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Chapter 27: Evolution of Life

Overview of Evolutionary Concepts

• Early Evolutionary Thought

• Darwin and Natural Selection

• Evidence of Evolution

• Microevolution

• Evolutionary Processes

• Macroevolution and Speciation

Page 1: Foundations of Evolutionary Thought

• Evolution concepts predate Darwin.

  • Farmers and breeders (dog/flower) noted variations in species and bred for desired traits.

  • Colonialism exposed Western scientists to diverse new species.

  • Darwin’s grandfather, Erasmus Darwin, proposed early evolutionary ideas.

Page 2: Influential Ideas for Evolution

• Jean-Baptiste Lamarck's Hypothesis:

  • Proposed that acquired characteristics could be inherited (e.g., giraffe necks).

  • Did not provide a mechanism for inheritance, leading to flaws in the theory.

• Charles Lyell:

  • Proposed the Earth is much older than previously thought, gradual geological change continues.

• Thomas Malthus:

  • Suggested human population growth is exponential, while resources grow linearly.

Page 3: Charles Darwin's Early Life

• Darwin (1809-1882) came from a wealthy family, with a physician father and a Wedgwood mother.

• His career path was uncertain; he shifted from medical school to clergy at Cambridge, ultimately becoming a naturalist on the H.M.S. Beagle at age 22.

Page 4: Darwin’s Voyage

• Key locations explored:

  • North America, South America, Galápagos Islands, Australia, New Zealand.

• Observations made during voyages contributed to his evolutionary theories.

Page 5: Development of Natural Selection

• Darwin's studies on the Beagle revealed:

  • Diverse habitats and species with adaptive traits.

  • Similar species in different regions fit local environments.

• Collaboration with Alfred Russell Wallace on the concept of natural selection.

Page 6: Darwin’s Theory of Evolution by Natural Selection

• Components of Darwin’s Theory:

  1. Heritable variation exists within populations.

  2. Organisms compete for finite resources (food, habitat, mates).

  3. Some variations enhance reproductive success.

  4. Traits that promote reproductive success spread, leading to adaptation.

  • Adaptation: characteristic advantageous for survival; also a process of fitting to the environment.

Page 7: Evidence of Evolution: Theoretical Context

• The Theory of Evolution by Natural Selection is supported by comprehensive evidence from different fields.

• Not merely a theory; incorporates extensive research findings from:

  • Fossils

  • Biogeography

  • Anatomy

  • Biochemistry

Page 8: Fossil Evidence

• Fossil ages can be dated using:

  • Relative dating (rock layers)

  • Isotope dating

• Fossilization conditions are scarce; specific requirements include hard body structures and appropriate burial conditions.

• Transitional fossils (e.g., Ambulocetus, Archaeopteryx) illustrate evolutionary links.

Page 9: Geological Timescale

• Earth is 4.6 billion years old, divided into four eons:

  1. Hadean: no life, high CO2, water vapor atmosphere.

  2. Archean: emergence of rock, first prokaryotic fossils (3.5 BYA).

  3. Proterozoic: eukaryotes appear (2.1 BYA).

  4. Phanerozoic: divided into

     • Paleozoic: marine life to land colonization.

     • Mesozoic: age of reptiles, rise of birds/mammals.

     • Cenozoic: diversification of modern life forms.

Page 10: Early Life Patterns

• Major periods and events:

  • Cambrian (488.3 MYA): emergence of non-vascular land plants.

  • Ordovician (443.7 MYA): marine algae flourish.

  • Devonian (416.0 MYA): first terrestrial vertebrates.

• Mass Extinctions:

  • Great Dying: 83% species extinction.

  • Other significant events affecting biodiversity.

Page 11: Evolutionary Patterns and Extinctions

• Mass Extinctions:

  • A significant decline in biodiversity.

  • Adaptive radiation follows extinctions.

  • Notable extinctions (e.g., Cretaceous asteroid impact killing dinosaurs).

  • Current species decline believed to be driven by human activity.

Page 12: Biogeography: Patterns in Species Distribution

• Biogeography examines species distribution patterns and adaptation.

• Continental drift has caused species lineage to spread and diverge.

• Historical climates have shaped current distribution.

Page 13: Biogeography Continued

• Island species show evidence of colonization and adaptation.

• Darwin’s finches and Hawaiian silver swords illustrate adaptive radiation based on environmental pressures.

