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BIO 121
Jason Wiles

Chapter 23: The Evolution of Populations ▯ Evolutionary impact of natural selection is only apparent in the charges in a population of organisms over time o Not the individual o Microevolution: evolution on smallest scale; as change in allele frequencies in a population over generations ▯ 3 main mechanisms in causing change in allele frequencies 1. natural selection 2. genetic drift (chance events that alter allele frequencies) 3. gene flow (the transfer of alleles between populations) ▯ Genetic variation: differences among individuals in the composition of their genes or other DNA segments o Molecular traits vary among individuals o Some phenotype variation is not all heritable, environmental factors take a part too ▯ Only genetically determined phenotypes can have evolutionary consequences ▯ Characters can be discrete or quantitative o Discrete characters: “either-or” basis (purple or white flowers of Mendel’s pea plants) ▯ Many determined by single gene locus with different alleles that produce distinct phenotypes o Quantitative characters: (involved in most heritable variation), vary along a continuum within a population ▯ Influence of two or more genes on a single phenotypic character ▯ Measuring genetic variation o Gene variability: whole – gene level o Nucleotide variability: molecular level of data ▯ Average heterozygosity: average % if loci that are heterozygotes o Quantifying gene variability ▯ Geographic variation: differences in the genetic composition of separate populations ▯ Cline: a grade change in a character along a geographic axis o Result from natural selection Sources of genetic information ▯ Mutations, gene duplications or other processes produces new alleles & new genes ▯ New allele formation o Mutation: change in mucleotide sequence in DNA ▯ Always random, unpredictable ▯ Only mutations in gamete cells can be passes to offspring ▯ Unlikely that is would improve the organism ▯ Mutations are the original source of all genetic variation ▯ Altering gene number or position o Deleting, disturbing or rearranging many loci on chromosomes are usually harmful o When genes are duplicated – important source of variation, - due to errors in meiosis, slippage during DNA replication o Duplications of large chromosomal segments – harmful ▯ Rapid Reproduction o Prokaryotes typically have short generation spans so mutations can quickly generate genetic variation in populations of these organisms ▯ True also of viruses, ex. HIV ▯ Sexual Reproduction o Genetic variation: results from unique combination of alleles that individuals receive from parents ▯ Crossing over, independent assortment, and fertilization ▯ Population: is a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring o Different populations of the same species may be isolated geographically ▯ Gene pool: consists of all copies of every type of allele at every locus in all members of population o Some alleles are “fixed” to a specific population Hardy – Weinberg Principle ▯ Predicts allele and genotype frequencies for a population that is not evolving ▯ Can determine a gene pool of a population that is not evolving ▯ The frequencies of alleles and genotypes in a population will remain constant from generation to generation provided that only Mendelian segregation and recombination of alleles are at work o Hardy-Weinberg equilibrium ▯ p : frequency of dominate allele ▯ q : frequency of recessive allele ▯ p : expected frequency of dominate homozygotes 2 ▯ q : expected frequency of the recessive homozygotes ▯ 2pq : exp2cted frequ2ncy of the heterozygote p + 2pq + q = 1 p + q = 1 Conditions of Hardy-Weinberg Equalibrium 1. No mutations ▯ ultimate source of new alleles 2. Random mating ▯ no sexual selection 3. No natural selection 4. Extremely large population size 5. No gene flow ▯ Natural Selection: constantly leads to adaptive evolution o Individuals in a population exhibit variations in their heritable traits, and those w/ traits that are better suited to their environment tend to produce more offspring than those w/ traits not as well suited (decreases genetic variation) o Environment effects natural selection ▯ Genetic Drift o Process where chance events can also cause allele frequencies to fluctuate unpredictably from one generation to the next, especially in small populations ▯ Responsible for bottleneck effect and founder effect ▯ Flounder effect o When a few individuals become isolated from a larger population this smaller group may establish a new population whose gene pool differs from the source population ▯ Bottleneck effect o Can cause a severe drop in population size o Population passes through a “bottleneck” that drastically reduces its size ▯ Effects of genetic drift 1. Genetic drift is significant in small populations 2. Genetic drift can cause allele frequencies to change at random 3. Genetic drift can lead to a loss of genetic variation within populations 4. Genetic drift can cause harmful alleles to become fixed ▯ Gene flow o Transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes o Can introduce new alleles into a population gene pool o Most important factor that holds a gene pool of species together and prevents speciation ▯ Relative fitness o The contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals ▯ Directional selection occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, shifting a populations frequency curve ▯ Disruptive selection favors variants at both ends of the distribution – increases genetic variation ▯ Stabilizing selection: removes extreme variants from the population and preserves intermediate types Sexual selection: a form of selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates ▯ Can result in, sexual dimorphism: a differences between the two sexes