1. The Origin of Species

2. Speciation:
Formation of new species

5. What is a Species?
“morphospecies” – species are defined by their anatomical features
-- used by Linnaeus

6. Biological Species Concept
Proposed by Ernst Mayr (1942)
Biological species: groups of interbreeding natural populations that are reproductively isolated from other such groups
Reproductive barriers preserve species integrity
Largest unit in which gene flow is possible
Defined by reproductive isolation from other species in natural environments

7. Cannot be applied to:
1. organisms that are completely asexual in their reproduction
2. extinct organisms represented only by fossils
3. geographically segregated populations

8. Reproductive Barriers
Any factor that impedes two species from producing fertile hybrids
Prevents interbreeding between closely related species

9. Prezygotic Barriers Impede mating or fertilization
1. habitat isolation: 2 species living in different habitats within same area may not encounter each other very often

10. 2. temporal isolation: 2 species that breed at different times of day, seasons or years
(e.g. brown trout breed in fall, rainbow trout breed in spring)

11. 3. behavioral isolation: species-specific behavior and elaborate courtship behavior;
(e.g. blinking patterns in fireflies,
-- pheromones (insect scents)
-- bird songs
pie-billed grebe
great blue heron

12. 4. mechanical isolation: anatomical incompatibility

13. 5. gametic isolation: the gametes of different species are rarely able to fuse

14. Postzygotic Barriers Prevent development of viable, fertile hybrid
1. reduced hybrid viability: genetic incompatibility may abort hybrid as an embryo or hybrid is very frail

15. 2. reduced hybrid fertility: hybrids are usually sterile

16. 3. hybrid breakdown: first generation hybrid is fine, but next generation is feeble or sterile

18. Allopatric Speciation
Geographical barrier physically isolates the population
(mountain ranges emerge, glaciers move, land bridges form)

19. Conditions Favoring Allopatric Speciation:
Small, isolated population
Geographical isolation of a small population usually occurs at fringe of parent population’s range (“peripheral isolate”)
Gene pool of peripheral isolate is probably different from parent pool
Genetic drift, natural selection may go in different direction

20. Sympatric Speciation
Formation of new species within the range of the parent population
Reproductive isolation evolves without geographical isolation
Many plant species have evolved from improper cell division that results in extra sets of chromosomes (“polyploidy”)

21. A) autopolyploidy: an organism that has more than 2 chromosome sets, all derived from a single parent species
-- mutants can’t breed with diploid parents because hybrids would have odd number of chromosomes  postzygotic barrier

22. B) allopolyploid: a polyploid hybrid resulting from contributions by two different species
More common than autopolyploidy
2 species interbreed to produce hybrid

23. Habitat Differentiation:
Another way for sympatric speciation to occur
A subpopulation uses a habitat or resource not used by the parent population

24. For example, the North American apple maggot fly’s original habitat was the hawthorn tree, but some populations colonized apple trees about 200 years ago
Apples mature more quickly than hawthorn fruit, so the natural selection has pressured the apple tree flies to develop more rapidly
Temporal isolation has now occurred between the populations

25. If 2 closely related populations come together after a lengthy separation,
They may:
1. interbreed freely (no speciation has occurred) OR
2. reproductive barriers may prevent interbreeding (speciation)
3. hybrid zone: region of interbreeding where geographical regions overlap

27. Hybrid Zones
Occur where two species have incomplete reproductive barriers
3 possible outcomes:

28. 1. Reinforcement When hybrids are less fit than parent species
Natural selection strengthens prezygotic barriers to reproduction
This reduces formation of unfit hybrids

29. 2. Fusion Occurs when barriers to reproduction are weak
Lots of gene flow occurs
The hybridizing species may eventually fuse into a single species

30. 3. Stability
Hybrids survive or reproduce better than members of either parent species, at least in certain circumstances

31. Punctuated Equilibrium
Eldredge and Gould
Long periods of stasis are punctuated with episodes of rapid speciation