Biology Sylvia S. Mader Michael Windelspecht Chapter 16 How Populations Evolve Lecture Outline See separate FlexArt PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes. Copyright © McGraw-Hill Education. Permission required for reproduction or display. 1

Biology, 9th ed,Sylvia Mader Chapter 18 Outline Processes of Evolution 16.1 Genes, Populations, and Evolution 16.2 Natural Selection 16.3 Maintenance of Diversity

MRSA, a Common Skin Bacterium Resistant to Antibiotics Biology, 9th ed,Sylvia Mader Chapter 18 MRSA, a Common Skin Bacterium Resistant to Antibiotics Processes of Evolution Methicillin-resistant Staphlococcus aureus is a strain of a common skin bacterium that can be deadly. It has become resistant to antibiotics. Overuse of antibiotics resulted in the evolution of resistant bacterial strains like MRSA. These strains are also known as “superbugs.” Understanding of evolutionary biology is helping fight the superbugs. It is also an example of why evolution is important in our lives.

16.1 Genes, Populations, and Evolution Biology, 9th ed,Sylvia Mader Chapter 18 16.1 Genes, Populations, and Evolution Processes of Evolution A population is a group of organisms of a single species occupying a particular area at the same time. Diversity exists among members of a population. Population genetics is the study of this diversity in terms of allele differences. It evaluates the diversity of a population by studying genotype and phenotype frequencies over time.

Genes, Populations, and Evolution Biology, 9th ed,Sylvia Mader Chapter 18 Genes, Populations, and Evolution Processes of Evolution In the 1930s, population geneticists began to describe variations in a population in terms of alleles. Microevolution pertains to evolutionary changes within populations. Various alleles at all the gene loci in all individuals make up the gene pool of the population. The gene pool of a population can be described in terms of: Genotype frequencies Allele frequencies

The Genetic Basis of Body Color in the Peppered Moth Biology, 9th ed,Sylvia Mader The Genetic Basis of Body Color in the Peppered Moth Chapter 18 Processes of Evolution Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phenotype: Genotype: DD Dd dd homozygous heterozygous homozygous Alleles: D D D d d d chromosome

Genes, Populations, and Evolution Biology, 9th ed,Sylvia Mader Chapter 18 Genes, Populations, and Evolution Processes of Evolution Allele Frequencies It is the proportion of each allele within a population’s gene pool. Frequencies of the dominant and recessive allele must add up to 1. This relationship is described by the expression p + q = 1. p is the frequency of one allele and q is the frequency of the other. Microevolution involves a change in these allele frequencies within populations over time. If the gene frequencies do not change over time, microevolution has not occurred.

How Hardy-Weinberg Equilibrium Is Estimated Biology, 9th ed,Sylvia Mader How Hardy-Weinberg Equilibrium Is Estimated Chapter 18 Processes of Evolution Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Equilibrium genotype frequencies: Population = 25 moths, 50 alleles Gene pool: Allele frequencies: D = 10 d = 40 DD DD Dd Dd Dd D D D D D d d d D Dd Dd Dd Dd Dd D D D D d d d d d dd p + q = 1 P2 + 2pq + q2 = 1 Dd D d p = frequency of D p = frequency of D q = frequency of d q = frequency of d dd dd dd dd d d d d d d d d Frequency of D Frequency of DD p = 10/50 alleles = 0.20 p2 = freq D2 = (0.2)(0.2) = 0.04 dd dd dd dd d d d d d d d d Frequency of d Frequency of Dd q = 40/50 alleles = 0.80 2pq = 2(freq D x freq d) = 2(0.20 x 0.80) = 0.32 dd dd dd dd d d d d d d d d Frequency of dd q2 = freq d 2 = (0.08)(0.80) = 0.64 dd dd dd dd d d d d d d d d 1.00 1.00

Genes, Populations, and Evolution Biology, 9th ed,Sylvia Mader Chapter 18 Genes, Populations, and Evolution Processes of Evolution The Hardy-Weinberg Equilibrium states that: Allele frequencies in a population will remain constant assuming No Mutations No Gene Flow Random Mating No Genetic Drift No Selection

Hardy-Weinberg Equilibrium Biology, 9th ed,Sylvia Mader Hardy-Weinberg Equilibrium Chapter 18 Processes of Evolution Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. eggs 0.20 D 0.80 d 0.20 D sperm 0.04 DD 0.16 Dd 0.80 d 0.16 Dd 0.64 dd 10 Offspring

Genes, Populations, and Evolution Biology, 9th ed,Sylvia Mader Chapter 18 Genes, Populations, and Evolution Processes of Evolution Hardy-Weinberg Equilibrium: Required conditions are rarely (if ever) met. Deviations from a Hardy-Weinberg equilibrium indicate that evolution has taken place. Analysis of allele changes in populations over time determines the extent to which evolution has occurred.

