Chapter 11 Part 3: Evolution – Evidence & Speciation

Evolution = change over time. This lecture focuses on the evidence that convinces biologists that evolutionary change is real, measurable, and foundational to the life sciences.
Four major lines of evidence covered:
1. Fossil record
2. Homologous structures (and the contrasting concept of analogous structures)
3. Vestigial structures
4. Speciation patterns revealed by classical observation & modern molecular genetics

Definition: the cumulative catalog of once-living organisms preserved in rock.
Key caveat: highly incomplete
- Requires a specific sequence of events to fossilize:
1. Organism dies in or near a depositional environment (e.g.
  stream, river bed, shallow sea, floodplain).
2. Rapid burial by sediment → shields remains from scavengers/decay.
3. Sediment undergoes compaction & mineral replacement → lithification → fossil.
4. Geological uplift/erosion later exposes rock for discovery.
- Because of those strict conditions, only ≈ $1\%$ of all species that ever lived are represented.
Common criticisms (“missing links”) stem from this incompleteness rather than conceptual error.
Illustrative fossils & anecdotes
- Archaeopteryx – earliest fairly complete “bird” fossil; shows reptile-like teeth & tail + avian feathers → famous transitional form; instructor viewed an original in the British Museum.
- Trilobites – marine arthropods present for hundreds of Myr; 99 % of fossils preserve only the dorsal exoskeleton; ventral soft parts rarely fossilize.
- Mammoth bones in Waco canal & Triceratops horn in Montana—examples of modern exposure events after erosion.
Fossil sequences reveal gradual transformation
- Example: jaw evolution in fishes – gill-support bones migrated forward & became articulated, forming functional jaws.

Share common ancestry, even if modern function differs.
Classic limb example (forelimb pentadactyl plan):
- Human arm → $\text{humerus} \to \text{radius/ulna} \to \text{carpals} \to \text{metacarpals} \to \text{phalanges}$
- Bat wing – elongated fingers support flight membrane.
- Whale flipper – shortened, thickened bones for paddling.
- Turtle forelimb – same bone order despite reptilian shell.
Pattern shows divergent evolution: one ancestral blueprint radiated into different adaptive outcomes.

Superficially similar because of similar selective pressures, not because of recent shared ancestry.
Flight evolved at least four independent times: birds (dinosaurs), bats (mammals), pterosaurs (extinct reptiles), and insects (arthropods).
Hydrodynamic “torpedo” body evolved convergently in penguins, fish, sharks, whales, etc.
Porcupine quills – African vs. North-American porcupines: similar defense but genetically distant; discovered via modern genetic work.
Process = convergent evolution: distinct lineages “converge” on similar adaptations.

Definition: anatomical remnants inherited from ancestors but no longer serve original function (or any function).
Examples:
- Human coccyx – tiny tail vertebrae; primate lineage lost external tail yet bones remain.
- Cetacean pelvic & femur remnants – whales descended from land mammals; hind limbs reduced because they increased drag; pelvic vestiges visible in museum skeletons.
- Appendix once classed as vestigial; now known to play an immune role → caution: some “vestiges” can regain or retain function.

Isolation of gene pool – geographic barrier (e.g.
river splits beetle population).
Independent evolution – each sub-population adapts to local conditions; allele frequencies diverge.
Reproductive isolation develops – when contact resumes, crosses fail or hybrids are non-viable/infertile → new species A & B.

A species = group of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups.
Fertile hybrids required for same species status.
Illustrative cases:
- Horse $\times$ Donkey → Mule (infertile) ⇒ horses ≠ donkeys.
- Domestic dog now treated as Canis lupus familiaris (subspecies of wolf) because fertile back-crossing occurs.
- Warbler songbirds form fertile hybrids yet still taxonomically split → demonstrates gray areas.

Uses DNA similarity to delineate species & reconstruct phylogenies.
Fast-evolving genes (e.g.
mitochondrial DNA) resolve recent splits; maternally inherited; mutates rapidly.
Slow-evolving genes (nuclear, conserved) resolve deep divergences.
Example genus Pyrrhura (South American parakeets)
- Morphologically similar birds split into multiple species & subspecies by mitochondrial data.
- “Clade” term used for monophyletic genetic groupings.
Taxonomic culture: “lumpers” (prefer fewer, broader species) vs. “splitters” (recognize many narrowly defined species).

Slide showed two bird species:
- Each has $22$ autosome pairs + sex chromosomes $X$ and $Y$ .
- Comparative karyotype coloring reveals large-scale similarity (homologous blocks) yet differences such as inversions & extra unsequenced regions.
- Despite near-identity, they’re still classified as distinct species—underscores subjectivity of species boundaries.

Apparent “gaps” in fossil evidence expected; not valid grounds to reject evolution.
Homology provides predictive power: knowing bone layout in mammals helps interpret new fossils.
Vestigial features are historical “footnotes” demonstrating descent with modification.
Species definitions affect conservation (legal protection depends on taxonomic status), agriculture (breed vs.
species), and ethics (e.g.
reintroducing wolves).