Evolutionary Genetics: Selection, Frequencies, and Speciation

Role of Evolutionary Mutations

Mutations themselves are not evolutionary necessities; they must be favored by natural selection to persist. Predispositions occur by chance, not necessity, and can be:

• Advantageous: Selected and allowed to spread.
• Unfavorable: Eliminated by natural selection.
• Neutral: The majority; their persistence depends on random chance.

Gene or Allelic Frequency

This is the proportion of a specific gene within the overall population.

Allele Frequency Calculation: (Number of subjects with AA × 2 + Number of individuals with Aa) / (Total number of alleles).

Genotype Frequency

This is the proportion of a population exhibiting a particular genotype.

Frequency Calculation for AA Genotype: (Number of AA individuals) / (Total number of subjects).

From allele frequencies (where p is the frequency of the dominant allele and q is the frequency of the recessive allele):

• F(AA) = p²
• F(Aa) = 2pq
• F(aa) = q²

Hardy-Weinberg Law

In a population in equilibrium, gene frequencies remain constant. This hypothetical situation occurs only if:

• No mutations occur.
• No migration (gene flow) exists.
• No natural selection is present.
• Mating is random.
• The population is infinitely large.

Since these conditions are rarely met, evolutionary agents cause gene frequencies to alter the balance of stocks, leading to evolution.

Biological Fitness

This measures the impact of natural selection on a genotype—the success of a genotype in persisting through generations in a given environment.

Types of Natural Selection

• Directional: Extreme phenotypes are favored.
• Disruptive: Individuals at both phenotypic extremes are favored.
• Stabilizing: Intermediate phenotypes are favored.

Speciation

The process of species creation:

• Phyletic Evolution (Anagenesis): One species evolves into another.
• Cladogenesis: One species splits into two or more different species.

Species Definition

Populations of individuals that are reproductively isolated from one another.

Reproductive Isolation Mechanisms

Prezygotic Barriers (Preventing Fertilization)

• Behavioral Isolation: Lack of sexual attraction.
• Mechanical Isolation: Physical problems preventing mating.
• Ecological Isolation: Populations inhabit different territories.
• Temporal Isolation: Different mating or flowering periods.
• Gametic Isolation: Reproductive cells are not viable or cannot unite in the genital tract.

Postzygotic Barriers (After Fertilization)

• Hybrid Inviability: Hybrid zygotes fail to develop.
• Hybrid Sterility: Hybrids fail to produce viable gametes.
• Hybrid Breakdown: Subsequent generations of hybrids show reduced viability.

Speciation Processes

• Allopatric Speciation (Geographical):
  1. Genetic exchange between populations ceases due to a barrier.
  2. Accumulation of genetic changes occurs in the separate populations.
  3. A reproductive isolation mechanism develops.
• Sympatric Speciation: Occurs without geographical separation, often driven by instantaneous genetic or chromosomal mutations.