Evolution Study Guide

Reproductive Barriers

Pre-zygotic Barriers

• Habitat Isolation: Species occupy different habitats (terrestrial, aquatic, etc.) and do not meet, leading to reduced chances of mating.

• Temporal Isolation: Species breed at different times or seasons (e.g., one species breeds in spring while another breeds in summer).

• Behavioral Isolation: Differences in mating behavior or rituals (e.g., courtship displays) lead to isolation as individuals of different species fail to recognize each other as mates.

• Mechanical Isolation: Structural differences in reproductive organs prevent successful mating between species (e.g., size or shape mismatch).

• Gametic Isolation: Sperm and egg are incompatible due to specific signaling proteins that prevent fertilization, even if mating occurs.

Post-zygotic Barriers

• Reduced Hybrid Viability: Hybrids may not develop properly or have lower survival rates due to genetic incompatibilities, leading to weaknesses that prevent them from reaching maturity.

• Reduced Hybrid Fertility: Hybrids, such as mules (offspring of horses and donkeys), are sterile due to uneven chromosome numbers, preventing them from producing viable offspring.

• Hybrid Breakdown: First-generation hybrids are viable and fertile, but their offspring (second generation) are inviable or sterile, usually due to genetic incompatibilities re-emerging over generations.

Natural Selection
Definition: Mechanism of evolution where organisms better adapted to the environment tend to survive and produce more offspring, leading to gradual changes in population traits.

• Artificial Selection: Humans choose which plants or animals to breed based on desired traits. Example: Breeding dogs for specific sizes or colors.

• Directional Selection: Nature favors one extreme version of a trait over others. Example: Bigger bugs surviving better, leading to a population of mostly large bugs over time.

• Disruptive Selection: Nature favors two extreme traits and not the middle one. Example: Light and dark-colored rabbits are more common, while medium-colored rabbits are less likely to survive.

• Stabilizing Selection: Nature favors average traits and not extremes. Example: Babies of average size are more likely to survive than very tiny or very large babies.

• Sexual Selection: Traits that help animals attract mates become more common. Example: Male birds with brighter feathers are chosen more often by females than duller males.

Hardy-Weinberg Principle
Equation: p^2 + 2pq + q^2 = 1
Where:

• p^2 = frequency of homozygous dominant (AA) individuals.

• 2pq = frequency of heterozygous (Aa) individuals.

• q^2 = frequency of homozygous recessive (aa) individuals.

• Allele Frequency: p + q = 1

  • p = frequency of dominant allele (A).

  • q = frequency of recessive allele (a).
    Conditions for Hardy-Weinberg Equilibrium:

• No mutations, meaning no new alleles introduced.

• Random mating, ensuring that all individuals have an equal chance of mating.

• No natural selection, where all individuals have the equal chance of survival and reproduction.

• Extremely large population size, minimizing effects of genetic drift.

• No gene flow (migration), meaning no individuals entering or leaving the population.

Genetic Drift
Definition: Random changes in allele frequencies in a population due to chance events, which can cause alleles to become more or less common regardless of their adaptive value.
Examples:

• Founder Effect: A new population is established by a small number of individuals from a larger population, leading to reduced genetic variation (e.g., when a few individuals colonize a new habitat).

• Bottleneck Effect: A sudden reduction in population size (e.g., due to a natural disaster) decreases genetic diversity, as the surviving population may not represent the genetic diversity of the original population.

Speciation

• Allopatric Speciation: Occurs due to geographic isolation of populations (e.g., mountain range formation) which leads to divergence and eventually the formation of new species.

• Sympatric Speciation: Occurs without geographic isolation, often due to factors like polyploidy (common in plants), habitat differentiation (some individuals exploit different niches), or sexual selection (where mate choice becomes the main driver of reproductive isolation).

Phylogeny and Classification

(abbreviated as DKPCOFGS).
Human Classification:

• Domain: Eukarya

• Kingdom: Animalia

• Phylum: Chordata

• Class: Mammalia

• Order: Primates

• Family: Hominidae

• Genus: Homo

• Species: Homo sapiens.
  Cladogram: A diagram that illustrates the evolutionary relationships among various biological species based on shared characteristics, visually representing ancestral lineages.

Three Domains of Life

• Bacteria: Prokaryotic, unicellular organisms without a nucleus.

• Archaea: Prokaryotic, unicellular organisms, often found in extreme environments, distinct from bacteria.

• Eukarya: Organisms with complex cells containing a nucleus (includes all plants, animals, fungi, and protists).