Evolution & Gene Frequencies

Evolution & Gene Frequencies

Paleontology

  • Fossil records provide insights into evolutionary transitions.
  • Example: Transition from early reptiles to dinosaurs and early birds (e.g., Stagonolepis, Deinonychus, Archeopteryx).

Fossil Records

  • Fossils in younger strata are more recent, while older strata contain older fossils.
  • Fossils document transitions, such as the evolution of cetaceans from land to sea.

Biogeography

  • Biogeography, the geographic distribution of species, offers evidence of evolution.
  • Continents were once united as Pangaea and have since separated via continental drift.
  • Understanding continent movement helps predict when and where different groups evolved.
  • Examples of biogeographic regions: Sahara Desert, Ethiopian region, Palearctic region, Oriental region, Australian region, Nearctic region, Neotropical region.

Continental Drift

  • Distributions of Triassic fossils (e.g., Cynognathus, Mesosaurus, Lystrosaurus, Glossopteris) illustrate biogeography via continental drift across South America, Africa, Antarctica, India, and Australia.

Endemic Species

  • Endemic species are unique to specific geographic locations.
  • Islands often have endemic species closely related to mainland species.
  • Island species adapt to new environments, giving rise to new species.
  • Example: Cuatrocienegas Valley, where not all "islands" are in the ocean.

Homologies and Evolutionary Trees

  • Evolutionary trees are hypotheses about relationships among groups.
  • Homologies form nested patterns in evolutionary trees.
  • Trees can use anatomical and DNA sequence data.
  • Phylogenetic tree example includes Fish, Amphibians, Reptiles, Dinosaurs, Birds, and Mammals.
  • Key traits:
    • Vertebrae
    • Terrestrial locomotion
    • Amniotic egg
    • Synapsid skull
    • Diapsid skull
    • Adaptations for flight
  • Groups:
    • Lepidosauria
    • Diapsida
    • Archosauria
    • Squamata
  • Diapsids have a diapsid skull with two pairs of temporal openings.

Speciation

  • Speciation, the origin of new species, is central to evolutionary theory.
  • Evolutionary theory explains how new species originate and how populations evolve.
  • Microevolution: Changes in allele frequency in a population over time.
  • Macroevolution: Broad patterns of evolutionary change above the species level.

Biological Species Concept

  • Emphasizes reproductive isolation.
  • Species are grouped by comparing morphology, physiology, biochemistry, and DNA sequences.
  • A species is a group of populations with the potential to interbreed and produce viable, fertile offspring; they do not breed successfully with other populations.
  • Gene flow maintains the phenotype of a population.

Reproductive Isolation

  • Reproductive isolation involves biological factors that prevent different species from producing viable, fertile offspring.
  • Hybrids are offspring from crosses between different species.
  • Isolation can occur before (prezygotic) or after (postzygotic) fertilization.

Prezygotic Barriers:

  • Block fertilization by:
    • Impeding mating attempts.
    • Preventing successful mating.
    • Hindering fertilization if mating is successful.
  • Types:
    • Habitat isolation: Species in different habitats rarely interact.
    • Temporal isolation: Species breed at different times.
    • Behavioral isolation: Unique courtship rituals.
    • Mechanical isolation: Morphological differences prevent mating.
    • Gametic isolation: Sperm and eggs are incompatible.

Postzygotic Barriers:

  • Prevent hybrid zygotes from developing into viable, fertile adults.
  • Types:
    • Reduced hybrid viability: Impaired hybrid development due to gene interaction.
    • Reduced hybrid fertility: Hybrids may be sterile.
    • Hybrid breakdown: Fertile first-generation hybrids produce feeble or sterile offspring in the next generation.

Limitations of Biological Species Concept

  • Cannot be applied to fossils or asexual organisms.
  • Emphasizes absence of gene flow, but gene flow can occur between distinct species (e.g., grizzly bears and polar bears producing "grolar bears").
  • Species delineations can be difficult with distinct-seeming populations (e.g., Ring salamander, Ensatina eschscholzii).

