Evolution and Population Ecology: Study Guide Notes

Evolution and Population Ecology

Key Vocabulary and Concepts

• Adaptation: A heritable trait that increases an organism's fitness in a particular environment.

• Homologous structures: Anatomical features in different species that are similar due to shared ancestry.

• Natural selection: The process by which individuals with advantageous traits survive and reproduce more successfully, leading to evolutionary change.

• Population: A group of individuals of the same species living in the same area and interbreeding.

• Gene pool: The total collection of genes and their alleles in a population at any one time.

• Hardy-Weinberg equilibrium: A theoretical state in which allele and genotype frequencies remain constant from generation to generation in the absence of evolutionary forces.

• Microevolution: Small-scale evolutionary changes within a population, typically observable over a few generations.

• Genetic drift: Random changes in allele frequencies in a population, especially significant in small populations.

• Gene flow: The movement of alleles between populations due to migration of individuals or gametes.

• Founder effect: Genetic drift that occurs when a small group establishes a new population, leading to reduced genetic variation.

• Bottleneck effect: A sharp reduction in population size due to environmental events, resulting in decreased genetic diversity.

• Directional selection: Selection that favors one extreme phenotype, causing a shift in the population's trait distribution.

• Disruptive selection: Selection that favors both extreme phenotypes over intermediate forms.

• Stabilizing selection: Selection that favors intermediate phenotypes and reduces variation.

• Sexual selection: Selection for traits that increase mating success, often leading to pronounced differences between sexes.

• Speciation: The formation of new and distinct species in the course of evolution.

• Prezygotic barriers: Reproductive barriers that occur before fertilization, preventing mating or fertilization between species.

• Postzygotic barriers: Barriers that occur after fertilization, reducing hybrid viability or fertility.

• Behavioral isolation: Differences in mating behaviors that prevent interbreeding.

• Mechanical isolation: Physical differences that prevent successful mating.

• Habitat isolation: Populations live in different habitats and do not meet.

• Temporal isolation: Species breed at different times.

• Gametic isolation: Gametes (egg and sperm) are incompatible.

• Reduced hybrid viability: Hybrids fail to develop or reach maturity.

• Reduced hybrid fertility: Hybrids are sterile or have reduced fertility.

• Hybrid breakdown: Hybrids are fertile but their offspring are inviable or sterile.

• Adaptive radiation: The rapid evolution of many diverse species from a common ancestor.

• Gradualism: The hypothesis that evolution proceeds chiefly by the accumulation of gradual changes.

• Sympatric speciation: Speciation occurring within the same geographic area, often due to behavioral or genetic differences.

• Allopatric speciation: Speciation that occurs when populations are geographically separated.

• Population growth: The change in the number of individuals in a population over time.

• Carrying capacity (K): The maximum population size that an environment can sustain.

• Exponential growth: Population growth under ideal conditions, producing a J-shaped curve.

• Logistic growth: Population growth that slows as it approaches carrying capacity, producing an S-shaped curve.

• Survivorship curve: Graph showing the number or proportion of individuals surviving at each age (Type I, II, III).

• K-selected species: Species that produce few offspring with high parental investment.

• r-selected species: Species that produce many offspring with little parental care.

Adaptation and Natural Selection

How Adaptations Benefit Organisms

• Adaptations increase an organism's ability to survive and reproduce in its environment.

• They arise through the process of natural selection acting on heritable variation.

Natural Selection: Definition and Requirements

• Natural selection is the differential survival and reproduction of individuals due to differences in phenotype.

• It requires:

  • Variation: Individuals in a population differ in their traits.

  • Heritability: Traits must be genetically passed to offspring.

  • Differential survival/reproduction: Some traits confer advantages, leading to more offspring.

• Advantageous traits become more common in the population over generations.

Populations and Evolution

Population and Gene Pool

• A population is a group of interbreeding individuals of the same species in a given area.

• The gene pool is the sum of all genetic information in a population.

• The population is the smallest unit that can evolve.

• Microevolution refers to changes in allele frequencies within a population over time.

Hardy-Weinberg Equilibrium

Definition and Implications

• A population is in Hardy-Weinberg equilibrium if allele and genotype frequencies remain constant across generations.

• This implies:

  • No evolution is occurring.

  • No differences in survival or reproduction among genotypes.

