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Processes of Evolution Chapter 16 Impacts and Issues “Rise of the Super Rats” Rats Are Notorious Pests The rodenticide warfarin was very effective when it was first introduced in the 1950s Within a few years, rats developed resistance – the chemical would no longer kill Rats Evolved The resistant rats happened to inherit a gene, which made the chemical ineffective The survivors passed on the gene to their offspring Soon resistant rats were the norm This is an example of evolution Section 16.1 Early Beliefs, Confounding Discoveries 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? Travel Generates Ideas When global voyages of the 16th century revealed unusual species not known in Europe, the students of biogeography began to question, “Where do all these species ‘fit’ in the great Chain of Being” Species’ Distribution Furthermore, if all species had been created at the same time and place, why were certain species found in only some parts of the world but not others And, how did so many species get from the center of creation to islands and isolated places? Big Birds Similar-looking species, like the South American rhea, the African ostrich and the Australian emu are native to three continents Why are they so much alike? African Ostrich Fig. 16-2c, p.238 Australian Emu Fig. 16-2b, p.238 South American Rhea Fig. 16-2a, p.238 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 fossilized ankle bone Fig. 16-3b, p.239 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 Section 16.2 A Flurry of New Theories 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 Lamarck - inheritance of acquired characteristics Natural Selection Defined These are Darwin’s key observations and conclusions about evolution: 1. Natural populations have an inherent reproductive capacity to increase in numbers through successive generations 2. No population can indefinitely grow in size, because its individuals will run out of food, living space, and other resources 3. Sooner or later, individuals will end up competing for dwindling resources Natural Selection Defined These are Darwin’s key observations and conclusions about evolution: 4. Those individuals generally have the same genes encoding the same shared traits. Genes are the population’s pool of heritable information. 5. Mutations have given rise to alleles, or slightly different molecular forms of genes, which are a source of differences in phenotypic details. Natural Selection Defined These are Darwin’s key observations and conclusions about evolution: 6. Some phenotypes are better than others at helping an individual compete for resources, and to survive and reproduce. Alleles for those phenotypes increase in the population, and other alleles decrease. In time the genetic changes lead to increased fitness – an increase in adaptation to the environment. 7. Natural selection among individuals of a population is an outcome of variation in traits that affect which individuals survive and reproduce in each generation. This microevolutionary process results in adaptation, or increased fitness to the environment. 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 Galapagos Islands Darwin Wolf • Volcanic islands far off coast of Ecuador Pinta Genovesa Marchena • All inhabitants are descended from species that arrived on islands from elsewhere Santiago Bartolomé Fernandia Seymour Baltra Rabida Pinzon Santa Cruz Santa Fe Tortuga San Cristobal Isabela Española Floreana Section 16.3 Darwin’s Theory Takes Form The Theory of Uniformity Lyell’s Principles of Geology proposed a theory of uniformity –the notion of a gradual, lengthy molding of the earth’s geologic structure Challenged the view that Earth was only 6,000 years old Allowed for enough time for evolution to work The “Species Problem” Darwin returned after five years at sea and began pondering the “species problem”: What could explain the remarkable diversity among organisms? Glyptodonts & Armadillos In Argentina, Darwin observed fossils of extinct glyptodonts Animals resembled living armadillos Of all places on Earth, armadillos live only where glyptodonts had lived Would it be reasonable to think that armadillos had “descended with modification” from glyptodonts Fig. 16-6b, p.242 Fig. 16-6a, p.242 A Key Insight – Variation in Traits Thomas Malthus wrote an essay that Darwin read that suggested that as a population outgrows its resources, its members must compete for what is available Some won’t make it Those that do… Darwin felt that if some normally variant members of a population had traits that increased their survival, then nature would select those same individuals to survive, reproduce, and possibly change future populations’ traits 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 Fig. 16-7b, p.243 Fig. 16-7a, p.243 Fig. 16-7c, p.243 Fig. 16-7d, p.243 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 Fig. 16-8, p.243 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 Section 16.4 The Nature of Adaptation Adaptation: Defined Variously 1. Short-term adaptations, such