The Process of Speciation Chapter 16-3 .

Species A species = a group of organisms with similar characteristics that breed with one another and produce fertile offspring.

Speciation Recall: A species = a group of organisms that breed with one another and produce fertile offspring. Speciation = The evolutionary process by which new biological species arise from old ones. reproductive isolation must occur.

Reproductive Isolation = Separation / isolation of populations so that the two groups cannot interbreed to produce offspring. Genes are not exchanged so gene pools are separated. Can lead to the genesis of distinct species. What are the mechanisms of reproductive isolation?

1) Behavioural Isolation (e.g. Sexual Selection) Some mates are more desirable than others (nicer song, more spectacular feathers). Disturbs random breeding. Results in directional selection. May result in extreme characteristics

Pre-zygotic (Fertilization) Barriers – Temporal barriers (different mating seasons) Mechanical barriers (incompatible genetilia) Incompatible gametes (sperm cannot digest egg to enter)

2) Geographical barriers – Rivers, mountain ranges, roads separate a population.

After isolation occurs, natural selection usually increases the differences between the separated populations. Gene pools of the two populations become more dissimilar. New species result.

Darwin’s Finches An Example of Speciation Darwin’s finches evolved on the Galapagos islands.

Background: The finches evolved from a single ancestral species.

Each of the 14 species has body structures and behaviors adapted to different niches. Some eat seeds, others eat ticks and some are vampire finches. What caused these adaptations?

Steps in the Speciation of Darwin’s Finches

Step 1: Founding Fathers and Mothers Mainland finches were lost / blown off course by storms and arrived at island A.

2. Separation of Populations Some of the birds from island A crossed to another island (B) by storm. Island B birds and island A birds were isolated from each other.

3. Changes in a Gene Pool Over many generations, each group of birds became adapted to the needs of the environment.

4. Reproductive Isolation Each group prefers to mate with those of their own kind. Even if reintroduced to each other, they will not breed with each other.

5. Sharing the Same Island 3 possibilities: Coexistence if the two populations have different niches. Extinction if one species is better at making a living. Further evolution if enough genetic variation exists. Example One group may change to eat a different food.

Patterns of Evolution Chapter 17 - 4

How are Darwin’s finches an example of divergent or adaptive radiation? A number of different species diverged, or moved away, from a common ancestral form. Evolved a variety of characteristics enabling them to survive in different niches.

An Example of Adaptive Radiation (also known as divergent evolution)

Other Patterns of Evolution We have spoken about adaptive radiation or divergence – will now review divergence and rates of evolutionary change.

Convergent Evolution Occurs when different organisms produce species that are similar in appearance and behavior but DO NOT share a common ancestor. Why: evolve similarly due to similar environments. e.g. Shark, Ichthyosaur, Dolphin

Convergent Evolution Other examples:

Rate of Evolution Two patterns of evolution:

1. Gradualism States that evolution occurs slowly and gradually with numerous tiny advantageous modifications over long periods of time. Favored by Darwin. Supported by Lyell’s work.

2. Punctuated Equlibrium States that there are long periods of little change (stability) interrupted by brief periods of rapid change. Rapid evolution may result from dramatic changes on Earth such as changes to global climates. Can result in mass extinctions. E.g. dinasaurs Supported by the work of other scientists.

Which theory more likely explains the rate of evolutionary change? Probably both, depending on the species. Horses – gradualism Horseshoe crabs – punctuated equilibrium

Two different models for rate of evolutionary change: