Natural Selection

Charles Dawin

English naturalist who researched on the Galapagos Islands

• Biogeography

Biogeography

The geographic distribution of species

Darwin’s hypothesis

Organisms left South America and colonized the Galapagos Islands where they then diversified and gave rise to new species

Modern definition of evolution

descent with modification

Evolution

change in the genetic makeup of a population over time

• Heritable traits change from generation to generation

• To explain evolution, Darwin proposed natural selection

Natural selection

A process in which individuals that have certain traits tend to survive and reproduce at higher rates than other individuals because of those traits

What does natural selection act on

Phenotypic variations in populations

• Some phenotypes will increase or decrease an organism’s fitness

Evolutionary fitness is…

measured by reproductive success

Effects of environments on populations

• Changes in environments causes selective pressures to populations

• Biotic and abiotic factors can fluctuate and change, affecting the rate and direction of evolution

• Alleles helpful in one environment may be harmful in another because selective pressures change

Finches in Galapagos

• Abiotic change: Drought

• Changed vegetation and types of seeds available to the finches

Genetic diversity

The more genetic diversity in a population, the better it can respond to changes in the environment (more likely that individuals can withstand changes)

Low genetic diversity

Species with this are at risk of decline or extinction

Two observations of Natural Selection

1. Traits are heritable

2. More offspring are produced than can survive

Traits are heritable

• Characteristics can be passed from parent to offpspring

• Adaptations

Adaptations

Inherited characteristics of organisms that enhance their survival and reproduction

More offspring are produced than can survive

• Leads to competition for limited resources, which results in differential survival

Differential survival

The traits that lead to survival will accumulate in the population

Artificial selection

The selective breeding of domesticated plants and animals to encourage the occurrence of desirable traits

Natural vs Artificial Selection

Natural: nature selects traits that are better suited for survival and reproduction

Artificial: Humans select traits that are desirable - domestication of plants and animals

Both lead to evolutionary change