Types of Selection

Introduction

Key Concepts

1. Overview of Natural Selection

Natural selection, a cornerstone of Charles Darwin's theory of evolution, describes the process by which traits that enhance survival and reproduction become more common in successive generations of a population. It operates through variations, differential survival and reproduction, and heritability of advantageous traits.

2. Types of Natural Selection

There are primarily three types of natural selection: directional selection, stabilizing selection, and disruptive selection. Each type influences the distribution of phenotypes in a population in distinct ways.

2.1 Directional Selection

Directional selection occurs when natural selection favors one extreme phenotype over the mean or other extremes. This type of selection shifts the population's trait distribution in one direction.

Example: The increase in antibiotic resistance in bacteria is a result of directional selection. Bacteria with mutations that confer resistance survive antibiotic treatments, leading to a population predominantly composed of resistant individuals.

Mechanism: Selection pressure is applied consistently in one direction, resulting in a shift of allele frequencies.

Mathematical Representation: If the average trait value in the population is μ, and directional selection favors traits greater than μ, the new average will be μ' > μ.

2.2 Stabilizing Selection

Stabilizing selection favors intermediate phenotypes, reducing variation and maintaining the status quo within the population.

Example: Human birth weight is subject to stabilizing selection. Babies with very low or very high birth weights have higher mortality rates, favoring mothers who produce infants of average weight.

Mechanism: This selection type reduces the frequency of extreme traits, favoring individuals with average characteristics.

Mathematical Representation: The trait distribution remains centered around the mean, μ' ≈ μ.

2.3 Disruptive Selection

Disruptive selection favors both extreme phenotypes over intermediate ones, potentially leading to speciation.

Example: In certain bird populations, individuals with either very large or very small beaks may have advantages in accessing different food sources, while those with medium-sized beaks are less successful.

Mechanism: Selection pressure favors individuals at both ends of the trait spectrum, increasing phenotypic variance.

Mathematical Representation: The trait distribution may become bimodal, with two distinct peaks at different trait values.

3. Balancing Selection

Balancing selection maintains genetic diversity within a population by keeping multiple alleles in balance.

Example: The sickle cell trait in humans is maintained by balancing selection. While homozygous individuals suffer from sickle cell disease, heterozygous individuals have resistance to malaria, providing a survival advantage in malaria-endemic regions.

Mechanism: Both homozygous and heterozygous genotypes have selective advantages, preventing any single allele from becoming fixed.

Mathematical Representation: Allele frequencies remain stable over time due to the selective pressures favoring multiple alleles.

4. Sexual Selection

Sexual selection is a form of natural selection arising from differential reproductive success. It often leads to the development of secondary sexual characteristics.

Example: The extravagant plumage of male peacocks serves to attract females, despite being a potential disadvantage in terms of survival.

Mechanism: Traits are favored not for survival benefits but for increasing mating success, often leading to pronounced sexual dimorphism.

Mathematical Representation: The fitness function in sexual selection includes components related to mating success, not just survival.

5. Artificial Selection

Artificial selection, also known as selective breeding, is the human-driven process of breeding organisms with desired traits.

Example: The breeding of domestic dogs has led to a wide variety of breeds, each with specific physical and behavioral traits.

Mechanism: Humans select which individuals reproduce based on specific desired characteristics, intentionally shaping the trait distribution.

Mathematical Representation: Similar to directional selection, but the selection pressure is imposed by human choice rather than natural environmental factors.

6. Stabilizing vs. Disruptive Selection

While both stabilizing and disruptive selection act to reduce genetic variation, they do so in different ways. Stabilizing selection favors intermediate phenotypes, maintaining population stability, whereas disruptive selection favors extreme phenotypes, potentially leading to speciation.

Implications: Understanding these selection types helps in comprehending how populations adapt to changing environments and how new species may arise.

7. Adaptive Landscapes and Selection Types

The concept of an adaptive landscape illustrates how different types of selection navigate the fitness peaks and valleys. Directional selection moves populations towards higher fitness peaks, stabilizing selection maintains populations on existing peaks, and disruptive selection can split populations into different peaks.

Visualization: An adaptive landscape graph plots fitness against trait variation, showing how selection pressures shift populations across the landscape.

8. Environmental and Genetic Factors Influencing Selection

Various factors influence the direction and intensity of natural selection, including environmental changes, genetic mutations, gene flow, and genetic drift.

Example: Climate change can alter selection pressures, favoring traits that confer advantages under new environmental conditions.

Interaction with Genetic Drift: In small populations, genetic drift can counteract selection by randomly changing allele frequencies.

9. Case Studies of Selection Types

Numerous case studies illustrate the different types of selection. For example, the classic case of the peppered moth demonstrates directional selection, where darker moths became more prevalent during the Industrial Revolution due to pollution-darkened trees.

Another example is the Galápagos finches, which exhibit both disruptive and directional selection based on available food sources and environmental conditions.

10. Measuring Selection

Scientists measure natural selection by assessing changes in allele frequencies over time and determining the fitness associated with different phenotypes.

Equation for Change in Allele Frequency: $$ \Delta p = \frac{p'(w) - p}{\bar{w}} $$ where $p$ is the initial allele frequency, $p'(w)$ is the allele frequency weighted by fitness, and $\bar{w}$ is the average fitness of the population. $$ \Delta p = \frac{p'(w) - p}{\bar{w}} $$

Fitness Measurements: Fitness can be quantified by reproductive success, such as the number of offspring produced by individuals with specific traits.

Comparison Table

Summary and Key Takeaways