Evolutionary Theory Notes

Module 14: Evolutionary Theory

Lesson 1: Darwin and Evolution

  • What is Evolution?

    • Evolution is the process by which all species change over time.

    • Though the idea of evolution existed before, Charles Darwin, in 1859, proposed an explanation for how evolution happens, supported by various types of evidence.

    • Prior to Darwin, most people believed that species did not evolve.

  • Lyell’s Influence on Darwin

    • Lyell's work suggested that the Earth is very old.

    • Geologic processes take a long time and are still happening today.

    • Rocks have layers, with older rocks located deeper.

  • Darwin's Voyage: Observations in the Galápagos Islands

    • Darwin observed tortoises and finches in the Galápagos Islands, noting that each island has different species.

    • These species were similar to those on the mainland.

    • Claim: After mainland species populated the islands, they changed over time into different species.

  • Artificial Selection

    • Darwin observed that breeders of pigeons or dogs could use artificial selection to intentionally promote certain traits in offspring.

    • Question: If humans can cause artificial selection, what happens in nature?

  • Darwin’s Theory of Natural Selection & Malthus' Influence

    • Darwin used evidence from Malthus to develop his theory.

    • Food growth is linear, while population growth is exponential.

    • Question: Why are we still here?

    • War, disease, and famine keep population numbers lower.

  • Principles of Natural Selection

    • Variation: There are variations within any species.

    • Heritability: Variations can be inherited and passed along.

    • Overproduction: More offspring are produced than can survive.

    • Reproductive Advantage: Some individuals will survive, reproduce, and have more offspring due to advantageous traits.

  • Examples Illustrating Natural Selection

    • Variations in any population, even if small.

    • Variations are inherited and passed to offspring.

    • Populations produce too many offspring, more than can survive to maturity.

    • Individuals with an advantage survive, reproduce, and pass on advantageous traits.

  • Peppered Moths Example

  • Moth Gizmo Critique:

    • The moth gizmo isn't a perfect model for natural selection of moths by birds.

    • Birds can't do everything that humans can do.

Lesson 2: Evidence of Evolution

  • The evidence consists of:

    • fossils

    • anatomy

    • embryology

    • molecular biology

    • geographic distribution

  • Fossil Record

    • Fossils are traces of organisms from the past.

    • Comparing extinct fossils and existing organisms provides evidence for evolution.

    • Darwin predicted that whales evolved from land mammals, which has been supported by fossil evidence.

  • Derived vs. Ancestral Traits

    • Derived traits are new in the fossil record.

    • Ancestral traits are older in the fossil record.

    • Conclusion: Species change over time.

  • Anatomy

    • Homologous Structures: Structures that look the same but have different purposes, like arms in vertebrates.

      • Conclusion: Shows common ancestry.

    • Vestigial Structures: Structures with a different function than the original or no function at all, such as the appendix in humans or legs in snakes.

      • They persist because they don’t prevent survival and reproduction.

      • Conclusion: They show ancestry.

    • Analogous Structures: Structures with the same purpose but very different forms, such as wings in birds, bats, and butterflies.

      • Conclusion: Natural selection favored the same adaptation in different species.

  • Embryology

    • Embryology is the study of how embryos develop in different species.

    • Related species have similar-looking embryos.

    • Conclusion: Embryos show ancestry.

  • Molecular Biology

    • Comparison of DNA, amino acid sequences, or proteins of different species.

    • Conclusion: The more similar the DNA, the more closely related the species.

  • Geographic Distribution (Biogeography)

    • Biogeography is the study of the distribution of life at different locations on Earth. Example: Rhea (South America), Ostrich (Africa), Emu (Australia).

    • These are 3 similar birds on 3 continents in different grasslands.

    • Conclusion: Similar environments shape evolution in similar ways.

  • Adaptation and Fitness

    • Natural selection leads to adaptations.

    • If an organism is well-adapted to its environment, it will have more offspring than those that aren’t.

