Evolution

General Biology

Chapters 15, 16 & 17

Darwin’s Journey• Darwin made

numerous observations and collected evidence that led him to propose what would become the theory of evolution.

Patterns of Diversity

• Darwin noticed that plants and animals seemed perfectly suited for their environment.

• He also noticed that while similar habitats existed all over the world, they were often inhabited by different types of animals (I.e. no rabbits in Australia’s grasslands- even though it was the perfect environment for them)

Lamarck’s Evolution Hypothesis

• Lamarck proposed that by selective use or disuse of organs, organisms gained or lost certain traits within their lifetime. They would pass these traits onto their offspring. Why was he wrong?

Evolution by Natural Selection• Each organism must struggle to exist- needs food,

living space, water, and ability to avoid predators• Survival of the fittest- the individual that is best

suited for its environment will survive and reproduce most successfully

• Adaptation- any inherited characteristic that increases an organism’s chance of survival

• Over time, Natural Selection results in changes in the inherited characteristics of a population. These changes increase a species’ fitness for its environment.

• Descent with Modification- each species has descended, with changes, from other species over time. All species derived from common ancestors.

Evidence of Evolution

• The Fossil Record• Geographic Distribution

of Living Species• Homologous Body

Structures (different mature form but develop from same embryonic tissue)

• Similarities in Embryology

Homologous Structures

Evolution of Populations

• Gene Pool- all genes (including different alleles) that are present in a population

• Evolution is any change in the relative frequency of alleles in a population

Sources of Genetic Variation• Mutations- Changes in

sequence of DNA (can be caused by mistakes during DNA replication or radiation/chemicals in the environment)

• Gene Shuffling- Genes are shuffled during gamete formation (Independent Assortment and Crossing-over during Meiosis)

Natural Selection on Single-Gene Traits

• If one gene controls a trait (I.e. color), and that trait is selected for or against, then the frequency of the alleles for that trait will change over time. What do you expect to happen

to the frequency of the green allele over time?

Natural Selection on Polygenic Traits

• Directional Selection: When individuals at one end of the curve have a higher fitness then those in the middle or at the other end, the population will evolve in the direction of the greatest advantage

• Stabilizing Selection: When individuals near the center of the curve have higher fitness then those at either end of the curve, the population will evolve so that the curve narrows over time (more individuals will be near the center of the curve)

• Disruptive Selection: When individuals at the upper and lower end of the curve have higher fitness than individuals near the middle, the population will evolve into two distinct phenotypes (and may eventually evolve into two different species)

Genetic Drift

• Genetic Drift: Random changes in allele frequencies due to small population size

• Founder Effect: Change in allele frequencies as a result of the migration of a small subgroup of a population in to a new area