Speciation and Extinction

Species

A group able to interbreed and produce viable, fertile offspring

Speciation

Formation of a new species - results in diversity of life forms

Geography

Has an impact on speciation

Two modes of Speciation

• Allopatric

• Sympatric

Allopatric Speciation

• Physical barrier divides population or a small population is separated from the main population

• Populations are geographically isolated

  • Prevents gene flow

  • Often caused by natural disasters

Sympatric Speciation

• A new species evolves while still inhabiting the same geographic region as the ancestral species

  • Usually due to the exploitation of a new niche

Speciation occurs due to reproductive isolation

• Prezygotic barriers

• Postzygotic barriers

Both types maintain isolation and prevent gene flow between the populations

Prezygotic barriers

Prevent mating or hinder fertilization

Five types of prezygotic barriers

• Habitat isolation

• Temporal isolation

• Behavioral isolation

• Mechanical isolation

• Gametic isolation

Habitat isolation

Species live in different areas or the occupy different habitats within the same area

• Ex) in western North America, the mountain bluebird lives and high elevation while the eastern bluebird lives at low elevation

Temporal isolation

Species breed at different times of the day, year, or season

• Ex) the western spotted skunk mates in late summer, while the eastern spotted skunk mates in late winter

Behavioral isolation

Unique behavioral patterns and rituals separate species

• Ex) the blue footed boobies will only mate after a courtship ritual

Mechanical isolation

The reproductive anatomy of one species does not fit with the anatomy of another species

• Ex) snails can have varying spirals on shells, which prevent mating

Gametic isolation

Proteins on the surface of gametes do not allow for the egg and sperm to fuse

• Ex) the sperm and eggs of red and purple sea urchins are released in the water, but they cannot fertilize each other

Postzygotic barriers

Prevent a hybrid zygote from developing into a viable, fertile adult

Three types of postzygotic barriers

• Reduced hybrid viability

• Reduced hybrid fertility

• Hybrid breakdown

Reduced hybrid viability

The genes of different parent species may interact in ways that impair the hybrid’s development or survival

• Ex) domestic sheep can fertilize domestic goats, but the hybrid embryo dies early on

Reduced hybrid fertility

A hybrid can develop into a healthy adult, but it is sterile - usually results due to differences in number of chromosomes between parents

• ex) a male donkey and a female horse can mate to produce a mule, but mules are sterile

Hybrid breakdown

The hybrid of the first generation may be sterile, but when they mate with a parent species or one another, their offspring will be sterile

• ex) farmers have tried crossing different types of cotton plants, but after the first generation the plants do not produce viable seeds

microevolution

Change in allele frequencies within a single species or population

Macroevolutioin

large evolutionary patterns (adaptive radiation, mass extinction)

• stasis: no change over long periods of time

Evolution and speciation can occur at different speeds

• Punctuated equilibrium

• Gradualism

Punctuated equilibrium

when evolution occurs rapidly after a long period of stasis

Gradualism

When evolution occurs slowly over hundred, thousands, or millions of years

Divergent evolution

Groups with the same common ancestor evolve and accumulate differences resulting in the formation of a new species

Adaptive radiation

If a new habitat or niche becomes available, species can diversify rapidly

Convergent evolution

Two different species develop similar traits despite having different ancestors

• Analogous traits

Extinction

The termination of a species

Mass extinctions

5 mass extinctions

• Human activity has affected extinctions rates

• Anytime there is ecological stress, extinction rates can quicken

• If a species does go extinct, it opens up a niche that can be exploited by a different species