Page 14: Anatomical Evidence

• Comparison of species anatomy reveals:

  • Homologous Structures: similar underlying structure due to common ancestry despite different functions.

  • Analogous Structures: similar function but different evolutionary origin.

Page 15: The Role of Developmental Biology

• Early embryonic development shows similarities among species.

  • Pharyngeal structures; human gills vs. fish.

  • Demonstrates common ancestry despite different adult forms.

Page 16: Biochemical Evidence

• Genetic similarities across species:

  • All organisms share DNA, ATP usage, and many proteins.

  • Mutations accumulate, leading to diversification over time.

Page 17: Population Genetics: Mechanisms of Microevolution

• Natural selection, mutations, migrations, and chance events drive microevolution.

• Population genetics studies allele frequencies and distributions.

Page 18: Hardy-Weinberg Equilibrium

• Established to explain variation in populations:

  • Conditions for equilibrium: no mutation, no migration, random mating, large population, no natural selection

• Shift away from HWE indicates evolutionary change.

Page 19: Evaluating Hardy-Weinberg

• Investigating allele frequencies provides a basis for formulating hypotheses.

• Practical applications in identifying potential evolutionary pressures within a population.

Page 20: Hard Weinberg Calculations

• Understanding allele frequencies in a population:

  • Simple dominant/recessive traits: A and a determination of frequencies (p and q).

Page 21: Troubleshooting HWE Problems

• Using phenotype frequencies to derive genotype frequencies:

  • Homozygous recessives (aa) provide a starting point for calculations.

Page 22: Agents of Evolution

• Key agents influencing variations in allele frequencies:

  1. Mutations: source of genetic variation, critical for selection.

  2. Genetic Drift: random shifts in allele frequencies.

  3. Gene Flow: movement of alleles between populations.

  4. Non-Random Mating: influences genotype frequencies.

  5. Natural Selection: primary mechanism aligning traits to environment.

Page 23: Genetic Drift and Its Effects

• Founder Effect: Loss of genetic diversity when new populations form.

• Bottleneck Effect: Population reduction leading to loss of variation impacting evolution.

Page 24: Gene Flow Dynamics

• Allows populations to become more alike; may introduce new traits.

• Essential for maintaining genetic diversity through crossover of alleles.

Page 25: Non-Random Mating Impacts

• Influences trait distributions in predictable ways:

  • Assortative mating increases homozygosity, while disassortative increases heterozygosity.

Page 26: Natural Selection Explained

• Mechanism requiring variation, competition, and reproductive success.

• Adaptations emerge and spread over generations; unfavorable traits are reduced.

Page 27: Measuring Natural Selection

• Fitness assessment as a measure of reproductive success:

  • High fitness associated with surviving offspring; factors like lifespan and mate attraction included.

Page 28: Patterns of Natural Selection

• Selection pressures on traits result in diverse effects, including

  • Disruptive, directional, and stabilizing selection pressures affecting phenotype distributions.

Page 29: Maintenance of Variation

• Variation persists despite natural selection, due to:

  • Frequency-dependent selection and oscillating selection strategies that shield against fixation.

Page 30: Heterozygote Advantage

• Maintains genetic diversity by favoring heterozygotes in certain environments, as seen with sickle-cell anemia in malaria-prone regions.

Page 31: Macroevolution and Species Definition

• Defining species through the Biological Species Concept (BSC): capable of mating and producing fertile offspring.

• Maintained through reproductive isolation mechanisms.

Page 32: Mechanisms of Reproductive Isolation

• Prezygotic isolation: includes ecological, behavioral, temporal, mechanical, and gametic barriers.

• Postzygotic isolation: zygote mortality, hybrid sterility, and reduction in F2 fitness impact species continuation.

Page 33: Paths of Speciation

• Allopatric speciation: geographic separation leading to genetic divergence.

• Sympatric speciation: new species formation without geographic isolations, often through mechanisms like polyploidy.

Page 34: Adaptive Radiation

• Evolution of diverse species from a common ancestor in varying habitats, following extinction events or colonization of new environments.

Page 35: Evolutionary Timeframes

• Gradualism: conception of slow, gradual evolution over time.

• Punctuated equilibrium: rapid bursts of evolution interspersed with longer periods of stability in species forms.

Conclusion

• Evolutionary theory is a dynamic framework grounded in evidence from various biological fields, illustrating the intricate processes that shape life on Earth.