Speciation & Phylogeny: Comprehensive Study Notes

Speciation – Core Definition and Requirements

  • Speciation: evolutionary process that yields new, distinct species.

    • Requires BOTH:

    • Interruption of gene flow between sub-populations.

    • Subsequent genetic divergence that leads to reproductive isolation (RI).

  • RI = two groups can no longer exchange genes; once complete, the lineages are different species.

  • Not every evolutionary change = speciation; it only occurs when RI is established.

Species Concepts – Multiple Perspectives

  • Morphological Species Concept (MSC)

    • Origin: Linnaeus; species delimited by shared morphological traits.

    • Rationale: phenotypic similarity reflects many shared alleles.

    • Limitations:

    • Sexual dimorphism, ontogenetic change, cryptic species.

    • Members of same species may not always “look alike.”

  • Cryptic Species

    • Two (or more) lineages are morphologically indistinguishable yet reproductively isolated (do not interbreed).

  • Biological Species Concept (BSC)

    • Species = populations that actually or potentially interbreed in nature and produce viable, fertile offspring.

    • Key foundation: reproductive isolation, not similarity of appearance.

    • Potential limitations:

    • Asexual organisms, fossils, geographically separated (“allopatric”) populations whose interbreeding potential is unknown.

  • Lineage Species Concept (LSC)

    • Species = distinct branches on the tree of life (ancestor–descendant lineages followed through time).

    • Useful in paleontology & molecular phylogenetics.

    • Limitations: requires phylogeny; may over-split recently diverged populations.

  • Summary: each concept highlights different speciation attributes; biologists often triangulate across multiple definitions.

Geographic Relationships & Core Modes of Speciation

(visual in transcript p.11)

  • Allopatric – physical barrier.

  • Peripatric – small peripheral isolate (founder effect emphasized).

  • Parapatric – populations occupy adjacent, partially isolated niches.

  • Sympatric – divergence within the same geographic area (no extrinsic barrier).

1. Allopatric Speciation (Geographical Isolation)

  • Barriers form via continental drift, sea-level fluctuation, glaciation, mountain uplift, etc.

  • Divergence drivers: genetic drift, differing selection regimes in separate environments.

  • Founder-effect variant: a few individuals cross an existing barrier → isolated population (e.g., Darwin’s finches, Grand Canyon squirrels, Panama snapping shrimp).

  • Example data (Fig. 17.6): Pliocene isolations produced sister fish pairs east vs. west of Mississippi drainage (bleeding vs. warpaint shiners, etc.).

2. Sympatric Speciation (Reproductive / Ecological Isolation)

  • Occurs without geographic barriers.

  • Mechanisms:

    • Disruptive selection + assortative mating by micro-habitat or resource use (e.g., apple- vs. hawthorn-maggot flies; Lake Apoyo cichlids; resident vs. transient killer whales).

    • Polyploidy—doubling of chromosome sets.

    • Most common route in plants; can be instantaneous.

    • Example: gray treefrog complex \textit{Hyla chrysoscelis} (diploid) vs. \textit{Hyla versicolor} (tetraploid).

Reinforcement – Strengthening Isolation after Secondary Contact

  • If incipient species re-meet while RI is incomplete, hybrids often exhibit reduced fitness.

  • Natural selection favors traits that avoid hybridization, thereby reinforcing prezygotic barriers.

  • Outcomes of secondary contact (p.35-36):

    1. Fusion – lineages merge.

    2. Reinforcement – selection against hybrids increases RI.

    3. Stable / transient hybrid zone – hybrids persist locally; parental species remain distinct.

    4. Hybrid speciation – hybrids form a new, reproductively isolated lineage.

Prezygotic Isolating Mechanisms (Often Reinforced in Sympatry)

  • Mechanical isolation – mismatched genitalia or floral structures.

    • Snail chirality (left- vs. right-handed): genital mismatch prevents copulation.

    • Plant example: Cryptostylis orchids and specialized wasp pollinators.

  • Temporal isolation – different breeding seasons/times (Eastern vs. Western spotted skunks).

  • Behavioral (Ethological) isolation – divergence in courtship signals (Eastern vs. Western meadowlark songs).

  • Habitat (Ecological) isolation – preference for different microhabitats despite overlap (aquatic vs. terrestrial garter snakes).

  • Gametic isolation – egg–sperm incompatibility, common in broadcast spawners (purple vs. red sea urchins).

  • Empirical pattern: prezygotic barriers are stronger in sympatric species pairs (e.g., Rana frogs) than in allopatric pairs.

Postzygotic Isolating Mechanisms

  • Low hybrid zygote viability – fertilized eggs fail to develop.

  • Low hybrid adult viability – hybrids survive poorly.

  • Hybrid infertility – hybrids sterile (e.g., mule = horse × donkey).

  • Hybrid breakdown – F₁ fertile but F₂ inviable/infertile (found in some copepods).

