Types and Modes of Natural Selection: Genetics Study Guide

Natural Selection: Definition and Principles

Introduction

Natural selection is a fundamental mechanism of evolution, describing how organisms better adapted to their environment tend to survive, reproduce, and pass on their genes. This process leads to changes in the genetic composition of populations over generations.

• Definition: Natural selection is any consistent difference in fitness among different classes of biological entities.

• Fitness: The number of offspring an individual leaves in the next generation. It has two components: survival and reproduction (also called reproductive success).

• Example: In a species of annual plant, if only 1 out of 1000 seeds survives to reproductive age and produces 3000 seeds, the average fitness is $0.001 imes 3000 = 3$.

Causes and Requirements of Natural Selection

Genetic Basis

For evolution by natural selection to occur, there must be a change in the population across generations, requiring inherited phenotypic differences.

• Correlation between an individual's phenotype and its fitness.

• Variation in phenotype must be correlated between parents and offspring.

Fitness Example

• In asexual plants with different resistance to herbicide, genotype B (fitness 4) increases faster than genotype A (fitness 3).

• After 7 generations, genotype B dominates ~90% of the population.

Natural Selection and Chance

Distinction from Other Evolutionary Processes

• Natural selection ≠ evolution: Selection can maintain stability or cause change.

• Genetic drift: Random fluctuations in allele frequencies.

• Neutral alleles: Alleles with no fitness differences; change only by chance.

• Natural selection causes consistent differences in reproductive success, while drift is random.

Levels of Selection

Hierarchy of Selection

• Selection can occur at multiple levels: genes, cell types, individual organisms, populations, and species.

• This forms a hierarchy: Genic Selection → Individual Selection → Group Selection → Species Selection.

Genic Selection

Selfish Genetic Elements

Genic selection refers to genes that selfishly propagate, such as transposable elements and segregation distorters.

• Transposable Elements: "Jumping genes" are DNA sequences that can copy and insert themselves into new locations in the genome.

• Segregation Distortion: Occurs when alleles are inherited in unequal proportions due to processes like meiotic drive.

Example: The T Locus in House Mouse (Mus musculus)

• A male carrying both t (selfish allele) and T (normal allele) produces sperm; the t allele kills sperm that carry the normal T allele, so most sperm carry t.

• Homozygous t embryos die or become sterile, but t persists due to efficient transmission in heterozygotes.

Selfish Genes and Kin Selection

Mechanisms

• A gene increases in frequency if it leaves more copies of itself, by any means (e.g., plants producing more pollen).

• Kin selection: Alleles can spread if they help relatives survive, since relatives share genes.

• Parental care: Parents help offspring survive, ensuring the same alleles persist.

Individual Selection

Whole-Organism Fitness

• Individual selection is the differential survival or reproduction of individuals with different traits within a population.

• Traits evolve because they increase the fitness of the individual, not necessarily the group.

• Individual selection is usually faster and more efficient than group selection.

• Example: Fast-running gazelles avoid predators and leave more offspring than slower gazelles.

Group Selection

Success of Groups

• Group selection is the differential survival or reproduction of groups with different compositions.

• Example: Selfish groups overconsume resources and die; altruistic groups survive longer.

• Most biologists agree individual selection dominates, but group selection may play a limited role.

Species Selection

Speciation and Extinction Rates

• Species selection occurs when species with certain traits speciate or go extinct at different rates.

• Changes the proportion of species with certain traits over time.

• Example: Sexual species are more common and persistent; asexual lineages often go extinct faster.

Modes of Selection

Overview

• Disruptive Selection

• Directional Selection

• Stabilizing Selection

• Balancing Selection

• Artificial Selection

Disruptive Selection

Definition and Mechanism

Disruptive selection favors individuals at both extremes of a trait while making intermediate individuals less fit. "Average" traits are disadvantageous.

• Extremes are often better adapted to different niches or resources.

• Example: In black-bellied seedcracker (Pyrenestes ostrinus), birds with very large or very small bills have higher survival than those with intermediate bills.

Effect on Population

• More individuals at the extremes, fewer in the middle.

• Variance increases, mean may stay the same.

• Example: Red crossbills (Loxia curvirostra): Small bills open western hemlock cones, large bills open lodgepole pine cones; intermediate bills have low fitness.

Directional Selection

Definition and Mechanism

Directional selection favors individuals at one extreme of a continuous trait distribution, causing a shift in the population mean toward the favored extreme.

• Occurs when fitness consistently increases or decreases with the value of a trait.

• Can lead to evolutionary change if the trait is heritable.

• Example: Evolution of larger bill depth in Galapagos finches after drought.

Measuring Strength: Selection Gradient

• The selection gradient ($\beta$) quantifies how strongly selection acts on a trait.

• $\beta > 0$: favors larger trait values; $\beta < 0$: favors smaller values; $\beta = 0$: no directional selection.

• Relative fitness = individual fitness / population mean fitness.

• Example: In guppies (Poecilia reticulata), females prefer orange males; estimated $\beta = 3.8$.

Evolution by Directional Selection: Breeder's Equation

• If selection acts on a heritable trait, evolution occurs.

• Described by the Breeder's Equation:

$\Delta \bar{Z} = h^2 S$

• $\Delta \bar{Z}$: evolutionary change in mean trait value

• $h^2$: heritability (strength of inheritance)

• $S$: selection differential (difference between mean before and after selection)

Stabilizing Selection

Definition and Mechanism

Stabilizing selection favors intermediate phenotypes and acts against extremes, maintaining the mean phenotype near an optimum and reducing phenotypic variance.

• Individuals with average traits have higher fitness.

• Extreme phenotypes suffer lower survival or reproduction.

• Population becomes more uniform; genetic variation decreases unless maintained by mutation or gene flow.

Features and Outcomes

• Reduces diversity by eliminating individuals with extreme trait values.

• Keeps traits near optimal values for a given environment.

• Can reduce evolutionary potential due to less genetic variation.

• Acts on both genetic and environmental variance.

• Example: Human birth weight: infants with average birth weight have higher survival rates.

Summary Table: Modes of Selection

Mode	Favored Phenotypes	Effect on Population	Example
Disruptive	Both extremes	Increases variance, reduces intermediates	Bill size in seedcrackers
Directional	One extreme	Shifts mean, reduces variance	Finch bill depth
Stabilizing	Intermediate	Reduces variance, maintains mean	Human birth weight

Additional info:

• These notes expand on the original slides by providing definitions, equations, and examples for each mode and level of selection, as well as a summary table for comparison.