Evolution in Populations and the Origin of Species

Evolution in Populations

Genetic Drift

Genetic drift refers to random changes in allele frequencies in a population, especially significant in small populations. These changes are due to chance events rather than natural selection.

• Definition: Change in allele frequencies of a gene pool due to chance alone.

• Significance: More pronounced in small populations, leading to greater deviations from expected results.

• Outcomes:

  • Reduces genetic variation within a small population through the random loss of alleles.

  • Tends to increase variability between populations.

• Bottleneck Effect:

  • Population size is drastically reduced by a sudden change (e.g., natural disasters).

  • Surviving population may not represent the genetic makeup of the original population.

  • Examples: Northern Elephant Seals, Black-footed Ferret, Cuvier's gazelle, Red-cockaded Woodpecker, Cheetahs.

• Founder Effect:

  • Few individuals colonize a new habitat, leading to a gene pool that may not represent the original population.

  • Rare alleles or combinations of alleles can occur at higher frequency in the new population.

  • Examples: Amish population (higher incidence of certain genetic disorders).

Gene Flow

Gene flow is the transfer of alleles or genes from one population to another, often through the movement of fertile individuals or gametes.

• Definition: Movement of alleles between populations.

• Effects:

  • Increases genetic variation within populations.

  • Can reduce differences between populations over time.

  • In the long run, may reduce variation if populations become more similar.

• Example: Pollen transfer between plant populations by wind or animals.

Natural Selection

Natural selection is the process by which individuals with advantageous heritable traits are more likely to survive and reproduce, increasing the frequency of those traits in the population.

• Relative Fitness: The contribution an individual makes to the gene pool of the next generation relative to others.

• Selection acts on phenotypes: Natural selection favors certain genotypes by acting on the phenotypes of individuals.

• "Survival of the fittest": Refers to reproductive success, not just survival.

Types of Natural Selection

Natural selection can alter the frequency distribution of heritable traits in three main ways:

• Directional Selection: Favors individuals at one extreme of the phenotypic range.

• Disruptive Selection: Favors individuals at both extremes of the phenotypic range.

• Stabilizing Selection: Favors intermediate variants and acts against extreme phenotypes.

Sexual Selection

Sexual selection is a form of natural selection in which individuals with certain inherited traits are more likely to obtain mates.

• Sexual Dimorphism: Differences in secondary sexual characteristics between males and females (e.g., size, color, ornamentation, behavior).

• Intra-sexual Selection: Competition among individuals of one sex (often males) for mates.

• Intersexual Selection: Mate choice, often by females, selecting males with desirable traits.

• "Good Genes" Hypothesis: Females select males with traits that indicate genetic quality or overall health.

Balancing Selection

Balancing selection maintains genetic diversity in a population by keeping two or more alleles at higher frequencies than would be expected by chance alone.

• Frequency-dependent Selection: Fitness of a phenotype depends on how common it is in the population.

• Heterozygote Advantage: Heterozygotes have higher fitness than either homozygote, maintaining multiple alleles in the population.

• Example: Sickle-cell allele in regions with malaria.

Table: Example of Frequency-Dependent Selection in Cichlids

Trait	Alleles	Frequency Pattern	Selective Advantage
Jaw orientation in scale-eating cichlids	Left-jawed, Right-jawed	Oscillates around 50%	Rarer form has advantage as prey learn to avoid the more common form

Case Study: Sickle-Cell Allele and Malaria

The sickle-cell allele provides a classic example of heterozygote advantage in human populations exposed to malaria.

• Homozygous recessive (ss): Sickle-cell anemia (disease).

• Homozygous dominant (SS): Susceptible to malaria.

• Heterozygous (Ss): Resistant to malaria and generally healthy.

• Distribution: High frequency of the sickle-cell allele in regions where malaria is prevalent.

Speciation and the Origin of Species

How Do New Species Originate?

Speciation is the evolutionary process by which populations evolve to become distinct species.

• Definition: The process by which one species splits into two or more species.

• Significance: Produces the tremendous diversity of life and explains the unity of life.

Microevolution vs. Macroevolution

• Microevolution: Changes in allele frequency in a population over time.

• Macroevolution: Broad patterns of evolutionary change above the species level.

Biological Species Concept

The biological species concept defines a species as a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring.

• Emphasizes reproductive isolation.

• Species are reproductively isolated from other such groups.

• Confirmed by morphological, physiological, and DNA sequence differences.

Reproductive Isolation

Reproductive isolation prevents gene flow between species and maintains species boundaries.

• Prezygotic Barriers: Prevent mating or fertilization between species.

• Postzygotic Barriers: Prevent the development of viable, fertile offspring after fertilization.

Table: Types of Reproductive Barriers

Barrier Type	Prevents	Examples
Prezygotic	Mating or fertilization	Habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, gametic isolation
Postzygotic	Viable, fertile offspring	Reduced hybrid viability, reduced hybrid fertility, hybrid breakdown

Additional info: Prezygotic barriers usually act before fertilization to prevent different species from mating, while postzygotic barriers act after fertilization to prevent hybrids from surviving or reproducing.