Biology, 9th ed,Sylvia Mader Chapter 18 Processes of Evolution

Mechanisms of Microevolution Biology, 9th ed,Sylvia Mader Mechanisms of Microevolution Chapter 18 Processes of Evolution Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. F1 generation F2 generation Allele frequencies: Genotype frequencies: Allele frequencies: Genotype frequencies: Conclusion: DD DD D D Dd Dd dd d If... dd d Random mating No selection No migration No mutation No change in allele frequencies No change in genotype frequencies Evolution has not occurred p2 + 2pq + q2 = 1 p2 + 2pq + q2 = 1 0.20 D 0.20 D 0.80 d DD = 0.04 ...then we expect 0.80 d DD = 0.04 Dd = 0.32 Dd = 0.32 dd = 0.64 dd = 0.64 DD If... Nonrandom mating D ...then we observe Dd d dd No change in allele frequencies Genotype frequencies change Evolution has not occurred X p2 + 2pq + q2 = 1 DD X Dd 0.20 D 0.80 d DD = 0.10 or Dd = 0.20 Dd X DD dd = 0.70 dd If... Selection ...then we observe d Dd D DD Allele frequencies change Genotype frequencies change Evolution has occurred p2 + 2pq + q2 = 1 0.80 D 0.20 d DD = 0.64 Dd = 0.32 dd = 0.04

Genes, Populations, and Evolution Biology, 9th ed,Sylvia Mader Chapter 18 Genes, Populations, and Evolution Processes of Evolution Causes of Microevolution Genetic mutations Genetic mutations are the raw material for evolutionary change. They provide new alleles. Some mutations might be more adaptive than others. Example: Genetic mutations affecting pigment color in peppered moths have provided the variation needed for natural selection to occur.

Genes, Populations, and Evolution Biology, 9th ed,Sylvia Mader Chapter 18 Genes, Populations, and Evolution Processes of Evolution Causes of Microevolution Gene Flow (gene migration) Movement of alleles between populations when: Gametes or seeds (in plants) are carried into another population Breeding individuals migrate into or out of population Continual gene flow reduces genetic divergence between populations. If migration between populations doesn’t occur, the gene pools become more different over time. The differences in the genetic makeup of these populations become so large that they become reproductively isolated and incapable of interbreeding. This is the first step in species formation.

Biology, 9th ed,Sylvia Mader Chapter 18 Gene Flow Processes of Evolution

Genes, Populations, and Evolution Biology, 9th ed,Sylvia Mader Chapter 18 Genes, Populations, and Evolution Processes of Evolution Causes of Microevolution Genetic Drift Changes in the allele frequencies of a population due to change rather than selection by the environment Does not necessarily lead to adaptation to the environment Occurs by disproportionate random sampling from population Can cause the gene pools of two isolated populations to become dissimilar Some alleles are lost and others become fixed (unopposed). Likely to occur: After a bottleneck When severe inbreeding occurs, or When founders start a new population Stronger effect in small populations

Biology, 9th ed,Sylvia Mader Chapter 18 Processes of Evolution Genetic Drift Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 10% of population natural disaster kills five green frogs 20% of population

Genes, Populations, and Evolution Biology, 9th ed,Sylvia Mader Chapter 18 Genes, Populations, and Evolution Processes of Evolution Bottleneck Effect A random event prevents a majority of individuals from entering the next generation. The next generation is composed of alleles that just happened to make it. It is mostly due to natural disasters or habitat loss.

