• Defined: the process of biological change by which descendants come to differ from their ancestors.

o 5 factors that lead to evolution:1. Genetic drift2. Gene flow3. Mutations4. Sexual selection5. Natural Selection

Species• Species: group of organisms that can

interbreed and produce fertile offspring– Different species rarely breed– Interspecies (between different

species) breeding often results in sterile offspring

• Ex: Horse x Donkey = Mule• Ex: Lion x Tiger = Liger

• Acquired Inheritance: Early theory where organisms could acquire and pass traits after birth.– Cause: environmental changes forced individuals to

change– Problem: Traits acquired after fertilization cannot be

passed to offspring• Early Belief: Giraffes & Long Necks

– Long necks are result of stretching to reach leaves– Extra length was passed on to offspring

Geologic Change• Early belief:

– Earth ~6,000 years old– Life remained unchanged

• “New” Observations– Rock layers (strata) contained

differing fossils– Deeper fossils (older) less

complex– Environmental changes

thought to affect life characteristics

Charles Darwin

• Observed:– Organisms have variations based upon environment– Similarities between extinct fossils & modern life– Marine fossils atop mountains

• Natural Selection: Process where organisms with favorable traits survive and reproduce

• Major concept in biology published in The Origin of Species (1859)

• Finches are another example.– Finches with strong, thick beaks live in areas with a

lot of large, hard shell nuts.– Finches with delicate beaks live in areas where

small insects and fruits are readily available.

Adaptation & Variation

• Variation: Inherited traits that make an individual different from others– Green, flat wings of the insect above

• Adaptation: Variation increases the chance of survival– Ability to blend with environment

Causes of Genetic Variation

• Genetic variation increases chances some organisms survive in changing environments

• Gene Pool: All alleles within a population• Two main causes of genetic variation:

1) Mutations: Random genetic changes may affect phenotypes2) Recombination: During meiosis, genes recombine in varying patterns

Variation: 4 Different Gametes

Chromatids

copied

Chromatids

copied Crossing-

over

Crossing-

over Crossing-

Over

finished

Crossing-

Over

finished

Genetic variation!

Four differing chromatids

Genetic variation!

Four differing chromatids

• Features of a species can evolve, or change over time

• Natural selection is when favorable traits allow survival & reproduction

• Variations allow an organism to be better suited to its environment