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Transcript
Chapter 8: Evolution and Natural Selection Darwin’s dangerous idea: evolution by natural selection Lectures by Mark Manteuffel, St. Louis Community College Learning Objectives Be able to explain evolution in action. Be able to describe and explain the four mechanisms that can give rise to evolution. Be able to explain how populations of organisms become better suited to their environment through natural selection. Be able to explain the scientific evidence supporting the occurrence of evolution. Evolution in Action 8.1 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? Here we have controlled the selective environmental pressure (1). This is really “artificial” selection The genetic variability (2) is still random. What’s the 3rd driving force of evolution? What happened? Evolution – the result • a genetic change in the population Natural selection – a process • 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 Artificial selection – when human beings exert the selective pressure and control reproduction. Does this still result in evolution? 8.14 Artificial selection is just a special case of natural selection. 8.2 Before Darwin, most people believed that all species had been created separately and were unchanging. People used to think that the earth was 6,000 years old and that species were unchanging. In the 18th and 19th centuries, scientists began to change their beliefs. 8.4 Observing geographic similarities and differences among fossils and living plants and animals, Darwin developed a theory of evolution. Two important and unexpected patterns: 1. Finch species on the Galapagos Islands differed in small but significant ways. 2. Similarity between the fossils of extinct species and the living species in that same area Scientists are still studying evolution on Galapagos. Four mechanisms can give rise to evolution. 1. Mutation – one source of genetic variability 2. Genetic drift 3. Migration – Gene Flow 4. Natural selection – whole process Evolution is genetic change in a population. The population is the smallest unit of evolution. 8.6 Evolution occurs when the allele frequencies in a population change. As the generations go by… Higher proportion of white tigers Individuals may adapt but do NOT evolve. Populations evolve Allele frequencies involve percentages of dominant and recessive traits within a population. • The Hardy-Weinberg formula: – Is a mathematical representation of a gene pool. – p2 + 2pq + q2 = 1 – Adds up all of the genotypes in a population. Natural Selection An efficient mechanism of evolution Describes the overall process But there are other agents of evolutionary change Mutations Genetic Drift Migration – Gene Flow 8.10 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 Variation is all around us. Variation is the raw material on which evolution feeds. Variation in meiosis and fertilization. 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 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. An animal naturally-suited for its environment… 8.7 Mutationa direct change in the DNA of an individual - is the ultimate source of all genetic variation. What causes mutations? The process of DNA replication or cells dividing can go awry. Environmental phenomena • Mutagens (chemical or physical agents) • i.e. Tanning beds bombard the body with ultraviolet radiation. • i.e. Benzene, a known carcinogen Mutations are generally random • Beneficial – sometimes • Detrimental – more likely to lower the reproductive success of the organism 8.8 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. The impact of genetic drift is much greater in small populations (microevolution) than in large populations. Genetic Drift… Only 5 of 10 plants leave offspring Generation 1 p (frequency of R) = 0.7 q (frequency of r) = 0.3 Only 2 of 10 plants leave offspring Generation 2 p = 0.5 q = 0.5 Generation 3 p = 1.0 q = 0.0 Fixation Genetic drift can lead to fixation for one allele for a gene in a population. If this happens, there is no more variability in the population for this gene. Genetic drift reduces the genetic variation in a population. Founder Effect: A special case of Genetic Drift A small number of individuals may leave a population and become the founding members of a new, isolated population. 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? Bottleneck Effect: A special case of Genetic Drift 8.9 Migration into or out of a population may change allele frequencies. Also called Gene Flow Gene Flow – Causes genetic exchange with another population – Tends to reduce genetic differences between populations – May offer stronger combinations of genes – This magazine cover celebrates our changing gene pools with a computer generated image blending facial features from several different races. Most agricultural pests evolve resistance to pesticides. How does this happen? Spraying crops with insecticides ends up favoring the reproductive success of insects with genetic resistance to the poison. Will GMO’s that are modified to contain pesticides in their cells favor resistance? Figure 13.1 8.12 Organisms in a population can become better matched to their environment through natural selection. Take-home message 8.12 Adaptation—the process by which organisms become better matched to their environment and the specific features that make an organism more fit—occurs as a result of natural selection. Individuals can also adapt through changes in behavior, but this is not a result of natural selection. Alex has adapted to his environment, but can he pass this on to the next generation? 8.13 Natural selection does not lead to perfect organisms. Environments change quickly. 8.15 Natural selection can change the traits seen in a population in several ways. 1. Directional selection, in which the average value for the trait increases or decreases 2. Stabilizing selection, in which the average value of a trait remains the same while extreme versions of the trait are selected against 3. Disruptive selection, in which individuals with extreme phenotypes have the highest fitness Evolution occurs in only one direction. 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. Co-evolution = mutually beneficial pressures drive evolution Five primary lines of evidence: 1. The fossil record 2. Biogeography 3. Comparative anatomy and embryology 4. Molecular biology 5. Laboratory and field experiments 8.17 The fossil record documents the process of natural selection. Radiometric dating confirms that the earth is very old and makes it possible to determine the age of fossils. 8-18. Geographic patterns of species’ distributions reflect their evolutionary histories. Convergent Evolution = similar traits found in unrelated populations in different geographical areas Another example of convergent evolution 8.19 Comparative anatomy and embryology reveal common evolutionary origins. Homologous Structures A common vertebrate ancestor… Vestigial Structures Convergent Evolution Analogous structures all developed from different original structures. These animals are not linked by a common ancestor. 8.20 Molecular biology reveals that common genetic sequences link all life forms. The genetic code provides our fourth line of evidence that evolution occurs. The more distantly you and another individual are related, the more your DNA differs. 8.21 Laboratory and field experiments enable us to watch evolution in progress. A fifth line of evidence for the occurrence of evolution comes from multigeneration experiments and observations.