Genes, Populations, and Evolution Biology, 9th ed,Sylvia Mader Chapter 18 Genes, Populations, and Evolution Processes of Evolution Founder Effect – a type of genetic drift A new population is started from just a few individuals. The alleles carried by population founders are dictated by chance. The gene pool of the small population is much different from the gene pool of the original population.

Genes, Populations, and Evolution Biology, 9th ed,Sylvia Mader Chapter 18 Genes, Populations, and Evolution Processes of Evolution Founder Effect – a type of genetic drift (cont’d) The greater the reduction in population size, the greater the effects on allele frequencies. Formerly rare alleles will either: Occur at a higher frequency in the new population, or Be absent in new population Higher-than-normal occurrence of inbreeding (mating between relatives) occurs. Rare recessive disorders emerge more frequently in inbred populations.

Consequences of Bottleneck and Founder Effects Biology, 9th ed,Sylvia Mader Consequences of Bottleneck and Founder Effects Chapter 18 Processes of Evolution Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a. b. c. d. Original population gene pool = 3,800 alleles* Remnant population gene pool = 90 alleles* 13% 11% 8% 26% 44% 45% 53% *1 marble = 10 alleles

Genes, Populations, and Evolution Biology, 9th ed,Sylvia Mader Chapter 18 Genes, Populations, and Evolution Processes of Evolution Nonrandom Mating When individuals do not choose mates randomly Assortative mating: Individuals select mates with the same phenotype with respect to a certain characteristic. Individuals reject mates with differing phenotype. It increases the frequency of homozygotes for certain loci. It can play an important role in the evolution of a population. Example: On island of Pingelap, nonrandom mating resulted in an increased frequency of colorblindness.

Biology, 9th ed,Sylvia Mader Chapter 18 16.2 Natural Selection Processes of Evolution Natural selection is the adaptation of a population to the biotic and abiotic environment. Requires: Variation – The members of a population differ from one another. Inheritance – Many differences are heritable genetic differences. Differential Adaptiveness – Some differences affect survivability. Differential Reproduction – Some differences affect the likelihood of successful reproduction. Natural selection favors the variant that is most adaptive to the present environmental conditions.

Biology, 9th ed,Sylvia Mader Chapter 18 Natural Selection Processes of Evolution Results in: A change in allele frequencies of the gene pool Improved fitness of the population Major cause of microevolution 25

Biology, 9th ed,Sylvia Mader Chapter 18 Natural Selection Processes of Evolution Most traits are polygenic; variations in the trait result in a bell-shaped curve. Three types of selection occur: (1) Directional Selection An extreme phenotype is favored. The curve shifts in one direction. Bacteria become resistant to antibiotics. Body size increases in horse evolution. 26

Biology, 9th ed,Sylvia Mader Chapter 18 Natural Selection Processes of Evolution Three types of selection occur (cont’d): (2) Stabilizing Selection An intermediate phenotype is the most adaptive for the given environmental conditions. The peak of the curve increases and tails decrease. Example: Human babies with low or high birth weight are less likely to survive. (3) Disruptive Selection Two or more extreme phenotypes are favored over the intermediate phenotype. The curve has two peaks. Example: British land snails vary because a wide geographic range causes selection to vary. 27

Three Types of Natural Selection Biology, 9th ed,Sylvia Mader Three Types of Natural Selection Chapter 18 Processes of Evolution Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Number of Individuals Phenotype Range Phenotype Range Phenotype Range stabilizing selection directional selection disruptive selection Peak narrows. Peak shifts. Two peaks result. Number of Individuals a. b. c. 28

Human Birth Weight (stabilizing selection) Biology, 9th ed,Sylvia Mader Chapter 18 Human Birth Weight (stabilizing selection) Processes of Evolution Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 20 100 70 15 50 30 20 Percent of Births in Population Percent Infant Mortality 10 10 7 5 5 3 2 .9 1.4 1.8 2.3 2.7 3.2 3.6 4.1 4.5 Birth Weight (in kilograms) 29

Directional Selection Biology, 9th ed,Sylvia Mader Directional Selection Chapter 18 Processes of Evolution Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Initial Distribution After Time After More Time Number of Individuals Number of Individuals Number of Individuals Body Size Body Size Body Size a. Hyracotherium Merychippus Equus b. 30

Biology, 9th ed,Sylvia Mader Disruptive Selection Chapter 18 Processes of Evolution 31

