Natural Selection Concepts

Natural Selection

• Definition: The process where heritable traits that improve survival and reproduction become more common in a population over time, changing the population's genetic makeup.
• Mechanism: Organisms adapt to environmental pressures; those with advantageous traits survive and reproduce, passing on those genes.
• Four Postulates:
  • Individuals within a population exhibit genetic variation.
  • This variation is heritable.
  • More offspring are produced than can survive, leading to competition.
  • Individuals with favorable traits survive and reproduce more successfully.

Types of Natural Selection

• Directional Selection: Favors one extreme phenotype. Individuals with that extreme have higher fitness.
• Disruptive Selection: Favors both extreme phenotypes. Individuals with extreme traits have higher fitness.
• Stabilizing Selection: Favors the intermediate phenotype. Individuals with the average trait have higher fitness.

Sexual Selection

• Definition: A special form of natural selection focused on mating success.
• Intrasexual Selection: Competition within the same sex (e.g., males fighting for mates).
• Intersexual Selection: Choice by one sex (often females) of mates based on traits or behaviors of the other (often males).
• Sexual Dimorphism: Differences in appearance between males and females of a species, often due to intrasexual competition and female choice.
• Female Choice: Females' higher reproductive investment leads them to be selective in mate choice.
• Male Ornamentation: Costly displays signify good health and genes, attracting females.
• Runaway Selection: Female preference for exaggerated male traits can lead to their constant enhancement, but only under specific conditions (low predation, unlimited resources).
• Natural Constraints: Natural selection usually limits the exaggeration of such traits.

Genetic Drift

• Definition: Random changes in gene frequencies in a population.
• Effect: Usually leads to a loss of genetic variation.
• Impact of Population Size: Smaller populations are more strongly affected by genetic drift.
• Fitness vs. Luck: Survival isn't necessarily based on fitness, but sometimes on luck.

Specific Genetic Drift Examples

• Bottleneck Effect: Catastrophes drastically reduce population size, altering allele frequencies. Some alleles may increase, others disappear.
• Founder Effect: A few individuals from a population colonize a new area, establishing a new population with potentially skewed allele frequencies.

Punctuated Equilibrium

• Definition: Evolutionary change happening relatively quickly, often associated with speciation events.
• Environmental Changes: Environmental pressures can drive speciation events.

Speciation

• Definition: The process of forming new species, resulting in reproductively isolated populations.
• Reproductive Isolation: Mechanisms preventing successful reproduction between species.
• Prezygotic Isolation: Prevents mating or fertilization:
  • Behavioral Isolation: Differences in mating rituals.
  • Temporal Isolation: Different mating times.
  • Geographic Isolation: Separation by physical barriers.
  • Gametic Isolation: Egg and sperm are incompatible.
  • Mechanical Isolation: Genitals are incompatible.
• Postzygotic Isolation: Prevents the formation of viable offspring after fertilization:
  • Zygotic Mortality: Zygote fails to develop.
  • Hybrid Inviability: Hybrid offspring don't survive to adulthood.
  • Hybrid Infertility: Hybrid offspring are sterile.

Modes of Speciation

• Allopatric Speciation: Formation of new species following geographic isolation, leading to adaptation and reproductive isolation. Often linked to genetic drift.
• Peripatric Speciation: Similar to allopatric, but the isolated group is very small, leading to a rapid impact of genetic drift. Often related to the founder effect.
• Parapatric Speciation: No geographic barrier, but mating isn't random, e.g., different flowering times.
• Sympatric Speciation: Formation of new species within the same geographic area.

Defense Mechanisms

• Passive Defense: Mechanisms to avoid predation.
  • Crypsis: Camouflage or hiding.
  • Aposematism: Warning coloration.
  • Camouflage: Matching the environment for concealment.
  • Concealing coloration: Matching the environment's color.
  • Distributive coloration: Breaking up body outlines.
  • Disguise: Resembling something non-living.
  • Mimicry: Resembling another organism.
    • Batesian mimicry: A harmless organism mimicking a harmful one.
    • Müllerian mimicry: Two or more harmful organisms mimicking each other.

Competition

• Interspecific Competition: Competition between different species for resources.
• Intraspecific Competition: Competition within the same species for resources, often more intense.
• Resource Partitioning: Species dividing resources to avoid competition and coexist.
• Benefits of Resource Partitioning: Increased survival for all species involved.
• Costs of Resource Partitioning: Potential reduction in population size for each species.

Adaptations

• Adaptations Types:
  • Structural: Related to body parts (e.g., thick fur).
  • Behavioral: Related to actions (e.g., hibernation).
  • Physiological: Related to bodily functions (e.g., venom production).

Evolutionary Processes

• Analogous Structures: Anatomically different structures with similar functions in unrelated organisms, due to similar environmental pressures. (e.g., bird and bat wings)
• Convergent Evolution: Traits evolving similarly in unrelated species due to similar environmental pressures.
• Homologous Structures: Anatomically similar structures in related organisms, potentially with different functions, due to divergent evolution. (e.g., human forearm and dolphin flipper)
• Divergent Evolution: Traits diverging from a common ancestor due to differing selective pressures.