Unit 7 (Speciation and Phylogeny)

Species

A group able to inerbreed and produce viable, fertile offspring

Speciation

Formation of new species

• Results in diversity of life forms

• Geography has an impact on speciation

• Two modes of speciation: Allopatric and Sympatric

Allopatric Speciation

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

Reproductive isolation

• Allows speciation to occur

• Two types: Prezygotic barriers and Postzygotic barriers

• Both maintain isolation and prevent gene flow between populations

Prezygotic barriers

Prevent mating or hinder fertilization

1. Habitat isolation

2. Temporal isolation

3. Behavioral isolation

4. Mechanical isolation

5. Gametic isolation

Postzygotic barriers

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

1. Reduced hybrid viability

2. Reduced hybrid fertility

3. Hybrid breakdown

Reduced hybrid viability

The genes of different parent species may interact in ways that impair the hybrids 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 fertile, 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 (natural and sexual selection, genetic drift, and gene flow)

Macroevolution

Large evolutionary patterns (adaptive radiation, mass extinction)

• Stasis: no change over long periods of time

Punctuated equilibrium

When evolution occurs rapidlly after a long period of stasis

Gradualism

When evolution occurs slowly over hundreds, 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

• Have occurred throughout the Earth’s history (5 mass extinctions)

• Every time 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