Biology, 9th ed,Sylvia Mader Chapter 18 Natural Selection Processes of Evolution Sexual selection – Adaptive changes in males and females lead to an increased ability to secure a mate. With males there is an increased ability to compete with other males for a mate. Females choose to select a male with the best fitness (ability to produce surviving offspring). 32

Biology, 9th ed,Sylvia Mader Natural Selection Chapter 18 Processes of Evolution Female Choice Choice of a mate is a serious consideration because females produce few eggs. Good genes hypothesis: Females choose mates on the basis of traits that improve the chance of survival. Runaway hypothesis: Females choose mates on the basis of traits that improve male appearance. Sexual dimorphism: Males and females differ in size and other traits. Female choice can explain why male birds are more ornate than females. Remarkable plumes of males may signify health and vigor. 33

Biology, 9th ed,Sylvia Mader Natural Selection Chapter 18 Processes of Evolution Male Competition Males can father many offspring because they continuously produce sperm in great quantity. They compete to inseminate as many females as possible. Cost-benefit analysis: Is access to mating worth the competition? 34

Biology, 9th ed,Sylvia Mader Chapter 18 Dimorphism Processes of Evolution The drab females tend to choose flamboyant males as mates. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 35

Sexual Selection: Competition Biology, 9th ed,Sylvia Mader Chapter 18 Sexual Selection: Competition Processes of Evolution 36

Sexual Selection: Competition Between Male Red Deer Biology, 9th ed,Sylvia Mader Chapter 18 Sexual Selection: Competition Between Male Red Deer Processes of Evolution 37

16.3 Maintenance of Diversity Biology, 9th ed,Sylvia Mader Chapter 18 16.3 Maintenance of Diversity Processes of Evolution Genetic Variability Populations with limited variation may not be able to adapt to new conditions. Maintenance of variability is advantageous to the population. 38

Maintenance of Diversity Biology, 9th ed,Sylvia Mader Maintenance of Diversity Chapter 18 Processes of Evolution Natural selection causes imperfect adaptations. It depends on evolutionary history. Imperfections are common because of necessary compromises. Evolution doesn’t start from scratch. The processes of development restrict the emergence of novel features. Imperfections are common. The benefit of freeing our hands outweighed the cost of spinal injuries from assuming an erect posture. 39

Maintenance of Diversity Biology, 9th ed,Sylvia Mader Maintenance of Diversity Chapter 18 Processes of Evolution The environment plays a role in maintaining diversity. Disruptive selection due to environmental differences promotes polymorphisms in a population. If a population occupies a wide range, it may have several subpopulations designated as subspecies. The environment includes selecting agents that help maintain diversity. Maintenance of variation among a population’s members has survival value for the species. Example: Average beak size of Galápagos finches depended on the available food supply. 40

Subspecies Help Maintain Diversity Biology, 9th ed,Sylvia Mader Subspecies Help Maintain Diversity Chapter 18 Processes of Evolution 41

Maintenance of Diversity Biology, 9th ed,Sylvia Mader Chapter 18 Maintenance of Diversity Processes of Evolution Recessive Alleles: Heterozygotes shelter recessive alleles from selection. Heterozygotes allow even lethal alleles to remain in the population at low frequencies virtually forever. 42

Maintenance of Diversity Biology, 9th ed,Sylvia Mader Maintenance of Diversity Chapter 18 Processes of Evolution Heterozygote Advantage It assists the maintenance of genetic, and therefore phenotypic, variations in future generations. Sometimes recessive alleles confer an advantage to heterozygotes. In sickle-cell disease heterozygous individuals don’t die from sickle-cell disease, and they don’t die from malaria. The sickle-cell anemia allele is detrimental in homozygote. However, heterozygotes are more likely to survive malaria. The sickle-cell allele occurs at a higher frequency in malaria-prone areas. Malaria is caused by a protozoan parasite that lives in and destroys red blood cells of a homozygous normal individual (no sickle-cell allele). Heterozygotes (one sickle-cell allele) have an advantage. This is a form of stabilizing selection. 43

Biology, 9th ed,Sylvia Mader Sickle-Cell Disease Chapter 18 Processes of Evolution Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. malaria sickle-cell overlap of both 44