Microevolution and Mating Strategies

Microevolution

• Microevolution is a change in allele frequency in a population or species across generations. Focuses on variation within populations/species and evolutionary change over shorter time periods.
• Macroevolution is evolution above the species level; focuses on variation among species and questions related to diversification. Macroevolution is the result of microevolution writ large. It considers the longer term and higher taxonomic consequences of microevolution within populations.
• Four processes can cause microevolution: mutation, gene flow, genetic drift, and natural selection.
• Microevolution requires genetic variation (i.e., more than one allele segregating at a locus in a population).

Learning Objectives

• Distinguish between micro and macroevolution, and list processes causing the latter. Identify the source of genetic variation.
• Contrast random and non-random mating, explain the effects of different forms of mating on allele/genotype frequencies, and define inbreeding/inbreeding depression. Identify mechanisms evolved to reduce inbreeding.
• Identify different types of mutations and their classification in terms of fitness (beneficial, neutral, deleterious), and discuss relative frequencies of these types. Discuss effects of mutation on allele frequencies and role in creating genetic variation.
• Define gene flow, compare/contrast it with mutation's roles in altering allele frequencies, and describe its role as a source of genetic variation in a population. Outline how gene flow and spatially varying selection interact to impact local adaptation.
• Define genetic drift, explain how it varies with population size, and summarize effects on allele frequencies, genetic variation, and population divergence. Summarize population bottlenecks and founder events.
• Outline the human health implications of drift via a founder event and subsequent gene flow.
• Detail understanding of natural selection, fitness, and its different components, different forms of natural selection, and approaches to detecting natural selection.
• Outline how genetic variation can be maintained, including processes discussed.

Introduction

• (micro)evolution: a change in allele frequency in a population or species across generations. Focus is on variation within populations/species and evolutionary change over shorter time periods.
• Macroevolution: evolution above the species level; focuses on variation among species and on questions related to diversification. Macroevolution is the result of microevolution writ large. The longer term and higher taxonomic consequences of microevolution within populations.

Mathematics of Microevolution

• Population and quantitative genetics provide rigorous frameworks to study the impacts of assortative mating and the processes of mutation, gene flow, genetic drift, and selection on Mendelian variation and quantitative traits. Equations given to show relationships between relevant components are also described.

Outline

• The outline is given in a hierarchical format that organizes multiple topics and/or sub-topics. This outlines mechanisms of microevolution including non-random mating, mutation, gene flow, genetic drift, and natural selection.

Random Mating

• Random mating in a population is also termed panmixia. Some species are panmictic, but most have geographic structuring.

Non-random Mating

• Mating is often non-random because relatives may mate more often or less often than expected, individuals may self-fertilize, or mate more often with individuals that are similar or dissimilar to them in phenotype (assortative vs disassortative).
• Non-random mating affects how alleles are organized into genotypes, alters genotype frequencies, and influences homozygosity and heterozygosity. Note that this is not sexual selection, which is affected by differences in mating success.

Inbreeding

• Inbreeding is when mating takes place between related individuals, producing inbred offspring.
• Inbreeding causes an increase in the frequency of homozygotes (decrease of heterozygosity) across the genome, deviating from Hardy-Weinberg expectations.
• The effect of inbreeding on genotype frequencies can be temporary; random mating can restore Hardy-Weinberg expectations.

Inbreeding Depression

• Inbreeding depression is the reduction in fitness associated with inbreeding. The increase in homozygosity due to inbreeding tends to reduce fitness. This is widespread, but not limited to royal dynasties.
• Inbreeding depression can exacerbate the loss of genetic variation (allelic diversity) and result in human health problems.

Mendelian Causes of Inbreeding Depression

• Two non-mutually exclusive hypotheses are responsible for inbreeding depression. Dominance hypothesis: alleles that decrease fitness (deleterious) tend to be recessive. Heterozygote advantage: heterozygotes have higher fitness than either homozygote.

