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The Mechanisms of Evolution. 22 The Mechanisms of Evolution. 22.1 What Facts Form the Base of Our Understanding of Evolution? 22.2 What Are the Mechanisms of Evolutionary Change? 22.3 What Evolutionary Mechanisms Result in Adaptation?
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22 The Mechanisms of Evolution • 22.1 What Facts Form the Base of Our Understanding of Evolution? • 22.2 What Are the Mechanisms of Evolutionary Change? • 22.3 What Evolutionary Mechanisms Result in Adaptation? • 22.4 How Is Genetic Variation Maintained within Populations? • 22.5 What Are the Constraints on Evolution? • 22.6 HowHave Humans Influenced Evolution?
22.1 What Facts Form the Base of Our Understanding of Evolution? The young Charles Darwin was passionately interested in geology and natural science. In 1831, he was recommended for a position on the H.M.S. Beagle, for a 5-year survey voyage around the world.
22.1 What Facts Form the Base of Our Understanding of Evolution? Darwin often went ashore to study rocks and collect specimens, and make observations about the natural world. In the Galapagos Islands he observed that species were similar to, but not the same as, species on the mainland of South America. He also realized that species varied from island to island.
22.1 What Facts Form the Base of Our Understanding of Evolution? Darwin postulated that species had reached the islands from the mainland, but then had undergone different changes on different islands. Part of the puzzle was determining what could be a mechanism for such changes.
22.1 What Facts Form the Base of Our Understanding of Evolution? These observations, and many others, led Darwin to propose an explanatory theory for evolutionary change based on two propositions: Species change over time. The process that produces the change is natural selection.
22.1 What Facts Form the Base of Our Understanding of Evolution? Darwin continued to amass evidence to support his ideas until 1858, when he received a letter from another naturalist, Alfred Russel Wallace. Wallace proposed a theory of natural selection almost identical to Darwin’s. A paper with the work of both men was presented in 1858 to the Linnean Society of London.
22.1 What Facts Form the Base of Our Understanding of Evolution? Darwin published his book, The Origin of Species in 1859. The book provided exhaustive evidence from many different fields to support evolution and natural selection.
22.1 What Facts Form the Base of Our Understanding of Evolution? Darwin and Wallace were both influenced by economist Thomas Malthus, who published An Essay on the Principle of Population in 1838. Populations of all species have the potential for rapid increase. But this does not occur in nature, so death rate must also be high.
22.1 What Facts Form the Base of Our Understanding of Evolution? Darwin observed that, though offspring tended to resemble their parents, they are not identical. He suggested that slight variations among individuals affect the chances of surviving and producing offspring: natural selection.
22.1 What Facts Form the Base of Our Understanding of Evolution? Natural selection: Differential contribution of offspring to the next generation by various genetic types belonging to the same population.
22.1 What Facts Form the Base of Our Understanding of Evolution? Darwin had observed variation and artificial selection of certain desirable traits in plants and animals by breeders. Darwin himself bred pigeons.
Figure 22.2 Many Types of Pigeons Have Been Produced by Artificial Selection
22.1 What Facts Form the Base of Our Understanding of Evolution? Individuals do not evolve. Populations do. Population: A group of individuals of the same species that live and interbreed in a particular geographic area. Members of a population become adapted to the environment in which they live.
22.1 What Facts Form the Base of Our Understanding of Evolution? Adaptations: The processes by which useful characteristics evolve; and the characteristics themselves. An organism is considered to be adapted to a particular environment when it can be demonstrated that a slightly different organism survives and reproduces less well in that environment.
22.1 What Facts Form the Base of Our Understanding of Evolution? For a population to evolve, its members must possess heritable genetic variation. The phenotype is the physical expression of an organism’s genes. Features of a phenotype are the characters (e.g., eye color), specific form of a character is a trait (e.g., blue).
22.1 What Facts Form the Base of Our Understanding of Evolution? A heritable trait is at least partly determined by genes. Genetic makeup of an organism is the genotype.
22.1 What Facts Form the Base of Our Understanding of Evolution? Population genetics has three main goals: Explain the origin and maintenance of genetic variation Explain patterns and organization of genetic variation Understand mechanisms that cause changes in allele frequencies
22.1 What Facts Form the Base of Our Understanding of Evolution? Different forms of a gene are called alleles. The gene pool is the sum of all copies of all alleles at all loci in a population.
