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Population Genetics. Mind Your P’s and Q’s. I. Introduction. A. Old definition 1. Change in the look of the species over time in response to a changing environment 2. Difficult to work with experimentally 3. Hard to measure slow and gradual change. B. New Defintion of Evolution.
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Population Genetics Mind Your P’s and Q’s
I. Introduction • A. Old definition • 1. Change in the look of the species over time in response to a changing environment • 2. Difficult to work with experimentally • 3. Hard to measure slow and gradual change
B. New Defintion of Evolution • 1. changing allelic frequencies over time • 2. represents a marriage of genetics and evolutionary theory • 3. new area of biology is born • 4. population genetics • 5. allows evolution to be quantified • 6. independently arrived at by two investigators-Hardy and Weinberg
C. Definitions of Population Genetics • 1. Species • a group of organisms made up of different populations whose individuals have the ability to interbreed and produce fertile offspring
2. Population • a. localized group of individuals belonging to the same species • b. Members of the same population tend to be more closely related to eachother than to other populations • c. result of inbreeding and proximity • d. A population is the smallest unit of living organisms that can undergo evolution.
3. Gene pool • a. sum of all of the alleles present in a population • b. each individual organism donates two alleles for each gene as they are diploid • c. walk down to gym and release genes • d. If homozygous for the gene-allele is said to be fixed
4. Allelic frequencies • a. P = the frequency of the dominant allele in the gene pool • b. Q = the frequency of the recessive allele in the gene pool • c. Calculate the p and q values from the population on the right.
5. Genotypic frequencies • a. Construct punnet square • b. Fishing example • c. Values total 1
6. Example • need an example to illustrate-population of students with freckles • freckles is the dominant allele and the condition of no freckles is the recessive allele • F = freckles f = no freckles • 500 individuals in our population and let’s have 20 individuals with no freckles, 160 heterozygous with freckles, and 320 of the individuals with homozygous dominant for the trait. • we can calculate the percentage of recessive and dominant alleles in the gene pool • 500 individuals donate 1000 alleles to the gene pool • 20 individuals are ff = 40 alleles are there any other recessive alleles to be concerned with? • 160 Ff = 160 recessive alleles from this source brings up the total to 200 alleles for the recessive condition • if the recessive is 200 alleles, it must be that the dominant type makes up the rest of the 1000 which is 800 • verify • -320 FF = 640 alleles • -160 Ff = 160 alleles and the grand total is 800 alleles
7. PKU • one in 10,000 babies is born with this condition • knowing this number we can predict all of the other frequencies • this represents q2 = .0001 take the square root of both sides and you have q = .01 or 1% • p would then be equal to .99 or 99% • what are the chances of being a carrier for this trait?
II. Hardy Weinberg Equilibrium • A. Definition • B. Conditions • 1. Must have very large population sizes
3. Must be no mutation-or if mutations occur-they must be equal in the forward as well as in the backward direction
6. No natural selection essentially means that all organisms have the same fitness
C. Value of H-W equilibrium • 1. Static allelic and genotypic frequencies • 2. Represents a situation of no evolution • 3. Idealized situation • 4. Can compare natural populations to idealized • 5. Zero in on source of evolutionary pressures
A. Natural selection • Hair color comes in two varieties the dominant white color (R) and the recessive red color (r) • Make up the gene pool with 50 red pieces of plastic representing the recessive and 50 white which represent the dominant allele • What are the initial p and q values for this population • 1. p = .5 • 2. q = .5
2. Genotypic frequencies • a. Homozygous dominant • b. Heterozygous • c. Homozygous recessive
3. Conditions change-new predator arrives • the predator is a hawk who has keen vision • he can see the red rabbits very well and completely depletes their number every generation • this would represent total selection against the red color • 13 WW 25WR 12RR • 12 red rabbits are annihilated this generation
4. Recalculate p and q • compute the number of surviving rabbits = 38 rabbits • how many alleles are in the gene pool = 38 x 2 = 76 • of those 76 alleles, how many are the recessive type • this number is simply the single copy of the recessive allele that each heterozygote possesses • 25/76 = 33% is equal to the new q value • if q is equal to 33%, the new p is equal to 67% • renew the gene pool to 100 alleles reflecting the new p and q values • draw again
Sample problem • A liver disease is caused by a recessive allele. One person in one hundred possesses the condition. • What are the values of p and q in this population? What is the percentage of carriers in the population?
Sample problem • Tall is dominant to short. A group of individuals left to colonize an island probably wishing to get away from the rat race. Two short women and a short man join three heterozygous tall men on the raft. • a. What are the p and q values for the pioneering group? • b. After several years of inhabiting the island, its population grows to 2500 people. If H-W conditions were in effect, how many homozygous tall people would you find in the population? • c. A decree is handed down from the governing body which rules that all short people have to leave the island tomorrow because it is getting too crowded. What are the new p and q values of the population?
B. Mutations • 1. occur very infrequently • 2. in a large population, the values of p and q are not shifted rapidly due to mutation • 3. this is the source of evolutionary change as it produces the raw material on which natural selection operates • 4. new alleles arise as mutations
C. Migration • 1. another name for migration is gene flow-either immigration or emigration • 2. usually the numbers of individuals who move out or into a population do not change the p and q • 3. the inertia of a large population mass buffers changes brought about by gene flow • 4. obviously migration has a homogenizing effect on two separate populations if enough occurs Time_Fall_1993.jpg • 5. populations evolving under different local conditions will look very different
D. Population size • 1. population size can produce a sampling error • 2. coin flip • 3. any population that is over 10,000 individuals is relatively free from sampling error • 4. if a population becomes very small, the p and q values of the population can change by chance alone • 5. chance change in p and q values due to small population size is called genetic drift.
Sample calculations • a. allele “a” makes up 1% of the gene pool or q = .01 • b. in a population of 1,000,000 people the gene pool is 2,000,000 • c. there would be 20,000 recessive alleles in the pool • d. in a population of 100 individuals the gene pool is 200 alleles • e. there are two recessive alleles in this pool • f. you can see how it is easier to lose the two alleles than the 20,000 alleles
1. inbreeding • a. Inbreeding depression • b. Increase in homozygosity
IV. Different types of selection • A. Directional selection
D. Clinal variation • 1. Bergmann’s rule • 2. Allen’s Rule
