1. Working with the Biological Species Concept Speciation is a two-part process –1. Identical populations must diverge –2. Reproductive isolation must evolve to maintain these differences Speciation occurs much more readily in the absence of gene flow –This much more likely in geographically isolated populations Populations can become geographically isolated for several reasons

2. Consider how the amount of genetic divergence (change) forms a continuum: Microevolution Macroevolution small changes large changes Microevolution = adaptation Macroevolution = speciation

3. The Evolution of Species Significant changes in the gene pool could lead to the evolution of a new species over time. The evolution of new species, a process called speciation occurs when members of similar populations no longer interbreed to produce fertile offspring within their natural environment.

4. The Evolution of Species Geographic isolation occurs whenever a physical barrier divides a population. A new species can evolve when a population has been geographically isolated. Reproductive isolation can result in speciation Reproductive isolation occurs when formerly interbreeding organisms can no longer mate and produce fertile offspring A change in chromosome numbers- polyploidy can lead to speciation

5. Isolating Mechanisms Reproductive isolating mechanisms are the barriers that prevent genetic exchange between species –Prezygotic isolating mechanisms Prevent the formation of zygotes –Postzygotic isolating mechanisms Prevent the proper functioning of zygotes after they have formed

6. Prezygotic Isolating Mechanisms

9. Postzygotic Isolating Mechanisms Examples: –Hybridization between sheep and goats produces embryos that die in the earliest embryonic stages –Leopard frogs of the United States are a group of similar species, and NOT a single species as was long assumed

10. Obstacle to mating or to fertilization if mating occurs Prezygotic barriers: behavioral isolation geographic isolation ecological isolation temporal isolation mechanical isolation gametic isolation

11. Prezygotic barriers: Geographic isolation Species occur in different areas –physical barrier –allopatric speciation “other country” Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel Ammospermophilus spp

12. Prezygotic barriers: Ecological isolation Species occur in same region, but occupy different habitats so rarely encounter each other –reproductively isolated 2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial lions & tigers could hybridize, but they live in different habitats:  lions in grasslands  tigers in rainforest lions & tigers could hybridize, but they live in different habitats:  lions in grasslands  tigers in rainforest

13. Prezygotic barriers: Temporal isolation Species that breed during different times of day, different seasons, or different years cannot mix gametes –reproductive isolation –sympatric speciation “same country” Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer

14. Prezygotic barriers: Behavioral isolation Unique behavioral patterns & rituals isolate species –identifies members of species –attract mates of same species courtship rituals, mating calls reproductive isolation Blue footed boobies mate only after a courtship display unique to their species

15. Prezygotic barriers: Mechanical isolation Morphological differences can prevent successful mating –reproductive isolation Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen. Plants

16. Mechanical isolation For many insects, male & female sex organs of closely related species do not fit together, preventing sperm transfer –lack of “fit” between sexual organs: hard to imagine for us… but a big issue for insects with different shaped genitals! Damsel fly penises Animals

17. Prezygotic barriers: Gametic isolation Sperm of one species may not be able to fertilize eggs of another species –mechanisms biochemical barrier so sperm cannot penetrate egg –receptor recognition: lock & key between egg & sperm chemical incompatibility –sperm cannot survive in female reproductive tract Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse.

18. Postzygotic barriers: Prevent hybrid offspring from developing into a viable, fertile adult –reduced hybrid viability –reduced hybrid fertility –hybrid breakdown zebroid

19. Forms of Selection Selection is a statistical concept –One cannot predict the fate of any single individual –But it is possible to predict which kind of individual will tend to become more common in a population Three types of natural selection have been identified –Stabilizing selection Acts to eliminate both extreme phenotypes –Disruptive selection Acts to eliminate intermediate phenotypes –Directional selection Acts to eliminate a single extreme phenotype

20. Three kinds of natural selection

21. Natural selection acts on variations Stabilizing selection is a natural selection that favors average individuals in a population. Selection for average size spiders Normal variation

22. Natural selection acts on variations Directional selection occurs when natural selection favors one of the extreme variations of a trait. Normal variation

23. Natural selection acts on variations In disruptive selection, individuals with either extreme of a trait’s variation are selected for. Selection for light limpets Normal variationSelection for dark limpets