Introduction to Evolution Basics
Introduction to Evolution Basics BIO 20C
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BLAW 3311 - 001
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This 12 page Study Guide was uploaded by Holly Chen on Friday February 19, 2016. The Study Guide belongs to BIO 20C at University of California - Santa Cruz taught by Baldo Marinovic in Summer 2015. Since its upload, it has received 54 views. For similar materials see Ecology and evolution in Biology at University of California - Santa Cruz.
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Date Created: 02/19/16
Unit 1 -An Introduction, Science Before Darwin Introduction • Evolution is the transformation process of life on Earth. It is also: - Complex relationship between living organisms. - Central theme of biology. - Links all other disciplines. • The Gallup survey shows whether people believe evolution to exist, if so whether species change on their own or through divine guidance. - Organisms existed in the present form in the past: 42% - Evolution exists through natural processes: 19% - Evolution exists through divine guidance: 31% Eidos • Eidos - an early Western belief that species exists in their form past and present, and their forms are eternal and permanent. • Essentialism is very important aspect of Eidos. • Every species are created with a perfect essence and variation meant failure to achieve eidos. • Scala Naturae - the great chain of being (led from essentialism) which has God at the very top. The Breakthrough of Scientific Revolution to theAge of Exploration • Several facts trumps the biblical explains of how, why and when the world came to be and the way it exists. • In the field of astronomy, it’s discovered that we are not the center of the universe. • Geological finds show Earth to be much older than previously through in biblical senses. • Many advances in physics/ math/ chemistry all disagree with biblical accounts. • Important discoveries made by Galileo Galilei, Isaac Newton, James Hutton, Charles Lyell. • The age of exploration: - Discovery of unique faunas and floras. - Extensive biogeographic development. - New fossils. AHistory of Development of Evolutionary Theory • 1809 - Jean Baptiste Lamarck published early ideas on evolution (first comprehensive theory of organic evolution, viewed evolution as a drive to push animals from single to complex organisms). • 1830 to 1833 - Charles Lyell popularizing the idea that Earth was shaped by slow acting forces still in operation today. • 1859 - Charles Darwin publishes The Origin of Species. • 1900 to 1918 - The basic principles of inheritance genetics are established. 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. Unit 3 - Speciation Introduction to Speciation • Species - the basic unit for classifying organisms (a group of organisms that share a common gene pool). Other definitions for species include: - Similar morphological characteristics that can be observed and measured. - Ability to interbreed and genetically different from other species. - Evolutionary independent (smallest unit that can evolve is populations). • Speciation - the dividing of biological lineages and the beginning of reproductive isolation between those lineages. Species Concept • The term species is useful but common. It’s usages varies among biologists who are interested in different aspects of speciation. • The most important factor in the divergence of sexually reproducing lineages from one another is the evolution of reproductive isolation, a state in which two groups of organisms can no longer exchange genes. Biological Species: • Biological species - groups of actually or potentially interbreeding natural populations. - ‘Actually’says the individual live in the same area and interbreed with one another. - ‘Potentially’says that even though individuals do not live in the same area, and therefore do not interbreed, they would if they were able to get together. • Advantages: Widely used. • Disadvantages: does not apply to organisms that reproduce asexually, and is limited to a single point in evolutionary time because fossils are not accounted for. Morphological Species: • Morphological species - a concept that assumes that a species comprises individuals that look alike and that individuals that do not look alike belong to different species. • An organism can belong to several morphological species, but only to one biological species. - Linnaeus developed the morphological species concept, what he did not know when he developed this was the groups he classified to be one species looked alike because they share alleles. So unknowingly, he was grouping organisms together based on their genetic construction. • Disadvantages: members of the same species do not always look alike (e.g males, females, and young individuals don’t always resembled one another closely). Furthermore, morphology is of little use in the case of cryptic species, species that are morphologically indistinguishable but do not interbreed. • Advantages: biologists now use an addition of behavioral and genetic data to differentiate morphological species. Phylogenetic Species: • Phylogenetic species (lineage species concept) - a way of putting organisms into species based on ancestral analysis. • Advantages: looks at the long term view of species and it allows biologists to consider species over evolutionary time. • Disadvantages: involve the fact that very few thorough phylogenies are available. Fossils records often have gaps. Ecological Species: • Ecological species - a species concept defined by the environmental context (each species’s ecological niche).Advantages of this concept involve the fact that it avoids problems with morphologically similar, asexual reproducing and fossils. Disadvantages due to the fact that it’s difficult to get sufficient detail of every organism’s niche. Speciation • Speciation - the emergence of a new species from ancestral species. • For this to happen, gene flow must not occur. • Speciation starts with genetic (or reproductive) isolation, then there must be a form of genetic divergence (adds a new branch to the tree of life/evolution). - Allopatry: populations physically separated. - Sympatry: co-occuring population become reproductively isolated. • Genetic divergence - natural selection, gene flow, mutations. • Speciation has concentrated on geographic processes that can result in the division of an ancestral species. - Splitting of the geographic range of a species is one obvious way of achieving such division • Specific patterns in speciation include: - Anagenesis: one species gradually transforms into another species. There isn’t a new branch added to the tree of life, the species simply looks different. - Cladogenesis: one species give rise to two or more species.Anew branch is