Population Genetics PDF

P O P U L AT I O N GENETICS ( M I C R O E VO LU T I ON) Dr. Szuroczki Chapter 23 Key Concepts 1. Genes in Populations 2. Natural Selection 3. Sexual Selection 4. Genetic Drift 5. Migration and Nonrandom Mating Population Genetics The study of genes and genotypes in a population Comb...

P O P U L AT I O N GENETICS ( M I C R O E VO LU T I ON) Dr. Szuroczki Chapter 23 Key Concepts 1. Genes in Populations 2. Natural Selection 3. Sexual Selection 4. Genetic Drift 5. Migration and Nonrandom Mating Population Genetics The study of genes and genotypes in a population Combines natural selection, Mendel’s laws of inheritance, and newer studies in molecular genetics Population geneticists' study: Genetic variation within a gene pool How variation changes from one generation to the next Explanations to understand the variation Key Terminology 1. Gene: A DNA sequence that codes for an RNA or protein gene locus 1. Contributes to the characteristics and traits of an organism 2. A gene is found at a chromosomal locus 3. Different variants of a gene are called alleles 2. Diploid organisms typically have two copies every gene 1. If alleles are identical = homozygous 2. If alleles are different = heterozygous 3. Gene pool: All of the alleles for every gene in a given population Genotype vs. Phenotype Genotype: The combination of alleles that a person possesses at a single locus or number of loci Phenotype: Observable characteristics of a person, organ, or cell Populations Group of individuals of the same species that occupy the same environment and can interbreed with one another Some species occupy a wide geographic range and are divided into discrete populations Populations can change in size, geographic location, and genetic composition Genes Are Usually Polymorphic Many traits or genes display variation within a population = polymorphism A polymorphic gene has two or more alleles in a population, with an allele frequency >1% Most variation is due to single nucleotide polymorphisms (SNPs) = single nucleotide change Large, healthy populations exhibit a high level of genetic diversity Polymorphism in Horse Coat Colour Chestnut vs. black coat colour in horses (and many other species!) due to different alleles of the MC1R gene Affects accumulation of eumelanin (black) vs. pheomelanin (red) pigment MC1R gene Black …CTG GCC GTG TCC GAC CTG CTG… Chestnut …CTG GCC GTG TTC GAC CTG CTG…  YOU WILL NOT Allele and Genotype NEED TO KNOW HOW TO Frequencies CALCUALTE THIS FOR THE Genetic variation in populations can MIDTERM!!! be analyzed by assessing allele and genotype frequencies:  YOU WILL NOT NEED TO KNOW Example: Four o’clock plants HOW TO CALCUALTE THIS Consider a population of 100 FOR THE four-o-clock plants MIDTERM!!! 49 red-flowered plants with the genotype 42 pink-flowered plants with the genotype 9 white-flowered plants with the genotype For diploid populations, homozygotes have two copies of a given allele, and heterozygotes have one  YOU WILL NOT NEED TO KNOW Allele frequency calculations HOW TO CALCUALTE THIS FOR THE MIDTERM!!! 49 42 9  YOU WILL NOT NEED TO KNOW Allele frequency calculations HOW TO CALCUALTE THIS FOR THE MIDTERM!!! 49 42 9 Frequency of Frequency of Since there are only two alleles, we could also calculate this by 1.0 – 0.3 = 0.7 Genotype frequency YOU WILL NOT NEED TO KNOW calculations HOW TO CALCUALTE THIS Consider a population of 100 four-o-clock FOR plants THE MIDTERM!!! 49 42 9 Frequency of Frequency of Frequency of Or calculate by: 1.0 – 0.49 – 0.42 = 0.09  YOU WILL NOT NEED TO KNOW The Hardy-Weinberg Equation HOW TO CALCUALTE THIS Describes a mathematical FOR THE MIDTERM!!! relationship between allele frequencies and genotype frequencies in a population The Hardy-Weinberg equation states that: p2 and q2 are the genotype frequencies of the homozygotes 2pq is the genotype frequency of heterozygotes Multiplied by 2 because two different gamete combinations produce heterozygotes Hardy-Weinberg Equilibrium The HW equation predicts that allele and genotype frequencies will remain the same generation after generation, if the population is in equilibrium (= 0.5) No evolutionary mechanisms acting on the population Conditions that must be met for equilibrium: 1. No new mutations 2. No natural selection 3. Large population 4. No migration between different populations 5. Random mating YOU WILL NOT NEED TO KNOW HOW TO CALCUALTE THIS FOR THE MIDTERM!!! Disequilibrium In reality, populations rarely achieve equilibrium So why is Hardy-Weinberg equilibrium a useful concept? When researchers examine allele and genotype frequencies, and find that a population is not in equilibrium, this indicates a condition is being violated It acts as a baseline to identify if a population's allele frequencies are changing over time, indicating potential evolutionary forces at play Summary Video Microevolution Changes in a population’s gene pool from generation to generation, due to: Introduction of new genetic variation E.g. New mutations, gene duplication, horizontal gene transfer Generally, occur at a low rate and do not significantly disrupt HW Equilibrium Mechanisms that alter the prevalence of an allele or genotype: E.g. Natural selection, random genetic drift, migration, non-random mating Potential for widespread genetic change Factors That Govern Microevolution Natural Selection Process by which beneficial traits that are heritable become more common in successive generations Over time, natural selection results in adaptations Changes in populations of living organisms that promote their survival and reproduction in a particular environment Reproductive Success Likelihood of an individual contributing fertile offspring to the next generation Attributed to two categories of traits: 1. Traits that make organisms better adapted to their environment and more likely to survive to reproductive age 2. Traits directly associated with reproduction, such as those that affect the ability to find a mate or produce viable gametes and offspring Modern description of natural selection Allelic variation arises from random mutations Some alleles encode proteins that enhance survival or reproductive success Individuals with beneficial alleles are more likely to survive and pass these alleles to the next generation Over time, allele frequencies change through natural selection, altering the characteristics of a population Fitness Relative likelihood that a genotype will contribute to the gene pool of the next generation as compared to