Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Populations are the units of evolution Figure 13.6

1.What is evolving? gene pool, microevolution 2.Four agents of evolution 3. Types of natural selection

Evolution happens when populations of organisms with inherited variations are exposed to environmental factors that favor the reproductive success of some individuals over others Figure 1.6C

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Populations are the units of evolution A population is a group of interbreeding individuals A species is a group of populations whose individuals can interbreed and produce fertile offspring Figure 13.6

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings gene pool = total collection of genes in a population at any one time Microevolution is a change in the relative frequencies of alleles in a gene pool What is evolving?

Four agents of microevolution 1. Mutation changes alleles 2. Genetic drift = random changes in allele frequency Bottleneck Founder effect

LARGE POPULATION = 10,000 SMALL POPULATION = 10 allele frequency = 1,000 10,000 = 10%allele frequency = 1 10 = 10% 50% of population survives, including 450 allele carriers 50% of population survives, with no allele carrier among them allele frequency = 450 5,000 = 9% allele frequency = 0505 = 0% little change in allele frequency (no alleles lost) dramatic change in allele frequency (potential to lose one allele) Genetic drift - effects of population size :

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Founder effect Figure 13.11B, C Bottleneck effect

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings 3.Gene flow can change a gene pool due to the movement of genes into or out of a population ex. Migration 4.Natural selection leads to differential reproductive success Nonrandom mating changes genotype frequency but not allele frequency.

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Natural selection - results in the accumulation of traits that adapt a population to its environment - the only agent of evolution that results in adaptation.

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings What are sources of genetic variation? Recombination of genes in sexual reproduction Mutation can create new alleles, new genes. - homeobox genes can create major changes

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Natural selection tends to reduce variability in populations. Mechanisms which counteract: –The diploid condition preserves variation by “hiding” recessive alleles (Bb) –Balanced polymorphism (2+ phenotypes stable in population) may result from: 1. heterozygote advantage Aa > aa and AA 2. frequency-dependent selection 3. variation of environment for a population Why doesn’t natural selection eliminate all genetic variation in populations?

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Many populations exhibit polymorphism and geographic variation Figure 13.13

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Some variations may be neutral, providing no apparent advantage or disadvantage –Example: human fingerprints Not all genetic variation may be subject to natural selection Figure 13.16

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Low genetic variability may reduce their capacity to survive as humans continue to alter the environment –cheetah populations have extreme genetic uniformity Endangered species often have reduced variation Figure 13.17

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings An individual’s Darwinian fitness is the contribution it makes to the gene pool of the next generation relative to the contribution made by other individuals Production of fertile offspring is the only score that counts in natural selection What is an organism’s evolutionary fitness?

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings There are three general outcomes of natural selection Figure Frequency of individuals Original population Phenotypes (fur color) Original population Evolved population Stabilizing selectionDirectional selectionDiversifying selection

beak depth Average beak depth, 1978 Average beak depth, 1976 Beak depth (mm) Shift of average beak depth during drought Number of individuals

Figure 15.9 Medium ground finch Cactus ground finch Small tree finch Large ground finch Small ground finch Large cactus ground finch Sharp-beaked ground finch Vegetarian finch Seed eaters Ground finches Cactus flower eaters Bud eaters Tree finches Insect eaters Medium tree finch Large tree finch Mangrove finch Woodpecker finch Green warbler finch Gray warbler finch Warbler finches Common ancestor from South America mainland

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Sexual selection leads to the evolution of secondary sexual characteristics Sexual selection may produce sexual dimorphism Why do male and female animals differ in appearance? Figure 13.20A, B

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Male and female lions Figure 13.20x

Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings This is due to: –historical constraints –adaptive compromises –chance events –availability of variations Natural selection cannot fashion perfect organisms