Other Species Concepts

  • Morphological species concept: Defines species by structural features (subjective criteria).
  • Ecological species concept: Views species in terms of ecological niches (emphasizes disruptive selection).
  • Phylogenetic species concept: Defines a species as the smallest group of individuals on a phylogenetic tree (can be difficult to determine required degree of difference).

Gene Pools and Allele Frequencies

  • A population is a localized group of interbreeding individuals producing fertile offspring.
  • A gene pool includes all alleles for all loci in a population.
  • A locus is fixed if all individuals are homozygous for the same allele.

Hardy-Weinberg Equilibrium

  • States that allele and genotype frequencies in a population remain constant from generation to generation.
  • In random mating, allele frequencies will not change.
  • Mendelian inheritance preserves genetic variation in a population.
  • Describes a population that is not evolving.
  • If a population does not meet Hardy-Weinberg criteria, it is evolving.

Allele Frequency Calculation

  • Frequency of an allele can be calculated.
  • With 2 alleles at a locus, p and q represent their frequencies.
  • The sum of all allele frequencies in a population equals 1: p + q = 1.

Example

  • Example:
    • If there are 16 red and 4 white beads p = frequency of C^W allele = 0.8
    • q = frequency of C^R allele = 0.2
    • p (0.8) + q (0.2) = 1

Hardy-Weinberg Equation

  • If p and q represent frequencies of two possible alleles: p^2 + 2pq + q^2 = 1.
  • p^2 and q^2 represent homozygous genotype frequencies.
  • 2pq represents heterozygous genotype frequency.

Example 2

  • Example: p=0.8 and q=0.2.
  • 0.8^2 + (20.80.2) + 0.2^2 = 1
  • 0.64 + 0.32 + 0.04 = 1
  • Approximately 64% homozygous dominant (AA), 32% heterozygotes (Aa), and 4% homozygous recessive (aa).

Estimating Allele Frequencies

  • Estimate p and q by measuring genotype frequencies.
  • If frequency of aa = 20%, then q^2 = 0.2.
  • You can find q by taking the square-root of 0.2 (\sqrt{0.2} = 0.44).
  • p = 1 - q, so p = 1 - 0.44 = 0.56.
  • Then the frequency of AA is 0.314 and Aa is 0.493.

Conditions for Hardy-Weinberg Equilibrium

  • The Hardy-Weinberg theorem describes a hypothetical non-evolving population.
  • In real populations, allele and genotype frequencies change over time.
  • Natural populations can evolve at some loci while being in Hardy-Weinberg equilibrium at others.

Conditions Not Met In Nature

  • Conditions for non-evolving populations are rarely met in nature:
    1. Extremely large population size (no effects of chance).
    2. No gene flow (movement).
    3. No mutations (or mutational equilibrium).
    4. Random reproduction (no differential success).

Genetic Drift

  • The smaller a sample, the greater the chance of deviation from a predicted result.
  • Genetic drift: Allele frequencies fluctuate unpredictably from one generation to the next.
  • Genetic drift reduces genetic variation through losses of alleles.
  • Examples:
    • The Founder Effect
    • The Bottleneck Effect

Founder Effect

  • Occurs when a few individuals become isolated from a larger population.
  • Allele frequencies in the small founder population differ from the larger parent population.

Bottleneck Effect

  • Sudden reduction in population size due to environmental change.
  • The resulting gene pool may no longer reflect the original population’s gene pool.
  • If the population remains small, it may be further affected by genetic drift.

Effects of Genetic Drift

  • Most significant in small populations.
  • Causes allele frequencies to change at random (Neutral Evolution).
  • Can lead to a loss of genetic variation within populations.
  • Can cause harmful alleles to become fixed.

Gene Flow

  • Gene flow: Movement of alleles among populations.
    • Emigration: Moving out of a population.
    • Immigration: Moving into a new population.
  • Alleles are transferred through the movement of fertile individuals or gametes.
  • Gene flow tends to reduce variation among populations over time.
  • Example: Gene Flow in Polar Bear (Ursus maritimus) Populations.

