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- 1. Chapter 24: Population Genetics Lesson 24.4: Genetic Drift
- 2. Learning Objectives • Describe the phenomena of the bottleneck effect and founder effect and explain how they contribute to changes in allelic frequencies. • Explain what is sometimes referred to as "survival of the luckiest."
- 3. Genetic Drift • So far, natural selection was the explanation for genetic change • What is genetic drift? • Changes in allele frequencies due to random chance • Observation that by chance, allele frequencies “drift” from one generation to the next.
- 4. Genetic Drift • Allele frequency changes occur regardless of those allele-carrying individuals and their fitness. • For example: • How is it known which alleles will be in the gametes that fuse together during fertilization? • It’s influenced by random chance!
- 5. Genetic Drift • Has a greater impact in small populations • Effects of Genetic Drift • Elimination or fixation of an allele • Depends on if allele frequency reaches 0 or 100% • Number of generations needed for this depends on population size
- 6. Genetic Drift and Population Size
- 7. Question • The relative effect of random chance (a.k.a. random sampling error) is much smaller/bigger when the sample size is large/small. • The relative effect of random chance (a.k.a. random sampling error) is much smaller when the sample size is large.
- 8. Genetic Drift Has a Greater Impact in Small Populations • In the end, regardless of population size, genetic drift = allele loss or fixation • More generations are needed in large populations than small ones • Decrease in population size could allow genetic drift to alter allele frequencies.
- 9. Bottleneck Effect • Different events can get rid of population members w/out regard to genetic composition • Events: earthquakes, floods, drought, human destruction of habitat • Bottleneck effect: allele frequencies in a population change due to genetic drift
- 10. Bottleneck Effect • 2 reasons for change: 1. Surviving population members have different allele frequencies than original population 2. When population is small, genetic drift reduces genetic variation faster a. Alleles could even get eliminated
- 11. Examples of the Bottleneck Effect
- 14. Founder Effect • Small group of individuals separates from large population, and establishes a colony in a new place. • Ex.) few people leave a population and become founders of a separate island.
- 15. Founder Effect
- 16. Founder Effect • 2 major consequences: 1. The smaller new population won’t have as much genetic variation as the original, larger population. 2. The allele frequencies in the new population will be different than the original population (by chance). • Ex. of the Founder effect: Old Order Amish population in Pennsylvania. • Group of 8,000 people left the original group. • That newer population had Ellis-van Creveld syndrome at 7% frequency rate (much higher compared to original population).
- 17. The Neutral Theory of Evolution • Majority of genetic variation is due to genetic drift instead of natural selection. • Reason: genetic drift can affect frequencies of helpful and detrimental alleles. • Genetic drift promotes neutral variation, which does not affect reproductive success.
- 18. The Neutral Theory of Evolution • Neutral theory of Evolution: most genetic variation is caused by the accumulation of neutral mutations that got high frequencies through genetic drift. • Ex.) a mutation in a gene that changes one codon won't affect the entire amino acid sequence of the encoded protein, so both genotypes are still equal in terms of fitness.
- 19. The Neutral Theory of Evolution • Non-Darwinian evolution: new mutations can spread throughout a population because of genetic drift. • a.k.a. "survival of the luckiest". • Darwin's idea: natural selection is responsible for adaptive changes during evolution is accepted • Ex.) giraffes with their long necks • Kimura's idea: most of DNA sequence variation is due to neutral variation. • At the genomic and molecular level, genome sequencing from different species is consistent with the neutral theory of evolution. • Ex.) changes of the coding sequences in structural genes
Hinweis der Redaktion
- genetic drift is a change in genetic variation from generation to generationallele frequency is a value determined by the number copies of a specific allele divided by the total number of alleles in a population
- Figure 24.10 illustrates the potential consequences of genetic drift in one large (N = 1,000) and two small (N = 10) populations. This simulation involves the frequency of hypothetical B and b alleles of a gene for fur color in a population of mice—B is the black allele, and b is the white allele.Genetic drift and population size. This graph shows three hypothetical simulations of genetic drift and their effects on small and large populations of black (B allele) and white (b allele) mice. In all cases, the starting allele frequencies are B = 0.5 and b = 0.5. The red lines illustrate two populations of mice in which N = 10; the blue line shows a population in which N = 1,000. Genetic drift eventually leads to either the elimination or fixation of alleles.At the beginning of this hypothetical simulation, which runs for 50 generations, all of these populations had identical allele frequencies: B = 0.5 and b = 0.5. In the small populations, the allele frequencies fluctuated substantially from generation to generation. Eventually, in one population, the b allele was eliminated, while in another, it was fixed at 100%. These small populations would then consist of only black mice or white mice, respectively. At this point, the gene has become monomorphic and cannot change any further. By comparison, the frequencies of B and b in the large population fluctuated much less.
- *do question first, then explain*Reviewing briefly from chapter 16The deviation between the observed and expected outcomes is called the random sampling error. With a small sample, the random sampling error may cause the observed data to be quite different from the expected outcome. By comparison, if we flipped a coin 1,000 times, the percentage of heads would be fairly close to the predicted 50%. With a larger sample, we expect the sampling error to be smaller.
- Population of frogs (shown in diagram to the right)Real-life example: the African cheetah population lost almost all genetic variation, probably due to bottleneck effect. Species came back to original numbers, but very low genetic variation
- Difference between founder effect and bottleneck effect is that the founder effect occurs in a new location, but both effects are related to a reduction in population size.
- -Japanese biologist Motoo Kimura came up with idea of “neutral theory of evolution”-neutral variation does not affect reproductive success
- - there are more nucleotides in a codon's 3rd base. 3rd base mutations are usually neutral, compared to the random, more harmful, and more likely to be eliminated mutations at the 1st or 2nd bases.
- Neutral evolution in a population. In this example, a mutation within a gene changes a glycine codon from GGG to GGC, which does not affect the amino acid sequence of the encoded protein. Each gene shown represents a copy of the gene in a member of a population. Over the course of many generations, genetic drift may cause this neutral allele to become prevalent in the population, perhaps even monomorphic.