Inbreeding Avoidance

• Inbreeding avoidance is a type of behavior in many plants and animals to reduce the chance/likelihood of inbreeding. Mechanisms include kin recognition, dispersal, delayed maturation/suppression of reproduction, and extra-pair copulations. Mechanisms in hermaphrodites or monoecious species include self-incompatibility and physical/temporal separation of reproductive organs.

Outbreeding

• Outbreeding is mating between individuals less closely related than expected by random mating. Outbreeding tend to increase heterozygosity and reduce homozygosity.
• A common outcome is an increase in fitness (heterosis or hybrid vigor) in non-outbred individuals; adaptation concerns are noted.
• Heterozygote advantage and inbreeding depression have analogous reasons.

Inbreeding/Outbreeding in Agriculture

• Nearly all corn grown in developed nations are F1 crosses between inbred lines.

Mutation

• Mutation is a change in genetic information (nucleotide sequence) of an organism's DNA. Errors occur during DNA replication, recombination, or repair of spontaneous DNA damage/errors induced by environmental or chemical mutagens (or from chemical mutagens).
• Mutations create new alleles and are the ultimate source of genetic variation. Some changes are created by the repeated creation of additional copies of alleles.
• Mutations are random, although the mutation rate can evolve. Mutations are transmitted (heritable) if found in the germ line. Mutations can have variable impacts on an organism's survival and reproductive success.

Types of Mutation

• Small-scale mutations (point mutations): include substitutions (one nucleotide is replaced by another - silent when amino acid and gene product remain unaffected; replacements change amino acid) and insertions/deletions (one or more nucleotides added or removed).
• Large-scale mutations: These occur in chromosome structure(including translocation, inversion, loss of a chromosome, duplication).

Mutation Rates

• Mutation rates are relatively low (10⁻⁷ to 10⁻¹¹ mutations/base pair/generation), although the total mutation rates for organisms can be large, considering the genome size and number of individuals involved, as demonstrated in the table of mutation rates.

Impacts of Mutation

• Most new mutations are deleterious (negative effect on fitness), with beneficial (positive effect on fitness) being much less common. Many deleterious mutations are selected against, but may appear at appreciable frequencies due to drift. Many mutations are likely neutral or nearly neutral (non-effect on fitness).
• Mutations (of neutral character in particular) are lost by drift but can remain (frequency increase) due to fixation.

Gene Flow

• Gene flow, also described as migration, is the movement of alleles between populations. This occurs as a result of the migration of individuals or gametes.
• Gene flow introduces and removes alleles. Gene flow rates are generally much greater than mutation rates, so there is a greater impact on allele frequencies.
• Gene flow homogenizes populations by reducing genetic differences between them; if sufficient, population differences can be abolished; a single panmictic population can occur.

Gene Flow and Local Adaptation

• Gene flow can impede local adaptation by constantly introducing maladaptive alleles from other populations (outbreeding depression), reducing fitness from matings.
• Gene flow can also help spread beneficial alleles among populations.

Example of Fugitive Atlantic Salmon

• See example of Lake Erie water snakes as referenced in Campbell biology in regard to genetic divergence.

Finite Populations

• All populations are finite, though some may be sufficiently large/behave as infinite, over short periods. Finite populations can be subject to random fluctuations in allele frequencies across generations.
• This is the process known as genetic drift. Genetic drift occurs through sampling variation, the difference between values in finite samples and true population values. Changes in allele frequency is due to the transmission from a parent generation to the next generation. There is an inherent sampling variation.

Effects of Genetic Drift

• Random (unpredictable) change in allele frequency occurs across generations. Size of population influences magnitude of random change; smaller populations exhibit stronger/larger effects of drift. Drift reduces genetic variation (as alleles are lost or their frequency decreases), decreasing heterozygosity.
• In small populations, genetic drift can overwhelm selection, potentially increasing the frequency or even fixation of deleterious alleles.
• Drift will cause populations to diverge if gene flow does not occur. If random mating occurs in each generation, drift-induced deviations from Hardy-Weinberg genotype expectations are typically smaller/weaker in larger populations.

