ANNOUNCEMENTS Thursday February 13, 2014 Chapter 8: Evolution and Natural Selection Dr. Traver returns next week. Movie on Tuesday afternoon What Darwin Never Knew. Midterm 2 is next week 2/20/2014 Use new chapter opening photo here I will be back to moderate the debate on Tuesday 2/25/2014. Please contact Dr. Traver if you would like to sign up. Darwin s dangerous idea: evolution by natural selection Learning objectives By the end of today s topic students should be able to: identify four agents of evolutionary change and describe how they contribute to changes in allele frequencies in a population. explain the three conditions required for evolution by natural selection to occur. list five primary lines of evidence of evolution and give an example of each. Evolution in Action We can see evolution occur right before us.
Could you breed fruit flies who could live longer than 20 hours on average? When these eggs hatch, do you think the flies in this new generation will live longer than 20 hours without food?
What happened? Does evolution occur? q Evolution A genetic change in the population q The answer is an unambiguous YES. q Natural selection The consequence of certain individual organisms in a population being born with characteristics that enable them to survive better and reproduce more than the offspring of other individuals in the population q We can watch it happen in the lab whenever we want. Evolution q How does evolution occur? Darwin s Journey to an Idea q What types of changes can cause evolution in a population? q Five primary lines of evidence Before Darwin, most people believed that all species had been created separately and were unchanging. q Evolution by natural selection
Jean-Baptiste Lamarck q Biologist, early 1800s q Living species might change over time. q Geologist Charles Lyell q 1830 book Principles of Geology Geological forces had shaped the earth and were continuing to do so. q Gradual but constant change
A job on a round-the-world survey ship allowed Darwin to indulge and advance his love of nature. q Age 16, University of Edinburgh, medical studies q Studied theology at Cambridge University q His real love: study of nature Observing geographic similarities and differences among fossils and living plants and animals, Darwin developed a theory of evolution. The Galapagos Islands q Lyell s Principles of Geology q The Galapagos Islands
Two important and unexpected patterns: 1. Traits exhibited by species 2. Similarity between the fossils of extinct species and the living species in that same area q Glyptodonts and armadillos q Economist Thomas Malthus The Book that Would Rock the World q Essay on the Principle of Population q Darwin realized that favorable variations are preserved. q 1842 first draft q 14 years in a drawer
In 1859, after decades of mulling and procrastinating, Darwin published his thoughts on natural selection. q After putting off publishing his thoughts on natural selection for more than 15 years, Darwin did so only after Alfred Russel Wallace independently came up with the same idea. q They published a joint presentation on their ideas in 1858 and Darwin published a much more detailed treatment in The Origin of Species in 1859, sparking wide debate and discussion of natural selection.
Poll Time Cheetahs (large African cats) are able to run faster than 60 miles per hour when chasing prey. How would a biologist explain how the ability to run fast evolved in cheetahs, assuming their ancestors could run only 20 miles per hour? Four mechanisms can give rise to evolution. Evolution occurs when the allele frequencies in a population change. Individuals do NOT evolve. q Populations evolve q Allele frequencies q It is helpful to think of each allele as having some market share of all of the alleles.
Natural Selection q An efficient mechanism of evolution q and a powerful force in adapting populations to their environment Agents of Evolutionary Change 1. Mutation 2. Genetic drift 3. Migration 4. Natural selection q Evolution and natural selection, however, are not the same thing. Evolution is genetic change in a population. Mutation a direct change in the DNA of an individual is the ultimate source of all genetic variation. Mutation is the ultimate source of genetic variation in a population. BUT only when mutated genes are present in the germline!
What causes mutations? q The process of cells dividing can go awry. q Environmental phenomena Mutagens q Mutations are random Beneficial? Detrimental? Nearly all mutations reduce an organism s fitness. q Suppose that you have written a ten-page paper. q Randomly select one letter in the paper and change it to another letter. q Is the change more likely to make your paper better or worse? Genetic drift is a random change in allele frequencies in a population.
