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Chapter 13 Lecture Outline See PowerPoint Image Slides for all figures and tables pre-inserted into PowerPoint without notes. The Scientific Concept of Evolution. Evolution is the change in the frequency of genetically determined characteristics within a population over time .
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Chapter 13 Lecture Outline See PowerPoint Image Slides for all figures and tables pre-inserted into PowerPoint without notes.
The Scientific Concept of Evolution • Evolution is the change in the frequency of genetically determined characteristics within a population over time.
Early Thinking About Evolution • Early 1800’s • Lamarck suggested a process by which the changes could occur. • He proposed that acquired characteristics could be passed on to offspring. • Example: Giraffes have long necks because their ancestors stretched to reach leaves in trees. • Lamarck was wrong, but his ideas stimulated thought about evolution.
The Theory of Natural Selection • 1858 • Darwin and Wallace suggested the theory of natural selection as a mechanism for evolution. • Darwin wrote Origin of Species by Means of Natural Selection. • Thetheory of natural selection • The idea that some individuals have genetic combinations that allow them to survive, reproduce and pass their genes on to the next generation
The Voyage of HMS Beagle 1831-1836 Charles Darwin set forth in 1831 at the age of 22. The voyage Lasted nearly five years. The ship visited South America, The Galapagos Islands, Australia, and many Pacific Islands.
Darwin in the Galapogos • http://www.youtube.com/watch?v=KUshLAzXJm4&feature=related • http://www.youtube.com/watch?v=l25MBq8T77w
Assumptions of the Theory of Natural Selection • All organisms produce more offspring than can survive. • No two organisms are exactly alike. • Among organisms, there is a constant struggle for survival. • Individuals that possess favorable characteristics for their environment have a higher rate of survival and produce more offspring. • Favorable characteristics become more common in the species. • Unfavorable characteristics are lost over time.
What is the major difference between Lamark’s idea and Darwin’s theory of natural selection?
Modern Interpretations of Natural Selection • Darwin and Wallace’s theories were not immediately accepted because meiosis, genes and inheritance were poorly understood. • Mendel’s discovery provided an explanation for how characteristics could be transmitted from one generation to the next. • Knowledge of mutation, gene flow and reproductive isolation supported Darwin/Wallace’s theory. • Now we can integrate Darwin and Wallace’s hypothesis with what we know about meiosis, genes and inheritance.
Darwin and Wallace’s Basic Assumptions can be Updated • An organism’s ability to overproduce results in surplus organisms. • Due to mutation, new traits enter the gene pool and due to meiosis new combinations of alleles can be generated. • Resources such as food, soil nutrients, water, etc. are in short supply, so some individuals will not survive. • Disease, predators, etc. will also affect survival. • These are called selecting agents.
Darwin and Wallace’s Basic Assumptions can be Updated • Selecting agents favor individuals with the best combinations of alleles. • They will be more likely to survive and reproduce and pass their alleles on to the next generation. • Allele combinations that favor survival and reproduction will be more common in a population.
The Role of Natural Selection in Evolution • Natural selection will select for individuals with certain alleles. • When allele frequencies change over time, evolution has occurred. • Natural selection works on individuals, but only populations evolve. • Three factors work together to determine how a population changes over time. • Environmental factors that affect individuals • Sexual reproduction among the individuals • Genetic diversity within the gene pool
Reproductive Success • Individuals that have the combinations of alleles that allow them to successfully reproduce will pass on their alleles. • This success is measured as fitness. • Fitness is a relative measure. • An individual can successfully reproduce, but be less fit than another individual. • Involves the number of offspring produced and their viability
The Importance of Genetic Diversity • A large gene pool with great genetic diversity is more likely to contain genetic combinations that will allow some individuals to adapt to changing environments. • Characteristics that may allow individuals to adapt include • Structural, behavioral, biochemical or metabolic characteristics
Common Misunderstandings About Natural Selection • What do we mean by survival of the fittest? • What do we mean by struggle for life?
Common Misunderstandings About Natural Selection • Survival of the fittest • Individual survival is not as important as reproductive success (# of descendants). • Struggle for life • Does not refer to conflict • Refers to finding and utilizing resources
Common Misunderstandings About Natural Selection • The significance of acquired characteristics • Phenotypes acquired during the life of the individual will not be passed on. • These will not be important during natural selection. • The relationship between the mechanism of natural selection and the outcomes of the selection process • The effects of natural selection are seen in the population. • The outcome of natural selection (death, reproductive success, mate choice) involves individuals.
What influences natural selection? • Genetic diversity within a species • Genetic recombination as a result of sexual reproduction • Gene expression • The ability of a species to reproduce excess offspring
Mechanisms that Affect Genetic Diversity • Natural selection cannot occur in a population in which all of the individuals are genetically identical. • Genetic diversity is essential for natural selection. • Several mechanisms generate genetic diversity in a population. • Mutation, migration and sexual reproduction
Mutations and Migration • Spontaneous mutations are changes in DNA that cannot be tied to a specific cause. • Mutations alter existing genes and generate new alleles. • Most mutations are harmful. • Some mutations are helpful. • Mutations are only relevant to natural selection if they happen in cells that give rise to gametes. • Migration results in alleles entering and leaving a population.
