Evolution of Populations PDF

The Evolution of Populations Chapter 23  Natural Selection Differential reproductive success Traits affect individual organism’s ability to survive & reproduce Individuals do NOT evolve Populations evolve and change  Microevolution Changes in allele frequen...

The Evolution of Populations Chapter 23  Natural Selection Differential reproductive success Traits affect individual organism’s ability to survive & reproduce Individuals do NOT evolve Populations evolve and change  Microevolution Changes in allele frequencies in a population 3 Main Causes Natural Selection Genetic Drift Gene Flow Before evolution of any kind can occur, we must have… Genetic Variation Differences among individuals Rock Pocket Mouse Variation within Population  Geographic Variation Differences in genetic composition of separate populations  Sources of Genetic Variation 1. Formation of New Alleles Arise through mutation A. New gene or allele created B. May be heritable, maybe not Sources of Genetic Variation 2. Sexual Recombination New genetic combination of every individual produced through sexual reproduction A. Independent Assortment ( or orientation) B. Crossing Over C. Random Fertilization  Populations Population evolve, not individuals So what is a population? Group of individuals of the same species, usually isolated, that interbreed and produce viable offspring  Gene Pool Each population = Gene pool All the copies of every type of allele at every locus existing in a population Fixed Alleles Everyone is homozygous Alleles exist in proportions in a population Frequency of alleles Wildflower Population n = 500 2 alleles exist for color Red allele and white allele Incompletely dominant Possible genotypes: C R CR C R CW C W CW  Wildflower Population n =320 n = 160 n = 20 Total n for population = 500 Alleles for color in population = 1000 CR Alleles: (320 x 2) + 160 = 800 CW Alleles: (20 x 2) + 160 = 200 Wildflower Population CR = p 800/1000 = 0.8 or 80% CW = q 200/1000 = 0.2 or 20% Frequencies must add up to 100% for a particular loci Hardy-Weinberg How can we tell if a population is evolving? Need some “baseline” measurement Hardy-Weinberg Principle Godfrey Wilhelm Hardy Weinberg Hardy-Weinberg Principle The frequency of alleles in a population remains constant from generation to generation At one locus Everything (alleles) stays the same Hardy-Weinberg Equilibrium Hardy-Weinberg Consider combinations all alleles for all crosses in a population Wildflower Population (n=500) Red flower allele = 0.8 White flower allele = 0.2  Hardy-Weinberg Calculate frequency of each genotype 2 CR alleles p x p = 0.8 x 0.8 = 0.64 64% chance CRCR Hardy-Weinberg 2 CW alleles q x q = 0.2 x 0.2 = 0.04 4% chance CWCW Hardy-Weinberg 1 CR and 1 CW allele p x q AND q x p 0.8 x 0.2 AND 0.2 x 0.8 2pq = 2 (0.8 x 0.2) = 2 (0.16) = 0.32 32% chance CRCW  Hardy-Weinberg 64% chance CRCR 4% chance CWCW 32% chance CRCW 100% total alleles Hardy-Weinberg Equation: p2 + 2pq + q2 = 1  Conditions for Hardy-Weinberg Hypothetical Populations! Based on 5 conditions: 1. No mutations 2. Random mating 3. No natural selection 4. Extremely large population size 5. No gene flow If any of the are violated, population is evolving Microevolution Change in allele frequencies within a population 3 main causes: Genetic Drift Gene Flow Natural Selection 1. Genetic Drift Unpredictable fluctuation of allele frequencies Particularly important in smaller populations Genetic Drift Can result in Bottleneck Effect Bottleneck Effect  2. Gene Flow Transfer of alleles into or out of a population Tends to reduce overall variation between populations 3. Natural Selection Only mechanism that consistently improves match between organism and environment Differential reproductive success Primary mechanism for “adaptive” evolution Survival of the Fittest 3. Natural Selection Relative Fitness Contribution an individual makes to future gene pool relative to other individuals Ability to find food Preferred territory Camouflage 3. Natural Selection Sexual Selection Certain traits increase reproductive success Can result in sexual dimorphism Size Color Ornamentation Behavior Evolutionary Changes Animation