other genotypes Measure of reproductive success Not necessarily equal to physical fitness Mean fitness of the population: Average reproductive success of members of a population As individuals with higher fitness values become more prevalent, natural selection increases the mean fitness of the population  YOU WILL NOT NEED TO KNOW Fitness example HOW TO CALCUALTE THIS Hypothetical gene with alleles A and FOR a THE Possible genotypes: AA, Aa, aa MIDTERM!!! Suppose average reproductive successes are: AA produces 5 offspring Aa produces 4 offspring aa produces 1 offspring The genotype with highest reproductive ability has a fitness (w) of 1.0, and other genotypes are assigned values relative to this Fitness of AA = wAA = 5/5 = 1.0 Fitness of Aa = wAa = 4/5 = 0.8 Fitness of aa = waa = 1/5 = 0.2 Natural Selection Patterns 1. Directional selection 2. Stabilizing selection 3. Disruptive/Diversifying selection 4. Balancing selection Directional Selection Individuals at one extreme of a phenotypic range have greater reproductive success in a particular environment Initiators: Prolonged environmental change New allele with higher fitness introduced by mutation Causes the favoured allele to eventually predominate in a population May lead to a monomorphic gene (no variation) Example: Directional Selection Example: Directional selection Stabilizing Selection Favours the survival of individuals with intermediate phenotypes and selects against those with extreme phenotypes Example: Clutch size Too many eggs  offspring die due to lack of care and food, strain may decrease parent survival Too few eggs  does not contribute enough to next generation Diversifying Selection Favours the survival of two or more different genotypes that produce different phenotypes Likely to occur in populations that occupy heterogeneous environments Fitness value of one genotype is high in one environment, but lower in a different environment, and vice versa Members of the populations can freely interbreed Example: Diversifying Selection Balancing Selection A type of natural selection that maintains genetic diversity in a population Over many generations results in balanced polymorphism: Two or more alleles are maintained in a population over the course of many generations Two common ways this occurs: 1. Heterozygote advantage: Heterozygotes for a trait have the highest fitness E.g. Sickle cell disease 2. Negative frequency-dependent selection: The fitness of a genotype decreases when its frequency becomes higher Rare individuals have a higher fitness than common individuals E.g. Predator & prey Example: Heterozygote advantage Example: Negative frequency-dependent selection Sexual Selection A type of natural selection, in which individuals with certain traits are more likely to engage in successful reproduction than other individuals In many species, affects male characteristics more intensely than it does female Males are more variable in their reproductive success than females Results in the prevalence of traits called secondary sex characteristics, that favour reproductive success Sexual Dimorphism A significant difference between the appearances of the two sexes within a species 37 Intrasexual Selection Between members of the same sex Males directly compete for mating opportunities or territories Example: Horns in male sheep, antlers in male moose, male fiddler crab enlarged claws Intersexual Selection Between members of the opposite sex aka mate choice Often results in showy characteristics in males Cool Videos of sexual selection  Cryptic Female Choice A type of intersexual selection that occurs by female-driven mechanisms at or after mating Leads to differential success of sperm in fertilizing the egg Female guppies will May sometimes function control to inhibit inbreeding copulation to receive less sperm from less colourful males The Cost of Reproduction Sexual selection can explain traits that decrease survival but increase reproductive success If trait increases predation, its frequency may be lower in environments where predators are abundant Genetic Drift Changes in allele frequencies due to random chance Unrelated to fitness Favours either loss or fixation of an allele Frequency reaches 0% or 100% Effect is strongest in small populations, where infrequently occurring alleles face a greater chance of being lost Reduces genetic diversity May quickly alter allele frequencies after population reduction, e.g.: Population bottleneck Formation of a founder population Genetic Drift and Population Size Genetic drift Bottleneck Effect Population size is reduced dramatically, and then rebuilds Randomly eliminates members without regard to genotype When the population is small, genetic drift may rapidly reduce the genetic diversity Surviving members may have allele frequencies different from original population Founder Effect Small group of individuals separates from a larger population and establishes a colony in a new location Relatively small founding populations are expected to have less genetic variation than original population By chance, allele frequencies in founding population may differ markedly from original population Migration Gene flow: transfer of alleles into or out of a population Occurs when fertile individuals move between populations having different allele frequencies Migration tends to: Reduce differences in allele frequencies between the two populations Increase genetic diversity within a population Nonrandom Mating Individuals choose their mates based on their genotypes or phenotypes Affects the balance of genotypes predicted by Hardy- Weinberg Occurs in two forms: 1. Assortative / disassortative mating 2. Inbreeding Assortative and Disassortative mating Assortative mating: Individuals with similar phenotypes are more likely to mate Increases the proportion of homozygotes Disassortative mating: Dissimilar phenotypes mate preferentially Increases heterozygosity Inbreeding Mating of two genetically related individuals Increases homozygosity and decreases heterozygosity May have negative consequences with regard to rare recessive alleles Inbreeding Depression If homozygous offspring have a lower fitness value, inbreeding lowers the mean fitness of a population As populations shrink, inbreeding becomes more likely, producing more homozygotes that are less fit Example: Florida panther suffers from inbreeding related defects which include poor sperm quality and quantity and morphological abnormalities  53 Science, 2010. 329: 1641. Summary Video