The important factor that distinguishes genetic drift from natural selection: The change in allele frequencies is not related to the alleles influence on reproductive success. Fixation q Genetic drift can lead to fixation for one allele for a gene in a population. q If this happens, there is no more variability in the population for this gene. q Genetic drift reduces the genetic variation in a population. Founder Effect Two special cases of genetic drift, the founder effect and population bottlenecks, are important in the evolution of populations. q A small number of individuals may leave a population and become the founding members of a new, isolated population. q The founders may have different allele frequencies than the original source population, particularly if they are a small sample.
Why are Amish people more likely to have extra fingers and toes? Population Bottlenecks The impact of genetic drift is much greater in small populations than in large populations. Migration into or out of a population may change allele frequencies. The third agent of evolutionary change
When three simple conditions are satisfied, evolution by natural selection occurs. 1. There must be variation for the particular trait within a population. 2. That variation must be inheritable. 3. Individuals with one version of the trait must produce more offspring than those with a different version of the trait. Condition 1: Variation for a Trait q Variation is all around us. q Variation is the raw material on which evolution feeds. Condition 2: Heritability We call the transmission of traits from parents to their children through genetic information inheritance or heritability.
Condition 3: Differential Reproductive Success Condition 3: Differential Reproductive Success 1. There are more organisms born than can survive. 2. Organisms are continually struggling for existence. Condition 3: Differential Reproductive Success 3. Some organisms are more likely to win this struggle and survive and reproduce. Differential Reproductive Success From all the variation existing in a population, individuals with traits most suited to reproduction in their environment generally leave more offspring than individuals with other traits.
Reproductive Success Traits causing some individuals to have more offspring than others become more prevalent in the population. q Fitness A measure of the relative amount of reproduction of an individual with a particular phenotype, compared with the reproductive output of individuals of the same species with alternative phenotypes
There are three important elements to an organism s fitness: 1. An individual s fitness is measured relative to other genotypes or phenotypes in the population. There are three important elements to an organism s fitness: 2. Fitness depends on the specific environment in which the organism lives. There are three important elements to an organism s fitness: Organisms in a population can become better matched to their environment through natural selection. 3. Fitness depends on an organism s reproductive success compared to other organisms in the population.
Natural selection does not lead to perfect organisms. Why doesn t natural selection lead to the production of perfect organisms? Factors that Prevent Populations from Progressing Inevitably Toward Perfection 1. Environments change quickly. 2. Variation is needed as the raw material of selection. Artificial selection is just a special case of natural selection. 3. There may be multiple different alleles for a trait, each causing an individual to have the same fitness.
q Because the differential reproductive success is determined by humans and not nature, this type of natural selection is also called artificial selection. Natural selection can change the traits in a population in several ways Directional Selection Individuals with one extreme from the range of variation in the population have higher fitness. Turkeys on poultry farms have such large breast muscles that they can t get close enough to each other to mate. How can such a trait evolve?
Stabilizing Selection Individuals with intermediate phenotypes are most fit. Disruptive Selection Individuals with extreme phenotypes experience the highest fitness, and those with intermediate phenotypes have the lowest. Natural selection can cause the evolution of complex traits and behaviors.
How can a wing evolve if 1% of a wing doesn t help an organism fly or glide at all? Often, structures appear because they serve some other purpose. Functional Shifts The evidence for evolution is overwhelming. It is indeed remarkable that this theory [evolution] has been progressively accepted by researchers, following a series of discoveries in various fields of knowledge. The convergence, neither sought nor fabricated, of the results of work that was conducted independently is in itself a significant argument in favor of this theory. Pope John Paul II, 1996
Five primary lines of evidence: 1. The fossil record 2. Biogeography 3. Comparative anatomy and embryology The fossil record documents the process of natural selection. 4. Molecular biology 5. Laboratory and field experiments
Comparative anatomy and embryology reveal common evolutionary origins.
Homologous Structures Vestigial Structures Convergent Evolution Analogous structures all developed from different original structures. Molecular biology reveals that common genetic sequences link all life forms. The genetic code provides our fourth line of evidence that evolution occurs.
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