Comparative Summary: Allopatric vs. Sympatric Speciation (p.33)

  • Barrier? Allopatric = extrinsic geographic; Sympatric = none.

  • Primary mechanism: Allopatric → natural selection + drift; Sympatric → polyploidy or disruptive selection.

  • Tempo: Allopatric usually slower; Sympatric can be rapid (especially autopolyploidy).

  • Taxonomic prevalence: Allopatric common across taxa; Sympatric well documented in plants, select animals.

  • Example taxa: Darwin’s finches vs. cultivated wheat, corn, tobacco, African tilapia.

Incipient Species & Hybrid Zones

  • Incipient species: diverging populations on the verge of full speciation.

  • Hybrid fitness often low → selection for reinforcement.

  • Hybrid zones can be stable, transient, or yield new hybrid species via recombination.

Patterns of Macroevolution

  • Divergent evolution – accumulation of differences → speciation.

  • Convergent evolution – unrelated lineages evolve similar traits (analogous structures) due to similar selection pressures.

  • Parallel evolution – closely related lineages evolve similar traits independently.

  • Coevolution – reciprocal evolutionary change between interacting species (e.g., crab claws ↔ snail shell thickness arms race).

  • Adaptive radiation – rapid diversification into multiple niches following ecological opportunity.

  • Gradualism vs. Punctuated Equilibrium

    • Gradualism: slow, uniform change.

    • Punctuated equilibrium: long stasis with brief bursts of rapid speciation.

    • Diagram (p.46) contrasts continuous vs. step-like lineage splitting.

Phylogeny – Reading & Building Trees

  • Phylogenetic tree terminology

    • Root, branch, node (common ancestor), clade, taxon.

    • Clade: ancestor + all descendants (monophyletic group).

  • Many graphical layouts convey identical relationships; branch rotation around nodes does not alter relationships (p.48).

  • Characters placed at nodes mark evolutionary origin (e.g., amniotic egg originates at ancestral node 1 and inherited by all amniotes).

  • Data sources for phylogenies:

    • Fossil record, morphological traits, molecular (DNA/RNA/protein) data.

  • Goals:

    • Reconstruct evolutionary relationships; test hypotheses about trait evolution, biogeography, and diversification rates.

  • Interpretation example (practice Q p.54): identify basal taxa, sister groups, relative relatedness.

Practice / Review Questions (p.50-55) – Key Answers

  1. C) Allopatric speciation.

  2. A) Sympatric speciation.

  3. D) Disruptive selection.

  4. A) Reinforcement (hybrids less fit than parents).

  5. A) Parapatric speciation (gradient environment).

  6. B) Understand evolutionary relationships.

  7. D) All of the above.

  8. A) A node = common ancestor.

  9. A) Clade.

  10. B) Homologous structures provide evidence of common ancestry.

  • Tree reading question (p.54): Incorrect statement = C) Lizards are more closely related to salamanders than to humans (because lizards share more recent ancestry with goats/humans than with salamanders).

  • Phylogram comparison (p.55): All trees with identical branching order represent same relationships; students must match topology, not graphic orientation.

Numerical / Statistical Elements (explicit)

  • No specific formulas presented, but reinforcement & hybrid fitness often modeled with selection coefficients, e.g. w{\text{hybrid}} < w{\text{parent}}.

  • Polyploidy involves chromosome doubling: 2n \rightarrow 4n (autopolyploid) or 2nA + 2nB \rightarrow 4n_{AB} (allopolyploid).

Connecting Concepts & Real-World Relevance

  • Speciation mechanisms explain global biodiversity patterns (e.g., island radiations, latitudinal diversity gradients).

  • Understanding RI helps in conservation: identifying cryptic species can change management units.

  • Phylogenetics underpins fields from epidemiology (viral phylogeography) to forensics (tracking illicit wildlife trade).

  • Ethical considerations: recognizing species boundaries influences legal protection (Endangered Species Act listings) and resource allocation.

Concept Map (verbal outline)

Speciation ⇨ (Barriers) ⇨ Allopatric (founder effect) / Sympatric (polyploidy) ⇨ Reproductive Isolation (pre- vs. post-zygotic) ⇨ Reinforcement ⇨ Patterns of Evolution (divergent, convergent, etc.) ⇨ Phylogenetic reconstruction ⇨ Applied questions (biodiversity, conservation, agriculture).

Exam Tips & Common Pitfalls

  • Do not confuse analogous (convergent) with homologous (common ancestry).

  • Remember: Prezygotic barriers often evolve faster than postzygotic ones when ranges overlap.

  • Sympatric speciation via polyploidy is almost instantaneous in plants but rare in animals.

  • Hybrid zones ≠ necessarily transitional stages toward fusion; they can be stable for millennia.

  • On tree reading, degree of relatedness = recency of common ancestor, not number of branch tips between taxa.