22.1 What Facts Form the Base of Our Understanding of Evolution? Populations have genetic variation for many characters. Artificial selection for different characters in a single species of wild mustard produced many crop plants.
22.1 What Facts Form the Base of Our Understanding of Evolution? In laboratory experiments with Drosophila, researchers selected for high or low numbers of body bristles from an initial population with intermediate numbers. After 35 generations, numbers for both high-bristle and low-bristle fell outside the range of the original population.
22.1 What Facts Form the Base of Our Understanding of Evolution? Locally interbreeding groups are called Mendelian populations. Allele frequencies, or their proportion in the gene pool, are estimated by counting alleles in a sample of individuals.
22.1 What Facts Form the Base of Our Understanding of Evolution? Allele frequency: If a locus has two alleles, A and a, there could be three genotypes: AA, Aa, and aa. The population is polymorphic at that locus.
Calculate genotype & allele frequencies: • 100 monsters = AA • 75 monsters = Aa • 2 monsters = aa
22.1 What Facts Form the Base of Our Understanding of Evolution? If p is the frequency of allele A, and q is the frequency of allele a, p + q = 1 q = 1 – p
22.1 What Facts Form the Base of Our Understanding of Evolution? Genotype frequencies may not be the same as allele frequencies. Frequencies of different alleles at each locus and the frequencies of genotypes in a Mendelian population make up the genetic structure of the population.
22.1 What Facts Form the Base of Our Understanding of Evolution? If certain conditions are met, the genetic structure of a population does not change over time. If an allele is not advantageous, its frequency remains constant. The Hardy-Weinberg equilibrium describes a model situation in which allele frequencies do not change.
22.1 What Facts Form the Base of Our Understanding of Evolution? Conditions that must be met: Mating is random. Population size is infinite. Large populations aren’t affected by genetic drift. No gene flow—no migration into or out of the population. No mutation. Natural selection does not affect survival of any genotypes.
22.1 What Facts Form the Base of Our Understanding of Evolution? If these conditions hold: Allele frequencies remain constant; after one generation, genotype frequencies occur in these proportions: Genotype AA Aaaa Frequency p2 2pq q2
Figure 22.7 Calculating Hardy–Weinberg Genotype Frequencies (Part 1)
Figure 22.7 Calculating Hardy–Weinberg Genotype Frequencies (Part 2)
22.1 What Facts Form the Base of Our Understanding of Evolution? For generation 1, probability of two A alleles coming together is: Probability of two a alleles:
22.1 What Facts Form the Base of Our Understanding of Evolution? There are two ways of producing a heterozygote: The Hardy-Weinberg equation:
22.1 What Facts Form the Base of Our Understanding of Evolution? Populations in nature never fit the conditions for Hardy-Weinberg equilibrium. But, it is useful in predicting genotype frequencies from allele frequencies; and, because the model describes conditions that would result in no evolution, patterns of deviation from the model help identify specific mechanisms of evolution.
22.2 What Are the Mechanisms of Evolutionary Change? Hardy-Weinberg equilibrium is a null hypothesis that assumes evolutionary forces are absent. Known evolutionary mechanisms: Mutation Gene flow Genetic drift Nonrandom mating Natural selection
H-W Practice! IB Questions
22.2 What Are the Mechanisms of Evolutionary Change? Mutation is the origin of genetic variation. Mutation is any change in DNA; it appears to be random with respect to the adaptive needs of an organism. Most mutations are harmful or neutral, but if conditions change, could become advantageous.
22.2 What Are the Mechanisms of Evolutionary Change? Mutations can also restore alleles that other processes remove. Mutation rates are low—about one per locus in a million zygotes. Creates a lot of variation because of the number of genes that can mutate, chromosome rearrangements that can change many genes simultaneously, and large numbers of individuals.
22.2 What Are the Mechanisms of Evolutionary Change? Because mutation rate is low, mutations in themselves result in only minor deviations from Hardy-Weinberg equilibrium. If large deviations are found, it is appropriate to look for other mechanisms.
22.2 What Are the Mechanisms of Evolutionary Change? Gene flow is a result of the migration of individuals and movements of gametes between populations. New alleles can be added to the gene pool, or allele frequencies changed.
22.2 What Are the Mechanisms of Evolutionary Change? Genetic drift results from random changes in allele frequencies. In large populations, genetic drift can influence frequencies of alleles that don’t affect survival and reproduction. If populations are reduced to a small number of individuals—a population bottleneck, genetic drift can reduce the genetic variation.
