Natural Selection Natural selection is a major mechanism of evolution.

Evolution—change in a population’s genetic makeup over time.

According to Darwin’s theory of natural selection, competition for limited resources results in differential survival.

Individuals with more favorable phenotypes are more likely to survive and produce more offspring, thus passing on their traits to future generations.

On the Origin of Species by Means of Natural Selection by Charles Darwin

Darwin hypothesized that all life descended from a common ancestor.

Natural Selection: Darwin’s proposed mechanism for evolution — a population can change over time if individuals with more fit traits leave more offspring than less fit individuals

Darwin’s background Loved nature, studied to be a clergyman Went on HMS Beagle for voyage around the world Loved nature, studied to be a clergyman Went on HMS Beagle for voyage around the world

Darwin’s Voyage

Darwin observed adaptations of plants and animals and saw fossils in South America

The fossils resembled modern animals. Darwin began to consider that fossils belonged to ancestors of modern species. The fossils resembled modern animals. Darwin began to consider that fossils belonged to ancestors of modern species.

In the Galapagos Islands, Darwin saw animals that were similar to the mainland but slightly different on each island.

Darwin inferred that adaptation to environment and origin of new species are related.

Descent with modification – Darwin’s way of referring to evolution

Mayr’s summary of Darwin's theory:

OBSERVATION 1: If all individuals born reproduce successfully, a population will increase exponentially.

OBSERVATION 2: But populations remain stable.

OBSERVATION 3: Resources are limited

OBSERVATION 4: Variation is heritable

INFERENCE 1: production of more individuals than can be supported by the environment leads to a struggle for existence, with only a fraction of offspring surviving each generation

INFERENCE 2: Fitness: Individuals whose inherited traits confer an advantage have a better chance of surviving in a given environment and will leave more offspring

INFERENCE 3: Unequal fitness will lead to gradual change in a population, with favorable traits accumulating over generation

Over time, a population might eventually accumulate enough change to become a new species

Evolutionary fitness is measured by reproductive success.

Genetic variation and mutation play roles in natural selection. A diverse gene pool is important for the survival of a species in a changing environment.

Environments can be more or less stable or fluctuating, and this affects evolutionary rate and direction.

Different genetic variations can be selected in each generation.

An adaptation is a genetic variation that is favored by selection and is manifested as a trait that provides an advantage to an organism in a particular environment.

In addition to natural selection, chance and random events can influence the evolutionary process, especially for small populations.

Measuring change in a population: The Hardy-Weinberg Equilibrium Model

Mathematical approaches such as Hardy Weinberg are used to calculate changes in allele frequency, providing evidence for the occurrence of evolution in a population.

Conditions for a population or an allele to be in Hardy-Weinberg equilibrium: (1) large population (2) no migration (3) no mutations (4) random mating (5) no natural selection. (1) large population (2) no migration (3) no mutations (4) random mating (5) no natural selection. *These conditions are seldom met

p 2 + 2pq + q 2 = 1 p + q = 1 p 2 + 2pq + q 2 = 1 p + q = 1

Example: Graphical analysis of allele frequencies in a population

Applications of the Hardy-Weinberg equilibrium equation

Evolution involves changes in the gene pool. A population in Hardy-Weinberg equilibrium shows no change. The law tells us that populations maintain a reservoir of variability so that if future conditions require it, the gene pool can change. If recessive alleles were continually tending to disappear, the population would soon become homozygous. Under Hardy-Weinberg conditions, genes that have no present selective value will nonetheless be retained.