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…
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).

2. Not all are mutually exclusive…

3. 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.

4. Morphological diffns ≠ Biological diffns
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 smithi L. 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
Next few slides explain how intermediaries are “prevented”.

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

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
Temporal/spatial separation – really 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 – Biological definition