Sexual Reproduction and Genetic Recombination • Sexual reproduction generates new combinations of alleles in individuals (genetic recombination). • Crossing over during meiosis generates new combinations of alleles in homologous chromosomes. • Independent assortment generates new combinations of alleles from non-homologous chromosomes. • Random fertilization results in a genetically unique individual. • Genetic recombination can generate a new combination of alleles that would give an individual a selective advantage.
Natural Selection Works on the Total Phenotype • One good allele is not enough to give an individual greater fitness. • Natural selection acts on the total phenotype which involves a combination of characteristics.
Processes that Drive Selection; Differential Survival • “Survival of the fittest” • Survival is a prerequisite of reproduction. • Individuals that do not survive cannot reproduce. • Darwin’s finches • The finch’s beak size correlated with the type of seeds they ate. • Small beaks - softer seeds • Larger beaks - harder seeds • During drought, as soft seeds became scarce, only birds with larger beaks survived.
Differential Survival of Herbicide-resistant Weeds • When herbicides are used on weeds • Only the individual plants that are resistant will survive. • As these reproduce, more and more individuals in the species are resistant to the herbicide. • Over time, the herbicide is ineffective.
Processes that Drive Selection; Differential Reproductive Rates • Survival is necessary to reproductive success, but does not guarantee reproductive success. • Reproductive success is measured by the number of offspring an individual leaves. • Clover fields • Cows eat taller plants first. • Flowers are at the top of the tall plants. • Tall plants didn’t die, but were not able to reproduce. • Therefore, shorter plants were more reproductively successful.
Processes that Drive Selection; Differential Mate Choice • Sexual selection occurs when some individuals are chosen as mates more often than others. • Relevant only in animal populations • Those that are chosen pass on more genes than those that are not chosen. • More frequently, chosen individuals are usually larger, more aggressive, more colorful, etc. • All of these features attract mates.
Examples of Sexual Selection • Male red-winged blackbirds establish territories in which they only allow females. • The ability to establish and maintain a territory determines which males will mate. • Male peacocks and chickens have elaborate tail feathers. • Males with the most spectacular tails are chosen by females more frequently.
Patterns of Selection • Natural selection changes allele frequencies in a population. • This actually reduces genetic diversity. • Over time, the individuals become more alike. • Selection can favor different phenotypes in different situations. • This leads to three different forms of selection. • Stabilizing, directional and disruptive
Stabilizing Selection • Occurs when individuals at the extremes of the range of characteristic are selected against • This means that the “average” individuals are selected for. • Brown mice selected for, white and black selected against • White and black are more likely to be noticed and eaten by predators. • Over time the population will have mostly brown mice. • Stable environments favor stabilizing selection.
What would the curve for stabilizing selection look like? • Another ex. Human birth weight
Directional Selection • Occurs when individuals of one extreme of the range of characteristic are selected for • Usually occurs when the environment is changing • Insecticide use will select for individuals that are resistant. • Originally there were probably only a few resistant individuals. • After several generations, the population will contain mostly resistant individuals.
Disruptive Selection • Occurs when both extremes of a range of characteristic is selected for • The intermediate is selected against. • Insects that live on plants with dark green or light green leaves • Medium green insects will be noticed and eaten by predators.
Evolution Without Selection —Genetic Drift • Genetic drift involves a significant change in allele frequency that is not a result of natural selection. • Results from chance events • More likely to impact small populations • In a population of 100 plants, 10 of them have red spots on their leaves. • If these 10 are randomly trampled by an animal or are killed by a late frost • The red spot allele would be lost, but not due to natural selection.
Genetic Drift • Cougars • Divided by human colonization • Small populations became isolated and experienced genetic drift. • Now the populations are endangered because their gene pools are not diverse.
Gene-frequency Studies and Hardy-Weinberg Equilibrium • G.H. Hardy and Wilhelm Weinberg described a simple mathematical relationship that can be used to study allele frequencies. • They recognized that under certain conditions, allele frequencies would notchange over time. • In this case, alleles would mix randomly and the same proportion of genotypes would result. • Their formula uses allele frequencies and genotypic frequencies to detect an equilibrium.
Hardy-Weinberg Equation • p2 + 2pq + q2 = 1 • p = frequency of the dominant allele • q = frequency of the recessive allele • p2 = frequency of homozygous dominants • 2pq = frequency of heterozygotes • q2 = frequency of homozygous recessives • If the proportion of genotypes matches this equation, then the population is not evolving. • If gene frequencies do not match this equation, then the population is evolving. • Certain conditions have to be met for a population to be in Hardy-Weinberg equilibrium.
The Conditions Necessary for H-W Equilibrium • If these five conditions are met, then gene frequencies will not change. • Random mating • No mutation • No migration • Very large population size • No natural selection • Hardy-Weinberg equilibrium provides a null hypothesis for evolution in a population. • Gives us a way to compare two populations, or two generations of a population to determine if evolution is occurring.
Determining Genotype Frequencies • Genotypic frequencies in a population can be calculated with a modified Punnett Square. • In this Punnett square, allele frequencies must be used to determine the likelihood of particular genotypes occurring in the next generation.
Why Hardy-Weinberg Conditions Rarely Exist • Since the gene pools of most populations are changing over time, H-W equilibrium rarely exists. • Why? • Random mating rarely occurs. • Spontaneous mutations occur. • Immigration and emigration introduce new alleles. • Populations are not infinitely large. • Natural selection does occur.
Using the Hardy-Weinberg Concept to Show Allele-frequency Changes
