Microevolution Chapter 17

Selective Breeding & Evolution Evolution is genetic change in a line of descent through successive generations Evolution is genetic change in a line of descent through successive generations Selective breeding practices yield evidence that heritable changes do occur Selective breeding practices yield evidence that heritable changes do occur

Domestication of Dogs Domestication of Dogs Began about 50,000 years ago Began about 50,000 years ago 14,000 years ago - artificial selection 14,000 years ago - artificial selection –Dogs with desired forms of traits were bred Modern breeds are the result Modern breeds are the result

Results of Artificial Selection Extremes in size Extremes in size –Great Dane and Chihuahua Extremes in form Extremes in form –Short-legged dachshunds –English bulldog Short snout and compressed face Short snout and compressed face Extreme traits lead to health problems Extreme traits lead to health problems

Evolutionary Theories Widely used to interpret the past and present, and even to predict the future Widely used to interpret the past and present, and even to predict the future Reveal connections between the geological record, fossil record, and organism diversity Reveal connections between the geological record, fossil record, and organism diversity

Early Scientific Theories Hippocrates - All aspects of nature can be traced to their underlying causes Hippocrates - All aspects of nature can be traced to their underlying causes Aristotle - Each organism is distinct from all the rest and nature is a continuum or organization Aristotle - Each organism is distinct from all the rest and nature is a continuum or organization

Confounding Evidence Biogeography Biogeography Comparative anatomy Comparative anatomy Geologic discoveries Geologic discoveries

Biogeography Size of the known world expanded enormously in the 15th century Size of the known world expanded enormously in the 15th century Discovery of new organisms in previously unknown places could not be explained by accepted beliefs Discovery of new organisms in previously unknown places could not be explained by accepted beliefs –How did species get from center of creation to all these places?

Comparative Morphology Study of similarities and differences in body plans of major groups Study of similarities and differences in body plans of major groups Puzzling patterns: Puzzling patterns: –Animals as different as whales and bats have similar bones in forelimbs –Some parts seem to have no function

Geological Discoveries Similar rock layers throughout world Similar rock layers throughout world Certain layers contain fossils Certain layers contain fossils Deeper layers contain simpler fossils than shallow layers Deeper layers contain simpler fossils than shallow layers Some fossils seem to be related to known species Some fossils seem to be related to known species

19th Century - New Theories Scientists attempt to reconcile evidence of change with traditional belief in a single creation event Scientists attempt to reconcile evidence of change with traditional belief in a single creation event Two examples Two examples –Georges Cuvier - multiple catastrophes –Jean Lamarck - inheritance of acquired characteristics

The Theory of Uniformity Lyell’s Principles of Geology Lyell’s Principles of Geology Subtle, repetitive processes of change had shaped Earth Subtle, repetitive processes of change had shaped Earth Challenged the view that Earth was only 6,000 years old Challenged the view that Earth was only 6,000 years old

Darwin’s Voyage At age 22, Charles Darwin began a five- year, round-the-world voyage aboard the Beagle At age 22, Charles Darwin began a five- year, round-the-world voyage aboard the Beagle In his role as ship’s naturalist, he collected and examined the species that inhabited the regions the ship visited In his role as ship’s naturalist, he collected and examined the species that inhabited the regions the ship visited

Voyage of the Beagle EQUATOR Galapagos Islands Figure 17.4e Page 275

Galapagos Islands Isabela Darwin Wolf Pinta Marchena Genovesa Fernandia Santiago Bartolomé Rabida Pinzon Seymour Baltra Santa Cruz Santa Fe Tortuga Española San Cristobal Floreana Volcanic islands far off coast of Ecuador All inhabitants are descended from species that arrived on islands from elsewhere Figure 17.4d Page 275

Malthus - Struggle to Survive Thomas Malthus, a clergyman and economist, wrote essay that Darwin read on his return to England Thomas Malthus, a clergyman and economist, wrote essay that Darwin read on his return to England Argued that as population size increases, resources dwindle, the struggle to live intensifies, and conflict increases Argued that as population size increases, resources dwindle, the struggle to live intensifies, and conflict increases

Galapagos Finches Darwin observed finches with a variety of lifestyles and body forms Darwin observed finches with a variety of lifestyles and body forms On his return, he learned that there were 13 species On his return, he learned that there were 13 species He attempted to correlate variations in their traits with environmental challenges He attempted to correlate variations in their traits with environmental challenges

Darwin’s Theory A population can change over time when individuals differ in one or more heritable traits that are responsible for differences in the ability to survive and reproduce.