Mutations

  • Mutations occur but are less likely in equilibrium (mutated alleles that mutate back to the original form as often as they mutate in the first place…).

Natural Selection

  • Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions.
  • Example: An allele that confers resistance to DDT increased in frequency after DDT was used widely in agriculture.

Sexual Selection

  • Natural selection for mating success.
  • Results in sexual dimorphism: Marked differences between the sexes in secondary sexual characteristics.
  • Can appear to have a negative impact on survival, but reproductive success compensates.
  • "Handicap Principle"- survival despite an obvious handicap indicates "good genes".

Types of Sexual Selection

  • Intrasexual selection: Competition among individuals of one sex (often males) for mates of the opposite sex.
  • Intersexual selection (mate choice): Individuals of one sex (usually females) are choosy in selecting their mates.
  • Non-random mating.

Modes of Selection

  • Three modes of selection:
    • Directional selection: Favors individuals at one end of the phenotypic range.
    • Disruptive selection: Favors individuals at both extremes of the phenotypic range.
    • Stabilizing selection: Favors intermediate variants and acts against extreme phenotypes.

Heterozygote Advantage

  • Heterozygotes have higher fitness than either homozygote condition.
  • Natural selection maintains two or more alleles at that locus.
  • Example: The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance, giving heterozygotes an advantage over either homozygote condition.

Speciation and Geographic Separation

  • Speciation can occur with or without geographic separation.
  • Two ways speciation can occur:
    • Allopatric speciation- separate places
    • Sympatric speciation- same place

Allopatric Speciation

  • In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations.
  • Example: The flightless cormorant of the Galápagos likely originated from a flying species on the mainland.

Process of Allopatric Speciation

  • The definition of barrier depends on the ability of a population to disperse.
  • Example: A canyon may create a barrier for small rodents, but not birds, coyotes, or pollen.
  • Sibling species of snapping shrimp (Alpheus) are separated by the Isthmus of Panama.
  • These species originated 9 to 13 million years ago, when the Isthmus of Panama formed and separated the Atlantic and Pacific waters.
  • Regions with many geographic barriers typically have more species than do regions with fewer barriers.
  • Reproductive isolation between populations generally increases as the distance between them increases.
  • Example: Reproductive isolation increases between dusky salamanders that live further apart.

Sympatric Speciation

  • In sympatric speciation, speciation takes place in geographically overlapping populations.
  • Mechanisms:
    • Polyploidy
    • Habitat Differentiation
    • Sexual Selection

Polyploidy

  • Polyploidy: The presence of extra sets of chromosomes due to accidents during cell division.
  • More common in plants than in animals.
  • An autopolyploid: An individual with more than two chromosome sets, derived from one species.
  • An allopolyploid: A species with multiple sets of chromosomes derived from different species.

Habitat Differentiation

  • Sympatric speciation can also result from the appearance of new ecological niches.
  • Example: The North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees.

Sexual Selection

  • Sexual selection can drive sympatric speciation.
  • Sexual selection for mates of different colors has likely contributed to speciation in cichlid fish in Lake Victoria.

Review of Allopatric and Sympatric Speciation

  • In allopatric speciation, geographic isolation restricts gene flow between populations.
    • Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations.
    • Even if contact is restored between populations, interbreeding is prevented.
  • In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species.
    • Sympatric speciation can result from polyploidy, natural selection, or sexual selection.

Rates of Evolution

  • Speciation can occur rapidly or slowly and can result from changes in few or many genes.
  • Many questions remain concerning how long it takes for new species to form, or how many genes need to differ between species.
  • Two major patterns observed:
    • Gradualism
    • Punctuated Equilibrium

Patterns in the Fossil Record

  • The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear.
  • Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibria to describe periods of apparent stasis punctuated by sudden change.
  • The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence.

Geological Time

  • Ways to look at geological time and how the diversity of life fits into it.
  • Note the overall scale vs. the time frames of organismal groups.
  • Note the advent of major groups of organisms.
  • Reminder of continental drift (at least from the latest supercontinent to the present).