Lecture 4 – Biogeography (196.101 Ecology, Evolution & Behaviour)

Announcements

No laboratories scheduled this week.
First assignment due Friday, $25\,\text{July}$ .
Next week’s practical: simulating evolution in R.
- Re-do last week’s R exercises to reinforce core commands and logic.

Learning Outcomes & Key Concepts

By the end of the lecture students should be able to:
- Define “adaptation” and identify examples.
- Explain why individuals do not act for “the good of the species.”
- Describe biogeography and its governing factors.
- Summarise the island-pattern evidence for natural selection.
- Locate and interpret Wallace’s Line.
Vocabulary to master:
- Adaptation; Group Selection; Endemism; Island Biogeography; Continental Drift.
Focal taxa/clades used as illustrative material: Vangas, Newtonias, Galápagos finches, mammals.

Review: Natural Selection in Four Postulates

1. Potential for organisms to increase geometrically, yet populations remain bounded ⇒ a “struggle for existence.”
2. Individuals within a species show variation.
3. Variations are heritable.
4. Variants that raise survival or reproduction become more common over time.

Empirical Example – Galápagos Finches

Study of Geospiza fortis (medium ground finch):
- Comparison of predicted vs. observed evolutionary responses following droughts $1976!–!1977$ and $1984!–!1986$ .
- Plot showed tight correspondence (data from Tables 3 & 5) ⇒ validates selection models.
Key message: Evolution lacks foresight; change is contingent on immediate selective pressures.

Defining Adaptation

An adaptation = a phenotypic trait “moulded” by natural selection to perform a specific function.
- Maintained because of the fitness advantages it confers.
Two cautionary questions:
- Do animals perfectly fit their environments?
- Can we over-interpret every trait as adaptive? (Risk of so-called “Just-So Stories.”)
Example slide: Rudyard Kipling reference signals dangers of unfalsifiable adaptation stories.

Interrogating Adaptive Hypotheses

“Selfish herd” vs. “defence against tsetse flies” ⇒ alternative functions for tight herd formation.
Extremely variable plumage in male red-billed quelea – multiple competing hypotheses (mate choice, predation confusion, etc.).
Ongoing Massey research: Isocladus armatus (intertidal isopod)
- Displays striking colour polymorphism ⟶ why maintained? (predation, thermoregulation, sexual selection?).
Sex ratios: across metazoans ~ $50!:!50$ ; explained by frequency-dependent selection (Fisherian logic).
- NZ population pyramid (Stats NZ, $30\,\text{June }2018$ ) illustrates real-world near-parity.
Horsehair worm / wētā: water-seeking behaviour benefits parasite, not host. Highlights need to identify the “unit of benefit.”

Evolutionary (Darwinian) Medicine

Applies evolutionary thinking to health/disease.
- Example phenomena: fever, pain – may be evolved defences not pathologies per se.

Group Selection vs. Individual Selection

Natural selection does not act for “species-level good.”
Group selection hypothesis: selection at group level; rare and contentious.
Iconic misinterpretations:
- Disney “lemmings” myth – staged mass mortality.
- Lion infanticide: occurs when new males take over a pride; individual fitness (inducing estrus), not population regulation.
Dobzhansky quote: “Nothing in Biology Makes Sense Except in the Light of Evolution” – underscores centrality of evolutionary framework.

Biogeography – Core Definition & Drivers

Scientific study of species’ distributions in space and time.
Influencing processes:
- Speciation, Extinction
- Continental Drift (plate tectonics)
- Glaciation cycles
- Isolation/barriers (water, mountains)
- Landmass area, elevation, latitude

Island Biogeography – Why Islands Are Ideal Natural Experiments

Islands express condensed, decipherable patterns.
High levels of endemism (species found nowhere else).
- Example: Hawaiian Happy-Face Spider.
Darwin’s Voyage of the Beagle (1831–1836) revealed pattern-based evidence for natural selection; devoted two Origin chapters to geographic distribution.