Alfred Wallace Naturalist who arrived at the same conclusions Darwin did Naturalist who arrived at the same conclusions Darwin did Wrote to Darwin describing his views Wrote to Darwin describing his views Prompted Darwin to finally present his ideas in a formal paper Prompted Darwin to finally present his ideas in a formal paper

Populations Evolve Biological evolution does not change individuals Biological evolution does not change individuals It changes a population It changes a population Traits in a population vary among individuals Traits in a population vary among individuals Evolution is change in frequency of traits Evolution is change in frequency of traits

The Gene Pool All of the genes in the population All of the genes in the population Genetic resource that is shared (in theory) by all members of population Genetic resource that is shared (in theory) by all members of population

Variation in Phenotype Each kind of gene in gene pool may have two or more alleles Each kind of gene in gene pool may have two or more alleles Individuals inherit different allele combinations Individuals inherit different allele combinations This leads to variation in phenotype This leads to variation in phenotype Offspring inherit genes, not phenotypes Offspring inherit genes, not phenotypes

What Determines Alleles in New Individual? Mutation Mutation Crossing over at meiosis I Crossing over at meiosis I Independent assortment Independent assortment Fertilization Fertilization Change in chromosome number or structure Change in chromosome number or structure

Genetic Equilibrium Allele frequencies at a locus are not changing Allele frequencies at a locus are not changing Population is not evolving Population is not evolving

Five Conditions No mutation No mutation Random mating Random mating Gene doesn’t affect survival or reproduction Gene doesn’t affect survival or reproduction Large population Large population No immigration/emigration No immigration/emigration

Microevolutionary Processes Drive a population away from genetic equilibrium Drive a population away from genetic equilibrium Small-scale changes in allele frequencies brought about by: Small-scale changes in allele frequencies brought about by: –Natural selection –Gene flow –Genetic drift

Gene Mutations Infrequent but inevitable Infrequent but inevitable Each gene has own mutation rate Each gene has own mutation rate Lethal mutations Lethal mutations Neutral mutations Neutral mutations Advantageous mutations Advantageous mutations

Hardy-Weinberg Rule At genetic equilibrium, proportions of genotypes at a locus with two alleles are given by the equation: p 2 AA + 2pq Aa + q 2 aa = 1 Frequency of allele A = p Frequency of allele a = q

Punnett Square AA(p 2 )Aa(pq) aa(q 2 ) A p a q A p a q In-text figure Page 280

Conditions for Hardy-Weinberg Single gene, there can be no sex-linkage or mutliple alleles Single gene, there can be no sex-linkage or mutliple alleles Mating must be random Mating must be random No migration into or out of population No migration into or out of population No gene changes through mutations No gene changes through mutations All genotypes must be viable, survive and produce the same number of offspring All genotypes must be viable, survive and produce the same number of offspring Population must be of infinite size Population must be of infinite size

Frequencies in Gametes AAAaaa 0.49 AA0.42 Aa0.09 aa A0.3a F 1 genotypes: Gametes: In-text figure Page 280

No Change through Generations STARTING POPULATION 490 AA butterflies Dark-blue wings 420 Aa butterflies Medium-blue wings 90 aa butterflies White wings 490 AA butterflies THE NEXT GENERATION 420 Aa butterflies 90 aa butterflies THE NEXT GENERATION 490 AA butterflies 420 Aa butterflies 90 aa butterflies NO CHANGE Figure 17.9 Page 281

Natural Selection A difference in the survival and reproductive success of different phenotypes A difference in the survival and reproductive success of different phenotypes Acts directly on phenotypes and indirectly on genotypes Acts directly on phenotypes and indirectly on genotypes

Reproductive Capacity & Competition Reproductive Capacity & Competition All populations have the capacity to increase in numbers All populations have the capacity to increase in numbers No population can increase indefinitely No population can increase indefinitely Eventually the individuals of a population will end up competing for resources Eventually the individuals of a population will end up competing for resources

Variation in Populations All individuals have the same genes that specify the same assortment of traits All individuals have the same genes that specify the same assortment of traits Most genes occur in different forms (alleles) that produce different phenotypes Most genes occur in different forms (alleles) that produce different phenotypes Some phenotypes compete better than others Some phenotypes compete better than others

Change over Time Over time, the alleles that produce the most successful phenotypes will increase in the population Over time, the alleles that produce the most successful phenotypes will increase in the population Less successful alleles will become less common Less successful alleles will become less common Change leads to increased fitness Change leads to increased fitness –Increased adaptation to environment

Results of Natural Selection Three possible outcomes: A shift in the range of values for a given trait in some direction A shift in the range of values for a given trait in some direction Stabilization of an existing range of values Stabilization of an existing range of values Disruption of an existing range of values Disruption of an existing range of values

