Evolutionary Concepts

Types of Selection

Types of Evolution

Rate of Evolution

Natural Selection & Populations

Review: Natural selection can affect the

distribution of phenotypes in populations If a particular version of a trait makes

individuals better adapted, then that version of the trait will become more common in the population over time.

Overview of Selection Types (select animation tab at the top of the box)

Stabilizing Selection

Occurs when individuals with the intermediate phenotype are better adapted than individuals with either extreme version of the phenotype

Stabilizing Selection, cont’d

Example: infant birth weight in humans tends to be around 7 lb – birth weights significantly higher or lower than this put baby and / or mom at danger

For humans, both very small and very large babies have higher mortality (or lower survivorship) – as shown in the graph above (red line, right “y” axis).

•Some of this data is based on studies conducted during the first half of the 20th century.

•More recent studies show that this trend toward stabilizing selection appears to be ending; why might that be?

Directional Selection

Occurs when individuals with one extreme version of the phenotype are best adapted to the environment

Directional Selection, cont’d

Example: Average beak depth and overall body size in a population of finches was larger in the years following a drought (1977) because of the changes in which seeds were available

Directional Selection, cont’d

Ex: When DDT is sprayed to stop the spread of malaria or West Nile, mosquitoes with a high resistance to DDT are favored – so the resistance becomes more common.

Animation

Disruptive Selection

Occurs when individuals with either extreme version of the phenotype are better adapted than individuals with the intermediate phenotype

Some scientists think that this type of selection can lead to the formation of two distinct species (speciation)

Disruptive Selection, cont’d Ex: British land

snails live in field and forest. Light-banded shells are better camouflage in fields, while dark, unbanded shells are better in the forest. Snails with “inbetween” shells are not successful in either habitat.

Short Animation

Types of Evolution

Convergent Evolution – two or more species become more similar over time

Divergent Evolution – two or more species become more different over time

Convergent Evolution

Becoming more similar Happens when species face similar

environmental pressures The similar structures that result are

considered to be analogous structures; they serve a similar function in different organisms with no common ancestry

convergent evolution video clip

The falcon, bat, and pterodactyl at left have different evolutionary ancestors; however, all developed similar wings to meet similar environmental conditions / pressures.

Below, the marsupial mammals on the bottom row – found only in Australia – closely resemble placental mammals found on other continents.

Divergent Evolution

Species that used to be similar become increasingly different

Happens when species (or populations) face different environmental pressures

Can stem from adaptive radiation – when one or a few populations / species diversify to fill different ecological roles

Rate of Evolution

Two conflicting hypotheses:

• Gradualism

• Punctuated Equilibrium

Gradualism

evolution occurs continuously at a slow, steady rate

Punctuated Equilibrium

there are long stretches of time with little or no evolutionary change, interrupted by (relatively) brief periods of significant / drastic change