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Download Unit 1 Notes #8 Other Mechanisms of Evolution - Mr. Lesiuk
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Unit 1 Notes #8 - Other Mechanisms Of Evolution - The number one mechanism that drives evolution is “NATURAL SELECTION”. Mutation that creates new alleles is also another significant driver of evolution. - There are some other factors/mechanisms that also disturb/change the allelic frequencies of a gene pool. 1. Genetic Drift: - Another force that drives evolution is Genetic Drift. - Genetic drift occurs when there is a random (by chance) change in the frequency of an allele in a gene pool. - Occurs most easily in small populations. - Chance mating, or luck can dramatically change the frequency of alleles in this small gene pool. Will take place more easily in the population on the left (2:4) Example: Imagine a bucket of 250 red marbles and 750 blue marbles. A handful of 30 or so marbles may or may not perfectly conform to the 3:1 ratio in the bucket. In fact, the smaller the sample, the less representative the sample will be of the normal distribution. Example: A handful of 12 marbles maybe 10 red : 2 blue, or another sample of 12 might be all blue! Small Sample Taken -Mutation along with Genetic Drift can work together to change small populations very quickly. -If the changed group is reunited with the main population, the two groups may now be too different for interbreeding. - If so, two new species have evolved from one. For Example: Imagine that you have a population of rabbits that you sampled near Enterprise Way. As a researcher you extracted DNA from 25 rabbits you trapped. In an effort to study the allelic ratios for fur coat colour you run DNA testing to determine the two alleles that each rabbit possesses. For fur coat colour there is one gene but two different alleles (forms) for this gene. One allele codes for the making of a black protein pigment. This allele is dominant and it is denoted as “B”. The other allele for this gene does not code for the production of this black pigment in the fur. This allele for white fur is recessive and is denoted as “b”. When the DNA was analyzed the following genotypes were identified: BB Bb Bb Bb BB Bb BB BB Bb Bb Bb Bb Bb Bb BB BB BB Bb Bb bb bb Bb BB BB Bb 1. What is the phenotypic ratio/percent for each phenotype in the sample? Black : ___84%____ White : ___16%____ 2. What is the genotypic ratio/percent for each genotype in this sample? BB : __36%__ Bb : __48%__ bb : __16%___ 3. As each rabbit has two alleles (one inherited from each parent), how many alleles are in the sample of 25 rabbits? Total number of alleles in sample : __50___ 4. What is the frequency/percentage of the “B” allele? _30/50 = 60%____ 5. What is the frequency/percentage of the “b” allele? _20/50 = 40%__ - Imagine that this sample of 25 rabbits is an accurate sample of the entire population of rabbits and that the actual population of rabbits numbers 200 rabbits. - In a small population like this it is very easy to get a random change to the allelic frequencies. Imagine that for no particular reason a number of black rabbits do not mate one season and a number of white rabbits mate twice that same season. The allelic frequency could totally flipflop in a very short time. This change in the gene pool was not due to natural selection or due to mutation but rather by chance. - Another example of this would be the odds of flipping a heads on a quarter versus the odds of flipping a tails on a quarter. If I give everyone a quarter and said that the quarter represented your genotype and that everyone has the following genotype: “Hh” - Where “H” = heads and “h” = tails Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh Hh - 30 Students with two alleles each, gives us a total of 60 alleles. 50% of the alleles are “H” while the other 50% of the alleles are “h”. - When each person flips their coin twice we can compare that to each person making two gametes (2 eggs or 2 sperm), if everything goes as expected we should get 30 Heads and 30 tails. If this were the case, the allelic frequency for the next generation should remain the same: “H” = 0.5 or 50% and the allelic frequency for “h” = 0.5 or 50%. - But there is a chance that we might get 42 Heads and only 18 tails. If that were the case, the gene pool of the next generation would quickly start to change as shown below: Hh HH HH hh Hh HH Hh hh HH Hh Hh HH HH Hh Hh Hh Hh Hh HH Hh HH HH HH Hh HH Hh HH HH HH Hh - Now the allelic frequency of “H” would now = 0.7 or 70% and the allelic frequency of “h” would only be 0.3 or 30%. Eventually the allele for “tails” may get weeded out all together, but for no real good reason. It just happened by chance. - In the above picture you can see that the allelic frequency shifted from 60% White : 40% Green all the way to 100% White : 0% Green. 2. Differential Migration -If a certain phenotype moves out of an area. The frequency of those alleles decreases. Emigrate – Move out Immigrate – Move in Ones that moved away: Frequency DECREASES Ones that stay: Frequency automatically INCREASES Example: Long necked giraffes on the plains. 3. Population Isolation -If a small group of individuals is separated from the main group, they may have a different frequency of alleles in their gene pool. -As the population grows, this frequency may become much different from the main group. 4. Gene Flow - Sometimes a barrier (mountain, river etc) between two different populations of a species has maintained two different gene pools. If that barrier is overcome a mixing of the two gene pools may occur causing a big disruption in the allelic frequencies that were found in the original gene pools.