Evolution and Population Ecology: Study Guide and Key Concepts

Evolution and Population Ecology

Key Vocabulary and Definitions

• 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 the accumulation of those traits in the population.

• 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 influences.

• Microevolution: Small-scale evolutionary changes within a population, typically involving changes in allele frequencies.

• Genetic drift: Random changes in allele frequencies in small populations due to chance events.

• 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 loss of genetic diversity.

• Directional selection: Natural selection that favors one extreme phenotype over others.

• Disruptive selection: Selection that favors individuals at both extremes of a trait over intermediate phenotypes.

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

• Sexual selection: Selection for traits that increase mating success.

• 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: Prezygotic barrier where differences in mating behaviors prevent interbreeding.

• Mechanical isolation: Prezygotic barrier where physical differences prevent successful mating.

• Habitat isolation: Prezygotic barrier where species occupy different habitats and do not meet.

• Temporal isolation: Prezygotic barrier where species breed at different times.

• Gametic isolation: Prezygotic barrier where gametes cannot fuse to form a zygote.

• Reduced hybrid viability: Postzygotic barrier where hybrids fail to develop or are frail.

• Reduced hybrid fertility: Postzygotic barrier where hybrids are sterile.

• Hybrid breakdown: Postzygotic barrier where hybrid offspring are viable and fertile, but their descendants are weak 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 that occurs without geographic separation.

• Allopatric speciation: Speciation that occurs due to geographic isolation.

• 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, with no limiting factors.

• Logistic growth: Population growth that slows as it approaches carrying capacity.

• Survivorship curve: Graph showing the number or proportion of individuals surviving at each age for a given species or group.

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

• r-selected species: Species that produce many offspring, with little parental care and high mortality rates.

Adaptation & Natural Selection

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

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

• Natural selection depends on:

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

  • Heritability: Traits must be heritable to be passed to offspring.

• Advantageous traits become more common in a population over generations due to increased reproductive success.

Populations & Evolution

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

• The gene pool includes all the alleles present in a population.

• The smallest unit that can evolve is the population, not the individual.

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

Hardy-Weinberg Equilibrium

• 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.

• The Hardy-Weinberg equation is:

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

$p + q = 1$

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

Mechanisms of Evolution

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

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

• Founder effect: When a new population is started by a small number of individuals, leading to reduced genetic diversity.

• Bottleneck effect: A drastic reduction in population size due to a sudden event, reducing genetic variation.

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

Examples:

• A small isolated group colonizing a new area experiences the founder effect.

• A population reduced by disaster undergoes the bottleneck effect.

• Migration between populations results in gene flow.

Types of Selection

• Stabilizing selection: Favors intermediate phenotypes, reducing variation.

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

• Disruptive selection: Favors both extremes over intermediate phenotypes, increasing variation.

• Sexual selection: Favors traits that enhance mating success, which may not always be advantageous for survival.

Effect on Variation: Stabilizing selection decreases variation, disruptive selection increases it, and directional selection shifts the population toward one extreme.

Speciation

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

• Allopatric speciation: Occurs when populations are geographically separated (e.g., by a mountain or river).

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

Examples:

• Geographic barriers (mountains, rivers) lead to allopatric speciation.

• Different mating behaviors in the same location can lead to sympatric speciation.

Reproductive Isolation

• Prezygotic barriers (before fertilization):

  • Behavioral isolation

  • Mechanical isolation

  • Habitat isolation

  • Temporal isolation

  • Gametic isolation

• Postzygotic barriers (after fertilization):

  • Reduced hybrid viability

  • Reduced hybrid fertility

  • Hybrid breakdown

Models of Evolution

• Gradualism: Evolutionary change occurs slowly and steadily over time.

• Recognize gradualism by descriptions of slow, continuous change rather than sudden shifts.

Interpreting Evolution Scenarios

• Identify causes of differences (e.g., isolation, changes in gene pool).

• Determine the mechanism of evolution (natural selection, genetic drift, etc.).

• Understand the role of mutation and selection in driving evolutionary change.

Population Growth Models

• Exponential growth: Population increases rapidly without limiting factors.

• Logistic growth: Population growth slows as it approaches carrying capacity (K).

Equations:

• Exponential growth: $\frac{dN}{dt} = rN$

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

Graph Interpretation: Exponential growth is a J-shaped curve; logistic growth is S-shaped, leveling off at carrying capacity.

Carrying Capacity

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

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

Survivorship Curves

Type	Description	Example
Type I	Low mortality early in life; most individuals survive to old age; high parental care	Humans, elephants
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

r-Selected vs. K-Selected Species

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	Fluctuates widely	Stable, near carrying capacity
Example	Insects, weeds	Polar bears, elephants

Human Population Growth and Demographic Transition

• Human population growth has shown exponential trends but is slowing in many regions.

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

Data & Graph Interpretation

• Be able to interpret population graphs, identify periods of highest growth rate, and analyze trends over time.

Additional info: This guide integrates key terms and concepts from evolutionary biology and population ecology, providing definitions, examples, and equations for exam preparation.