1. Speciation
Section 8.2

2. Speciation
Learning Goal: To explore the factors leading to the evolution of a new species.
Success Criteria:
I know I am succeeding when I can…
define the concept of speciation
identify the modes of speciation
explain how human activities affect the evolution of other species
Not all evolutionary changes are result of NS
Sometimes changes in genetic makeup of pop. not influenced by traits of individuals
These changes tend to reduce genetic diversity within pop

3. Speciation
recall… a species includes all the members of a population that can interbreed under natural conditions
microevolution: changes in gene (allele) frequencies and phenotypic traits within a population and species
speciation: the formation of new species
Individuals of different species cannot interbreed under natural conditions and are described as being reproductively isolated from one another.
Populations of different species do not exchange genetic information—they have different gene pools.

4. Reproductive Isolating Mechanisms
for a new species to form, individuals from the original species must evolve to become reproductively isolated from the remainder of the population and establish a new interbreeding population 
reproductive isolating mechanism: any biological factor that prevents the two populations from interbreeding when living in the same region
Prezygotic: prevents fertilization and zygote formation
Postzygotic: prevents a fertilized egg from growing into a viable and reproducing adult
Individuals of different species cannot interbreed under natural conditions and are described as being reproductively isolated from one another.
Populations of different species do not exchange genetic information—they have different gene pools.

5. Prezygotic Reproductive Isolating Mechanisms Cont’d…
behavioral isolation: different species use different courtship and other mating clues to find and attract a mate
ex. male frogs of different species have unique calls that attract only females of their own species

6. Prezygotic Reproductive Isolating Mechanisms Cont’d…
temporal isolation: different species breed at different times of the year
ex. pussy willows produce flowers in the early spring, other plant species flower at different times of the year

7. Prezygotic Reproductive Isolating Mechanisms Cont’d…
ecological isolation: very similar species may occupy different habitats within a region
the mountain bluebird lives at high elevations, while the eastern bluebird prefers lower elevations and does not encounter the mountain species

8. Prezygotic Reproductive Isolating Mechanisms Cont’d…
mechanical isolation: differences in morphological features may make two species incompatible
ex. the male and female genitalia of each species of damselflies are uniquely shaped and are physically incompatible with other species
Picture of damselfly penises.

9. Prezygotic Reproductive Isolating Mechanisms Cont’d…
gametic isolation: male gametes may not be able to recognize and fertilize an egg of a different species
Ex. Many marine animals including corals, clams, and sea cucumbers release their sperm and eggs into open water. The sperm recognize eggs of their own species through chemical markers on the surface of the eggs.
Picture of sea cucumber.

10. Postzygotic Reproductive Isolating Mechanisms
zygotic mortality: mating and fertilization are possible, but genetic differences result in a zygote that is unable to develop properly
ex. some species of sheep and goat are able to mate, but the zygote is not viable

11. Postzygotic Reproductive Isolating Mechanisms Cont’d…
hybrid inviability: a hybrid individual develops but either dies before birth or, if born alive, cannot survive to maturity
ex. when tigers and leopards are crossed, the zygote begins to develop but the pregnancy ends in a miscarriage or stillborn offspring

12. Postzygotic Reproductive Isolating Mechanisms Cont’d…
hybrid infertility: hybrid offspring remain healthy and viable but are sterile
ex. mules are the sterile hybrid offspring of a horse–donkey cross

13. Modes of Speciation
What causes these reproductive isolating barriers to form?
Two modes of speciation:
Allopatric Speciation
Sympatric Speciation

14. Allopatric Speciation
allopatric speciation is when new species forms after a single species is geographically separated into two isolated populations
ex. Anole Lizards

15. Allopatric Speciation Cont’d…
geographic splitting of a species into two populations can occur by mountain ranges, continental drift, isolation on remote islands etc.
The scene: a population of wild fruit flies minding its own business on several bunches of rotting bananas, cheerfully laying their eggs in the mushy fruit...
Disaster strikes: A hurricane washes the bananas and the immature fruit flies they contain out to sea. The banana bunch eventually washes up on an island off the coast of the mainland. The fruit flies mature and emerge from their slimy nursery onto the lonely island. The two portions of the population, mainland and island, are now too far apart for gene flow to unite them. At this point, speciation has not occurred — any fruit flies that got back to the mainland could mate and produce healthy offspring with the mainland flies.

16. Allopatric Speciation Cont’d…
any mutation that arises in one population is not shared with the other population
differences in the environments of the two populations will lead to different forms of natural selection
over many generations the populations will gradually become less and less alike
The populations diverge: Ecological conditions are slightly different on the island, and the island population evolves under different selective pressures and experiences different random events than the mainland population does. Morphology, food preferences, and courtship displays change over the course of many generations of natural selection.

17. Allopatric Speciation Cont’d…
once enough time has passed, there is a good chance that the two populations will have evolved some sort of reproductive isolating mechanism
Perhaps their courtship rituals will have changed, the time of year they breed or produce pollen will have shifted, or they may no longer be physically compatible.
So we meet again: When another storm reintroduces the island flies to the mainland, they will not readily mate with the mainland flies since they've evolved different courtship behaviors. The few that do mate with the mainland flies, produce inviable eggs because of other genetic differences between the two populations. The lineage has split now that genes cannot flow between the populations.

18. Sympatric Speciation
sympatric speciation is when a new species evolves from within a large population
occurs when individuals within a population become genetically isolated from the larger population

19. Sympatric Speciation Cont’d…
a mutation may occur within an individual or individuals of a species that prevents breeding
gene flow is reduced and over time genetic differences accumulate between the species
A mutation occurred and now some flies can feed on mango fruit. Flies find mates by hanging out on preferred foods, the flies that eat the mango will not end up mating with the flies that eat the bananas. Gene flow would be greatly reduced; and once gene flow between the two species is stopped or reduced, larger genetic differences between the species can accumulate.

20. Human Influence on Speciation
human activities, such as agricultural expansion and the construction of roads, are causing once large habitats to be fragmented into smaller areas that isolate populations
reduces gene flow and biodiversity amongst a species making them at greater risk of disease and changing environmental conditions
A recent study of timber rattlesnake populations in the state of New York revealed that roadways are a significant barrier to gene flow and that genetic diversity within each isolated population was low. Similar research in China has shown that the survival of giant pandas, a critically endangered species, is threatened by their separation into small isolated populations in patches of bamboo forest.

21. Human Influence on Speciation
gene flow can be maintained and enhanced if the connectivity between habitats is ensured (ex. fragmented habitats can be joined by wildlife corridors)

22. Speciation
Learning Goal: To explore the factors leading to the evolution of a new species.
Success Criteria:
I know I am succeeding when I can…
define the concept of speciation
identify the modes of speciation
explain how human activities affect the evolution of other species
Not all evolutionary changes are result of NS
Sometimes changes in genetic makeup of pop. not influenced by traits of individuals
These changes tend to reduce genetic diversity within pop

23. Activity: Monster Speciation
explore how mutations and natural selection lead to new species over time
A recent study of timber rattlesnake populations in the state of New York revealed that roadways are a significant barrier to gene flow and that genetic diversity within each isolated population was low. Similar research in China has shown that the survival of giant pandas, a critically endangered species, is threatened by their separation into small isolated populations in patches of bamboo forest.

24. Homework
Answer 8.2 questions #2-6 on page 340