1. Evolution and Zygotic Barriers (Part 5)
Ms. Day
AP Biology
Chapter 24: The Origin of Life

2. How are new species created?
Geographic isolation
Reproductive barriers
Change in chromosome numbers through mutation
Adaptive radiation (example of divergent evolution)
Speciation = formation of NEW species

3. Hello over there 
A. harrisi
A. leucurus
Geographic Isolation

4. Speciation and geographic separation
Speciation can occur in two ways:
Allopatric speciation (means “other”)
a genetic isolation WITH a geographical barrier; new group isolated from its parent population
Sympatric speciation (means “together”)
genetic isolation WITHOUT a geographical barrier; a reproductive barrier isolates population in SAME habitat

5. Allopatric speciation
Sympatric speciation

6. http://www. pbs. org/wgbh/nova/evolution/evolution-action-salamanders

7. Reproductive Isolation
biological factors (barriers) that stop 2 species from producing viable, fertile hybrids
Two types of barriers
Postzygotic “after the zygote”
Zygote can NOT develope
Prezygotic “before the zygote”
Sperm and egg can not fuse

10. Pre-Zygotic Barriers

11. Habitat Isolation
2 species encounter each other rarely, or not at all, because they live in different habitats, even though not isolated by physical barriers

12. Temporal Isolation
Species that breed at different times of the day, different seasons, or different years cannot mix their gametes
Late Summer
Late Winter

13. Behavioral Isolation
Courtship rituals and other behaviors unique to a species are effective barriers

14. Mechanical Isolation
Morphological differences can prevent successful mating
Related species may attempt to mate but CAN’T anatomically incompatible
Sperm = transfer
Mating organs don’t fit

15. Gametic Isolation
Sperm of one species may not be able to fertilize eggs of another species
Ex: specific molecules on egg coat adhere to specific molecules on sperm

16. Post-Zygotic Barriers

17. Reduced Hybrid Viability
Genes of the different parent species may interact and impair the hybrid’s development
Hybrids are very weak and/or underdeveloped
Salamander hybrid shows incomplete development

18. Reduced Hybrid Fertility
Even if hybrids may live and be strong, they may be sterile

19. Hybrid Breakdown
Some F1 hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation (F2) are weak or sterile
Cultivated rice have different recessive mutant alleles F2 accumlates many of these alleles

20. Polyploidy
Polyploidy is presence of EXTRA sets of chromosomes due to accidents during cell division
ex: “nondisjunction”
It has caused the evolution of some plant species
More common in plants than in animals