1. 16
The Origin of Species 1

2. 16.1 What Is a Species? Each species evolves independently
Today, a species is defined as a group of populations that evolves independently
Each species follows a separate evolutionary path because alleles rarely move between the gene pools of different species

3. 16.1 What Is a Species? Each species evolves independently (continued)
The biological species concept is based on the observation that reproductive isolation (inability to successfully breed outside the group) ensures evolutionary independence

4. 16.2 How Is Reproductive Isolation Between Species Maintained?
Traits that prevent interbreeding and maintain reproductive isolation are called isolating mechanisms
The benefit of isolating mechanisms is the production of no offspring that are unfit or sterile
Isolating mechanisms prevent the wasting of reproductive effort and failing to contribute to future generations
Natural selection favors traits that prevent reproduction across species boundaries

5. Table 16-1 5

6. 16.2 How Is Reproductive Isolation Between Species Maintained?
Premating isolating mechanisms prevent mating between species

7. 16.2 How Is Reproductive Isolation Between Species Maintained?
Premating isolating mechanisms include the following (continued)
Members of different species may be prevented from meeting
Geographical isolation prevents interbreeding between populations that do not come into contact because they live in different, physically separated places

8. 16.2 How Is Reproductive Isolation Between Species Maintained?
Premating isolating mechanisms include the following (continued)
Members of different species may be prevented from meeting (continued)
This type of isolation is usually considered to be a mechanism that allows new species to form rather than one that maintains reproductive isolation between species
(Albert moved into Grand Canyon area and made the Kaibab)

9. Figure 16-3 Geographic isolation
Kaibab squirrel
Abert squirrel 9

10. 16.2 How Is Reproductive Isolation Between Species Maintained?
Premating isolating mechanisms include the following (continued)
Different species may occupy different habitats
Ecological isolation occurs when species do not mate because they occupy different habitats
White-crowned sparrows inhabit fields and meadows, while white-throated sparrows inhabit dense thickets

11. 16.2 How Is Reproductive Isolation Between Species Maintained?
Premating isolating mechanisms include the following (continued)
Ecological isolation occurs when species do not mate because they occupy different habitats (continued)
In another example, each species of fig wasp breeds in the fruit of a different species of fig, and the wasps thus do not come into contact with one another

12. Figure 16-4 Ecological isolation12

13. 16.2 How Is Reproductive Isolation Between Species Maintained?
Premating isolating mechanisms include the following (continued)
Different species may breed at different times
Temporal isolation (time-based isolation) prevents breeding between two species occupying the same habitat because of different breeding seasons
The spring field cricket and the fall field cricket both occur in many areas of North America
They are unable to interbreed because of their varying breeding seasons

14. 16.2 How Is Reproductive Isolation Between Species Maintained?
Premating isolating mechanisms include the following (continued)
Different species may breed at different times (continued)
In nature, Bishop pines and Monterey pines do not interbreed
Bishop pine pollination occurs in summer
Monterey pine pollination occurs in early spring

15. Figure 16-5 Temporal isolation
Bishop pine
Monterey pine 15

16. 16.2 How Is Reproductive Isolation Between Species Maintained?
Premating isolating mechanisms include the following (continued)
Different species may have different courtship signals
Among animals, elaborate courtship colors and behaviors can prevent mating between species
Behavioral isolation is created by signals and behaviors that differ from species to species

17. 16.2 How Is Reproductive Isolation Between Species Maintained?
Premating isolating mechanisms include the following (continued)
Different species may have different courtship signals (continued)
The plumes and arresting pose of a courting male Raggiana bird of paradise are conspicuous indicators of this species
There is little chance females of another species will mate with him by mistake

18. Figure 16-6 Behavioral isolation18

19. 16.2 How Is Reproductive Isolation Between Species Maintained?
Premating isolating mechanisms include the following (continued)
Different species may have different courtship signals (continued)
Male frogs embrace any female regardless of species
Female frogs encountering males of a different species utter the “release call,” which causes the males to let go
As a result, few hybrids—offspring of parents of different species—are produced

20. 16.2 How Is Reproductive Isolation Between Species Maintained?
Premating isolating mechanisms include the following (continued)
Differing sexual organs may foil mating attempts
In animals with internal fertilization, male and female sexual organs may not fit together
Incompatible body shapes may make copulation between species impossible
For example, snails of species whose shells have left-handed spirals may be unable to copulate successfully with snails whose shells have right-handed spirals

21. 16.2 How Is Reproductive Isolation Between Species Maintained?
Premating isolating mechanisms include the following (continued)
Differing sexual organs may foil mating attempts (continued)
In plants, differences in flower size or structure may attract different pollinators
Mechanical incompatibility occurs when species cannot mate because their reproductive structures are incompatible

22. 16.2 How Is Reproductive Isolation Between Species Maintained?
Postmating isolating mechanisms limit hybrid offspring (continued)
If the resulting offspring die during development, the two species remain reproductively isolated from each other
Mechanisms that prevent the formation of vigorous, fertile hybrids between species are called postmating isolating mechanisms

23. 16.2 How Is Reproductive Isolation Between Species Maintained?
Postmating isolating mechanisms limit hybrid offspring (continued)
One species’ sperm may fail to fertilize another species’ egg
Gametic incompatibility occurs when sperm from one species cannot fertilize eggs of another
In animals with internal fertilization, fluids of the female reproductive tract may weaken or kill sperm of another species

24. 16.2 How Is Reproductive Isolation Between Species Maintained?
Postmating isolating mechanisms limit hybrid offspring (continued)
One species’ sperm may fail to fertilize another species’ egg (continued)
Gametic incompatibility may be an especially important isolating mechanism in certain species
Marine invertebrates and wind-pollinated plants reproduce by scattering gametes in the water or in the air

25. 16.2 How Is Reproductive Isolation Between Species Maintained?
Postmating isolating mechanisms limit hybrid offspring (continued)
One species’ sperm may fail to fertilize another species’ egg (continued)
Among plants, chemical incompatibility may prevent the germination of pollen from one species that lands on the stigma (pollen-catching structure) of the flower of another species

26. 16.2 How Is Reproductive Isolation Between Species Maintained?
Postmating isolating mechanisms limit hybrid offspring (continued)
Hybrid offspring may fail to survive or reproduce
If cross-species fertilization occurs, the resulting hybrid may be unable to survive, a situation called hybrid inviability

27. 16.2 How Is Reproductive Isolation Between Species Maintained?
Postmating isolating mechanisms limit hybrid offspring (continued)
Hybrid offspring may fail to survive or reproduce (continued)
The genetic instructions directing development of the two species may be so different that hybrids abort early in development
The hybrid may abort early in development or die shortly after birth

28. 16.2 How Is Reproductive Isolation Between Species Maintained?
Postmating isolating mechanisms limit hybrid offspring (continued)
Hybrid offspring may be infertile
Mule hybrids (a cross between a horse and a donkey) are sterile
Liger hybrids (a zoo-based cross between a male lion and a female tiger) are sterile
Hybrid infertility prevents hybrids from passing on their genetic material to offspring, thus blocking gene flow between the two parent populations