1. Chapter 24 – The Origin of Species

2. Macroevolution
Macroevolution: evolutionary change above the species level, the formation of new taxonomic groups

3. What is a Species?
Biological Species Concept (Ernst Mayr) - a population or group of populations whose members have the potential to interbreed and produce viable, fertile offspring (genetic exchange is possible and that is genetically isolated from other populations)

4. Reproductive Barriers
Biological factors that impede production of viable, fertile hybrids.
Both prezygotic and postzygotic barriers.
Prezygotic barriers impede mating or hinder fertilization if mating does occur
Postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult

5. Prezygotic Barriers

6. Prezygotic Barriers
Habitat isolation: 2 species that occupy different habitats within the same area will rarely, if ever, come into contact with each other
Example: 2 garter snakes (genus Thamnophis) live in same area but (a) lives in water while (b) lives on land.

7. Prezygotic Barriers
Temporal Isolation: species that breed during different times of the day, seasons, or years; prevents gamete mixing
Example: habitats of the the eastern spotted skunk (c) and the western spotted skunk (d) overlap but (c) mates in late winter while (d) mates in late summer.

8. Prezygotic Barriers
Behavioral Isolation: 2 species have different courtship rituals thereby attracting different mates
Example: the blue footed boobies – specific dance, bird calls, insect mating dances
Video

9. Prezygotic Barriers: mating attempt
Mechanical Isolation: when mating is attempted, but morphological differences prevent successful mating
Example: 2 species of snail whose shells spiral in different directions, prevent genital openings from being aligned
Gametic Isolation: one species cannot fertilize the eggs of another species
Example: common in sea creatures who discharge their gametes into the water column, like sea urchins and sea anemones

11. Postzygotic Barriers
Reduced hybrid viability: genes of different parent species may interact and impair the hybrid’s development; offspring usually die in the embryonic stage of development
Example: salamanders in genus Ensatina can hybridize, but hybrids do not complete development, to frail.

12. Postzygotic Barriers Reduced hybrid fertility: hybrid are sterile
Example: male donkey (i) and female horse (j) = a mule (k) which are sterile

13. Postzygotic Barriers
Hybrid breakdown: first generation hybrids are viable and fertile, but the second generation offspring are feeble or sterile
Example: cultivated rice, hybrid offspring are fertile (l, left and right), but the F2 generation offspring with too many localized recessive alleles are small and sterile (l, center)

14. Biological Species Concept
There are some limitations with the biological species concept:
No method to measure reproductive success of fossils or asexual organisms
Difficult to determine ability of one organism to mate with another
Other definitions of a species:
Morphological species concept – uses structural features
Palentological species concept – morphology of fossils
Ecological species concept – ecological niche
Phylogenetic species concept – genetic history
None of these are perfect, but a combination of each (especially biological and morphological) is most accepted.

15. Modes of Speciation
Must have some type of geographic separation of populations for speciation to occur; 2 different modes
1 – Allopatric Speciation
2 – Sympatric Speciation

16. Allopatric Speciation
“other country” or “other homeland”
Populations segregated by a geographical barrier
Can result in adaptive radiation
Example: white-tailed antelope squirrel and Harris’s antelope squirrel – separated by Grand Canyon

17. Sympatric Speciation “same country”
Speciation takes place in overlapping populations
Can result from various mechanisms including chromosomal changes (polyploidy in plants) and non-random mating that reduces gene flow (cichlids).
Pundamilia pundamilia
Pundamilia nyererei

18. Adaptive Radiation
The evolution of many diversely adapted species from a common ancestor

19. Tempo of Speciation
Gradual model – gradual divergence from a common ancestor
Punctuated model – changes in short bursts, mostly when it buds from a parent species, with long stretches of little change
Helped explain the non-gradual appearance of species in the fossil record

20. Hybrid Organism Assignment
Identify 10 different hybrid organisms.
For each organism identify
The mother and father species
The post zygotic barrier that causes them to be a hybrid and not all one species
One interesting fact about the hybrid or mating pair
You may identify/classify an additional 2 hybrids for extra credit
Assignment due on

21. Speciation Speciation: the origin of new species
Anagenesis (phyletic evolution): accumulation of heritable changes
Cladogenesis (branching evolution): budding of new species from a parent species that continues to exist (basis of biological diversity)