Genes Within Populations Chapter 20 Copyright © McGraw-Hill Companies Permission required for reproduction or display Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Gene Variation is Raw Material of Evolution Evolution – how an entity changes through time. Darwin used the phrase “Descent With Modification” Darwin’s phrase of Biological evolution: - All species arise from other pre-existing species. However through time, species accumulate differences such that ancestral and descendent species are not identical. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Gene Variation is Raw Material Natural Selection and Evolutionary Change Some individuals in a population possess certain inherited characteristics that play a role in producing more surviving offspring than individuals without those characteristics. As a result, the population gradually includes more individuals with advantageous characteristics. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Gene Variation In Nature Measuring Levels of Genetic Variation Blood Groups – 75 genetically variable genes in humans Enzymes – 5% are heterozygous Enzyme Polymorphism A Locus with more variation that can be explained is termed polymorphic. Natural populations tend to have more polymorphic loci than can be accounted for by mutation. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Hardy-Weinberg Principle Population Genetics - Study of the properties of genes in populations. Theory of Blending Inheritance (Phenotypic Inheritance) was widely accepted. If correct new genetic variants would quickly be diluted to point of disappearance in subsequent generations. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Hardy-Weinberg Principle Hardy-Weinberg Principle - proposed by G.H. hardy & G. Weinberg to explain why genetic variation persists. Hardy-Weinberg - Original proportions of genotypes in a population will remain constant from generation to generation as long as 5 assumptions are met. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Hardy-Weinberg Principle Necessary Assumptions : Population size is very large. Random mating is occurring. No mutation takes place. No gene input from external sources. (No immigration takes place) No selection occurs Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Hardy-Weinberg Principle Calculate Genotype Frequencies With a Binomial Expansion (p+q)2 = p2 + 2pq + q2 p = Individuals homozygous for dominant allele. 2pq = Individuals heterozygous for alleles. q = Individuals homozygous for recessive allele. Because there is only two alleles, p plus q must always equal 1. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Example: Consider a population of 100 cats with 84 black and 16 white cats. Assume white is homozygous recessive, calculate allelic frequencies of the two alleles in the population and genotype frequencies. If bb = .16, then q2 = .16 and q = .4 If p + q = 1, then p = .6 Therefore, p2 = (.6)2 = .36 and 2pq = 2(.6)(.4) = .48 B = Dominant allele b = Recessive allele p2 + 2pq + q2 Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Hardy-Weinberg Equilibrium Copyright © McGraw-Hill Companies Permission required for reproduction or display Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Example: Given the allele A is present in a large random-mating population at a frequency of 54 per 100 individuals, what is the proportion of individuals in that population expected to be p2 + 2pq + q2 Heterozygous for the allele? Homozygous dominant? Homozygous recessive? p = .54 q = .46 0 .4968 = 5/10 0.2916 = 3/10 0.2116 = 2/10 Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Establishes a convenient baseline against which to measure changes. Importance of Hardy-Weinberg Principle: Establishes a convenient baseline against which to measure changes. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Five Factors that alter proportions of homozygotes & heterozygotes Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Five Agents of Evolutionary Change 1. Mutation Mutation rates are generally so low they have little effect on Hardy-Weinberg proportions of common alleles. Ultimate source of genetic variation and thus makes evolution possible. 2. Gene Flow Movement of alleles from one population to another. Tend to homogenize allele frequencies. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Example: Inbreeding (mating w/relatives) 3. Nonrandom Mating phenomenon in which individuals with certain genotypes sometimes mate with one another more commonly then would be expected on a random basis Example: Inbreeding (mating w/relatives) - cause frequencies of a particular genotype to differ greatly. - increases proportions of homoygous individuals - increases the expression of recessive alleles Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Five Agents of Evolutionary Change 4. Genetic Drift In small populations frequencies of particular alleles may change drastically by chance alone. Two causes of decreases in population size: Founder Effect - Few individuals become founders of a new populations (Small allelic pool). Bottleneck Effect - Drastic reduction in population, and gene pool size. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Raven - Johnson - Biology: 6th Ed Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Five Agents of Evolutionary Change 5. Selection Artificial - Breeders exert selection. Natural - Nature exerts selection. - Variation must exist among individuals. Variation must result in differences in numbers of viable offspring produced. Variation must be genetically inherited. Natural Selection is a process, and Evolution is the historical record of change through time; it’s an outcome. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Five Agents of Evolutionary Change Selection Pressures: Avoiding Predators Matching Climatic Condition Pesticide Resistance Copyright © McGraw-Hill Companies Permission required for reproduction or display Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Evolutionary Forces Maintaining Polymorphism Adaptive Selection Theory Heterogeneous environments produce many alleles. Neutral Theory Balance between mutation and genetic drift. Gene Flow versus Natural Selection Gene flow may decrease frequency of an allele favored by selection. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Heterozygote Advantage Heterozygote Advantage will favor heterozygotes, and maintain both alleles instead of removing less successful alleles from a population. Example: Sickle Cell Anemia Homozygotes exhibit severe anemia, have abnormal blood cells, and usually die before reproductive age. Heterozygotes are less susceptible to malaria. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Raven - Johnson - Biology: 6th Ed Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

3 kinds of Natural Selection: Disruptive Selection Forms of Selection 3 kinds of Natural Selection: Disruptive Selection Selection eliminates intermediate types. Directional Selection Selection eliminates one extreme from a phenotypic array. Stabilizing Selection Selection acts to eliminate both extremes. Fitness - Number of surviving offspring passed to the next generation. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Types of Natural Selection Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Evolution Requires Genetic Variation Limits to Selection Evolution Requires Genetic Variation 1. Intense selection may remove variation from a population at a rate greater than mutation can replenish. Thoroughbred Horses Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

2. Selection Against Rare Alleles Limits to Selection 2. Selection Against Rare Alleles As selection only operates on phenotypes, rare recessives cannot be effectively selected against except in homozygote form. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Hardy Weinberg Principle Five Agents of Evolutionary Change Review Gene Variation Hardy Weinberg Principle Five Agents of Evolutionary Change Forces Maintaining Polymorphism Heterozygote Advantage Forms of Selection Limits to Selection Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

The End Copyright © McGraw-Hill Companies Permission required for reproduction or display Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies