Unit 7: Natural Selection

📚 Theory of Evolution

• Charles Darwin’s Theory of Evolution

  • Each species produces more offspring than can survive

  • These offspring compete with one another for the limited resources available to them

  • Organisms in every population vary

  • Evolutionary fitness is measured by reproductive success

  • The fittest offspring or those with the most favorable traits are the most likely to survive and therefore produce a second generation

    • These are the traits most likely to pass to subsequent generations

  • Fitness varies based on biotic and abiotic factors; different genetic variations can be selected for in different generations

• John Baptiste de Lamarck

  • Lamarck proposed that acquired traits were inherited and passed on to offspring

  • Lamarck believed that giraffes had long necks because they were constantly using it

    • “law of use or disuse“

    • Wrong lol

    • Gametes the body makes include copies of the regular old genome, not based on what you do during your life such as building muscles (does not pass big muscles to your kids)

🔎 Evidence for Evolution

• All living organisms share a common ancestor

• Support for the theory of evolution come from several areas

  • Paleontology - study of fossils

    • Fossils can show major lines of evolution

    • can be dated by age of nearby rocks, rate of decay of isotopes like carbon-14

    • geographical data

  • Biogeography (study of distribution of flora and fauna)

  • Embryology - study of development of an organism

    • In the early stages of vertebrate development, all embryos look alike!

  • Morphological homologies - study of anatomy of various animals (comparative anatomy)

    • A human’s arm, dog’s leg, bird’s wing, and a whale’s fin all have the same appendages even though they have different purposes

    • these homologous structures point to a common ancestor

  • Analogous features with the same function but are structurally different

    • Ex: Eyes in scallops, insects, and humans are thought to have evolved entirely independently of one another

  • Molecular biology - similarity at the molecular level

    • Nucleotide and amino acid sequences of different organisms

    • Organisms that are closely related have a greater proportion of sequences in common than distantly related species

    • All eukaryotes have membrane-bound organelles, linear chromosomes, and genes that contain introns

🌱 Natural Selection

• Members of a species are defined by the ability to reproduce fertile offspring

• Evolution and speciation occur when environmental pressures favor traits that permit survival and reproduction of selected individuals in a varied population

  • Favored traits may evolve to cause divergent or convergent evolution

• Adaptations - random variations in traits that end up being selected for until they become prominent in the population

• Genetic drift - when a population’s traits change due to random chance rather than natural selection

• Selection can be:

  • stabilizing if an average phenotype is preferred

  • disruptive if extremes are preferred

  • directional if one extreme is preferred

📊 Hardy-Weinberg Equations

• Can be used to determine genetic variation within a population

• p + q = 1

  • Frequency of the dominant (p) and recessive (q) alleles

• p²+2pq+q²=1

  • Frequency of homozygous dominants (p²), heterozygotes (2pq) and homozygous recessive (q²)

• Hardy-Weinberg equations describe a population that is not evolving but is instead said to be in genetic equilibrium

• A population will be this way only if it is not violating any of the following five conditions (almost all populations are evolving due to a violation of at least one of these factors)

  • large population

  • no mutations

  • no immigration or emigration

  • random mating

  • no natural selection

Key Terms

• Morphological Homologies: Similarities in the structure or form of organisms due to shared ancestry, indicating common origin.

• Homologous Structures: Body parts in different species that have a similar structure due to common evolutionary origin, even if they serve different functions.

• Analogous Structures: Body parts in different species that serve similar functions but do not have a common evolutionary origin.

• Continuing Evolution: The ongoing process of evolutionary changes in a population or species over time.

• Phylogenetic Tree (Cladogram): A branching diagram showing the evolutionary relationships between different species based on shared characteristics.

• Out-group: A species or group used as a reference point in a phylogenetic study to compare with the in-group (the group of interest).

• Genetic Variability: The diversity in gene frequencies within a population, which provides the raw material for evolution.

• Peppered Moths: A classic example of natural selection, where the frequency of dark-colored moths increased due to industrial pollution and predation by birds.

• Environmental Pressure: External factors such as predators, climate, or food sources that influence the survival and reproduction of organisms.

• Random Mutation: A spontaneous change in an organism's DNA that can lead to new traits and genetic variation.

• Adaptation: A trait or characteristic that increases an organism's chances of survival and reproduction in a specific environment.

• Evolutionary Fitness: The ability of an organism to survive, reproduce, and pass on its genes to the next generation.

• Sexual Selection: A form of natural selection where certain traits increase an individual's chances of attracting mates, even if those traits do not enhance survival.

• Genetic Drift: A random change in allele frequencies in a population, often more noticeable in small populations.

• Bottleneck (Founder Effect): A sharp reduction in the size of a population due to environmental events, leading to a loss of genetic diversity.

• Gene Flow: The transfer of genetic material between different populations of the same species, increasing genetic diversity.

• Directional Selection: A form of natural selection where one extreme phenotype is favored over others, shifting the population's traits in that direction.

• Stabilizing Selection: A type of natural selection where intermediate phenotypes are favored, reducing variability in a population.

• Disruptive Selection: A type of natural selection that favors both extreme phenotypes at the expense of intermediate phenotypes.

• Artificial Selection: The intentional breeding of organisms with desirable traits by humans, leading to changes in the population.

• Species: A group of organisms that can interbreed and produce fertile offspring under natural conditions.

• Reproductively Isolated: When different species cannot interbreed successfully due to various barriers, such as geographic, behavioral, or temporal factors.

• Divergent Evolution: The process by which two or more related species become more dissimilar over time, often due to different environmental pressures.

• Punctuated Equilibrium: A theory suggesting that evolution occurs in rapid bursts, followed by long periods of stability or little change.

• Gradualism: A model of evolution that proposes species evolve slowly and continuously over time through small, incremental changes.

• Adaptive Radiation: The rapid diversification of a species into a variety of forms to fill different ecological niches.

• Pre-zygotic Barriers: Reproductive barriers that prevent fertilization from occurring, such as differences in mating behaviors or timing.

• Post-zygotic Barriers: Reproductive barriers that occur after fertilization, leading to reduced hybrid viability or fertility.

• Convergent Evolution: The process where unrelated species evolve similar traits due to similar environmental pressures, despite having different ancestors.

• Speciation: The formation of new and distinct species in the course of evolution.

• Allopatric Speciation: The formation of new species due to geographic isolation, leading to reproductive isolation.

• Sympatric Speciation: The formation of new species without geographic isolation, typically through reproductive isolation mechanisms.

• Polyploidy: A condition where an organism has more than two complete sets of chromosomes, often leading to speciation in plants.

• Hardy-Weinberg Law: A principle stating that allele frequencies in a population will remain constant from generation to generation in the absence of evolutionary forces.

• Alexander Oparin and J.B.S. Haldane: Scientists who independently proposed the hypothesis that life originated from simple organic compounds in a "primordial soup" under early Earth conditions.

• Stanley Miller and Harold Urey: Scientists who conducted an experiment in 1953 demonstrating that amino acids could form under prebiotic conditions, supporting the idea of chemical evolution.

• RNA-World Hypothesis: A theory proposing that RNA molecules were the first self-replicating entities and could have been the precursors to current life forms.