curve becomes narrower. Example: Human birth weight; babies with average weight have higher survival rates.

• Disruptive Selection: Favors both extremes of the trait range, leading to a bimodal distribution.

Key Components of Disruptive Selection

Several factors usually contribute to disruptive selection:

1. Environmental Heterogeneity: The environment must contain different niches or resources that favor different traits.

2. Frequency-Dependent Selection: The fitness of a phenotype depends on its frequency in the population. In some cases, rare phenotypes might have an advantage.

3. Assortative Mating: Individuals with similar extreme phenotypes tend to mate with each other, further reinforcing the differences and potentially leading to reproductive isolation.

Disruptive Selection Example Scenarios

Let’s delve into specific disruptive selection example scenarios to illustrate how this evolutionary force operates.

The Black-Bellied Seedcracker Finch

This African finch provides a compelling disruptive selection example. Their beak size is directly related to their ability to crack different types of seeds.

• Scenario: In regions where only small, soft seeds and very large, hard seeds are available, finches with either small beaks (good for small seeds) or large beaks (good for large seeds) thrive. Finches with intermediate beak sizes struggle to efficiently crack either type of seed.

• Outcome: Over time, the finch population evolves into two distinct groups: one with small beaks and one with large beaks. The intermediate beak size becomes less common.

Three-Spined Stickleback Fish

This fish species exhibits disruptive selection related to feeding habits.

• Scenario: In some lake environments, three-spined sticklebacks specialize in feeding on either small invertebrates in shallow water or larger prey in deeper water. Fish with either very short or very long gill rakers are more efficient at capturing their respective prey. Gill rakers are bony projections in the gills that filter food particles.

• Outcome: Disruptive selection favors the two extreme phenotypes (short and long gill rakers), while individuals with intermediate gill rakers are less efficient feeders and less likely to survive and reproduce.

Color Polymorphism in Butterflies

Butterflies can display different color morphs due to disruptive selection.

• Scenario: Consider a butterfly species where bright colors make them attractive to mates but also visible to predators. Dark colors provide better camouflage but make them less attractive to mates.

• Outcome: In certain environments, butterflies that are either extremely bright (high mating success, high predation risk) or extremely dark (low mating success, low predation risk) have better overall fitness than butterflies with intermediate coloration, which are moderately visible to both predators and potential mates. This drives the population towards two distinct color morphs.

The Case of Artificial Selection

While disruptive selection is a natural process, humans can also intentionally create disruptive selection pressures.

• Example: Imagine selectively breeding a plant for disease resistance. If the breeder only propagates plants that are either highly resistant to disease A or highly resistant to disease B, but not plants with moderate resistance to both, this creates disruptive selection.

• Outcome: Over generations, the plant population will likely split into two groups: one highly resistant to disease A and another highly resistant to disease B.

Consequences of Disruptive Selection

The most significant consequence of disruptive selection is increased phenotypic and genetic variation within a population. This can lead to:

• Speciation: If disruptive selection is strong enough and assortative mating occurs, the two diverging groups can eventually become reproductively isolated, leading to the formation of new species.

• Polymorphism: The maintenance of distinct morphs (forms) within a population. The butterfly color disruptive selection example illustrates this.

• Increased Adaptability: A population with greater genetic variation is generally better equipped to adapt to future environmental changes.

Table: Examples Summarized

So, next time you’re pondering how life’s diversity came to be, remember the disruptive selection example. It’s a fascinating reminder that evolution isn’t always about finding the middle ground!