Evolution Without Selection

1. Evolution Without Selection SBI3U0

2. Evolution • We have learned about how evolutionary change can be affected through natural selection • Directional selection • Disruptive Selection • Stabilizing Selection • Sexual Selection • Not all evolution is due to natural selection • Organisms can evolve without the selective pressures of nature

3. Genetic Drift • Genetic Drift • Changes in allele frequencies in a population as a result of chance • Genetic drift is most pronounced in small populations • If there is a small population it is much more likely that a particular allele will be lost or become more common • Eg: AA – Aaaa – AA Aa – aaAa – Aa F1: Aa , AaAa, Aaaa, aaaa, aa F1: AA, AA Aa, AaAa, Aa, AA, AA

4. Genetic Drift • Genetic drift ALWAYS results in a net loss of genetic diversity in a population

5. Genetic Drift

6. Genetic Drift • Under what conditions do we see genetic drift significantly impacting a population? • The two main conditions are • When populations are cut down to a small number very quickly • When small groups of organisms migrate to a new area and only breed with themselves

7. Bottlenecks • Bottleneck • When a population is drastically reduced in a short amount of time • This occurs under several natural conditions • Natural disasters (flood, earthquake, eruption, etc...) • Over hunting • The surviving members of the population will have specific alleles • The frequency of alleles in this population will likely not be identical to that of the original population

8. Bottlenecks • Since the population is so small: • Rare alleles may be more common than normal • Some alleles may not be present at all • Since the genetic diversity is so small: • Following generations will be very similar to the surviving individuals

10. Founder Effect • The founder effect occurs when a small group of individuals establishes a new population that is geographically isolated • Eg: A small group of birds flying to an island and living there • Since the new population is so small, the genetic diversity is small • Genetic drift becomes very important • Resulting generations will be genetically similar to the founders • Any alleles the founders did not have will not be present

11. Founder Effect • Since the new population is geographically isolated from other populations, they cannot interbreed • Meaning whatever alleles the founders brought are the only ones available to be passed around • Any new mutations that occur will only be present in the new population, not in the original population (before isolation)

12. Hardy-Weinberg Principle • Evolution is defined as “the changing of allele frequencies in a population” • Allele frequency refers to how often a specific allele occurs in a population • Ie: if there are 5 people, they together have 10 alleles for eye colour (2 each). If 6 of these alleles are for blue eyes, we say the allele frequency for the blue allele is 6/10, or 60% • This means that if the allele frequencies aren’t changing, a species is not evolving

13. Hardy-Weinberg Principle • Two mathematicians, Godfrey Hardy and Wilhelm Weinberg developed a mathematical relation to describe evolutionary equilibrium (when allele frequencies do not change) • Equilibrium can only occur when the following conditions are met • No natural selection (also means there must be random mating) • Small population sizes • No mutation • No immigrations or emigration • No horizontal gene transfer

14. Hardy-Weinberg Principle • In nature these conditions are NEVER met • The value of the HWP is that it gives us a standard to gage evolutionary change by • If we look at one gene with two alleles A and a • We call p the frequency of A in the population • We call q the frequency of a in the population • Thus: p + q = 1 • Therefore, if p is 0.75 (or 75%) than q is 0.25 (or 25%)

15. Hardy-Weinberg Principle • The following equation predicts the frequency of different genotypes in the population p2 + 2pq + q2 = 1 p2 = AA pq = Aa q2 = aa • Why does this equation make sense? • It is based entirely on probability

16. Hardy-Weinberg Principle • If p is the frequency of A in the gene pool, it is also the probability that a child will receive the A allele • A child with the AA genotype received both A alleles by chance • There is a p probability to receive the first A allele and a p probability to receive the second A allele • Thus a p x p or p2 probability • If we use numbers. If p = 0.75: • The probability of being AA is (0.75)(0.75) = 0.56 or 56%

17. Hardy-Weinberg Principle • A similar thing is true for aa, the probability is q2 • The heterozygous genotype Aa is more interesting • There are two ways to get this genotype • If your first allele is A, then you are Aa • If your second allele is A, then you are aA • These are functionally the same • The probability of getting either combination is pq • Thus the total probability is pq + pq or 2pq

18. Hardy-Weinberg Principle • Thus the equation is p2 + 2pq + q2 = 1 • The addition of the probabilities adds up to 1 (or 100%) since there are only three genotypes • Let’s see how we can use the Hardy-Weinberg Principle in practice

19. Example • Approximately 1 in 1700 North American Caucasian children are born with cystic fibrosis, a genetic disorder • The allele CN is normal and dominant over the cystic fibrosis allele c • What percent of the population are carriers of the cystic fibrosis allele (and not affected)? What do we know? • The frequency of cystic fibrosis, or the cc genotype, is 1 in 1700 or 1/1700 = 0.000588 (0.0588%) • This means that q2 = 0.000588 • Thus, q = √0.000588 = 0.0243 or 2.43%

20. Example • If q = 0.0243 then p = 1 – 0.0243 = 0.977 • Now that we know the frequency of the two alleles, we can calculate the probabilities of any of the three genotypes • The question asks for the heterozygous genotype CNc • The probability of having this genotype is 2pq 2pq = 2(0.0243)(0.977) = 0.0475 Or 4.75%