BIOL 20C Baldo (Week 2 Mechanism of Evolution)
BIOL 20C Baldo (Week 2 Mechanism of Evolution) 20C
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This 6 page Class Notes was uploaded by Holly Chen on Saturday October 1, 2016. The Class Notes belongs to 20C at University of California - Santa Cruz taught by Baldo Marinovic in Fall 2016. Since its upload, it has received 4 views.
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Date Created: 10/01/16
Unit 2 - Mechanisms of Evolution Introduction to Evolution • Evolution - the process of change in heritable characteristics that accumulates over time. Evolution can also be defined as: - Overtime, if enough change occurs in a population, new species can appear. - Common origins: by accepting that species evolve and species appear, we can trace all life on Earth a common origin. - Evolution is genetic change occurring in a population. - Agroup of individuals of a single species that live and interbreed in a particular geographic area. - The smallest unit capable of evolution is a population. • Darwin sees evolution as changes in the characteristics of a population (heritable traits) and the changes in the allele frequency in a population. • Vertical evolution is a lineage through time that produces a branching, tree-like phylogeny. - Ortholog: a gene in divides species that share a common ancestry. - Paralog: a gene copied within the same species. • Horizontal evolution is a much rarer form of evolution that involves the direct transfer of genes between different species. - Most common in bacteria - The way eukaryotes evolved from prokaryotes ancestors. • Gene ancestry - organisms that share genes suggest the common ancestors of organisms on Earth. Key Terms to Know for Evolution: • DNA(deoxyribonucleic acid) - the base of hereditary material of all living organisms. • Gene - a single unit of heredity.Aregion of the DNAthat codes for a specific polypeptide or RNA. • Gene pool - the sum of all the alleles found in a population or at a particular locus in that population. • Allele - the alternate form of a genetic character found at a given locus on a chromosome. • Locus - a location on a chromosome. • Genetic diversity - the number and relative frequencies of alleles in a population. - The lack of genetic diversity usually decreases the ability of a population to respond to environmental change. • Heritable trait - a trait that is at least partly determined by the organism’s genes. • Fitness - the ability of an individual to produce offspring relative to other individuals in the population. • Adaptation - any trait that increases fitness. • Genotype - an exact description of the genetic constitution of an individual, either with respects to a single trait or with respect to larger set of traits. • Phenotype - the observable properties of an individual resulting from both genetic and environmental factors. - Example: eye color, height, skin tone. Mechanisms of Evolution • The main processes that are the fuel to evolution are: natural selection, mutation, gene flow, genetic drift, and non-random mating. Natural Selection: • Natural selection has two main components: - Struggle for existence: populations can reproduce beyond the resources needed to support them. Struggle for existence is the competition for limited resources. Not all individuals will survive or reproduce. - Survival of the fittest: members of a population show variations for heritable traits. Some traits give individuals an advantage over others and those individuals tend to leave more offspring in the next generation. • Populations will gradually look more like the individuals who are fittest. • Only the subset of each generation survive to reproduce. • Natural selection acts directly on the phenotype. - The reproductive contribution of a phenotype to subsequent generations relative to the contributions of other phenotypes is called fitness. • Changes in numbers of offspring are responsible for increases and decreases in the size of a population, • Only changes in the relative success of different phenotypes in a population will lead to changes in allele frequencies from one generation to the next. Mutation: • Mutation - the production of a new allele and the source of genetic variation. • In the broader sense, mutation is any change in the nucleotide sequence of an organism’s DNA (DNAreplication is not perfect). • Mutations occur randomly with respect to their costs or benefits to the organism. • Most mutations are harmful or have no effect but few have been beneficial. • Mutation can restore genetic variation that other evolutionary mechanisms have removed. • Mutation both creates and helps maintain genetic variation in populations. • Natural selection acts on mutations (increases frequency of alleles that increases fitness, vice versa). • Fitness of a phenotype is determined by the relative rates of survival and reproduction of individuals without that phenotype. Gene Flow: • Gene flow - the movement of gametes between populations. • It can change the allele frequency in a population. • When individuals survive and reproduce in the new location, they can add new alleles to the population’s gene pool or change the allele frequency of the original population). • Amechanism that prevents new species from arising because gene flow equalizes allele frequencies between populations. • Donor population loses genetic diversity while recipient population increases genetic diversity. • Allele frequencies become more similar between populations. Genetic Drift: • Genetic drift - random changes in allele frequencies from one generation to the next. • This phenomenon is especially significant when a population is reduced dramatically in size over a short period of time (e.g a previously large population passes through environmental disasters with only small numbers survive). • Three aspects of genetic drift: - Random with respect to fitness. - Most pronounced with small populations. - Overtime can lead to lost or fixed alleles. • Populations forced through a bottleneck is likely to lose much of it’s genetic variation. - An environmental change causes