Microevolution Concepts and Definitions

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 across relatively long periods of time
• Four processes cause microevolution: mutation, gene flow, genetic drift, and natural selection
• Microevolution requires genetic variation (more than one allele segregating at a locus in a population)

Learning Objectives

• Differentiate micro and macroevolution and the processes causing the latter, and the sources of genetic variance
• Contrast random and non-random mating, explaining the effects on allele and genotype frequencies
• Define inbreeding and inbreeding depression, outlining causes and mechanisms that reduce inbreeding likelihood
• Identify types of mutations, their classification in terms of fitness effects (beneficial, neutral, deleterious) and how relative frequencies differ
• Discuss how mutation impacts allele frequencies and the creation of genetic variation
• Define gene flow and compare it with mutation in terms of their roles in altering allele frequencies and creating genetic variation
• Examine how gene flow and spatially varying selection interact to affect local adaptation
• Define genetic drift, explaining how population size affects allele frequencies and genetic variation/population divergence; also summarize effects of bottlenecks and founder events
• Outline the human health implications due to genetic drift
• Define natural selection and fitness
• Summarize the approaches to detecting natural selection, and the associated problem with correlated traits
• Detail how genetic variation can be maintained

Introduction

• Microevolution : a change in allele frequency in a population over generations that focuses on changes within a single species
• Macroevolution: a larger scale change (evolving to a new species, or related species) across longer time scales than microevolution
• Four processes that can cause microevolution: mutation, gene flow, genetic drift and natural selection
• Microevolution requires genetic variation

Mathematics of Microevolution

• Population and quantitative genetics provide rigorous mathematical frameworks for studying the impacts of assortative mating and the microevolutionary processes of mutation, gene flow, genetic drift and selection on Mendelian variation and quantitative traits

Outline

• 4.1 Non-random mating
• 4.2 Mutation
• 4.3 Gene flow
• 4.4 Genetic drift
• 4.5 Natural selection

Random Mating

• Individuals mate randomly with respect to their genotype at a particular locus of interest
• Also called panmixia
• Some species exhibit panmixia, but most have geographic structuring into populations

Non-random mating

• Mating may occur between relatives more or less frequently/more or less often than expected by chance (called inbreeding and outbreeding respectively)
• Individuals may self-fertilize more or less often than expected
• Individuals may mate more frequently with other individuals that are more or less similar in phenotype (called assortative and disassortative mating, respectively)
• Non-random mating affects allele frequencies - homozygosity or heterozygosity

Inbreeding

• Inbreeding is mating between related individuals
• Inbreeding increases the frequency of homozygotes and decreases heterozygosity
• Inbreeding depression: a decrease in fitness that may result from inbreeding

Inbreeding Depression

• The increase in homozygosity resulting from inbreeding tends to lower fitness
• Widespread phenomenon (not just seen in royals)
• Can exacerbate the loss of genetic variation (allele diversity) that occurs in small populations due to genetic drift
• Has implications impacting conservation biology and human health

Mendelian Causes of Inbreeding Depression

• Dominance Hypothesis: Deleterious alleles are recessive. Heterozygotes hide the effects of the allele, so inbreeding leads to a higher expression of negative effects
• Heterozygote Advantage: Heterozygotes are more fit than either homozygote. Inbreeding reduces heterozygosity, leading to lower fitness

Inbreeding Avoidance

• Many plants and animals have evolved traits that reduce the likelihood of inbreeding
• These include:
  • Kin recognition
  • Dispersal
  • Delayed maturation/reproductive suppression
  • Extra-pair copulations
• Hermaphrodites/monoecious species have features that prevent self-fertilization

Outbreeding

• Outbreeding occurs when individuals are less related than expected by random mating
• Increase in heterozygosity and decrease in homozygosity
• Can improve fitness compared to non-outbred individuals

Inbreeding/Outbreeding in Agriculture

• Nearly all corn in developed nations is F1 cross between inbred lines
• These populations are maintained by repeated inbreeding across many gens
• This ensures that almost all genetic variation is lost and single genotypes are fixed
• Selection of specific genotypes can be maintained for better performance

Mutation

• Mutation is a change in the genetic information (DNA)
• Arises from DNA replication, recombination or repair errors, environmental or chemical mutagens
• Creates new alleles
• Is an ultimate source of genetic variation
• Random (not directional)
• Can be transmitted if present in the germ line
• Can have variable effects on an organism