added. Genetic Isolation: • Prezygotic isolation - when a zygote is never formed. - The sperm of one species may not attach to the eggs of another species because the eggs do not release the appropriate attractive chemicals. - The sperm may be unable to penetrate the eggs because the two gambits are chemically incompatible. - Extremely important to for many aquatic species that spawn (releasing gametes directly into the environment). - Causes of prezygotic isolation are disruptions such as temporal, spatial, behavior barrier or gamete barriers (incompatibility to form a zygote). • Postzygotic isolation - zygote is formed. - Low hybrid zygote viability: hybrid zygotes may fail to mature normally, either dying during development or developing phenotypic abnormalities that prevent them from surviving. - Low hybrid adult viability: offspring may have low survivorship than non-hybrid offspring. - Hybrid infertility: hybrid offspring can mature into infertile adults (e.g mule). - Causes postzygotic isolation are when the result of sufficient genetic divergence of populations has occurred. Allopatric Speciation (Reproductive Isolation): • Allopatric speciation - the result when a population is divided by a physical barrier. - This is thought to be the dominant mode of speciation in most groups of organisms. • Populations must be geographically isolated (no gene flow) • Diverge genetically. • Over time, the two populations will separate.And if enough time has passed, they may become two separate biological species that can no longer interbreed. • The populations separated by such barriers are often large initially. • Once a barrier to gene flow is established, reproductive isolation will begin to develop through genetic divergence. • Over generations, differences accumulate in the isolated lineages, reducing the probability that individuals from each lineage will mate successfully with individuals in the other when they come back into contact. - Reproductive isolation can evolve as a by-product of the genetic changes in the two diverging lineages. • There are two basic type of allopatric speciation: - Disperse: when individuals disperse to the new habitat (founder’s effect increases the likelihood of genetic drift) and a sudden environmental change (physical barrier) causes them to be unable to return. Environmental difference between the two habitat will eventually cause selective pressure to change. This is most prevalent in islands. - Example of disperse allopatric speciation: Darwin’s finches. - Vicariance: large populations split into two or more subpopulations. This happens usually due to emerging geographic barriers (e.g river changes course) that results in no gene flow. - Example of vicariance allopatric speciation:Ancestors of modern day ratites that separates into ostrich, kiwis..etc Sympatric Speciation (Reproductive Isolation): • Sympatric speciation - speciation that occurs without the existence of a physical barrier. - Usually a gradual process. • Polyploidy - results from the duplication of sets of chromosomes within individuals. - This can arise either from chromosome duplication in a single species or from the combining of the chromosomes of two different species. - More than two homologous chromosomes. - This doesn’t always kill the organism but they may not be able to reproduce with other offsprings of their parent’s generation. - This often occurs in the zygote stage. - Autopolyploid: an individual originates when two accidentally unreduced diploid gambits (each with two sets of chromosomes) combine to form a tetraploid individual (with four sets of chromosomes). - Allopolyploid: when individuals from two different (but closely related) species interbreed. • In sympatric speciation, natural selection overwhelms gene flow. • Sympatric speciation may occur with some forms of disruptive selection if individuals with different genotypes have a preference for distinct microhabitats where mating takes place. Hybrids • Hybrids - the offspring of genetically different/dissimilar parents. They are formed when isolated populations connect again. • Hybrid zones - areas of separated species (morphologically) interbreed - When hybrid zones first form, most hybrids are offspring of crosses between purebred individuals of the two hybridizing species. - Subsequent generations include a variety of individuals with varying proportions of their genes derived from the original two species. - Hybrid zones often contain recombinant individuals resulting from many generations of hybridization. • If separation is recent, they may be similar enough to produce viable offspring. • Lack of genetic divergence should allow gene flow. • If viable hybrid forms, fitness is usually lower than the parent species or higher than the parent species. • It is possible that the hybrids outperform other offspring from their parent’s generation but it’s not common. • If the hybrid is weaker in fitness than either parent species, the hybrid zone is narrow. Hybridize Toads in Europe: • The fire-bellied toad lives in Eastern Europe, they are very closely related to the yellow-bellied toad. • The range of the two species overlap a long but very narrow hybrid zone (4800 kilometers). • Hybrids between the two species suffer from a range of defects, many of which are lethal. • Surviving hybrids have skeletal abnormalities, such as misshapen mouths, ribs that are fused to the vertebrae and a reduced number of vertebrae. • Investigators found that the hybrid toads, on average, is only half as fit as the purebred. • Hybrid zone remains narrow because there is strong selection against hybrids. - Adult toads do not move over long distances. - The zone has persisted for hundreds of years because individuals of both species continue to move short distances into it, continually replenishing the hybrid population. Isolating Mechanisms Preventing Hybridization: • Reproduction may be incomplete when incipient species come back into contact. • If hybrid individuals are less fit than non-hybrids, selection will favor parents that do not produce hybrid offspring. • Selection will result in reinforcement of mechanisms that prevent hybridization. • These mechanisms are also processes that lead to sympatric speciation. • Temporal isolation - when two closely related species breed at different times of the year (or different times of the day), the two species will not have an opportunity to hybridize. - Example: field crickets that breed in spring and fall. • Behavioral isolation - individuals may reject, or fail to recognize, individuals of other species as potential mating partners. • Habitat (spatial) isolation - when two closely related species evolve preferences for living or mating in different habitats, they may never encounter one another.
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