1. Species - a population(s) whose members have the potential to interbreed with one another in nature to produce fertile offspring, and cannot with members of other species. Ex: (Left) These birds look similar, but cannot interbreed with one another. Q: Are they two different species? Yes. Ex: Breeds of dogs look different from one another, but all can interbreed. Q: Are they all different species? No. They are all the same species.

2. Ex: These humans may live in different geographical areas, and may never get together. But if they did, could they interbreed? Are they the same species? Yes, all humans belong to the same species. Q: Are humans and chimpanzees the same species? A: No Q: What about dogs and wolves? A: No.

3. Q: What prevents two species from reproducing with each other? A: Reproductive Barriers! 2 types of Reproductive Barriers: I. Pre-zygotic barriers - impedes mating or hinders fertilization of eggs. A. Temporal isolation - time based. Ex: Western spotted skunks breed in the fall, but eastern species breed in late winter. B. Habitat isolation - spatially segregated. Ex: one species of garter snake lives in water, and a closely related species lives on land. C. Behavioral isolation - courtship rituals. Ex: in bird species, courtship is so elaborate that individuals are unlikely to mistake a bird of a different species as one of their kind. Courtship rituals of Blue-footed boobies in Galapagos Islands. Male does “high-step,”showing bright blue feet.

4. D. Mechanical isolation - male and female sex organs of different species are anatomically incompatible. Ex: 2 insects’ copulatory organs may not fit together correctly; no sperm is transferred. E. Gametic isolation - individuals may copulate, but their gametes are incompatible and fertilization does not occur. Ex: mammal sperm may not survive in female of a different species.

5. Horse+Donkey= Mule (but is sterile) II. Post-zygotic barriers - if mating actually occurs between different species and a zygote is formed, these mechanisms affect the hybrid offspring. (baby) A. Hybrid inviability - the hybrid offspring die before reaching reproductive maturity. Ex: certain frogs B. Hybrid sterility - the hybrid offspring may become vigorous adults, but are infertile. Ex: see below