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… Holotype: problem was supposed to be an “exact” definition but too much variation within species. Why do we want a definition of species? Use in the field, hard to study species biology without knowing what individuals we are talking about. Conservation measures. Evolutionary history. When study evolution, can’t really do so without knowing what constitutes a species (e.g. to measure selection).

Not all are mutually exclusive…

Phenetic Species Concept Clusters in morphospace Extension of how spp. defined in practice Works for sexual, asexual & extinct spp. Sp. A Sp. C Sp. B Great in theory, generally need to look at a lot of phenotypic or genetic characters to get really good separation. Next slide for more problems.

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

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

4) Sibling Species Differ in reproduction but not morphology e.g. Leptobrachium smithi L. hendricksoni e.g. Uca borealis U. vocans

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

How define BSC in measurable terms? Most spp. rarely observed during breeding Look for intermediaries…evidence of interbreeding Next few slides explain how intermediaries are “prevented”.

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

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

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

Temporal Isolation

Ethological Isolation Acoustic Visual Displays

Mechanical Isolation Scotch broom Bumblebees – weight determines ability to pollinate

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

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 RIMS – arise fortuitously, epistatically whereas SRMS (species recognition mechanisms arise by NS).

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

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 Temporal/spatial separation – really species??

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…

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

Overall Difficulty Competing effects of selection & gene flow Selection – Ecological definition Gene flow – Biological definition