1. Chapter 24 The Origins of the Species

2. Biological Species Concept
Species: population whose members can interbreed and produce viable, fertile offspring
(reproductively compatible)
These happy face spiders look different, but since they can interbreed,
they are considered the same species: Theridion grallator

3. Speciation
lineage-splitting event that produces two or more separate species
Original species cannot produce viable offspring together or that they avoid mating with members of the other group

4. speciation and extinction
Microevolution: changes within a single gene pool
Macroevolution: evolutionary change above the species level
Speciation explains similarities and differences among species.
speciation video
speciation and extinction

5. New species are formed by being isolated from each other
New species are formed by being isolated from each other. Reproductive isolation barriers that prevent members of 2 species from producing viable, fertile hybrids

6. Reproductive Barriers/Isolation
Limits gene flow between species
and formation of hybrids

7. Types of Reproductive Barriers
Prezygotic: impede mating or hinder fertilization if mating occurs

8. Habitat Isolation Ex: garter snakes
Two species occupy different habitats
rarely encounter each other
Ex: garter snakes
aquatic or terrestial

9. Temporal Isolation
Species that breed during different times of day, different seasons, or different years cannot mix gametes.
eastern spotted skunk western spotted skunk
mates in late winter mates in late summer

10. Behavioral Isolation blue footed boobie
Unique behavioral patterns / rituals isolate species
- identifies members of species
- attract mates of same species
courtship rituals, mating calls
blue footed boobie

11. Mechanical Isolation
Mating is attempted, but morphological differences prevent its successful completion different direction spirals prevent genital allignment

12. Gametic Isolation
Sperm of one species not able to fertilize eggs of another species Sea urching egg and sperm can’t fuse due to different proteins on gamete surface

13. Types of Reproductive Barriers
Postzygotic: prevent a hybrid zygote from developing into viable, fertile adult

14. Reduced Hybrid Viability
Genes of different parent species may interact in ways that impair hybrid’s development or survival in its environment. Hybrids don’t complete development and are frail

15. Reduced Hybrid Fertility
Robust hybrids are sterile
- parent species chrom. # differ
- meiosis does not form normal gametes
Horse + donkey = mule
mules are sterile

16. Hybrid Breakdown
Some first generation hybrids are viable and fertile, but when they mate with one another, offspring of next generation are feeble or sterile.
Hybrid offspring of rice are vigorous and fertile
Next generation are small and sterile
Species have begun separation

17. Haldane’s Rule
In the early stage of speciation, if in a species hybrid only one sex is sterile
more likely to be the heterogametic sex. (one with two different sex chromosomes)
major form of post-zygotic reproductive isolation

18. Other Definitions of Species
Morphological : by body shape, size, and other structural features Ecological : by niche/role in community Phylogenetic : smallest group of individuals that share common ancestry, branch on tree of life * holds true for sexual and asexual species*

19. Two Main Modes of Speciation
Allopatric Speciation
“other” “homeland”
Geographically isolated populations
Caused by geologic events or processes
Mutations occur- evolution by natural selection & genetic drift
Reproductive isolation
Eg. Squirrels on N/S rims of Grand Canyon
Sympatric Speciation
“together” “homeland”
Overlapping populations within home range (new species within original area-not isolated)
Gene flow between subpopulations blocked by:
polyploidy
sexual selection
habitat differentiation
Eg. polyploidy in crops (oats, cotton, potatoes, wheat)

20. Modes of Speciation

21. Allopatric Speciation by Geographical Isolation
Population of wild fruit flies minding their business munching on their bananas
Hurricane washes bananas and unhatched fruit flies out to sea.
Bananas washes up on island off coast of mainland
Fruit flies hatch and live on island
Two population too far apart for gene flow between them

22. Populations diverge due to different selective pressures on populations
Morphology
Food preferences
Courtship behaviors
Rivers, mountains, continental drift, migration, etc.

23. Allopatric speciation of antelope squirrels

24. Sympatric Speciation by Reduction of Gene Flow
Populations not totally isolated
Populations live at distant locations in geographic range
Mating throughout range is not random
Causes reduced gene flow

25. Sympatric Speciation by Polyploidy
Polyploidy: extra sets of chromosomes (2, 3n, 4n)
More common in plants
Types
Autoploid: 2+ sets of chrom. from a single species
- produce fertile offspring
Alloploid: 2+ sets of chrom. from different species
- produce sterile hybrids- propagate asexually
- can become fertile and reproduce with each other but not parent species
- represent new biological species

26. Sympatric speciation: autoploidy
Same species
Error in meiosis

27. Sympatric Speciation: alloploidy
chromosomes not homologous
(different species)
can’t undergo meiosis
undergo asexual reproduction
new species - diploid
number equal to
two parent species

28. Sympatric speciation by Sexual Selection
Under normal light, two cichlid species are different in coloration, and females mate only with males of their own species. Under orange light, the two species appear identical, and females mate with males of either species, yielding fertile hybrids. Intersexual selection by females based on coloration is the reproductive barrier

29. What happens when new or partially formed species come into contact with each other?

30. Notice difference in allele frequencies
Hybrid Zone: region where different sp. mate and produce hybrid offspring
- Incomplete reproductive barriers
Notice difference in allele frequencies

31. Change in Hybrid Zones Over Time
Result:
1. reinforcement: barriers strengthened (limit hybrid formation)
2. fusion: barriers weakened (formation of single sp.)
3. stability: barriers same (continuation of hybrids)
1. Three populations
connected by gene flow
barrier to gene flow
established
Separated population
begins to diverge from
other 2 populations
Gene flow is re-established
in a hybrid zone

32. Weakened barriers: grizzly and polar bears  hybrid

33. Rate of Speciation/Evolution
Punctuated Equilibium
Eldridge & Gould
Long period of stasis punctuated by short bursts of rapid, significant change
Gradualism
Common ancestor
Slow, constant change

34. Remember Evolution begins with small changes in a species that become more and more pronounced eventually leading to macroevolution.