Natural Selection

• In science, theories are statements or models that have been tested and confirmed many times.

In science, the term "Theory" does not express doubt.

• They explain a wide variety of data and observations

• They can be used to make predictions

• They are not absolute, can be changed as new evidence is found

Natural Selection

• Lamarke's Theory of Acquired Characteristics

• Some thought that you would gain or lose features if you overused or didn't use them,

•  PROVEN TO BE WRONG!

Photo courtesy of ucumari, creative commons, flickr

Example:A giraffe acquired its long neck

because its ancestor stretched higher and higher into the trees to reach leaves, and that the animal’s increasingly lengthened neck was passed on to its offspring.

Summary of Darwin’s Theory1. Organisms differ; variation is inherited

2. Organisms produce more offspring than survive

3. Organisms compete for resources

4. Organisms with advantages survive to pass those advantages to their children

5. Species alive today are descended with modifications from common ancestors

•Darwin’s main ideas can be summarized in three points.• Natural selection is differential success in

reproduction (unequal ability of individuals to survive and reproduce).

• Natural selection occurs through an interaction between the environment and the variability inherent among the individual organisms making up a population.

• The product of natural selection is the adaptation of populations of organisms to their environment.

1. Variation exists among individuals in a species.2. Individuals will compete for resources (food, mates, and space)3. Competition would lead to the death of some individuals while others would survive4. Individuals that had advantageous variations are more likely to survive and reproduce.

This process came to be known as Natural SelectionThe favorable variations are called Adaptations Photo courtesy of

digitalART2, flickr creative commons

Natural Selection

The traits that help an organism survive in a particular environment are “selected” in natural selection

Natural selection acts on the phenotype, or the observable characteristics of an organism, such that individuals with favorable phenotypes are more likely to survive and reproduce than those with less favorable phenotypes.

The phenotype's genetic basis, the genotype associated with the favorable phenotype, will increase in frequency over the following generations. Over time, this process may result in adaptations that specialize organisms for particular ecological niches and may eventually result in the emergence of new species.

• While natural selection involves interactions between individual organisms and their environment, it is populations, not individuals that evolve.

• Populations are defined as a group of interbreeding individuals of a single species that share a common geographic area.

Natural Selection and Species Fitness

Overtime, natural selection results in changes in the inherited characteristics of a population.

These changes increase a species fitness (survival rate)

• Say in a species of blob….there exists blobs of all shapes and sizes (variation)

Blobs eat the little purple organisms that live underground and on the surface.

During a particularly hot year, food became less abundant (competition), blobs that had the ability to dig into the soil to get food had a better chance of survival.

Many blobs died that year…….

The ones that survived mated and passed their genes to the next generation. (reproduction)

The next generation had move blobs with the pointed noses. That is NATURAL SELECTION.

1. Variation2. Competition3. Survival4. Reproduction

Genetic DriftGenetic drift or allelic drift is the change in

the frequency of a gene variant (allele) in a population due to random sampling.[1] The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces. A population's allele frequency is the fraction of the copies of one gene that share a particular form.[2] Genetic drift may cause gene variants to disappear completely and thereby reduce genetic variation.