Evolution of Populations

A. Genetic drift is a change in allele frequencies due to chance.

• Genetic drift causes a loss of genetic diversity.• It is most common in small populations.• A population bottleneck can lead to genetic drift.

– It occurs when an eventdrastically reducespopulation size.

– The bottleneck effect isgenetic drift that occursafter a bottleneck event.

The founder effect is genetic drift that occurs after start of new population

–It occurs when a few individuals start a new population.

B. Sexual selection occurs when certain traits increase mating success.

• Sexual selection occurs due to higher cost of reproduction for females.– males produce many

sperm continuously– females are more

limited in potential offspring each cycle

C. Isolation can cause speciation.

1. Geographic isolation

• physical barriers divide population

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.