Nine Classic Oceanic-Island Patterns (after Darwin)

Fewer species in total vs. continental areas of equal size.
- Galápagos ( $8000\,\text{km}^2$ ): $26$ terrestrial bird spp.
- Isle Royale ( $535\,\text{km}^2$ ): $261$ terrestrial bird spp.
High proportion endemic.
- Galápagos: $21/26$ birds endemic.
- Isle Royale: $0/261$ endemic.
  3a. Entire higher taxa absent; ecological niches taken by substitutes (e.g., reptiles replace mammals in Galápagos; giant moa in NZ filled large-herbivore role).
  3b. Terrestrial mammals absent on islands > $300$ mi from mainland, yet bats (aerial mammals) present almost everywhere (NZ’s two endemic bats).
  3c. Amphibians virtually absent; sea-water lethal to eggs/larvae ⟶ exception: few NZ frogs.
Endemics often retain non-useful traits (flightless cormorant; barbed seeds adapted for mammal fur yet mammals absent).
Endemics may evolve novel adaptive traits (Galápagos marine iguana algae-grazing).
Strong affinity to nearest mainland fauna/flora, though not identical.
Within an archipelago, island species are more closely related to each other than to mainland relatives (e.g., Galápagos finches, tortoises).
Principle of “colonisation from the nearest source.”
Implied requirement: at some past time, a single ancestral population spanned source and island; subsequent isolation drove divergence.

Darwin analogised Time and Space: divergence among modern spatially separated species mirrors temporal descent.

Summary Statement

Observations: endemicity, dispersal filters, mainland affinities, intra-archipelago relatedness.
Conclusion: islands are colonised by mainland founders that then diversify via natural selection and drift.

Case Study – Newtonias & Vangas of Madagascar

Newtonia genus (4 spp.) long placed in Sylviidae/Muscicapidae.
Molecular phylogeny (Yamagishi et al. 2001; $882$ bp of mt 12S + 16S rRNA) shows:
- Monophyly of Vangidae includes Tylas (formerly Pycnonotidae) and Newtonia.
- Radiation is in situ; diversity not due to multiple colonisations.
- Adaptive explosion into vacant niches parallels Darwins’ finches.
Representative species list (partial): Xenopirostris damii, Artamella viridis, Oriolia bernieri, Falculea palliata, Vanga curvirostris, Euryceros prevostii.

Alfred Russel Wallace & the Malay Archipelago

Conducted extensive surveys of Amazon Basin & SE Asia; deemed “father of biogeography.”
Independently formulated natural selection; biogeography central to his evidence.
Proposed Wallace’s Line: sharp faunal turnover between Bali–Lombok & Borneo–Sulawesi regions.

Understanding Wallace’s Line

Reflects underlying continental shelves:
- Sunda Shelf (Asia): Borneo, Sumatra, Java, Bali—all once connected during low sea levels.
- Sahul Shelf (Australia): Australia & New Guinea connected.
- Islands in between (Wallacea) were never land-bridges, limiting terrestrial dispersal.
Continental drift + fluctuating sea levels produced persistent biogeographic barrier.
Multiple demarcations: Huxley’s original line, Mayr’s modification, Weber & Lydekker lines refine transitional zones.

Global Biogeographic Realms (Wallace, later formalised)

Nearctic, Neotropical, Palearctic, Ethiopian, Oriental, Australian.
Wallace’s Line separates Oriental and Australian realms.

Convergent Ecologies vs. Historical Contingency

Similar environments at great distances often house ecological analogues (e.g., marsupial vs. placental predators) that are distantly related.
Evolutionary history explains “who lives where” better than environment alone.

Final Thoughts & Further Reading

Species distributions make coherent sense only when interpreted through evolution by natural selection.
Recommended: David Quammen – The Song of the Dodo: Island Biogeography in an Age of Extinction; integrates theory with conservation crises.

Numerical & Quantitative References (LaTeX formatted)

Assignment due date: $25\,\text{July}$ .
Island–mainland dispersal limit for mammals: >300\,\text{miles}.
Areas: Galápagos $8000\,\text{km}^2$ ; Isle Royale $535\,\text{km}^2$ .
Species counts: Galápagos birds $26$ (endemic $21$ ); Isle Royale birds $261$ (endemic $0$ ).
Sex ratio expectation: $1:1$ (i.e., $50\%$ males, $50\%$ females).
mtDNA alignment length: $882\,\text{bp}$ .
Study periods for finch selection: $1976!–!1977$ ; $1984!–!1986$ .

These bullet-point notes encapsulate every salient argument, example, statistic, and conceptual link presented across the 53-slide transcript, providing a stand-alone study resource for the upcoming exam.