Mechanisms of Evolution and Natural Selection: Study Notes

Mechanisms of Evolution

Introduction to Evolutionary Mechanisms

Evolution is the process by which populations of organisms change over generations. Several mechanisms drive evolutionary change, including mutation, gene flow, genetic drift, and natural selection. Understanding these mechanisms is fundamental to the study of biology.

• Mutation: Random changes in DNA that introduce new genetic variation into a population.

• Gene Flow: Movement of genes between populations, often through migration, which increases genetic diversity.

• Genetic Drift: Random changes in allele frequencies, especially in small populations, that can lead to significant evolutionary change.

• Natural Selection: Differential survival and reproduction of individuals due to differences in phenotype.

Example: The image provided shows how each mechanism can alter the genetic makeup of a population over time.

Natural Selection

Definition and Requirements

Natural selection is a process that causes change in lineages over time. It is the primary mechanism by which populations adapt to their environments.

• Limits on Population Growth: More individuals are born than can survive, leading to competition for resources.

• Variation in the Population: Individuals differ in traits that affect their ability to survive and reproduce.

• Differential Reproductive Success: Some individuals leave more offspring than others.

• Inheritance: Traits that influence reproductive success are passed from parents to offspring.

Equation:

$w = \frac{N_{offspring}}{N_{average}}$ Where $w$ is individual fitness, $N_{offspring}$ is the number of offspring produced by an individual, and $N_{average}$ is the average number of offspring in the population.

Fitness and Relative Fitness

Fitness refers to the impact an individual has on its population into the future, measured by the number of offspring it leaves. Relative fitness compares the reproductive success of one individual to others in the population.

• Absolute Fitness: Total number of offspring produced.

• Relative Fitness: Ratio of an individual's fitness to the highest fitness in the population.

Outcomes of Natural Selection

Natural selection leads to adaptation, where traits that enhance survival and reproduction become more common in the population. Traits that hinder survival and reproduction decrease in frequency.

• Adaptation (verb): Changes in a lineage that increase individuals’ survival and/or reproduction.

• Adaptation (noun): A trait that influences an individual's survival and reproduction.

Example: The evolution of beak shape in Galapagos finches is a classic example of natural selection in action.

Genetic Drift

Definition and Effects

Genetic drift is a mechanism of evolution that refers to random changes in allele frequencies in a population. It is most significant in small populations and can lead to the loss of genetic variation.

• Founder Effect: Occurs when a new population is established by a small number of individuals, carrying only a fraction of the original population's genetic variation.

• Bottleneck Effect: Occurs when a population is drastically reduced in size due to a catastrophe, resulting in reduced genetic diversity.

Example: Dwarfism in the Pennsylvania Amish population is a result of the founder effect.

Additional info: Genetic drift does not lead to adaptation, but it can cause significant evolutionary change, especially in small populations.

Speciation

Phyletic Evolution vs. Speciation

Evolution can cause a lineage to change over time (phyletic evolution) or create new species (speciation).

• Phyletic Evolution: Gradual transformation of a species into a new form without branching.

• Speciation: The process by which new species are generated, often through reproductive isolation.

Allopatric Speciation

Allopatric speciation occurs when a population is divided geographically, leading to the formation of new species as genetic differences accumulate in isolated groups.

• Geographic Barriers: Ice sheets, mountain chains, rivers, sea level changes, climate shifts, and island formation can separate populations.

• Example: Antelope squirrels in the Grand Canyon have diverged into separate species due to geographic isolation.

Adaptive Radiation

Definition and Examples

Adaptive radiation is the rapid evolution of diversely adapted species from a common ancestor when new environments become available.

• Small-scale: Galapagos finches and Hawaiian honeycreepers.

• Large-scale: Evolution of mammals after the End-Cretaceous extinction.

Example: The diversification of mammals into various ecological niches after the extinction of dinosaurs.

Convergent Evolution

Definition and Causes

Convergent evolution occurs when unrelated species evolve similar traits independently, often as adaptations to similar environments, rather than inheriting them from a common ancestor.

• Example: The streamlined bodies of sharks (fish), ichthyosaurs (reptiles), and dolphins (mammals) are adaptations to aquatic life.

• Cause: Natural selection acting on species in similar environments.

Additional info: Convergent evolution is distinct from parallel evolution, which occurs in closely related species.

Summary Table: Mechanisms of Evolution

Mechanism	Description	Effect on Population	Example
Mutation	Random changes in DNA	Introduces new alleles	Sickle cell mutation
Gene Flow	Movement of genes between populations	Increases genetic diversity	Migration of birds between islands
Genetic Drift	Random changes in allele frequencies	Loss of genetic variation	Founder effect in Amish population
Natural Selection	Differential survival and reproduction	Adaptation to environment	Beak shape in Galapagos finches