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Evolution, Natural Selection, and Speciation D. L. A. Underwood Biology 316 - Entomology A. Adaptation 1. Three definitions: a. “Any behavioral, morphological, or physiological trait that is assumed to be the result of natural selection.” b. “Any physiological or morphological feature or form of behavior used to explain the ability of an organism to live where it does.” c. “A change in the population mean of a physical, physiological, or behavioral trait that results from some current environmental pressure.” 2. Team project -D takes notes; B speaks. a. Find three things wrong with the following two statements: Individuals evolve to become better adapted to their environment. This evolution occurs because it is good for the species. 3. Populations are adapted to historical conditions. a. They appear to be adapted to their present environment because environmental conditions usually do not change rapidly. b. What is adaptive today may not be tomorrow if the environment changes. 4. “Adaptedness” is at the phenotypic level. B. Phenotypic Variation 1. The phenotype is what you see in an organism. It includes all of the characteristics of that individual. For example: a. Color of skin, feather, exoskeleton, etc. b. Behavior c. Physiological responses to stimuli d. Metabolism e. Abilities 2. Types of variation a. Discontinuous or discreet characters have definite states. i. Number of toes ii. Number of flower petals iii. Some fur, plumage, and petal colors iv. Generally under the control of a few genes b. Continuous or quantitative characters vary along a wide range of states that blend into one another. i. Human height Biology 316 – Evolution, Natural Selection, and Speciation – Page 1 of 6 ii. Human weight iii. Generally under the control of many genes c. Can the phenotype of an individual change over time? Yes! d. Does this mean the individual is evolving? NO! i. An individual cannot evolve in the biological sense. ii. Evolution is the change in allele frequencies in a population over time. e. Can a particular aspect of the phenotype be a product of something other than genotype (i.e. genes)? Yes! f. Phenotypic plasticity i. The expression of the genotype is often influenced by environmental variables leading to a variable phenotype. • Snowshoe rabbits change fur color as winter sets in. • Flower petal color can be determined by soil pH. • The behavior of monarch butterflies to either fly north or south. 3. Team project - C takes notes, A speaks a. Why can't an individual evolve in a strict biological sense (not in the colloquial sense)? 4. Phenotype is the product of genetics and environment 5. Environmental conditions a. What is an individual's "environment?" i. Abiotic factors - anything not alive. ii. Biotic factors - anything alive. b. Diurnal variability - Changes in the environment that occur over the course of the day/night. c. Seasonal variability - Changes in the environment that occur over the course of a year. d. Spatial variability - Within an area there are differences due to the physical outlay, abiotic, and biotic factors. e. The size and habits of the organism will influence the relative importance of any particular aspect of the organism’s environment. f. An organism’s environment is defined by the organism. This is referred to as the individual's habitat. C. Artificial Selection 1. Humans have induced changes in animal and plant phenotypes over the centuries a. Breeds of dogs, chickens, cats, horses, etc. b. Milk production in cattle c. Leanness of pork Biology 316 – Evolution, Natural Selection, and Speciation – Page 2 of 6 D. Natural Selection 1. Definition of evolution a. Evolution is the change in allele frequencies in a population over time. b. Technically, this is the definition of "micro-evolution." Speciation and extinction events are considered "macro-evolution." We won't worry about this distinction. 2. Review - Conditions necessary for evolution by natural selection a. Individuals vary with respect to phenotype. b. Offspring phenotypes resemble those of their parents due in part to shared genetics. c. Different phenotypes exhibit differential survival and reproduction. 3. The classic case of natural selection - Industrial Melanism and the Peppered Moth a. The peppered moth exists in two color morphs, peppered and black. These two morphs are the same species and freely interbreed. b. Before the Industrial Revolution (before air pollution), most trees were covered with grayish lichens and the most frequent color morph of the moth was peppered. c. By the late 1800's, industrial pollution in the form of soot blackened the bark of trees and killed the lichens in areas near cities. The frequency of the black morph increased during this period, with some populations becoming 100% of the black morph. d. Industrial practices now produce less pollution and many areas now host trees with lichens once again. The frequency of the peppered morph is now increasing. 4. The evolution of insecticide resistance 5. The phenotypes that are most represented in the next generation are said to be the most fit in the population. 6. Fitness a. Individual fitness is the proportionate contribution of alleles to future generations relative to other individuals in the population. The fittest individuals are those that leave the greatest number of descendants relative to other individuals in the population. b. In practice, fitness is often applied not to a single individual, but to a phenotype or to the alleles of a particular aspect of the phenotype. Biology 316 – Evolution, Natural Selection, and Speciation – Page 3 of 6 i. We may say that in sand dunes, yellow-colored snakes have a higher fitness than brown-colored snakes (i.e. they are more likely to survive and leave larger numbers of descendants than brown -colored snakes). ii. We may also say that the alleles that lead to yellow color have a greater fitness than those that lead to a brown color because these alleles increase in frequency through time. c. Fitness is a relative term. Numbers of offspring produced is not a direct measure of fitness. Fitness is the proportionate contribution; you must compare numbers of offspring produced within types. 