1. Patterns of Selection and Speciation
Topic 3
Patterns of Selection and Speciation 1

2. Stabilizing Selection
Natural selection acts against the extreme phenotypes in a population and favors the middle ground.
Example: birth weight in humans.
In a study done in the past, human babies weighing less than 4.5 lb had a 41.4% survival rate.
Babies between lbs had a 98.5 survival rate.
Babies greater than 10.0 lb had 90.5% survival rate. 2

3. Directional selection
Natural selection favors one extreme for phenotype over the other, shifting the prevalence of the characteristic from the middle of the range to one extreme.
Example: medium ground finch (Galapagos) 3

4. Disruptive selection
Natural selection favors the extreme phenotype characteristics over the intermediate.
Example: African swallowtail butterflies 4

5. Pea fowl 5

6. Birds of paradise
Video 1 6

7. Sexual selection
The selection of individuals for secondary sex characteristics
eg bird coloration, elk horns
Can be intrasexual or intersexual
Male-male competition
Female choice
Often appears at odds with natural selection

8. Does natural selection make perfect organisms?
Selection only acts on existing variations
Evolution is limited by historical constraints
Adaptations are compromises
Chance, natural selection, and the environment are all in play

9. Prezygotic Barriers Temporal isolation Habitat isolation
Behavioral isolation
Mechanical isolation
Gametic isolation 9

10. Postzygotic Barriers Reduced hybrid viability Reduced hybrid fertility
Hybrid breakdown 10

11. Patterns of speciation

12. The pace of evolution: Punctuated equilibrium vs Gradualism12

13. Evolution is often gradual…13

14. …but stasis and rapid evolution is also common
Bryozoan example.
From Sepkoski 1989. 14

15. Divergent evolution
the accumulation of differences between groups which can lead to the formation of new species
Via geographic isolation or adaptive radiation

16. Geographic Isolation
…can lead to reproductive isolation, divergence of gene pools and speciation. 16

17. Background matching and local adaptation
Hoekstra, HE (2006) Genetics, development and evolution of adaptive pigmentation in vertebrates. Heredity 97:222–234. doi: /sj.hdy ; published online 5 July 2006 17

18. Adaptive Radiation
“…adaptive radiation…refers to more or less simultaneous divergence of numerous lines all from much the same ancestral adaptive type into different, also diverging adaptive zones.”
Simpson 1953
Seeds
Bark & Crevice Insects
Evolution of many species from one common ancestor in response to variations in available environments and resources.
Buds
Nectar and gleaned insects
From Petren et al. 1999 18

19. Coevolution Two species evolve in response to each other over time
Pollinators and flowering plants
Hosts and parasites
Predators and prey
Hammer orchid 19

20. Predation drives evolution of both predators and prey
Predators adaptations
Locate, sneak up on & subdue prey
Prey adaptations
elude & defend
horns, speed, coloration
spines, thorns, toxins
Predation provides a strong selective pressure on prey populations. Ay feature that would decrease the probability of capture should be strongly favored. 20

22. African Rift Lake Cichlids22

23. Model of Cichlid Fish Diversification23

24. Convergent Evolution
The evolution of similar characteristics in unrelated (or extremely far removed) species as a result of similar ecological opportunities and environments
Analogous structures are due to similar selection pressures 24

25. Convergent evolution