Directional Selection Allele frequencies shift in one direction Allele frequencies shift in one direction Number of individuals in the population Range of values for the trait at time 1 Range of values for the trait at time 2 Range of values for the trait at time 3 Number of individuals in the population Number of individuals in the population Figure Page 282

Peppered Moths Peppered Moths Prior to industrial revolution, most common phenotype was light colored Prior to industrial revolution, most common phenotype was light colored After industrial revolution, dark phenotype became more common After industrial revolution, dark phenotype became more common

Pesticide Resistance Pesticides kill susceptible insects Pesticides kill susceptible insects Resistant insects survive and reproduce Resistant insects survive and reproduce If resistance has heritable basis, it becomes more common with each generation If resistance has heritable basis, it becomes more common with each generation

Antibiotic Resistance First came into use in the 1940s First came into use in the 1940s Overuse has led to increase in resistant forms Overuse has led to increase in resistant forms Most susceptible cells died out and were replaced by resistant forms Most susceptible cells died out and were replaced by resistant forms

Stabilizing Selection Intermediate forms are favored and extremes are eliminated Intermediate forms are favored and extremes are eliminated Number of individuals in the population Range of values for the trait at time 1 Range of values for the trait at time 2 Range of values for the trait at time 3 Figure Page 284

Selection for Gall Size Selection for Gall Size Gall-making fly has two major predators Gall-making fly has two major predators Wasps prey on larvae in small galls Wasps prey on larvae in small galls Birds eat larvae in large galls Birds eat larvae in large galls Flies that cause intermediate-sized galls have the highest fitness Flies that cause intermediate-sized galls have the highest fitness

Disruptive Selection Forms at both ends of the range of variation are favored Forms at both ends of the range of variation are favored Intermediate forms are selected against Intermediate forms are selected against Number of individuals in the population Range of values for the trait at time 1 Range of values for the trait at time 2 Range of values for the trait at time 3 Number of individuals in the population Number of individuals in the population Figure Page 285

African Finches Selection favors birds with very large or very small bills Selection favors birds with very large or very small bills Birds with intermediate-sized bill are less effective feeders Birds with intermediate-sized bill are less effective feeders Number of individuals Widest part of lower bill (millimeters) nestlings drought survivors Figure Page 285

Sexual Selection Selection favors certain secondary sexual characteristics Selection favors certain secondary sexual characteristics Through nonrandom mating, alleles for preferred traits increase Through nonrandom mating, alleles for preferred traits increase Leads to increased sexual dimorphism Leads to increased sexual dimorphism

Balanced Polymorphism Balanced Polymorphism Polymorphism - “having many forms” Polymorphism - “having many forms” Occurs when two or more alleles are maintained at frequencies greater than 1 percent Occurs when two or more alleles are maintained at frequencies greater than 1 percent

Sickle-Cell Trait: Heterozygote Advantage Allele Hb S causes sickle-cell anemia when heterozygous Allele Hb S causes sickle-cell anemia when heterozygous Heterozygotes are more resistant to malaria than homozygotes Heterozygotes are more resistant to malaria than homozygotes less than 1 in 1,600 1 in 400-1,600 1 in in in more than 1 in 64 Malaria case Sickle-cell trait Figure Page

Gene Flow Physical flow of alleles into a population Physical flow of alleles into a population Tends to keep the gene pools of populations similar Tends to keep the gene pools of populations similar Counters the differences that result from mutation, natural selection, and genetic drift Counters the differences that result from mutation, natural selection, and genetic drift

Genetic Drift Random change in allele frequencies brought about by chance Random change in allele frequencies brought about by chance Effect is most pronounced in small populations Effect is most pronounced in small populations Sampling error - Fewer times an event occurs, greater the variance in outcome Sampling error - Fewer times an event occurs, greater the variance in outcome

Bottleneck A severe reduction in population size A severe reduction in population size Causes pronounced drift Causes pronounced drift Example Example –Elephant seal population hunted down to just 20 individuals –Population rebounded to 30,000 –Electrophoresis revealed there is now no allele variation at 24 genes

Founder Effect Effect of drift when a small number of individuals starts a new population Effect of drift when a small number of individuals starts a new population By chance, allele frequencies of founders may not be same as those in original population By chance, allele frequencies of founders may not be same as those in original population Effect is pronounced on isolated islands Effect is pronounced on isolated islands

Inbreeding Nonrandom mating between related individuals Nonrandom mating between related individuals Leads to increased homozygosity Leads to increased homozygosity Can lower fitness when deleterious recessive alleles are expressed Can lower fitness when deleterious recessive alleles are expressed Amish, cheetahs Amish, cheetahs