7. Natural selection as a process or an outcome, not a 'thing' or a force a. If conditions x, y, and z are present, A is the outcome. b. A does not force or influence conditions x, y, or z. c. A is merely the product of these events happening. 8. An analogy - the events a. Individuals vary phenotypically. b. Reproduction and survival vary with respect to some phenotypic characters. c. Some proportion of the phenotype is genetically based (h2). d. OUTCOME: Alleles that code for the phenotype that reproduces more than other phenotypes will increase in a population through time. This is natural selection. e. The phrase "selection favors a given trait" implies some thing is choosing individuals with the trait. This is incorrect!! f. Natural selection is the result of heritable biological differences among individuals that result in some individuals leaving more offspring than others in the population. 9. The relationship between evolution and the mechanisms resulting in evolution. a. Natural selection can occur without evolution. b. Genetic drift or mutation or non-random mating can occur without evolution. c. Evolution can occur by natural selection or other processes like mutation, meiotic drive, genetic drift, etc. d. A population may have natural selection, genetic drift, and evolution occurring at the same time for different traits. Biology 316 – Evolution, Natural Selection, and Speciation – Page 4 of 6 10. Team Project - C takes notes, D speaks a. Identify what is wrong with the following statement: Polar bears evolved to become white because they needed to be camouflaged against the white snow to ensure successful hunting. E. Speciation 1. What is a species? a. A species is a population or a series of populations whose members can interbreed and produce viable offspring. b. Actual interbreeding is not necessary, only the potential of interbreeding is necessary for a group of populations to be called the same species. c. Members of populations separated geographically may never encounter one another; for many species, we have no way to actually test whether interbreeding and the production of viable offspring is possible or not. d. Different species are characterized by being reproductively isolated from one another. 2. Reproductive isolation a. Members of a population of the same species must recognize one another as potentially viable mates. b. Breeding with an individual not within the same species will not lead to viable offspring. Hence, interspecific mating is at an extreme selective disadvantage. c. Species recognition is accomplished by a variety of mechanisms. i. Species specific courtship dances ii. Species specific pheromones iii. Species specific coloration d. For many species, interbreeding with a different species is not possible due to physical differences in the genitalia. 3. Interspecific hybridization a. Members of closely related species may still be able to interbreed. b. Sometimes this leads to offspring that cannot reproduce, e.g. mules. c. Sometimes these matings do produce reproductive offspring, e.g. gulls. 4. Team Project - B takes notes, D speaks a. Can interspecific hybridization lead to two species becoming one? How? 5. Population differentiation a. In order for speciation to occur, two populations of the same species must become reproductively isolated. b. Generally, geographic separation over a long period of time is required. i. The different environments of the two places leads to selection for different traits in the two populations. ii. Over time the accumulation of different traits will produce populations whose members are so different that if they encounter one another, they do not recognize each other as potentially viable mates. Biology 316 – Evolution, Natural Selection, and Speciation – Page 5 of 6 iii. Sometimes the traits that differ are related to the timing of mating, one may mate in the spring while the other mates in the fall. Interspecific mating is prevented by temporal differences in the timing of reproduction. c. There are four general models. i. Allopatric speciation – a population is split into two whose members do not encounter each other. ii. Founder effect – a small number of individuals become physically separated from the original population. This could be viewed as a special case of allopatric speciation. iii. Parapatric speciation – a population of organisms that do not move much experiences different selection pressures along its range until differentiation occurs. iv. Sympatric speciation – a population splits into two along ecology, e.g. hosts. A classic example of this is speciation that occurred in flies that feed on apples. F. Extinction 1. Extinction occurs on multiple levels a. Local extinction is when a population disappears. b. Regional extinction is when numerous populations occupying a region disappear. c. Global extinction is when all populations of a species disappear. 2. Populations become extinct due to a variety of factors. a. All populations must contain a sufficient number of reproductively active members to overcome natural death rates. b. Not all individuals in a population mate. Some are too young while others are too old. c. The density of reproductively active members must also be sufficient for individuals to successfully find one another during the mating season. d. Specific resources required of the young must also me in sufficient supply. For example, monarch caterpillars only eat the leaves of milkweed plants. Even if there are enough adult monarchs for successful mating, not enough milkweed plants in the area for larval growth can cause a local extinction. Biology 316 – Evolution, Natural Selection, and Speciation – Page 6 of 6