Evolution of Populations

Microevolution

• Microevolution is the change in allele frequencies within a population over generations.

Macroevolution

• Macroevolution refers to broad patterns of evolutionary change above the species level.

Populations (1)

• Populations are the smallest biological unit that can evolve.
• A population consists of individuals of the same species living in the same area and interbreeding with each other more often than with individuals from other populations.
• Individuals at the edge of the population range may breed infrequently or not at all.
• The boundaries of a population are dependent on space and time but are often arbitrarily defined for study purposes.

Populations (2)

• Populations are limited by a specific geographic range.
• Even with overlapping ranges, distinct populations can exist if interbreeding is more common within a population than between populations. This is called exclusion breeding.

Gene Pool

• A gene pool is the total collection of genes/alleles in a population at any one time.

Allele Frequencies of the Gene Pool (1)

• Detecting microevolution requires determining allele frequencies in a population.
• Allele frequency is the proportion of a specific allele compared to all other alleles for the same gene.
• Allele frequency is expressed as a decimal ranging from 0.0 to 1.0.

Allele Frequencies of the Gene Pool (2)

• Allele frequency is calculated by dividing the number of a specific allele by the total number of alleles for that gene. For example, if there are 56 A alleles out of a total of 78, the frequency of A is $56/78 = 0.72$.
• An allele is "fixed" if its frequency is 1.0, meaning all individuals are homozygous for that allele.
• An allele is considered "lost" or "extinct" if its frequency is 0.0, meaning it is not present in the gene pool.

Allele Frequencies of the Gene Pool (3)

• The sum of all allele frequencies for a gene must equal 1.0.
• Example: Gene A has three alleles (A1, A2, A3) in a population of 67 diploid individuals.
  • 25 A1 alleles: Frequency = $25/134 = 0.187$
  • 67 A2 alleles: Frequency = $67/134 = 0.5$
  • 42 A3 alleles: Frequency = $42/134 = 0.313$
  • The sum of the frequencies: $0.187 + 0.5 + 0.313 = 1.0$

Three Major Agents of Microevolution

• Genetic drift: Random changes in allele frequencies due to chance events.
• Gene flow: Transfer of alleles between populations.
• Natural selection: Differential survival and reproduction based on traits.
• Mutations introduce new alleles, but most are neutral and rare.
• Non-random mating changes genotype frequencies but not allele frequencies, as it only redistributes existing alleles.

Genetic Drift as an Agent of Microevolution (1)

• Genetic drift is the random fluctuation of allele frequencies in a population due to chance events.
• These chance events can randomly preserve or remove alleles, regardless of their adaptive value.
• Genetic drift causes allele frequencies to drift unpredictably between 0 and 1.
• It can increase or decrease genetic diversity temporarily but generally decreases it over time.

Effect of Genetic Drift on Allele Frequency

• Random events influence changes in allele frequencies.

Genetic Drift as an Agent of Microevolution (2)

• Genetic drift is unlikely to promote the spread of adaptive traits because it is random.
• The effects of genetic drift are more pronounced in small populations, where the random loss of a few individuals can significantly alter allele frequencies.

Genetic Drift as an Agent of Microevolution (3)

• The founder effect and the bottleneck effect are two processes that lead to small, homogenous populations where genetic drift is significant.

The Founder Effect

• The founder effect occurs when a few individuals from a larger population establish a new population with a different allele frequency.

Genetic Drift Due to the Founder Effect

• A small group separates from the original population and forms a new population.
• The new population likely has different allele frequencies compared to the original population.
• Founder populations are small and susceptible to genetic drift, potentially leading to the fixation of certain alleles (frequency = 1.0).

Bottleneck Effect

• The bottleneck effect occurs when a population size is drastically reduced due to a random event.

Genetic Drift Due to a Bottleneck

• A genetic bottleneck happens when a random event severely reduces the population size.
• The surviving individuals are unlikely to represent the original population's genetic makeup.
• This results in subsequent generations having different allele and genotype frequencies than the pre-bottleneck population.

Effect of Genetic Drift on Prairie Chicken Reproduction

• Reduced reproduction can be caused by inbreeding depression, which is more common in small populations that have experienced a genetic bottleneck.

Gene Flow

• Gene flow is the movement of alleles between populations.

Gene Flow as an Agent of Microevolution (1)

• Gene flow is the transfer of alleles or genes between populations of the same species.
• It occurs through the movement of fertile individuals, seeds, or gametes.
• Gene flow tends to homogenize the gene pools of connected populations, making them genetically more similar.
• The amount and frequency of gene flow can vary and may not be equal in all directions.

Effect of Gene Flow on Allele Frequency

• Illustrates how allele frequencies change in populations after gene flow occurs.

Gene Flow as an Agent of Microevolution (2)

• The extent of gene flow depends on:
  1. Distance between populations: Shorter distances result in greater gene flow.
  2. Population size or density: Larger populations have more individuals translocating.
  3. Physical or geographical barriers: Substantial barriers reduce gene flow.

