Microevolution Chapter 16

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

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

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

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

Early Scientific Theories 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

Confounding Evidence Biogeography Comparative anatomy Geologic discoveries

Biogeography 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 –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 Puzzling patterns: –Animals as different as whales and bats have similar bones in forelimbs –Some parts seem to have no function

Comparative Anatomy Python Human

Geological Discoveries Similar rock layers throughout world Certain layers contain fossils Deeper layers contain simpler fossils than shallow layers 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 Two examples –Georges Cuvier - multiple catastrophes –Jean Lamark - inheritance of acquired characteristics

The Theory of Uniformity Lyell’s Principles of Geology Subtle, repetitive processes of change, had shaped Earth 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 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

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

Glyptodonts & Armadillos In Argentina, Darwin observed fossils of extinct glyptodonts Animals resembled living armadillos

Malthus - Struggle to Survive 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

Galapagos Finches Darwin observed finches with a variety of lifestyles and body forms On his return he learned that there were 13 species 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 Wrote to Darwin describing his views Prompted Darwin to finally present his ideas in a formal paper

On the Origin of Species Darwin’s book Published in 1859 Laid out in great detail his evidence in support of the theory of evolution by natural selection

Missing Links If one species can evolve into another, there should be transitional forms When Darwin published his work, no such forms were known First fossil Archaeopteryx found in 1860

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

The Gene Pool All of the genes in the 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 Individuals inherit different allele combinations This leads to variation in phenotype Offspring inherit genes, NOT phenotypes

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

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

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

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

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

Frequencies in Gametes AAAaaa 0.49 AA0.42 Aa0.09 aa A0.3a F 1 genotypes: Gametes:

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

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

Reproductive Capacity & Competition All populations have the capacity to increase in numbers No population can increase indefinitely 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 Most genes occur in different forms (alleles) that produce different phenotypes Some phenotypes compete better than others

Change Over Time Over time, the alleles that produce the most successful phenotypes will increase in the population Less successful alleles will become less common 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 Stabilization of an existing range of values Disruption of an existing range of values

Directional Selection 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

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

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

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

Stabilizing Selection 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

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

Disruptive Selection Forms at both ends of the range of variation are favored 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

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

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

Balanced Polymorphism Polymorphism - “having many forms” 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 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

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

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

Computer Simulation AA in five populations allele A lost from four populations Generation (25 stoneflies at the start of each)

Computer Simulation allele A neither lost nor fixed Generation (500 stoneflies at the start of each)

Bottleneck A severe reduction in population size Causes pronounced drift 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 start a new population By chance, allele frequencies of founders may not be same as those in original population Effect is pronounced on isolated islands

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