• Conditions required:

  • Large population size

  • No mutation

  • No migration (gene flow)

  • Random mating

  • No natural selection

• Hardy-Weinberg equation:

$p^2 + 2pq + q^2 = 1$

$p + q = 1$

• Where p and q are the frequencies of two alleles in the population.

Mechanisms of Evolution

Types and Examples

• Natural selection: Differential survival and reproduction based on inherited traits.

• Genetic drift: Random changes in allele frequencies, especially in small populations.

• Founder effect: Small group colonizes a new area, leading to reduced genetic diversity.

• Bottleneck effect: Population size is drastically reduced, causing loss of genetic variation.

• Gene flow: Movement of alleles between populations through migration.

Scenarios:

• Small isolated group → Founder effect

• Disaster reduces population → Bottleneck effect

• Migration between populations → Gene flow

Types of Selection

Definitions and Effects on Variation

• Stabilizing selection: Favors intermediate phenotypes, reduces variation.

• Directional selection: Favors one extreme phenotype, shifts population mean.

• Disruptive selection: Favors both extremes, increases variation.

• Sexual selection: Favors traits that enhance mating success.

Speciation

Definition and Types

• Speciation is the process by which one species splits into two or more species.

• It is important for generating biodiversity.

• Allopatric speciation: Occurs due to geographic separation (e.g., mountain, river).

• Sympatric speciation: Occurs without geographic separation, often due to behavioral or genetic differences.

Examples:

• Geographic barriers → Allopatric speciation

• Same location, different behaviors → Sympatric speciation

Reproductive Isolation

Prezygotic and Postzygotic Barriers

• Prezygotic barriers (before fertilization):

  • Behavioral isolation: Different mating behaviors prevent mating.

  • Mechanical isolation: Physical differences prevent mating.

  • Habitat isolation: Species occupy different habitats.

  • Temporal isolation: Species breed at different times.

  • Gametic isolation: Gametes are incompatible.

• Postzygotic barriers (after fertilization):

  • Reduced hybrid viability: Hybrids do not survive well.

  • Reduced hybrid fertility: Hybrids are sterile.

  • Hybrid breakdown: Hybrids are fertile, but their offspring are weak or sterile.

Models of Evolution

Gradualism

• Gradualism proposes that evolution occurs slowly and steadily over long periods.

• Contrasts with punctuated equilibrium (not listed), which suggests rapid bursts of change.

Population Growth Models

Exponential and Logistic Growth

• Exponential growth: Population increases rapidly under ideal conditions.

• Equation:

$\frac{dN}{dt} = rN$

• Logistic growth: Growth slows as population approaches carrying capacity (K).

• Equation:

$\frac{dN}{dt} = rN \left(1 - \frac{N}{K}\right)$

• Where N = population size, r = intrinsic rate of increase, K = carrying capacity.

• Exponential growth produces a J-shaped curve; logistic growth produces an S-shaped curve.

Carrying Capacity

• Carrying capacity (K): The maximum number of individuals an environment can support.

• If population exceeds K, resources become limited, leading to increased mortality or decreased birth rates.

Survivorship Curves

Types and Characteristics

Type	Description	Example
Type I	Low mortality early in life; most individuals survive to old age; high parental care	Humans, large mammals
Type II	Constant mortality rate throughout life	Birds, some reptiles
Type III	High mortality early in life; few survive to adulthood; little parental care	Oysters, many fish, plants

r-Selected vs. K-Selected Species

Comparison

Characteristic	r-Selected Species	K-Selected Species
Offspring number	Many	Few
Parental care	Little or none	High
Survivorship curve	Type III	Type I
Population stability	Unstable, boom-bust cycles	Stable, near carrying capacity
Example	Insects, weeds	Polar bears, elephants

Human Population Growth and Demographic Transition

Trends and Concepts

• Human population growth has historically been exponential but is slowing in many regions.

• Demographic transition: Shift from high birth and death rates to low birth and death rates as a country develops.

Data and Graph Interpretation

Skills for Analysis

• Be able to interpret population growth graphs (identify exponential vs. logistic growth).

• Identify when growth rate is highest (steepest slope on graph).

• Analyze trends over time and relate them to ecological and evolutionary concepts.

Additional info: Some explanations and examples have been expanded for clarity and completeness beyond the original study guide prompts.