Comprehensive Study Notes on Natural Selection, Population Genetics, Speciation, Phylogeny, and Origins of Life

Natural Selection

• Charles Darwin

  • English naturalist known for his research at the Galapagos Islands.

• Darwin's Research

  • Interested in biogeography (geographic distribution of species).
  • Hypothesized that organisms from South America colonized the Galapagos Islands and diversified into new species.
  • Focused on finches.

• Evolution

  • Descent with modification: change in the genetic makeup of a population over time.
  • Heritable traits change from generation to generation.
  • Darwin proposed natural selection to explain the pattern of descent with modification.

• Natural Selection

  • Process where individuals with certain traits survive and reproduce at higher rates due to those traits.
  • Acts on phenotypic variations in populations.
  • Some phenotypes increase or decrease an organism's fitness (ability to survive and reproduce).
  • Fitness is measured by reproductive success.
  • Environments can change, causing selective pressures on populations.
  • Based on two main observations:
    1. Traits are heritable.
    2. More offspring are produced than can survive.

• Traits are Heritable

  • Characteristics passed from parent to offspring.
  • Adaptations: inherited characteristics that enhance survival and reproduction.

• More Offspring are Produced Than Can Survive

  • Leads to competition for limited resources, resulting in differential survival.
  • Traits that lead to survival ("favorable" traits) accumulate in the population.
  • Populations evolve, not individuals.

• Artificial Selection

  • Selective breeding of domesticated plants and animals to encourage desirable traits.
  • Used by Darwin to support natural selection, which he thought was similar.
    • Nature "selects" traits suited for survival and reproduction.
    • Humans select desirable traits.
    • Both lead to evolutionary change, but natural selection occurs without human influence.

Population Genetics

• Population

  • A group of individuals of the same species that live in the same area and interbreed to produce fertile offspring.

• Gene Pool

  • A population’s genetic makeup.
  • Consists of all copies of every type of allele at every locus in all members of the population.
  • If there is only one allele present for a particular locus in the population it is fixed.
  • Many fixed alleles=less genetic diversity

• Population Genetics

  • A population’s allele frequencies will change over time.
  • Populations evolve, not individuals.
  • Microevolution: small-scale genetic changes in a population.
    • Evolution is driven by random occurrences
      • Mutations.
      • Genetic Drift.
      • Migration/gene flow.
      • Natural selection.

• Mutations

  • Can result in genetic variation.
  • Can form new alleles.
  • Natural selection can act on varied phenotypes.
  • Mutation rates tend to be slow in plants and animals and fast in prokaryotes due to faster generation time.
  • Mutations can be harmful, neutral, or beneficial. Most mutations are neutral to harmful.
  • Not all mutations lead to evolution.

• Genetic Drift

  • Chance events that cause a change in allele frequency from one generation to the next.
  • Most significant to small populations.
  • Can lead to a loss of genetic variation.
  • Can cause harmful alleles to become fixed.
  • Does NOT produce adaptations.
  • Two types:
    • Bottleneck effect.
    • Founder effect.

• Bottleneck Effect

  • A large population is drastically reduced by a non-selective disaster.
  • Floods, famine, fires, hurricanes, hunting, etc.
  • Some alleles may become overrepresented, underrepresented, or absent.

• Founder Effect

  • A few individuals become isolated from a large population and establish a new small population with a gene pool that differs from the large population.
  • Lose genetic diversity.

• Gene Flow

  • The transfer of alleles into or out of a population due to fertile individuals or gametes.
  • Alleles can be transferred between populations, for example pollen being blown to a new location.

• Natural Selection

  • Reproductive success is measured by relative fitness.
  • The number of surviving offspring that an individual produces compared to the number left by others in the population.
  • Effects of natural selection can be measured by examining the changes in the mean of phenotypes.
  • There are three modes of natural selection.
    1. Directional selection.
    2. Stabilizing selection.
    3. Disruptive selection.

