MICROEVOLUTIONMechanisms of

POPULATIONS• Populations are groups of individuals that

can breed with one another and are localized in certain regions.

• Some populations are isolated from others.

• Still others can easily mix with other members of a population.

POPULATIONS• Within a population, all of the genes are called

the gene pool and it consists of all alleles at a given locus.

• If only one allele exists in a population, it is said to be fixed and all individuals are homozygous.

• If more than one allele exists, then individuals are either homozygous or heterozygous.

Remember Genetics?

Let’s recap Allele Frequency &

Hardy-Weinberg

Allele Frequency

• Consider a population of 500 with 2 alleles, CR and CW

• CRCR gives Red• CWCW gives White• CRCW gives Pink

Allele Frequency

• Our Population Breakdown:

• 320 red, CRCR

• 160 Pink, CRCW

• 20 White, CWCW

• There are still only 2 alleles within our population for this trait

5 Conditions that result in Evolution:

1. Small population size2. Non-Random mating3. Mutations4. Natural selection 5. Gene Flow

These 5 conditions results in EVOLUTION, more specifically MICROEVOLUTION

Mutation and Sexual Recombination

• These provide variety within gene pools.

• Mutations are changes in the nucleotide sequences that give rise to new genes and new alleles. – Sometimes they’re good, usually they are not.

• Most mutations occur in somatic cells and are never passed on.– Only a small percentage of gametes ever get into the

populations, so any mutation occurring in the gametes likely won’t get passed on.

Mutation and Sexual Recombination

• Mutation rates in general are low. The larger the organism the less likely a mutation will occur and vice-versa.

– For example: Plants and animals with long generation times are relatively large and have a much lower frequency of mutations than do microorganism and viruses.

Mutation and Sexual Recombination• Sexual recombination is the best way to

produce variation within a population on a generation-to-generation time scale.

• Evolution is a change in allele frequency brought on by mutation and sexual recombination

• There are 3 factors which cause the most evolutionary change by altering allele frequencies

1. Natural selection2. Genetic drift3. Gene flow

Organisms More Fit to

Survive

Organisms Pass on these Traits

Change in Allele Frequency

within a Population

1. Natural Selection

As you know, when organisms are more fit to survive, they are more likely to pass on the traits that make them better suited for survival and this often changes the allele frequency within a population.

Mutation Occurs in

Organism(s)

2. Genetic Drift• Genetic drift is an unexpected fluctuation in allele

frequency from one generation to the next. • This is often due to a chance event where a large

proportion of the population is wiped out.

2. Genetic Drift

• There are two situations which increase the likelihood of genetic drift that have a large impact on a population:

A. The bottle neck effectB. The founder effect

A. The Bottle Neck Effect• A sudden change in the environment

which drastically changes a population can have a profound impact on the genetic makeup of the population.

• It may change the population in such a way that the survivors no longer represent the original population.

• The survivors are said to have gone through a “bottleneck.”

The Bottle Neck Effect

B. The Founder Effect• Occurs when a few organisms become isolated

from a large population and establish a new population whose gene pool is not reflective of the large population.

• These founders pass through an isolation bottleneck and represent a gene pool with altered allele frequencies.

3. Gene Flow• Gene flow occurs when populations gain or lose

alleles as organisms come and go within a population

• Gene flow tends to reduce differences between populations.

• Also known as Migration