Microevolution and Genetic Variation: Study Notes for General Biology

Microevolution: The Evolution of Populations

Introduction to Microevolution

Microevolution refers to the changes in allele frequencies that occur within a population over time. These changes are the foundation of evolutionary processes and can be observed over relatively short timescales.

• Microevolution involves small-scale changes within populations, as opposed to macroevolution, which refers to large-scale evolutionary changes such as the formation of new species.

• It is driven by several mechanisms, including natural selection, genetic drift, gene flow, and mutation.

• Microevolution is measurable by tracking changes in the gene pool of a population.

Adaptation vs Phenotypic Plasticity

Definitions and Distinctions

Organisms can respond to their environment through adaptation or phenotypic plasticity. Understanding the difference is crucial for studying evolutionary biology.

• Adaptation: A heritable trait that increases an organism's fitness in a particular environment, resulting from natural selection over generations.

• Phenotypic Plasticity: The ability of an individual to change its phenotype in response to environmental conditions, without genetic change. These changes occur within an individual's lifetime and do not involve alterations to the DNA sequence.

• Phenotypic plasticity does not have a genetic component and is not inherited by offspring.

Examples of Phenotypic Plasticity

• Leaf shape changes in response to sunlight exposure.

• Butterfly wing coloration changes depending on temperature during development.

• Water flea morphology changes in response to predator presence.

• Horn size in beetles influenced by nutrition.

• Flowering time in plants affected by temperature or day length.

Example: Some plants can alter leaf thickness or color depending on the amount of sunlight they receive, allowing them to optimize photosynthesis in different environments.

Genetic Material: Genes, Chromosomes, and Genomes

Key Definitions

• Gene: A sequence of DNA on a chromosome that codes for a specific protein or functional RNA.

• Chromosome: A structure composed of DNA and proteins that contains many genes.

• Genome: The complete set of all genetic material (genes and chromosomes) in an organism.

• Locus: The specific physical location of a gene on a chromosome.

• Allele: Different versions of a gene that may produce distinguishable phenotypic effects.

Example: The human genome consists of 23 pairs of chromosomes, each containing many genes.

Homologous Chromosomes and Alleles

• Homologous chromosomes carry the same sequence of genes, but may have different alleles for those genes.

• An individual can be homozygous (having two identical alleles for a gene) or heterozygous (having two different alleles for a gene).

Genotype and Phenotype

• Genotype: The collection of all alleles that an organism contains for a given gene or set of genes.

• Phenotype: The observable physical or physiological traits of an organism, determined by its genotype and environment.

Gene Pool and Population Genetics

Definition of Gene Pool

• Gene pool: All copies of all alleles at every locus in all members of a population.

• Represents the total genetic diversity found within a population.

Example: In a population of flowers, the gene pool includes all the alleles for flower color present in that population.

Microevolution in Populations

• Evolution occurs within populations as a change in the relative frequencies of alleles in the gene pool over time.

• Microevolution can be measured by tracking changes in allele frequencies from one generation to the next.

Mechanisms of Microevolution

Natural Selection

Natural selection is a non-random process where individuals with traits better suited to their environment have higher reproductive success.

• Leads to adaptive evolution, increasing the frequency of beneficial alleles.

• Can result in different patterns: stabilizing, directional, or disruptive selection.

Genetic Drift

Genetic drift is the change in allele frequency in a population due to random sampling effects, especially significant in small populations.

• Can lead to the loss or fixation of alleles by chance.

• Two main types:

  • Bottleneck effect: A sharp reduction in population size due to environmental events, leading to a loss of genetic diversity.

  • Founder effect: When a new population is established by a small number of individuals, resulting in a gene pool that may not represent the original population.

• Effects are more extreme in small populations.

Example: If a natural disaster drastically reduces a population, the surviving gene pool may have different allele frequencies than the original population.

Gene Flow

Gene flow is the transfer of alleles from one population to another due to the movement of individuals or gametes.

• Decreases genetic differences between populations.

• Can introduce new alleles into a population, increasing genetic variation.

Example: Pollen carried by wind from one population of plants to another introduces new genetic material.

Mutation

• Mutation is the original source of genetic variation, creating new alleles by altering DNA sequences.

• Most mutations are neutral or harmful, but some can be beneficial and subject to natural selection.

Random vs Non-Random Processes

• Natural selection: Non-random

• Genetic drift: Random

• Gene flow: Dispersive (can be random or non-random depending on context)

• Mutation: Random

Key Terms and Concepts

Term	Definition
Adaptive evolution	Evolution that results in a better match between organisms and their environment
Bottleneck effect	Genetic drift resulting from a drastic reduction in population size
Gene flow	Transfer of alleles between populations
Gene pool	All alleles at all loci in all individuals of a population
Genetic drift	Random changes in allele frequencies in a population
Genotype frequency	Proportion of a specific genotype among all individuals in a population
Heterozygous	Having two different alleles for a gene
Homozygous	Having two identical alleles for a gene
Mutation	Change in DNA sequence, source of new alleles
Phenotypic frequency	Proportion of a specific phenotype in a population
Population	Group of individuals of the same species living in the same area
Sexual dimorphism	Differences in secondary sexual characteristics between males and females
Sexual selection	Natural selection for mating success
Stabilizing selection	Selection that favors intermediate variants
Directional selection	Selection that favors one extreme phenotype
Disruptive selection	Selection that favors both extreme phenotypes

Formulas and Equations

• Allele frequency in a population: $p = \frac{2N_{AA} + N_{Aa}}{2N}$ $q = \frac{2N_{aa} + N_{Aa}}{2N}$ Where $N_{AA}$, $N_{Aa}$, and $N_{aa}$ are the numbers of individuals with each genotype, and $N$ is the total number of individuals.

• Hardy-Weinberg equilibrium (for two alleles): $p^2 + 2pq + q^2 = 1$ Where $p$ and $q$ are the frequencies of the two alleles.

Additional info: Some context and examples were inferred and expanded for clarity and completeness, as the original notes and slides were fragmented and brief.