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Chapter 1 - Evidence for Evolution

Overview of Evolution Concepts

  • Evolution definition: all the changes that have occurred in living things since the beginning of time

  • Core ideas:

    • Common descent: all life shares a common ancestor

    • Adaptation: organisms become better suited to their environment

    • Evidence across multiple lines supports evolution: fossil, biogeographical, anatomical, biochemical, and molecular data

  • Key takeaway: DNA as the universal genetic code links all living things; we have evolved from shared ancestors from single-celled organisms to the diversity seen today

  • Example-driven framing: for each type of evidence, there are concrete examples discussed in the slides (Tiktaalik, Archaeopteryx, Galapagos finches, etc.)

Fossil Record

  • Definition: fossil evidence as a record of past life and transitions over geological time

  • Time scale (typical fossil strata, from older to newer):

    • Paleozoic era: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian

    • Mesozoic era: Triassic, Jurassic, Cretaceous

    • Cenozoic era: Paleogene (including Paleocene, Eocene, Oligocene), Neogene (including Miocene, Pliocene), Quaternary (Recent, Pleistocene, Holocene)

  • Primary ideas from fossils:

    • Transitional forms show gradual change over time

    • Fossils document major evolutionary steps (e.g., water-to-land transition, rise of flight)

  • Specific fossils mentioned:

    • Tiktaalik roseae (~$375$ million years ago) as a transition between fish and tetrapod vertebrates

    • Archaeopteryx (~$155$ million years ago) as a transition toward birds during the Jurassic

    • Xiaotingia zhengi (later transitional bird-like fossil from the Jurassic)

  • Significance: fossil evidence provides direct snapshots of how major body plans and features evolved over deep time

Tiktaalik roseae

  • Transitional fossil bridging fish and tetrapod features

  • Age: approx. 375 imes 10^6 ext{ years ago}

  • Fish-like traits: gills, scales, fins

  • Tetrapod-like traits: shoulder, elbow, wrist, mobile neck

  • Source: Shubin et al., Nature, April 6, 2006

  • Significance: demonstrates a gradual progression from aquatic to terrestrial locomotion and anatomy

Transition to Flight

  • Archaeopteryx as a classic transitional fossil toward birds

  • Age: approx. 155 imes 10^6 ext{ years ago} (Jurassic)

  • Dinosaur-like features: teeth, claws, long bony tail, abdominal ribs

  • Bird-like features: feathers, wings

  • Also noted: Xiaotingia zhengi (Nature, 2011) as another Jurassic avian fossil contributing to the flight-transition narrative

  • Significance: supports the idea that bird evolution involved gradual acquisition of flight-related traits from theropod dinosaurs

Biochemical Evidence

  • Molecular biology and genomics underpin evolution through DNA sequence comparison

  • Key points:

    • Humans and mouse share essentially all genes (near 100%), indicating close evolutionary relationship

    • Humans and fruit fly share ~60% of genes

    • Humans and nematode share ~40%

    • Humans and yeast share ~31%

  • Interpretation: the degree of gene sharing aligns with the expected evolutionary relationships from a common ancestry

  • Implication: large-scale genetic conservation across diverse lineages is consistent with descent with modification

Anatomical Evidence

  • Homologous structures:

    • Similar skeletal elements reflecting shared common ancestry

    • Example: mammalian forelimbs across human, cat, whale, bat show homologous arrangement despite different functions

  • Analagous structures:

    • Similar in appearance/ function but not due to shared ancestry

    • Example: wings of insects vs birds have similar function but different structural origins

  • Significance: homologous structures support common descent; analogous structures reflect similar selective pressures rather than shared ancestry

Biogeographical Evidence

  • Core idea: the geographic distribution of species reveals patterns consistent with evolutionary history

  • Key patterns:

    • Correlation between species similarity and geographic proximity

    • Historical breakup of supercontinents explains broad distribution (e.g., Pangaea, Laurasia, Gondwana)

  • Illustrative timeline:

    • ~300 million years ago: Pangaea existed; Laurasia and Gondwana separated over time

    • ~120 million years ago: Landmasses drifted to form modern continents (North America, South America, Eurasia, Africa, Antarctica, Australia)

  • Figure: distribution patterns in the Galapagos and other regions reflect vicariance and dispersal events tied to continental drift and isolation

  • Significance: biogeography supports the idea that species diverged after populations became geographically isolated, leading to speciation

Biogeography in Practice: Example with Galapagos Mockingbirds and Related Birds

  • Observations on the Galapagos archipelago (and related lineages in nearby islands) show how geography shapes speciation and divergence

  • Darwin’s inference: speciation can be driven by geographic separation and local ecological conditions

  • Conceptual takeaway: geographic context is a strong driver of evolutionary change, reflected in modern distributions of related species

Agents of Evolutionary Change

  • Mutation: introduces new alleles and genetic variation

  • Genetic drift:

    • Random changes in allele frequencies over generations

    • Founder effect: when a small group establishes a new population

    • Bottleneck effect: population decline followed by shifts in allele frequencies among survivors

  • Gene flow: movement of alleles between populations (e.g., migration of birds)

  • Non-random mating: assortative mating or inbreeding affecting allele frequencies

  • Natural selection (Darwin):

    • Variation exists within populations

    • Heritable traits pass to offspring

    • Differential survival and reproduction based on adaptation to environment

    • Result: alleles that confer better adaptation increase in frequency over generations

  • Summary: these mechanisms shape the genetic makeup of populations over time and drive evolution

Charles Darwin: Core Concepts and Reasoning

  • Publication: On the Origin of Species (1849)

  • Two main concepts:

    • Descent with modification: species arise from a succession of ancestors

    • Natural selection: differential survival and reproduction based on heritable variation

  • Inductive reasoning leading to conclusion:

    • Individual variation with heritable traits exists within populations

    • Populations produce more offspring than the environment can support (struggle for existence)

    • The most fit individuals contribute more offspring, leading to adaptation over generations

  • Practical implication: natural selection explains how evolution occurs through differential reproductive success

Darwin’s Examples: Galapagos Islands

  • Case study: finches and tortoises on the Galapagos isolated on different islands

  • Conclusion: ecological differences among islands drive divergence and speciation

  • Key idea: ecology helps explain why related species differ in traits across islands

Galapagos Finches: Evidence of Adaptive Radiation

  • Groups discussed: ground finches (Geospiza) and tree/warbler-type finches (Camarhynchus, Certhidea, etc.)

  • Beak variation across species reflects specialization for different food sources (seeds, insects, cactus, etc.)

  • Notable finch groups and beak associations:

    • Ground finches (Geospiza): large beaks for hard seeds (e.g., Geospiza fortis, Geospiza magnirostris), medium and small beaks for a range of seeds

    • Large vs small ground finches and cactus-eating finches reflect dietary diversification

    • Tree finches and warbler-type finches show insectivory and nectar/flower-related feeding

    • Mangrove finch and other specialized forms illustrate ongoing adaptive divergence

  • Genus-level notes:

    • Geospiza: main group of ground finches

    • Camarhynchus: warbler/nectar/seed eaters within tree finches

    • Certhidea: warbler-like finches within tree finches

  • Visual takeaway: beak size and shape change is a central adaptive response to food availability; this is a vivid modern example of natural selection in action

  • Overall message: the Galapagos finches illustrate how geographic isolation and ecological opportunity can drive diversification from a common ancestor

CHNOPS

  • Research prompt for Monday's class: What is CHNOPS?

  • CHNOPS stands for the six essential elements for life on Earth:

    • Carbon (C)

    • Hydrogen (H)

    • Nitrogen (N)

    • Oxygen (O)

    • Phosphorus (P)

    • Sulfur (S)

  • Significance: these elements form the backbone of biomolecules (carbohydrates, proteins, lipids, and nucleic acids) and are central to biochemistry and the study of life’s chemistry

Connections to Foundational Principles

  • Common descent is supported by multiple lines of evidence: fossils, comparative anatomy, biogeography, and modern genomics

  • The fossil record provides snapshots of failed and successful transitions (e.g., fish to tetrapod, non-avian dinosaurs to birds)

  • Biogeography ties distribution patterns to historical continental configurations and migration, reinforcing the role of geography in evolution

  • Molecular data (DNA sequences) align with evolutionary trees and provide quantitative backing for relationships inferred from morphology

  • Darwinian natural selection is integrated with other evolutionary mechanisms (mutation, drift, gene flow, non-random mating) to explain how populations evolve over time

Practical Takeaways for Study

  • Be prepared to explain, with examples, how each type of evidence supports evolution:

    • Fossil: Tiktaalik and Archaeopteryx as transitional forms

    • Biogeographical: Pangaea and subsequent continental drift shaping species distributions; Galapagos patterns

    • Anatomical: homologous vs analogous structures (forelimb example)

    • Biochemical: shared genes across diverse taxa and the percentages cited

  • Remember key dates and dates-relative figures for major transitions:

    • Fish-to-tetrapod transition: 375 imes 10^6 ext{ years ago}

    • Flight transition (dinosaurs to birds): 155 imes 10^6 ext{ years ago}

    • Pangaea breakup timing: around 300 imes 10^6 ext{ years ago}; Laurasia and Gondwana split around 120 imes 10^6 ext{ years ago}

  • Review Darwin’s arguments for descent with modification and natural selection, and be able to outline the four components Darwin highlighted for natural selection (variation, inheritance, differential adaptation, differential reproduction)

  • Understand CHNOPS as a shorthand for the elemental basis of life and its relevance to evolution and biology