    • Fitness: The number of offspring an organism has.

  • Examples of Adaptations

    • Camouflage: Adaptation to blend into the environment.

    • Mimicry: When a species adapts to look like another species.

  • Antibiotic Resistance

    • Mutation creates an antibiotic-resistant cell.

    • Antibiotics create selective pressure, killing non-resistant cells.

    • Antibiotic-resistant cells survive and proliferate.

Lesson 3: Shaping Evolutionary Theory

  • Mechanism of Evolution:

    • What is actually happening to cause a change in species over time?

    • Answer: Change in allele frequency.

  • Hardy-Weinberg Principle

    • No change in allele frequency in a population means evolution is not happening, which indicates genetic equilibrium.

    • What conditions are necessary for genetic equilibrium?

  • Conditions for Genetic Equilibrium & Their Violations

    • Condition #1: Large population

      • Violation: Small population size leads to genetic drift.

      • Random natural events (fire, volcano, disease) can cause certain alleles to disappear, especially in small populations.

    • Condition #2: No movement of a population

      • Violation: Migration leads to gene flow.

      • Migration can add or remove alleles from a population.

    • Condition #3: Random mating

      • Violation: Mate selection leads to sexual selection.

      • Mates may be selected based on behavior, appearance, timing, or scents.

    • Condition #4: No mutations

      • Violation: Mutations

      • Mutations are rare but may add new alleles to a population.

      • Sometimes, the new alleles are an advantage.

    • Condition #5: No natural selection

      • Violation: Natural selection

      • Not all individuals in the population will survive and reproduce.

      • Natural selection can increase or decrease certain alleles in a population.

      • Alleles that help survival and reproduction are passed on.

  • Three Ways Natural Selection Changes Allele Frequencies

    • Stabilizing Selection: Natural selection selects for the middle phenotype.

    • Directional Selection: Natural selection selects for the extreme phenotype.

    • Disruptive Selection: Natural selection selects against the middle phenotype.

  • Possibility of Genetic Equilibrium

    • Is genetic equilibrium even possible? Can all 5 conditions happen at the same time? No.

    • The mechanisms of evolution are genetic drift, gene flow, nonrandom mating, mutations, and natural selection.

  • Speciation

    • What causes new species to form?

    • Members of a population can no longer reproduce with each other, leading to reproductive isolation.

    • What causes reproductive isolation?

  • Prezygotic Isolation

    • Happens before sperm and egg meet.

    • Caused by geographic barriers, behavior, or timing.

  • Postzygotic Isolation

    • Happens after sperm and egg meet.

    • Why isn’t there a new species?

    • The hybrid zygote dies or is weak.

    • The hybrid zygote is sterile (no gametes).

  • Speciation Process

    • When new species form, a population is separated, no longer reproduces with others, and reproductive isolation occurs.

    • How does this happen?

  • Allopatric Speciation

    • Starts with a physical barrier, dividing a population into two groups.

    • Each changes so much that if they are together again, they will be too different and have reproductive isolation.

  • Sympatric Speciation

    • There is no physical barrier separating the population.

    • The barrier is behavior.

    • The two groups become so different that they no longer reproduce with each other and are reproductively isolated.

  • Evolutionary Patterns

    • Adaptive Radiation

      • When a species splits into two or more new lineages (branches).

      • Faster when a new niche opens up, such as after a volcano or meteor event.

      • This has happened with insects and mammals.

    • Coevolution

      • Species evolve in response to each other.

      • Examples: predator/prey relationships (cheetah/gazelle) and pollinator/plant relationships.

    • Convergent Evolution

      • Similar evolution in similar environments.

      • Examples: Arctic animals and flightless birds on different continents.

  • Rates of Speciation

    • Gradualism

      • When many small genetic changes add up over long periods of time to cause speciation.

      • Example: wild horses.

    • Punctuated Equilibrium

      • When speciation occurs abruptly, then there is no change for a long time.

      • Due to a drastic change in environment.

      • Example: elephants.