• Modes of Natural Selection

  • Directional Selection
    • Selection towards one extreme phenotype
  • Stabilizing Selection
    • Selection towards the mean and against the extreme phenotypes.
  • Disruptive Selection
    • Selection against the mean.
    • Both phenotypic extremes have the highest relative fitness.

• Sexual Selection

  • A type of natural selection that explains why many species have unique/showy traits.
  • Males often have useless structures (i.e. colorful male peacock feathers) simply because females choose that trait.
  • Can produce traits that are harmful to survival.
  • Example: colorful feathers in male peacocks make them easier to spot by predators.

Hardy Weinberg Equilibrium

• Hardy Weinberg Equilibrium

  • A model used to assess whether natural selection or other factors are causing evolution at a particular locus.
  • Determines what the genetic make up of the population would be if it were NOT evolving.
  • This is then compared to actual data:
    • If there are NO differences, then the population is NOT evolving.
    • If there ARE differences, then the population MAY BE evolving.

• The Hardy Weinberg principle states

  • The frequencies of alleles and genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.
  • Remember: this is a hypothetical situation where no evolution would take place. In real populations the allele and genotype frequencies DO change over time.

• Five conditions must be met to be in Hardy Weinberg equilibrium

  1. No mutations.
  2. Random mating.
  3. No natural selection.
  4. Extremely large population size.
  5. No gene flow.

• If any of these conditions are not met, then microevolution occurs (i.e. mutation, gene flow, genetic drift, natural selection, and non-random mating).

• Two formulas

  • p + q = 1
    • Frequency of the dominant allele in a population.
    • Frequency of the recessive allele in a population.
  • p^2 + 2pq + q^2 = 1
    • Percentage of the homozygous dominant individuals.
    • Percentage of the heterozygous individuals.
    • Percentage of the homozygous recessive individuals.

Speciation and Extinction

• Speciation

  • Species: a group able to interbreed and produce viable, fertile offspring.
  • Speciation: formation of new species.
  • Results in diversity of life forms.

• Geography has an impact on speciation

  • Two modes of speciation
    • Allopatric speciation
    • Sympatric speciation

• Allopatric Speciation

  • Physical barrier divides population or a small population is separated from main population.
  • Populations are geographically isolated.
    • Prevents gene flow.
  • Often caused by natural disasters.

• Sympatric Speciation

  • A new species evolves while still inhabiting the same geographic region as the ancestral species.
  • Usually due to the exploitation of a new niche.

• Speciation occurs due to reproductive isolation

  • Two types:
    • Prezygotic barriers
    • Postzygotic barriers
  • Both types maintain isolation and prevent gene flow between the populations

• Prezygotic Barriers

  • Prevent mating or hinder fertilization.
  • Five types
    • Habitat isolation
    • Temporal isolation
    • Behavioral isolation
    • Mechanical isolation
    • Gametic isolation

• Postzygotic Barriers

  • Prevent a hybrid zygote from developing into a viable, fertile adult.
  • Three types
    • Reduced hybrid viability
    • Reduced hybrid fertility
    • Hybrid breakdown

• Micro and Macroevolution

  • Speciation is a bridge between the concepts of microevolution and macroevolution
    • Microevolution: change in allele frequencies within a single species or population (natural and sexual selection, genetic drift, gene flow)
    • Macroevolution: large evolutionary patterns (adaptive radiation, mass extinction).
      • Stasis: no change over long periods of time

• Pace of Speciation

  • Evolution and speciation can occur at different speeds
    • Punctuated equilibrium: when evolution occurs rapidly after a long period of stasis
    • Gradualism: when evolution occurs slowly over hundreds, thousands, or millions of years

• Evolution and Speciation

  • Divergent evolution: groups with the same common ancestor evolve and accumulate differences resulting in the formation of a new species
    • Adaptive radiation: if a new habitat or niche becomes available, species can diversify rapidly
  • Convergent evolution: two different species develop similar traits despite having different ancestors
    • Analogous traits

