Evolution of Populations

Five factors that can lead to evolution

1. Genetic drift

2. Gene flow

3. Mutation

4. Sexual selection

5. Natural selection

bald eagle migration

Gene flow: the movement of alleles between populations.

• Gene flow occurs when individuals join new populations and reproduce.

• Lots of gene flow: similar populations

• Limited gene flow: higher chance of new species

Genetic drift: change in allele frequencies due to chance.

• Genetic drift: small populations more affected

a. Bottleneck effect – occurs when an event

drastically reducespopulation size.

Example: In the 1800’s, northern elephant seals were overhunted. The population was reduced to about 20 individuals. Hunting has ended, and there are now about 100,000 seals. However, the population has little genetic variation.

B. Founder effect: genetic drift that occurs after start of new population

• Genetic drift has negative effects on a population.

– less likely to have some individuals that can adapt

– harmful alleles can become more common

Sexual Selection: occurs when certain traits increase mating success.

• How? Females prefer males that possess certain traits.

• There are two types of sexual selection.

– intrasexual selection: competition among males– intersexual selection: males display certain traits

to females

• Evolution and fitness

• Reproductive isolation: can occur between isolated populations.

– members of different populations cannot mate successfully

• Speciation is the rise of two or more species from one existing species.

Reproductive isolation can be caused by

1. Geographic isolation

• physical barriers divide population

• Geographic isolation

2. Temporal isolation

• timing of reproductive periods prevents mating

3. Behavioral isolation

• includes differences in courtship or mating behaviors

Example: female fireflies only respond to light patterns of males of their own species

History of Life

Fossils can form in several ways. • Permineralization occurs when minerals carried

by water are deposited around a hard structure.

• Trace fossils record the activity of an organism.

• Amber-preserved fossils are organisms that become trapped in tree resin that hardens after the tree is buried.

• Preserved remains form when an entire organism becomes encased in material such as ice.

• Specific conditions are needed for fossilization.

• Only a tiny percentage of living things became fossils.

PETRIFIED WOOD – These trees thrived in a sub tropical forest ecosystem 180-220 million of years ago. When they fell they were covered by volcanic ash that seeped into their cells. Over millions of

years the ash crystallized into a silica-quartz composite that gives it the “stone-like” quality.

Arizona - World’s largest collection of petrified wood

• Relative dating estimates the time during which an organism lived.– It compares the placement

of fossils in layers of rock.– Scientists infer the order in

which species existed.

• Radiometric dating uses decay of unstable isotopes.– Isotopes are atoms of an element that differ in their

number of neutrons.

neutrons protrons

– A half-life is the amount of time it takes for half of the isotope to decay.

• Radiometric dating uses decay of unstable isotopes.

– Isotopes are atoms of an element that differ in their number of neutrons.

Several sets of hypotheses propose how

life began on Earth. • There are two organic molecule hypotheses.

– Miller-Urey experiment

– meteorite hypothesis

electrodes

heat source amino acids

water

“atmosphere”

“ocean”

Microbes have changed the physical and chemical composition of Earth. • The oldest known fossils are a group of marine

cyanobacteria. – prokaryotic cells– added oxygen to

atmosphere– deposited minerals

Eukaryotic cells may have evolved through endosymbiosis.

• Endosymbiosis is a relationship in which one organism lives within the body of another.

• Mitochondria and chloroplasts may have developed through endosymbiosis.