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Transcript
11/24/2015 Species Variation Biology 102 Lecture 15: Population Genetics • Natural variation occurs in every population of every species • Due to different alleles for genes • Evolution is measured as changes in allele and genotype frequency in a population HardyHardy -Weinberg Principle • If evolution is not at HardyHardy -Weinberg Principle • Without evolution, alleles in a population will quickly reach equilibrium • Once the frequency of alleles in a population is known, we can use the HardyHardy-Weinberg equation work, allele and genotype frequencies will stay the same from one generation to the next • p = frequency of the dominant allele in the population • q = frequency of the recessive allele in the population Where does it come from? • Assume that 2 heterozygous individuals produce offspring p q p pp (p2) pq q pq qq (q2) What does it mean? • We can calculate what proportion of individuals in the next generation will have a given genotype and phenotype • You will do this in Lab 9 1 11/24/2015 Example Example • Two alleles exist for color in a certain type of beetle. Red (R) is dominant to blue (r). • In a specific population of beetles, 51% are are red and 49% are blue. = RR and Rr = rr only p2 and pq q2 only q2 = 0.49 • Assuming the population is in HH-W equilibrium, what are the frequencies of the red and blue alleles in the gene pool? q = 0.7 p+q=1 1 – 0.7 = p p = 0.3 Example Example • Cystic fibrosis is an autosomal recessive disorder that affects 1/2500 (0.0004) Caucasians. • Assuming the population is in HH-W equilibrium, what percentage of Caucasians are carriers? = CC and Cc = cc only p2 and pq q2 only q2 = 0.0004 q = 0.02 p+q=1 1 – 0.02 = p p = 0.98 Example Check Your Work! = CC and Cc = cc only p2 + 2pq + q2 = 1 p = 0.98 p2 and pq q = 0.02 q2 only (0.98)2 + 2(0.98)(0.02) + (0.02)2 = ? Frequency of heterozygotes = 2pq 2(0.98 2(0.98)(0.02) )(0.02) = 2pq 1 0.0392 or 3.92% 2 11/24/2015 Problems • The HardyHardy-Weinberg equation makes some pretty big assumptions Problems • Adjustments can be made for these assumptions, but not for the biggest problem of all… • Organism is diploid • Only sexual reproduction is used • Only 2 alleles exist for a given gene • Complete dominance • Very large sample size • Not a sexsex-linked trait • No migration • No mutation • Random mating • No differential reproductive success IT ASSUMES NO EVOLUTION!! • Assumes… Problems • • The HardyHardy-Weinberg equation is great for… • Getting a snapshot of alleles in time • Tracking evolutionary changes over time Contibuting Factors • Five agents of evolutionary change • All bring about changes in allele frequency and disrupt HardyHardy-Weinberg equilibrium It is NOT good for predicting evolutionary changes • Genetic drift • Gene flow • Mutations • Sexual selection • Natural selection Genetic Drift • Changes in allel allele e frequency due to chance events in a small population • Example: Flipping a coin 3 out of 10 300 out of 1000 • Drift tends to reduce variation • 2 main types • Founder effect • Bottlenecking event Founder Effect • When a small group separates from a larger population • Small group might… • Carry alleles that are rare in the original population • Be missing alleles that are present in the original population 3 11/24/2015 Founder Effect • Example: Founder Effect • • A population of red and yellow beetles exists • The wind randomly blows only red beetles to a new island Example: • Ellis Ellis--Van Creveld syndrome is common among old order Amish in Lancaster County, Pennsylvania • All are descendents of one couple that founded the community in 1744 Bottlenecking Event • A catastrophe (e.g. earthquake, tsunami, etc) kills off most of a population • Handful of survivors may not represent the full genetic diversity of the original population • Reduction in variation and adaptability IMPORTANT!! • A bottlenecking event is random • A plague that kills off individuals lacking a particular allele is natural selection • Natural selection kills individuals due to genetic make make--up • Bottlenecking kills indiscriminately Bottlenecking Event • Example: • Endangered species Genetic Drift • It’s important to remember that all sources of genetic drift… • Genetic variation low • Occur in small populations • Suceptibility high • Are RANDOM 4 11/24/2015 Gene Flow • Transfer of alleles from one population to another • Due to individuals moving into or out of a population Gene Flow Examples: • Mutations • Permanent changes in the DNA sequence • Multiple sources (UV radiation, chemicals, etc) • We’ve talked about this before • A migrating bird changes flocks • Grolar bears (gene flow between species) Mutations • If mutations occur in the gametes, changes can be passed to the next generation • Frequency = 1/10,000 to 1/100,000,000 • Almost always leads to early death or lowered reproductive fitness Mutations Sexual Selection • But not always • Mating is not random • Examples: • Certain characteristics increase an organism’s chances of reproduction 5 11/24/2015 Sexual Selection • In many species, males and females look very different • Sexual dimorphism Sexual Selection • Some of these differences give a competitive advantage when competing with each other for mates • Intrasexual Intra sexual selection Sexual Selection • Some of these differences give a competitive advantage when attempting to attract mates • Intersexual Inter sexual selection 6