allele frequencies in the surviving population to differ from those in the original population (jelly beans forced through a bottleneck). - As the population grows following the bottleneck event, it’s allele frequencies reflect the surviving population. • The founder effect: - Colonizing populations is unlikely to possess all the alleles found in the gene pool of it’s original source. - When the colony grows in size, their allele frequency is going to resemble that of it’s founders. Measuring Evolutionary Change • Most of evolution happens through gradual changes in the frequency of different alleles in a population from one generation to the next. • Genetic structure - the frequencies of the different alleles at each locus and the frequencies of the different genotypes in a population. Hardy-Weinberg Equilibrium: • Hardy-Weinberg Equilibrium - a model in which allele frequencies do not change across generations, and genotype frequencies can be predicted from all allele frequencies. • Frequencies of all alleles in a population add up to 1 and if all the frequencies in a current generation is known, the genotype in the next generation can be predicted. • Conditions must be met for a population to be at Hard-Weinberg equilibrium: - There is no mutation (the alleles present in the population do not change and no new alleles are being added. - There is no selection among genotypes (no natural selection). - No gene flow (there is no movement of individuals or gametes into or out of the population/ reproductive contact). - Population size is infinite. - There must be random mating. • If the frequencies of alleles at a locus remains constant from generation to generation, there is no evolutionary changes occurring. • This allows biologists to evaluate which mechanism of evolution are acting on a particular population. - Deviation from Hardy-Weinberg equilibrium can help identify the various mechanisms of evolutionary change. Deviations from Hardy-Weinberg Equilibrium: • Natural selection can act on characteristics with quantitative variation. • Stabilizing selection - the average characteristics of a population is preserved by the favoring of average individuals (mean traits are valued). - Stabilizing selection reduces variation in populations, but it does not change the mean. - Way of countering increases in variation brought about by sexual recombination, mutation, or gene flow. - Rates of phenotypic change in many species are slow because natural selection is often stabilizing. - Example: human birth weight. • Directional selection - the characteristics of a population is changed by favoring individuals that vary in one direction from the mean of the population. - Allele frequency change in one direction and favors one extreme of the trait distribution. - Tends to reduce genetic diversity. - Usually the work of human artificial selection. - Results in an increase of the frequencies of alleles that produce the favored phenotype. - Example: Texas Longhorn cattle in theAmerican Southwest. • Disruptive selection - the characteristics of a population is changed by favoring individuals that vary in both directions from the mean of the population. - Opposite of stabilizing selection. - Operates when individuals at opposite extremes of a character distribution contribute more offspring to the next generation than do individuals closer to the mean. - Often results in two separate species. - Maintains a bimodal distribution. • Sexual selection - a special form of selection (results when individuals in a population differ in ability to attract mates). - Batman traits: sexual selection acts more strongly on males (fundamental asymmetry of sex). - Female choice: attraction to some aspect of the male phenotype (e.gAfrican long-tailed widow bird females tend to choose males with the longest tail). - Male-male competition: males compete for female attention. - Sexual selection often leads to sexual dimorphism. Genetic Variation Distribution and Maintenance • Genetic variation is the raw material on which mechanisms of evolution act upon. Neutral Evolution: • Neutral allele - does not affect the fitness of an organism. - Accumulate in populations. - Added to a population over time through mutation, which provides the population with more genetic variation. • Frequencies of neutral alleles are not affected directly by natural selection, but become fixed or lost purely by genetic drift. • Useful for reconstructing phylogenies. HeterozygoteAdvantage: • In some cases, different alleles of a particular gene (heterozygotes) are advantageous under different environmental conditions. • Environmental conditions usually vary over time, which leads to difficulty for one gene to perform under all conditions. - Heterozygous individuals (two different alleles) are likely to perform better/survive. - Homozygous individuals are not so lucky. - Example: sickle cell anemia, mating success in many flying insects. Sexual Recombination: • Sexual recombination creates endless variation of genotypic combinations which increases the evolutionary potential of populations. • Sexual reproduction permits the elimination of deleterious mutations. - Genetic recombination produces some individuals with more deleterious mutations, which are less likely to survive than the ones with less. Overtime, it eliminates particular deleterious mutations from the population. • Sexual recombination does NOT directly influence the frequencies of alleles. • It generates new combinations of alleles on which natural selection can act upon. • Expands number of variations in character influenced by alleles at many loci by creating new genotypes. Balancing Polymorphism: • Maintains a balance between different alleles in a population. - Example: Scale eating fish with either a right canted or left canted mouth will feed on the right or left side of their prey. When right canted mouth fishes are more abundant, prey will be more vigilant of their right side, which causes left canted mouth fishes to have an advantage. This process repeats itself back and force, thus creating balance.
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