• Extinction

  • Extinction: the termination of a species
  • Extinctions have occurred throughout the Earth’s history (5 mass extinctions)
    • Human activity has affected extinction rates
    • Anytime there is ecological stress, extinction rates can quicken
    • If a species does go extinct, it opens up a niche that can be exploited by a different species

Phylogeny

• Phylogeny

  • Systematics: classification of organisms and determining their evolutionary relationships
    • Taxonomy: naming and classifying species
    • Phylogenetics: hypothesis of evolutionary history
      • Use phylogenetic trees to show evolution
    • To determine evolutionary relationships, scientists use
      • Fossil records
      • DNA
      • Proteins
      • Homologous structures

• Phylogenetic Trees

  • Phylogenetic trees: diagrams that represent the evolutionary history of a group of organisms
  • Similar to cladograms, except trees show the amount of change over time measure

• Cladograms

  • Each line represents a lineage
  • Each branching point is a node
    • Nodes represent common ancestors
    • Nodes and all branches from it are called clades
    • Species in a clade have shared derived features
  • The root is the common ancestor of all the species
  • Two clades that emerge from the same node are sister taxa
  • A lineage that evolved from the root and remains unbranched is the basal taxon
  • Synapomorphy: a derived character shared by clade members
    • Derived characteristic: similarity inherited from the most recent common ancestor of an entire group
    • Ancestral characteristic: similarity that arose prior to the common ancestor

Origins of Life on Earth

• Origins of Life

  • Earth formed approximately 4.6 billion years ago (bya)
    • Early Earth was not suitable for life until 3.9 bya
      • Earliest fossil evidence is 3.5 bya
        • Cyanobacteria

• How Did Life Arise?

  • Early Earth contained inorganic molecules
    • These could have synthesized organic molecules due to free energy and abundant oxygen
      • Organic molecules could have also been transported to Earth via meteorites or other celestial events

• Experimental Data

  • Oparin and Haldane hypothesized that early Earth was primarily composed of hydrogen, methane, ammonia and water
    • Stanley Miller and Harold Urey tested the hypothesis in their lab
      • They found organic compounds and amino acids formed

• RNA World Hypothesis

  • RNA World Hypothesis: proposes that RNA could have been the earliest genetic material
    • Helps to explain the pre-cellular stage of life

Evidence of Evolution and Common Ancestry

• Evidence of Evolution

  • Overwhelming evidence supports the theory of evolution
    • Primary sources of evidence are
      • The fossil record
      • Comparative morphology
      • Biogeography

• Fossil Record

  • Fossils: remains or traces of past organisms
    • Fossil record: gives a visual of evolutionary change over time
      • Fossils can be dated by examining the rate of carbon 14 decay and the age of rocks where the fossils are found
      • Gives geographical data for the organisms found

• Comparative Morphology

  • Comparative morphology: analysis of the structures of living and extinct organisms
    • Homology: characteristics in related species that have similarities even if the functions differ
      • Embryonic homology: many species have similar embryonic development
      • Vestigial structures: structures that are conserved even though they no longer have a use
        • Example: tailbone and appendix in humans
      • Molecular homology: many species share similar DNA and amino acid sequences
        • Structural evidence indicates common ancestry of all eukaryotes
          • Many fundamental and cellular features and processes are conserved across organisms
          • Cellular examples
            • Membrane-bound organelles
            • Linear chromosomes
            • Introns in genes

• Biogeography

  • Biogeography: the distribution of animals and plants geographically
    • Example: Species on oceanic islands resemble mainland species
    • Example: species on the same continent are similar and distinct from species on other continents

• Putting It All Together

  • Populations continue to evolve
    • Genomes change
      • Examples
        • Antibiotic resistance in bacteria
        • Insect resistance to pesticides
        • Pathogens cause emerging (new) diseases