Chapter 16Evolution of Populations

Two main sources of genetic variation

1. Mutations—change in genes (DNA sequence) or chromosomes

2. Gene recombination—mixing of genes that result from meiosis

and sexual reproduction

Gene Pool—the combined genetic information of all the

members of a particular populaiton

Speciation—formation of a new species through reproductive

isolation

Example: Galapagos Island finches

Ground Squirrels

**Quick Speciation Activity**

Fig. 24-6

A. harrisi A. leucurus

Types of reproductive isolation1. Behavioral isolation (sympatric)

2. Geographical isolation (allopatric)

Which type did we demonstrate in our activity?

If one of the Earth’s plates moves 1.9 cm a yr., in 1 million years it would

move 12 miles

Geographic Isolation

Some birds from species A cross to a second island.

The two populations no longer share a gene pool.

Seed sizes on the second island favor birds with large beaks.

The population on the second island evolves into population B, with larger beaks.

Changes in the Gene Pool

Fig. 22-6

(a) Cactus-eater (c) Seed-eater

(b) Insect-eater

Species—a group of similar organisms that can breed and

produce fertile offspring

Fig. 24-2a

(a) Similarity between different species

Exit Slip

• List the conditions required for a population to become a species.

Types of selection

1. Disruptive Selection • Selection that splits a population into 2

groups.

• Removes individuals with average traits, but keeps those with more extreme traits.

2. Stabilizing Selection• Eliminates extreme expressions of a trait

when the average expression leads to higher fitness.

• Most common form of natural selection.

3. Directional Selection

• An extreme version of a trait makes an organism more fit.

Height Selection Activity:

•Only extremely tall and extremely short •Only medium•Only extremely tall

4. Sexual Selection

• Operates in populations where males and females look very differently.

• Typically, males will be larger and more colorful.

Types of Evolution

Adaptive radiation (divergent evolution)—

a single species evolves into several new species that live

in different ways

Convergent evolution—

unrelated species independently

evolve similarities when adapting to

similar environments

Coevolution— two species evolve in response to changes

in each other over time

Example: mutualism

Moth pollinates the comet orchid

Rate of Evolution

Catastrophism

• Evolution occurs after a catastrophy

Gradualism

• Evolution proceeds in small, gradual steps

Punctuated Equilibrium

• Rapid spurts of genetic change that cause species to diverge quickly.

• These periods disrupt much longer periods when the species exhibit little change.

• Instances of abrupt transitions.

Chapter 17The History of Life

If one of the Earth’s plates moves 1.9 cm a yr., in 1 million years it

would move 12 miles

Fossil record—information about past life that has been obtained

from fossils-it is incomplete

1. Most organisms are now extinct

2. fossils occur in a particular order

3. groups of organisms have changed over time

Extinct—a species that has died out

Where do most fossils form?

Most fossils form in sedimentary rock as weight compresses layers

of sediment in bodies of water

Index Fossil—an easily recognized species used to compare the relative ages of

fossils

Sedimentary rocks form in horizontal layers.

When part of Earth’s crust is compressed, a bend in a rock forms, tilting the rock layers.

As the surface erodes due to water, wind, waves, or glaciers, the older rock surface is exposed.

New sediment is then deposited above the exposed older rock surface.

Water carries small rock particles to lakes and seas.

Dead organisms are buried by layers of sediment, which forms new rock.

The preserved remains may later be discovered and studied.

Relative dating—the age of a fossil is determined by comparing its placement with fossils in other

layers

The oldest layers are on the bottom

Half-life—the length of time required for half of the

radioactive atoms in a sample to decay

Carbon-14 5770Uranium-235 713 million yrsPotassium-40 1.3 billion yrsUranium-238 4.5 billion yrs

Radioactive dating—scientists use half-life to calculate the age

of fossils based on the amount of remaining radioactive isotopes

Microfossils—microscopic fossil

Mass Extinction—many types of living things become extinct

in a short period of time

Example: Dinosaurs