Microevolution involves the evolutionary changes within a population. A population is defined as all the members of a single species occupying a particular area and reproducing with one another.

The members of a population vary from one another. Variation is the raw material for evolutionary change.

What Causes Variations? MUTATIONS CROSSING OVER INDEPENDENT ASSORTMENT RANDOM FERTILIZATION Mutations are the only way to introduce new traits/alleles into a population

Godfrey Hardy and Wilhelm Weinberg created a principle that explains the conditions necessary for evolution NOT to take place in a population.

HARDY-WEINBERG principle THE PRINCIPLE TRANSLATIO N When allele frequencies remain constant, a population is in genetic equilibrium. If you don’t add or subtract traits that exist in a population, then the population won’t evolve (change)

Hardy-Weinberg equilibrium is maintained in a population if five conditions are met. This means that evolution will not happen. No mutations No gene flow (migration of alleles in or out of the population) Random mating must occur No genetic drift No natural selection

These conditions are rarely if ever met in the real world These conditions are rarely if ever met in the real world. Thus allele frequencies continually change and microevolution occurs. The value of the Hardy-Weinberg principle is that it describes the factors that cause evolution.

Therefore for evolution to take place, the following has to happen. Mutations Gene flow Genetic drift Nonrandom mating Natural selection

1. Mutations will occur for evolution to take place. Mutations are the raw material of evolutionary change. (alteration in the DNA nucleotide sequence of an allele) Mutation introduces new variation into a population. They can be harmful or beneficial.

2. Gene Flow will occur for evolution to take place. Gene flow, or gene migration, occurs when breeding members of a population leave a population or new members enter. Gene migration can introduce new alleles to populations. When new individuals enter an existing population, they also introduce all of their genetic material (DNA, traits, etc…).

3. Genetic Drift will occur for evolution to take place. Chance events that cause the allele frequency to change is called genetic drift.

An example of genetic drift is the bottleneck effect. A bottleneck occurs when an event reduces the number of organisms in a population. The variation in that population is reduced, changing the allele frequencies within the population.

Bottleneck effect is caused by a severe reduction in population size due to natural disaster, predation, or habitat reduction. Bottleneck effect causes severe reduction in total genetic diversity of the original gene pool. The cheetah bottleneck causes relative infertility because of the intense interbreeding when populations were reduced in earlier times.

The founder effect is another example of genetic drift. The founder effect occurs when combinations of alleles occur at a higher frequency in a population that has been isolated from a larger population. This is due to founding individuals containing a fraction of total genetic diversity of original population. Which particular alleles are carried by the founders is dictated by chance alone.

Genetically inherited diseases like Ellis-van Creveld are more concentrated among the Amish because they marry within their own community, which prevents new genetic variation from entering the population.

4. Nonrandom Mating must occur for evolution to take place. When males and females reproduce together strictly by chance it is called random mating. Any behavioral activity that fosters the selection of specific mates is nonrandom mating.

5. Natural Selection must occur for evolution to take place. Natural selection is the process that adapts populations to the environment. Operates to select certain phenotypes, which are passed to different generations.

Three types of selection are: DIRECTIONAL SELECTION STABILIZING SELECTION DISRUPTIVE SELECTION

Directional selection When one extreme phenotype is favored by natural selection, the distribution of the phenotype shifts in that direction. It usually comes with a change in the environment. ie) change in coat color of deer This type of selection is therefore called directional selection.

Stabilizing Selection • Stabilizing selection occurs when the intermediate phenotype is favored. This means there are more average organisms than extemes.

Disruptive Selection In disruptive selection, natural selection acts upon both extremes of the phenotype. This creates a increasing division within the population which may ultimately lead to two different phenotypes. Disruptive selection is the process that leads to speciation (creation of a new species).

DIRECTIONAL DISRUPTIVE STABILIZING

Maintenance of Variations The preservation of variation in a population is important because it provides a foundation on which natural selection can act. Variation is preserved by a variety of mentioned processes. Mutations and genetic recombination Gene flow Natural selection