Natural Selection as an Agent of Microevolution

• Natural selection results in differential reproductive success, leading to an overrepresentation of certain traits or alleles in future generations.
• It is the only microevolutionary mechanism that consistently increases adaptation to the environment.

Natural Selection of Prey by a Predator

• Selective predation can lead to changes in prey population characteristics.

Effect of Natural Selection on Allele Frequency

• Shows how allele frequencies change due to natural selection.

Natural Selection and Gene Flow

• Gene flow can counteract the effects of natural selection on allele frequencies.

Relative Fitness (w)

• Relative fitness (w) is the contribution of a genotype to the next generation compared to the most fit genotype in the same population.
• The most fit genotype has a relative fitness of 1.0.
• The relative fitness of a less fit genotype is calculated as $w = 1 - s$, where s is the selection coefficient.
• The relative fitness of an allele depends on the overall success of the organism.

The Selection Coefficient (s)

• The selection coefficient (s) quantifies the reduction in reproductive success of less fit genotypes compared to the most fit genotype.
• For example, if a genotype produces 10% fewer offspring, its selection coefficient is 0.1.
• s ranges from 0 to 1. As s approaches 1, w (relative fitness) approaches zero.
• The most fit genotype has a selection coefficient of 0.
• The selection coefficient can be estimated from direct observation or genomic analysis.

Relative Fitness Example (1)

• Example: Calculating relative fitness for deer mouse coat color variants:
  • Dark brown: 6.7 offspring per female per litter
  • Medium brown: 6.0 offspring per female per litter
  • Light brown: 6.4 offspring per female per litter
• Calculating selection coefficients:
  • s11 (dark brown) = $0/6.7 = 0.0$
  • s12 (medium brown) = $0.7/6.7 = 0.1$ (10% fewer offspring)
  • s22 (light brown) = $0.3/6.7 = 0.045$ (4.5% fewer offspring)

Relative Fitness Example (2)

• Calculating relative fitness (w = 1 - s):
  • w11 (dark brown) = $1.0 - 0.0 = 1.0$ (most fit)
  • w12 (medium brown) = $1.0 - 0.1 = 0.9$ (least fit)
  • w22 (light brown) = $1.0 - 0.045 = 0.955$
• Interpretation: For every 1 offspring produced by dark-coated mice, medium brown-coated mice produce 0.9 offspring, and light brown-coated mice produce 0.955 offspring.

Types of Selection

• Directional selection: Favors one extreme phenotype.
• Disruptive (diversifying) selection: Favors both extreme phenotypes over intermediate phenotypes.
• Stabilizing selection: Favors intermediate phenotypes and reduces variation.

Balancing Selection

• Balancing selection maintains genetic diversity by preserving two or more phenotypes in a population.
• Two major forms of balancing selection:
  • Heterozygote advantage
  • Frequency-dependent selection

Heterozygote Advantage (1)

• Heterozygote advantage occurs when heterozygous individuals have higher fitness than either homozygous genotype.

Heterozygote Advantage Example: Sickle-Cell Disease

• Individuals heterozygous for the sickle-cell allele are resistant to malaria but do not suffer the full effects of sickle-cell disease, giving them an advantage in malaria-prone regions.

Frequency-Dependent Selection

• In frequency-dependent selection, the fitness of a genotype depends on its frequency in the population.
• As a genotype becomes less common, its fitness increases, preventing it from being driven to extinction.
• Example: Predators may switch their preference, targeting more common prey genotypes.

Additional Agents of Microevolution

• The three major agents are genetic drift, gene flow, and natural selection.
• Two additional processes that can change allele or genotype frequencies are:
  • Mutations
  • Non-random mating

Mutations as an Agent of Microevolution (1)

• A mutation is a change in the nucleotide sequence of DNA or RNA.
• Somatic cell mutations do not affect allele frequencies in future generations.
• Mutations in gametes directly alter the gene pool.
• Mutation is a rare and random event, occurring at a rate of about 0.00001 per gene per generation.

Mutations Spread in Population Via Selection or Drift

• Mutations can spread through a population through natural selection or genetic drift.

Mutations as an Agent of Microevolution (2)

• A single mutation has a small effect on allele frequencies in large populations, but the cumulative effect of multiple mutations can be significant.
• Most changes to fitness due to mutation are small, but mutations in regulatory genes can have larger effects.
• Types of mutations:
  • Advantageous: Increase fitness.
  • Deleterious: Decrease fitness.
  • Neutral: Have no effect on fitness.

Non-Random Mating as an Agent of Microevolution (1)

• Non-random mating involves mate selection based on specific criteria.
• Two major forms:
  • Sexual Selection: Mates are selected based on heritable traits or similarity (assortative mating) or dissimilarity (disassortative mating).
  • Inbreeding: Mating between genetic relatives, often due to social structure or proximity.