Types of Mutation

• Small scale (point mutation):
  • Substitution: one nucleotide is replaced with another (silent or replacement)
  • Insertion/Deletion: one or more nucleotides are added or removed (Frameshift)
• Large scale: mutations in chromosomal structure (translocations, inversions, loss, duplications)

Mutation Rates

• Relatively rare per nucleotide site, but rates vary among taxa (10⁻⁷ to 10⁻¹¹ mutations/base pair/generation)
• In humans, a large number of mutations still occur each generation due to genome size

Impacts of Mutation

• Most new mutations are deleterious

Gene Flow

• Gene flow is the movement of alleles between populations (migration of individuals or gametes)
• Introduces and removes alleles from a population
• Generally larger effect than mutation on allele frequencies
• Homogenizes populations by reducing genetic differences
• If gene flow is very high, population differences are lost

Gene flow and local adaptation

• Gene flow can impede adaptation by constantly introducing maladaptive alleles
• Can also promote adaptation by spreading beneficial alleles

Example: Fugitive Atlantic Salmon

• See example in Campbell (Fig. 23.11 and accompanying text)

Genetic Drift

• Finite populations are subject to random changes in allele frequencies across generations
• Process is called genetic drift
• Occurs due to sampling variation

Effects of Genetic Drift

• Random changes in allele frequencies
• Magnitude of change is inversely related to population size (smaller populations have stronger drift)
• On average reduces genetic variation because alleles are lost
• In small populations, drift can overwhelm selection
• Drift causes populations to diverge
• Drift-induced deviations from expected genotype frequencies are usually small in larger populations

Simulating Genetic Drift

• Use simulations to observe genetic drift in replicate populations across various scenarios (e.g., varying population size, frequency of allele A1, and migration)

Population Bottlenecks

• Severe (generally rapid) reductions in population size
• Reduces genetic variation and enhances genetic drift
• Caused by environmental factors, human activity or disease
• Can result from founder effects (small group of individuals colonizes a new area)

Mutation-drift: human health implications

• Population bottlenecks can result in increased frequency of deleterious mutations

Natural Selection

• Natural selection occurs when certain conditions are met:
  • Individuals vary in a trait
  • There's a non-random association between the trait and reproductive success (Darwinian fitness)
  • The trait is heritable
• Traits evolve when there's differential reproductive success based on the trait and traits are heritable

Fitness

• Darwinian fitness: the absolute contribution of an individual to the next generation
• Reporductive success measured as the number of offspring an individual produces
• Natural selection arises from variation in relative fitness: comparing the contribution of an individual to the next generation relative to other individuals

Genotype-Phenotype

• Natural selection acts on phenotypes
• Genetic variation underlies phenotypic variation
• Alleles associated with advantageous/fitness-enhancing phenotypes are passed more frequently
• Changes in allele frequencies across generations lead to changes in the distribution of traits among individuals

Example: DDT Resistance in Insects

• France's use of pesticides led to DDT resistance in insects
• Resistance occurred due to a single mutation in an esterase gene that allows breakdown of toxins like DDT
• The mutation rapidly spread throughout populations

Components of fitness & types of selection

• Fitness has components of
  • survivorship/viability
  • fecundity
  • mating success
• Types of selection
  • Viability
  • Fecundity
  • Sexual

Forms of Selection

• Linear/directional
• Stabilizing
• Disruptive

Detecting Natural Selection

• Direct measurement (observational and often experimental study)

Problem of Correlated Traits

• Traits are often correlated (due to pleiotropy or physical linkage)
• Direct selection on one trait can induce correlated response in other traits
• Selection isn't necessarily acting on a trait directly

Other Forms of Selection: frequency-dependent selection

• Fitness of a phenotype depends on its frequency in a population
• Positive frequency dependence: directional selection increases as the phenotype becomes more common
• Negative frequency dependence: directional selection is stronger when the phenotype is less common (balancing selection)

Other Forms of Selection: Heterozygote Advantage

• Heterozygotes have higher fitness compared to homozygotes which maintains genetic variation
• Example: sickle-cell anemia, where heterozygotes are resistant to malaria

Test your understanding of microevolution with this quiz that covers key processes, definitions, and relationships with macroevolution. Assess your knowledge of genetic variation, mating patterns, and the impact of gene flow and genetic drift. Perfect for biology students looking to deepen their grasp of evolutionary concepts.