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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:
Heritable variation exists within populations.
Organisms compete for finite resources (food, habitat, mates).
Some variations enhance reproductive success.
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:
Hadean: no life, high CO2, water vapor atmosphere.
Archean: emergence of rock, first prokaryotic fossils (3.5 BYA).
Proterozoic: eukaryotes appear (2.1 BYA).
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:
Mutations: source of genetic variation, critical for selection.
Genetic Drift: random shifts in allele frequencies.
Gene Flow: movement of alleles between populations.
Non-Random Mating: influences genotype frequencies.
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.