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Evolution → "descent with modification"
1. Historical Background
• Existence of life originally explained by creationism/religion/supernatural intervention
• Naturalistic models (based on logic, philosophy): new species are a result of natural causes only
◦ Plato (428BC): species are unchanging, fixed.
◦ Aristotle (384BC): developed "scale of nature" → natural world arranged on ladder, showing progression
from imperfect to most perfect (increasing complexity: inanimate matter to humans).
• Evolutionary ideas made comeback in 18th century.
• 1831 HMS Beagle: Charles Darwin travels to SA, Aus, South America.
◦ Studied, logged local plants/animals and geological formations
◦ Galapagos Islands, 950km from S. America:
‣ Mainland had 1 species, while islands had 14.
‣ Species were similar but nonidentical finches, each well-suited for its environment/role
• Ate large seeds = large, tough beaks. Ate insects = thin, sharp beaks, etc.
‣ Observed similar patterns with tortoises (differing in shell shape) and many plants
‣ "What is the mechanism by which each population acquired adaptations?"
◦ Argentina, S. America:
‣ Found fossils similar to modern forms
• e.g Glyptodonts → extant Armadillos
‣ "Why have species gone extinct only to be replaced by more successful ones?"
• Key influencers
◦ Carl Linnaeus: the founder of taxonomy, the nested system of classifications
◦ George Cuvier: recognised fossils as evidence of earlier life, established extinctions as fact
◦ Charles Lyell: friend of Darwin, influenced his thinking about gradual bio evolution
‣ promoted uniformitarianism: geological features caused by gradual change, not catastrophic events
◦ Jean Baptiste de Lamarck: the main evolutionary theorist before Darwin
‣ Stated evolution takes place, suggested an gradualism mechanism
• 'Species are plastic, can be modified'
• 'The disuse/use of a structure can be passed on', e.g giraffes have long necks due to ancestors
stretching them a lot
‣ Incorrect mechanism: acquired characteristics not inherited
◦ Thomas Malthus: interested in growth of human populations
‣ Stated factors that limit exponential population growth (disease - MALaria! - and food).
‣ Made Darwin realise that far more offspring are produced than can survive; competition for resources
• Influences (DADFGG)
◦ Diversity of life
◦ Adaptations
◦ Distributed species differ in different regions
◦ Fossil and extant forms
◦ Geology
◦ Galapogos Islands research
• Took a scientific approach: induction (form hypothesis, facts), deduction (if true, certain things should follow),
and testing
Notes: Don't need to know Charles Darwin biography.
Alfred Wallace independently reached similar conclusions to Darwin at roughly the same time.
, 2. Evidence for Evolution
Deduction phase of testing: certain things should be true, if evolutionary hypothesis is correct.
A) The Earth should be old
• Earth was created about 4.5 bn years ago. Dated via:
◦ Zircon rock in Australia (4.1 bn years old), moon rocks, meteorites, fossils
‣ Using radiometric dating: relies on half-life decay of radioactive elements to date rocks and materials
‣ Using stratigraphy: the study of rock layers from which relative dates can be extrapolated
• Life began nearly 4 bn years ago
◦ While the oldest known rocks/fossils are approximately 3.5 bn years old...
‣ 3.5byo Stromatolite (SA): layers of cyanobacteria that trap/precipitate sediments
‣ 2.1 byo Francevillian biota (Gabon): first multicellular life
◦ Miller & Urey generated amino acids, biomolecules under early-Earth conditions (suggesting it began earlier!)
‣ Organic molecules could have been introduced via primordial soup, meteorites (found with organic
traces), or hydrothermal events
‣ Panspermia: life exists universally, is distributed via space dust, meteoroids, comets
B) Fossil evidence should reflect evolution
• Fossil: preserved remains of ancient organisms
◦ Calcium/bone minerals replaced with rock via diagenesis (∴ only hard parts preserved)
◦ Gives info on patterns, successive evolutionary change, origin, diversity, extinction
‣ BUT very incomplete - most organisms never fossilize
• Records help document extinct species & histories of extant species, e.g Horse fossil records indicate changes in
lineage: reduction of toed feet to hooves
• Shows how new groups arise via gradual modification of existing
◦ Reflect change in composition
◦ Reflect change in complexity
◦ Reflect increasing diversity
C) Characteristics should reflect evolutionary tree
• 'Descent with modification" → history of life forms a branching tree, where all 3 domains of life are united by
LUCA.
◦ This evolutionary theory predicts more related groups have more recent common ancestors
‣ ∴ morphological, physiological, biochemical, and genetic similarities may reflect shared evolutionary
ancestry. Similar characteristics due to relatedness are homologies.
‣ Can work backwards to find relation based on shared features
• Morphological/anatomical
◦ Homologous characteristics: derived from the same structure or shared due to evolutionary history/
shared ancestor.
‣ Don't necessarily have same function, but same structure, relationship to other features, and
development
‣ E.g While forelimbs of whales, humans, birds, and dogs look different ( adapted to function in different
environments), the bone structure is very similar ∴ likely all share a common ancestor
‣ E.g the forelimb of tetrapods all share the same set of bones.
‣ Vestigial structures: reduced/non-functional structures that are homologous to useful structures in
other organisms. Evolutionary 'leftovers'.
• E.g human tailbone (vestigial tail), whale hind limbs, python limb bones
◦ Analagous characteristics: superficially similar but have independent evolutionary origins
‣ A result of convergent evolution: organisms not closely related evolving similar traits as a result of
having to adapt to similar environments/ecological niches.
• E.g Shark (evolved from fish), Ichthyosaur (from reptile), and Porpoise (from mammal) have very
similar body shape that helps rapid swimming; bat and bird wings; marsupial and placental mole
body structure
‣ Homoplasy: character thats shared by a set of species, but not present in their common ancestor