as an individual plant’s stunted growth on a windy plain, last only as long as the individual does 2. Long-term adaptations have some heritable aspect that improves the odds for surviving and reproducing Said a slightly different way… Long-term adaptation is any heritable aspect of form, function, behavior, or development that contributes to the fit between an individual and its environment An adaptive trait improves the odds of surviving and reproducing, or at least it did so under conditions that prevailed when genes for the trait first evolved Salt-Tolerant Tomatoes Tomatoes originated in South American soils with a high salt content – They were adapted to these conditions Commercial tomatoes in today’s markets will not tolerate salt However, gene transfers can yield a salt-tolerant tomato that will grow in irrigated plots (see pg. 244) Section 16.5 Individuals Don’t Evolve, Populations Do 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 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 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 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 What Determines Alleles in New Individual? Mutation Crossing over at meiosis I Independent assortment Fertilization Change in chromosome number or structure On to part II of “Galapagos” Genetic Equilibrium Allele frequencies at a locus are not changing Population is not evolving Five Conditions No mutation Random mating All members survive, mate and reproduce (no selection) Large population No immigration/emigration Genetic Equilibrium Seldom Attained Because these five conditions are rarely fulfilled in natural populations, any deviation from the reference point established by the “rule” will indicate evolution is occurring Microevolution is the change in allele frequencies brought about by mutation, natural selection, gene flow and genetic drift. Hardy-Weinberg Rule At genetic equilibrium, proportions of genotypes at a locus with two alleles are given by the equation: p2 (AA) + 2pq (Aa) + q2 (aa) = 1 Frequency of allele A = p Frequency of allele a = q Punnett Square p A q p A AA(p2) Aa(pq) q Aa(pq) aa(q2) a a Frequencies in Gametes F1 genotypes: Gametes: 0.49 AA A 0.42 Aa A A 0.09 aa a a a 0.49 + 0.21 0.21 + 0.09 0.7A 0.3a No Change through Generations STARTING POPULATION THE NEXT GENERATION 490 AA butterflies dark-blue wings 490 AA butterflies dark-blue wings 490 AA butterflies dark-blue wings 420 Aa butterflies medium-blue wings 420 Aa butterflies medium-blue wings 420 Aa butterflies Medium-blue wings 90 aa butterflies white wings 90 aa butterflies white wings THE NEXT GENERATION 90 aa butterflies white wings Microevolutionary Processes Drive a population away from genetic equilibrium Small-scale changes in allele frequencies brought about by: – Natural selection – Gene flow – Genetic drift Section 16.6 Mutations Revisited Gene Mutations Infrequent but inevitable Each gene has own mutation rate Lethal mutations Neutral mutations Advantageous mutations Number of individuals in population Directional Selection Allele frequencies shift in one direction Number of individuals in population Number of individuals in 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 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 Natural Selection A difference in the survival and reproductive success of different phenotypes Acts directly on phenotypes and indirectly on genotypes Number of individuals in population Stabilizing Selection Intermediate forms are favored and extremes are eliminated Number of individuals in population Number of individuals in population Range of values for wing-color trait at time 1 Range Range of values of values for wing-color for the trait trait at time at time 22 Range of values for wing-color trait at time 3 Number of individuals in population Disruptive Selection Number of individuals in population Number of individuals in population Range of values for wing-color trait at time 1 Range of values for wing-color trait at time 2 Forms at both ends of the range of variation are favored Intermediate forms are selected against Range of values for wing-color trait at time 3 Stabilizing Selection: Another Example • Weight distribution for 13,370 human newborns (yellow curve) correlated with death rate (white curve) 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 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 HbS Allele causes sickle-cell anemia when heterozygous Heterozygotes are more resistant to malaria than homozygotes Malaria case Sickle cell trait less than 1 in 1,600 1 in 400-1,600 1 in 180-400 1 in 100-180 1 in 64-100 more than 1 in 64 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 1.0 AA in five populations 0.5 allele A lost from four populations 0 1 5 10 15 20 25 30 35 40 45 50 Generation (25 stoneflies at the start of each) Computer Simulation 1.0 0.5 allele A neither lost nor fixed 0 1 5 10 15 20 25 30 35 40 45 Generation (500 stoneflies at the start of each) 50 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 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