Ecology & Evolution
Ecology & Evolution
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Date Created: 04/16/14
CHAPTER 2T MECHANISMS OF EVOLUTION RELATIONSHIP BETWEEN FACT AND THEORY IN EVOLUTION Evolution the change in the genetic makeup of biological populations over time Evolutionary Theory the resulting understanding of the mechanisms of evolutionary change Allows biologists to understand how life diversi es and how species interacts GENETIC VARIATION CONTRIBUTES TO PH ENOTYPIC VARIATION Phenotype the physical expressions of organisms genes The observable properties of an individual resulting from both genetic and environmental factors The features of a phenotype are its characters for example eye color Trait the speci c form of a character such as brown eyes A Heritable trait is a trait that is partly determined by the organisms genes Different forms of a gene Alleles may exist at a locus a particular site on a chromosome At any given locus a single diploid individual carries no more than 2 of all the alleles found in the population Alleles Different forms of a gene At any given locus a single diploid individual carries no more than 2 of all the alleles found in the population Gene Pool The sum of all copies of all alleles of all loci found in a population Evolution can be de ned as changes in the proportions of alleles in the gene pool over time Genotype An exact description of the genetic constitution of an individual either with respect to a single trait or with respect to a larger set of traits A given genotype can produce different phenotypes depending on the environmental conditions during the development MECHANISMS OF EVOLUTIONARY CHANGE Evolution is a genetic change occuring in a population a group of individuals in a single species that live and interbreed in a particular area Individuals do not evolve populations do Mutation Generates genetic variation Any change in the nucleotide sequence in the organisms DNA Most mutations are either harmful to their bearers deleterious mutations or have no effect neutral mutations The proportion of each allele in the gene pool is its allele frequency Similarly the proportion of each genotype of individuals in the population is its genotype frequency Arti cial Selection The purposeful selection of genotypes by humans Adaptation A favored trait that evolves through natural selection Describes both the trait itself and the process that describes the trait Positive Selection Selection for beneficial changes Purifying Selection Selection against deleterious changes Gamete The mature sexual reproduction cell egg sperm Gene Flow Migration of individuals and movements of gametes between populations that can change allele frequencies in the population Genetic Drift Random changes in allele frequencies from one generation to the next Population Bottleneck When a population goes through environmental conditions in which only a few individuals survive Founder Effect When a few pioneering individuals colonize a new region the small population is unlikely to possess all the alleles found in the gene pool in its source population resulting in the reduction of genetic variation equivalent to bottleneck Any time individuals mate preferentially with other individuals of the same genotype homozygous genotypes will increase and heterozygous genotypes will decrease in frequency over time Sexual Selection Occurs when individuals of one sex mate preferentially with articular individuals of the opposite sex rather than at random MEASURIN EVOLUTIONARY CHANE If there is only one allele in a given locus in a population its frequency is one The population is then monomorphic one form at that locus and the allele is said to be xed Genetic Structure the frequencies of the different alleles at each locus and the frequencies of the different genotypes in a population RESTRICTIVE CONDITIONS Hardy Weinberg Equilibrium constitutes a model in which allele frequencies do not change across generations and genotypes frequencies can be predicted from allele frequencies Apply to only sexually reproducing organisms Five principle mechanisms 1 there is no mutation 2 there is no selection among genotypes 3 there is no gene ow 4 population size is in nite 5 mating is random NATURAL SELECTION ON PHENOTYPES Fitness the reproductive contribution to a phenotype to subsequent generations relative to the contributions of other genotypes The tness of a phenotype is determined by the relative rates of survival and reproduction of individuals with that phenotype NATURAL SELECTION CAN CHANGE OR STABILIZE POPULATIONS Stabilizing selection preserves the average characteristics of a population by favoring average individuals Reduces variation in populations but it does not change the mean Natural selection is often stabilizing Directional selection changes the characteristics of a population by favoring individuals that vary in one direction from the mean of the population Directional selection is operating when individuals at one extreme of a character distribution contribute more offspring to the next generation than other individuals do shifting the average value of that character in the population toward the extreme Results in a increase of the frequencies of alleles that produce the favored phenotype If directional selection operates over many generations an evolutionary trend is seen in the population Disruptive selection changes the characteristics of the population by favoring individuals that vary in both directions from the mean of the population Operating when individuals at opposite extremes of a character distribution contribute more offspring to the next generation than do individuals close to the mean Increases variation in the population GENETIC VARIATION DISTRIBUTED AND MAINTAINED WITHIN POPULATIONS NEUTRAL MUTATIONS Neutral allele an allele that does not affect the tness of an organism is no better or worse than the alternative alleles at the same locus Added to a population over time through mutation Neutral alleles may be lost or increase in frequency purely by genetic drift SEXUAL RECOMBINATION AMPLIFIES OF POSSIBLE GENOTYPES Disadvantages of sex in short term recombination breaks up adaptive combinations of genes Sex reduces the rate at which females pass genes on to their offspring Dividing offspring into separate sexes greatly reduces the overall reproductive rate Advantages of sex breaks and other errors in DNA on one chromosome can be repaired by copying the intact sequence from the homologous chromosome Permits the elimination of deleterious mutations Allows natural selection to eliminate particular deleterious mutation from the population over time Sexual reproduction generates new combinations of alleles on which natural selection can act Muller s ratchet the accumulation of deleterious mutations in the nonrecombining genomes of asexual species Frequencydependent selection selection that changes in intensity with the proportion of individuals in a population having the trait HETEROZYGOTE ADVANTAGES MAINTAINS POLYMORPHIC LOCI Heterozygous individuals with two different alleles are likely to outperform individuals that are homozygous with only one of those two alleles The heterozygous genotype can never become xed in the population because the offspring of two heterozygotes will always include both classes of homozygotes in addition to heterozygotes GENETIC VARIATION IS MAINTAINED IN GEOGRAPHICALLY DISTINCT POPULATIONS Clinal variation pattern of gradual change in phenotype across a geographic gradient CONSTRAINTS ON EVOLUTION DEVELOPMENTAL PROCESSES CONSTRAIN EVOLUTION All evolutionary innovations are modi cations of loreoiously existing structures Current phenotypes of organisms are constrained by historical conditions and past selective pressures TRADEOFFS CONSTRAIN EVOLUTION Tradeoffs for an adaption to be favored the tness bene ts it confers must exceed the tness costs it imposes the tradeoffs must be worthwhile Many traits that are adaptive in one context may be maladaptive in another SHORTVS LONGTERM EVOLUTIONARY OUTCOMES SOMETIMES DIFFER Microevolutionary changes the shortterm changes in allele frequencies within populations These changes can be observed directly they can be manipulated experimentally and they demonstrate the actual processes by which evolution occurs Macroevolutionary changes longterm changes
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