Mechanisms of Evolution

Mechanisms of Evolution There are several: Natural Selection Gene Flow Genetic drift Mutations Non-random mating

Genetic Variation individuals in a species carry different alleles (An allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. Any change in gene (and allele) frequencies within a population or species is Evolution Allele Frequency – proportion of gene copies in a population of a given allele

1. Natural Selection: Affects variation in a population as the better adapted (more fit) individuals to their environment survive and reproduce Nature “selects” which organisms will be successful

Imagine that green beetles are easier for birds to spot (and hence, eat). Brown beetles are a little more likely to survive to produce offspring. They pass their genes for brown coloration on to their offspring. So in the next generation, brown beetles are more common than in the previous generation.

Natural Selection

4 Steps of Natural Selection: 1. In nature , more offspring are produced than can survive. 2. In any population, individuals have variation. 3. Individuals with advantageous variations survive and pass on their variations to the next generation. 4. Overtime, offspring with certain advantageous variations make up most of the population

2. Gene Flow: Is the movement of alleles into or out of a population (immigration or emigration). Gene flow can introduce new alleles into a gene pool Example: Plant pollen being blown into a new area Gene flow is what happens when two or more populations interbreed. This generally increases genetic diversity

Imagine two populations of squirrels on opposite sides of a river Imagine two populations of squirrels on opposite sides of a river. The squirrels on the west side have bushier tails than those on the east side as a result of three different genes that code for tail bushiness. If a tree falls over the river and the squirrels are able to scamper across it to mate with the other population, gene flow occurs. The next generation of squirrels on the east side may have more bushy tails than those in the previous generation, and west side squirrels might have fewer bushy tails.

3. Genetic Drift The change in allele frequencies as a result of chance processes. These changes are much more pronounced in small populations. Smaller population sizes are more susceptible to genetic drift than larger populations because there is a greater chance that a rare allele will be lost.

Imagine that in one generation, two brown beetles happened to have four offspring survive to reproduce. Several green beetles were killed when someone stepped on them and had no offspring. The next generation would have a few more brown beetles than the previous generation—but just by chance. These chance changes from generation to generation are known as genetic drift.

Examples of Genetic Drift A) The Founder Effect: A founder effect occurs when a new colony is started by a few members of original population. Small population that branches off from a larger one may or may not be genetically representative of the larger population from which it was derived. Only a fraction of the total genetic diversity of the original gene pool is represented in these few individuals.

For example, the Afrikaner population of Dutch settlers in South Africa is descended mainly from a few colonists. Today, the Afrikaner population has an unusually high frequency of the gene that causes Huntington’s disease, because those original Dutch colonists just happened to carry that gene with unusually high frequency. This effect is easy to recognize in genetic diseases, but of course, the frequencies of all sorts of genes are affected by founder events.

Examples of Genetic Drift B) Population Bottleneck: Occurs when a population undergoes an event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing. The event may eliminate alleles entirely or also cause other alleles to be over-represented in a gene pool.

Bottleneck = any kind of event that reduces the population significantly..... earthquake....flood.....disease.....etc.…

An example of a bottleneck:
Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s. Hunting reduced their population size to as few as 20 individuals at the end of the 19th century. Their population has since rebounded to over 30,000 but their genes still carry the marks of this bottleneck. They have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.

4. Mutations Are inheritable changes in DNA Mutations provide the raw material on which natural selection can act. Only source of additional genetic material and new alleles. Can be neutral, harmful or beneficial( give an individual a better chance for survival). Antibiotic resistance in bacteria is one form.

Mutation is a change in DNA the hereditary material of life Mutation is a change in DNA the hereditary material of life. An organism’s DNA affects how it looks, how it behaves, and its physiology—all aspects of its life. So a change in an organism’s DNA can cause changes in all aspects of its life. Somatic mutations occur in non-reproductive cells and won’t be passed onto offspring.

The only mutations that matter to large-scale evolution are those that can be passed on to offspring. These occur in reproductive cells like eggs and sperm and are called germ line mutations. A single germ line mutation can have a range of effects: No change occurs in phenotype.

2. Small change occurs in phenotype. 3. Big change occurs in phenotype.
 . A single mutation can also have strong negative effects for the organism. Mutations that cause the death of an organism are called lethals —and it doesn't get more negative than that.

5. Non-Random Mating In animals, non-random mating can change allele frequencies as the choice of mates is often an important part of behavior. Many plants self-pollinate, which is also a form of non-random mating (inbreeding).

Sexual selection occurs when certain traits increase mating success.

There are two types of sexual selection. intrasexual selection: competition among males intersexual selection: males display certain traits to females