Powerpoint: Feb 4

Learning Objectives

• Explain random fluctuations in genes may produce changes in genetic frequency
• Describe how the size of the population may reduce genetic diversity
• Describe how mutations introduce new traits into a population
• Explain why mutations are most likely to establish in large populations

And because I was half asleep when I wrote these learning objectives I’m going to rewrite them in a way that’s more understandable.

• Explain how genetic drift can produce changes in genetic frequency (evolution)
• Describe how allele frequencies fluctuate more dramatically in smaller populations than larger ones
• Describe how genetic drift tends to decrease genetic diversity
• Define the bottleneck effect and founder effect
• Describe how mutations introduce new alleles (and sometimes traits) into a population

Genetic drift is defined as random fluctuations (changes) in genetic frequency due to chance events. This is also known as sampling error (when the sample of the population does not represent the diversity of the population). To picture this, imagine a bag with 5 pokeballs, 15 ultra balls and 30 master balls.

Suppose you were to draw 10 balls out of the bag. There is only one way you could draw the balls in order to have a sub-population that is representative of the original (1 pokeball, 3 ultra ball and 6 master balls). In a natural population of organisms, where each organism has hundreds of genes and each gene may have many more alleles, the probability that you will draw a population that is exactly representative of the population is next to none.

There are two types of genetic drift: bottleneck effect and founder effect.

Bottleneck effect: The bottleneck effect occurs when the population is suddenly heavily reduced. The remaining individuals in the population are unlikely to be an exact representative of the original, meaning, allele frequencies has changed. Therefore, evolution has occurred, simply because the allele frequencies has changed.

The causes of bottleneck effects vary. Natural disasters, human factors and natural boom and bust in the population (for example, algae blooms and locusts, which naturally increase and decrease in number drastically in cycles) can all cause a sudden and dramatic reduction in the population.

Founder effect: occurs when members of one population “splits off” from the original population and founds their own population. Again, the sample is unlikely to exactly reflect the allele frequency of the original.

An example of a founder effect can be seen in human migration. When Europeans left the continent of Europe to travel to North America, the population that left Europe is unlikely to exactly reflect or properly represent the population of Europe.

Genetic drift is more pronounced in a small population than a large one. Let’s use the fuzzy buns as an example. If we had a population of 200 fuzzy buns, a total of 400 alleles (since each fuzzy bun has 2 copies of DNA), and one of the fuzzy buns died, that is a loss of 2 of 400 (0.5%) of the alleles in the population. However, if the population numbered just 2, a loss of one fuzzy bun is a loss of 2 of 4 of the alleles, which equates to a 50% drop in the alleles of the population.

In the case of Genetic drift, the genetic diversity can only stay the same or decrease. Since no new alleles are being made, the reduced population is more likely to have far less alleles than the original. Going back to our poke-ball example, if we drew out a few of the balls, we can only have a genetic diversity that is equal (we draw out a master ball, a poke ball and an ultra ball) or less than the original. We can’t draw out any new “alleles” to increase the diversity (e.g. a safari ball).

Since alleles are more likely to be lost, either due to genetic drift or due to selection against the allele, if no new alleles are added to the population, more and more alleles will be lost over time. Thankfully, mutations help to replenish the diversity by adding new alleles over time.

Pay attention 0:00 to 2:10

Mutation are any changes in the DNA of the organism.

Mutations tend to arise when mistakes are made in repairing a broken piece of DNA. When repair enzymes repair the DNA, they often replace the base pairs with something entirely different, or insert/delete new nitrogenous base pairs in place. In either case, new alleles are formed. This can then lead to changes in the characteristics of the organism (e.g. cancer) or offspring of the organism (albinism or hairlessness).

Since natural selection can only act on traits that are heritable, and only the gametes (egg or sperm cells) of an organism’s body are passed to the next generation (if the organism reproduces sexually), only mutations in the gametes lead to evolutionary change.

If the organism reproduces asexually however, the mutation will be propagated into the next generation.

Handouts

Group Activity – Investigating Genetic Drift

Notes Package – Study Package (Complete by next Wednesday!)