Simulating Genetic Drift

• Instructions/methodology on simulating genetic drift with regards to the program/software used. Relevant parameters to consider when setting up the simulations.

Population Bottlenecks

• A severe, rapid decrease in population size reduces genetic variation. This can be influenced by environmental factors, human activities, or disease. This is different from, and occurs in addition to, founder events (a small group of individuals colonizes a new geographic area).
• Population bottlenecks can have consequences for population persistence and future adaptation. Effects on current and future population persistence are noted.

Bottlenecks & Human Health

• This can result in more frequent or fixation of deleterious mutations (having human health implications) in isolated populations. Examples of conditions discussed include these relevant conditions: Ellis-van Creveld, Myotonic dystrophy, and Tay-Sachs.

Natural Selection

• Natural selection is a process that occurs when certain conditions are met; a deduction: if conditions are met, then natural selection will happen. Conditions include variation amongst individuals, non-random association between the trait and reproductive success (or survival/reproductive success), and heritability of the trait.
• The traits will evolve, and frequencies across generations will evolve as a result—allele frequencies at the loci affecting the trait will change across generations.
• 1 and 2 are necessary for natural selection to occur, and all (1, 2, and 3) are necessary for natural selection to produce evolutionary change, where selection causes allele frequencies to change across generations.

Fitness

• Darwinian fitness is the absolute contribution an individual makes to the next generation, measured by the number of offspring produced. Adaptive traits are associated with increased fitness in an environment.
• Natural selection arises from variation in relative fitness (contribution of one individual relative to other individuals).

Genotype-Phenotype

• Natural selection acts on phenotypes. Gene variation underlies phenotypic variation in many cases. Alleles associated with advantageous/fitness-enhancing phenotypes are more likely to be passed to the next generation. Changes in allele frequencies result in changes in the distribution of traits across generations.

Example - DDT Resistance in Insects

• Example of natural selection given for insect populations, including a description of DDT resistance with reference to a particular geographic area, the role of a specific gene and mutation, and patterns of frequency of selected alleles over time.

Components of Fitness & Types of Selection

• Fitness encompasses reproduction success alongside survivorship/viability and fecundity (or mating/fertilization success). Types of selection discussed include viability selection, fecundity selection, and sexual selection.

Forms of Selection

• Form of selection (linear/directional, stabilizing, disruptive) are shown in graphs/images and explanations/descriptions are made with reference to traits (and relative fitness or probability of survival).

Detecting Natural Selection

• Methodological approach to detecting natural selection.
  • Direct measurement (observational studies) - Weis et al. (1992) study mentioned with example reference.

The Problem of Correlated Traits

• Traits are often correlated due to pleiotropy or physical linkage. Direct selection on one trait can cause a correlated/indirect response in another. Direct and indirect selection, and an overview/example of the issue of correlated traits given.
• Selection can be more complex than directly favoring a change in one trait and indirect/correlated responses are evident; this can involve multiple traits and how they are correlated.

Other Forms of Selection: Frequency-Dependent Selection

• Fitness of a phenotype depends on its frequency within the population.
• Positive frequency-dependent selection when a phenotype becomes more common causes the phenotype to more strongly enhance fitness; examples are noted and explored.
• Negative frequency-dependent selection occurs when a phenotype is stronger when the frequency is low/less common. This is a form of balancing selection - where selection maintains genetic variation (multiple alleles at a locus).

Other Forms of Selection: Heterozygote Advantage

• Heterozygotes (Aa) exhibit higher fitness than either homozygote (AA, aa). Preserves genetic variation.
• Example of sickle cell anemia given, with a description of the role of genetic loci or location of genes related to this.

Additional Resources

• Provides resources (reading material - Campbell Biology chapter) relevant to understand microevolution and the various topics of microevolution. Resources for the simulation program/software used are given, and the resource pages for the Wikipedia about genetic drift and the HHMI sickle cell anemia video are also noted/included.