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Lecture 11: The Idea of Species

Lecture 11: The Idea of Species. Why do we need a definition? Species : fundamental unit of biology Earliest definition: HOLOTYPE Controversy about definition: Phenetic Species Concept Biological Species Concept Ecological Species Concept And more…. Not all are mutually exclusive….

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Lecture 11: The Idea of Species

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  1. Lecture 11: The Idea of Species Why do we need a definition? Species: fundamental unit of biology Earliest definition: HOLOTYPE Controversy about definition: • Phenetic Species Concept • Biological Species Concept • Ecological Species Concept And more…

  2. Not all are mutually exclusive…

  3. Sp. B Sp. A Sp. C Phenetic Species Concept • Clusters in morphospace • Extension of how spp. defined in practice • Works for sexual, asexual & extinct spp.

  4. Problems… Morphological diffns ≠ Biological diffns • Sexual Dimorphism e.g. Black Widow Spider • Life Cycle Stages e.g. Barnacle

  5. 3) Polymorphism Can get tons of morphological variation in one species e.g. Cattleya orchids e.g. Snow goose e.g. Ensatina salamanders

  6. 4) Sibling Species • Differ in reproduction but not morphology • e.g. Leptobrachium smithiL. hendricksoni • e.g. Uca borealis U. vocans

  7. Biological Species Concept • Most commonly used definition • Reproductively isolated group of interbreeding individuals • Species = Gene Pool • Must form viable offspring • Idea predates Darwin! • NOT better just different than phenetic def’n

  8. How define BSC in measurable terms? • Most spp. rarely observed during breeding • Look for intermediaries…evidence of interbreeding

  9. Sympatric Species Very closely related & present in same range but no offspring. Why not? • Premating Isolation • Prevent formation of hybrid zygotes • Postmating Isolation • Reduce viability/fertility of hybrids

  10. Premating Mechanisms • Ecological Isolation – wrong place • Seasonal/Temporal Isolation – wrong time • Ethological Isolation – no attraction • Mechanical Isolation – copulation/pollen transfer impossible • Gametic Isolation – gametes inviable

  11. Ecological Isolation • Overlap in range but niche differences e.g. Sceloporus clarkii (trees) vs. Sceloporus magister (open areas)

  12. Temporal Isolation

  13. Ethological Isolation • Acoustic • Visual Displays

  14. Mechanical Isolation • Scotch broom • Bumblebees – weight determines ability to pollinate

  15. Post-mating Mechanisms • Hybrid inviability (dies/does poorly) • Hybrid sterility (gen’lly heterogametic) F1 hybrids have non-functional gametes • Hybrid breakdown F2 hybrids have reduced viability/fertility

  16. Are RIM the cause or effect of speciation? • EFFECT if selection against hybrids Anatidae (surface-feeding ducks) vs. Anserini (geese) • anatids usually have complex diverse courtship displays • hybrids usually fertile Ho : courtship displays are R.I.M. that evolved because hybrids have reduced fitness

  17. Pair formation behav. divergence vs. pair maintenance behav. • selection favours early ID Diversity should correlate with degree of sympatry • risk of hybridization Alternative hypotheses: • sexual selection • dominance • predation risk Conclusion: they are RIM now but don’t know history

  18. Biological Species Concept Problems: • "ring" species (hybrids) • Only applicable to the present • No evolutionary dimension • Doesn’t apply to asexual organisms • Temporally/Spatially separated species

  19. Ecological Species Concept • Because niches are discrete, can be used to identify discrete species (phenetic clusters) Disruptive selection (against hybrids) Competitive exclusion: competitive spp. cannot coexist forever if have same limiting resources (only 1 sp. /niche) Leads to…

  20. Character displacement • Spp. that are sympatric differ more than allopatric spp. • Theory: allows exploitation of different niches e.g. Hawaiian Honeycreepers

  21. Overall Difficulty • Competing effects of selection & gene flow • Selection – Ecological definition • Gene flow – Biologicaldefinition

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