in secondary sexual characteristics o Differences in size, color, ornamentation and behavior ▯ Intrasexual selection: selection within the same sex, individuals of one sex compete directly for mates of opp. sex (many species, occurs among males) ▯ Interselection: individuals of one sex are choosy in selecting their mates from the other sex (females choice depends on showiness of males appearance/ behaviors) Neutral Selection: differences in DNA sequence that do not confer a selective advantage or disadvantage (genetic variation in populations) ▯ Diploidy, in eukaryotes, a considerable amount of genetic variation is hidden from selection in the form of recessive alleles ▯ Balancing selection: occurs when natural selection maintains two or more forms in a population ▯ Heterozygote advantage: when individuals who are heterozygous at a particular locus have greater fitness than do both kinds of homozygotes o Interms of genotype, not phenotype o Results in more genetic variation ▯ Frequency – dependent selection: the fitness of a phenotype depends on how common it is in the population Why natural selection cannot fashion perfect organisms 1. Selection can act only on existing variations 2. Evolution is limited by historical constraints 3. Adaptations are often compromises 4. Chance, natural selection, and the environment all interact ▯ Evolutionary forces include nonrandom mating, mutation, genetic drift, gene flow, and natural selection ▯ Evolution in populations has been and continuously observed among all kinds of organisms, evolution is real and on going Chapter 24: ▯ Speciation: the process by which one species splits into two or more species o Explains the differences and the similarities between species o Microevolution: changes over time in allele frequencies in a population o Macroevolution: the broad pattern of evolution above the species level ▯ Biological species concept o Primary definition of species o Species: a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring ▯ Reproductive compatability ▯ Reproductive Isolation o Existence of biological factors (barriers) that impede members of two species from interbreeding and producing viable, fertile offspring o Can limit ▯ hybrids: offspring that result from an interspecific mating (2 different species) ▯ Prezygotic barriers: block fertilization from occurring o Impeding members of different species from attempting to mate, preventing an attempted to make from being successful, or by hindering fertilization if mating is completed successfully ▯ Posyzygotic barriers: contribute to reproductive isolation after hybrid zygote is formed Reproductive barrier Prezygotic or Description Example Postzygotic Behavioral isolation Prezygotic Differences in mating Different birds display rituals or displays features of diff. colors Temporal isolation Prezygotic Different in timing of One plant species mating flowers before dawn, the other afterwards Habitat isolation Prezygotic Different in habitat Different ticks live on occupied different host species Mechanism isolation Prezygotic Reproductive Differently shaped structure snail shells prevent incompatibility alignment of genital openings Gametic isolation Prezygotic Sperm and egg are Different coral species incompatible release gametes into water, but only gametes of cospecifics can fuse Reduced hybrid Postzygotic Hybrid offspring are Two species of fish viability produced but are successfully feeble or unable to interbreed, but their compete for mates offspring don’t live to maturity Reduced hybrid Postzygotic Hybrid offspring are Horses can be mated fertility produced but are with zebras, but their sterile offspring are sterile Species concept Criteria Advantages Disadvantages Biological Ability to create, Objective criteria Not applicable to viable, fertile meshes well with extinct or asexual offspring gene flow concept species Morphological Similarities in body Easy to apply, works Researchers may structure for asexual, and disagree on which extinct species structures are important Phylogenetic Common evolutionary Based on actual Requires good history relationships, works evolutionary histories for asexual and extinct species Ecological Ecological niche, sum Works for sexual and of how many of the extinct organisms members of species interact w/ environment ▯ Allopatric speciation: gene flow is interrupted when a population is divided into geographically isolated subpopulations o Lake separating into two smaller lakes – first step o The scale of geographic barrier bepnds on ability of organism to move o Individuals contain more than 2 haploid sets of chromosomes ▯ Sympatic speciation: occurs in populations that live in the same geographic area o Can occur if gene flow is reduced by such factors as polyploidy, habited differentiation, and sexual selection o The appearance of a new species in same area of parent population ▯ Can occur in a single generation ▯ Polyploidy: an accident in cell division that results in an extra sets of chromosomes ▯ Occasionally in animals, more common in plants o Autopolyploid: an individual that has more than two (haploid) chromosome sets that are all derived from a single species ▯ Has been shown to cause speciation most rapidly o Tetraploid: can produce fertile tetraploid offspring by self-pollinating or mating with other tetraploids o Allopolyploid: a sterile hybrid that through various mechanisms changed into a fertile polyploidy ▯ Fertile when mating with each other only ▯ new species ▯ Habitat differentiation, when gentic factors enable a sub population to exploit a habitat or resource not used by parent population, can cause sympatric speciation to occur o Sexual selection can also drive sympatric speciation ▯ Hybrid zone: a region in which members of different species meet and mate, producing at least some offspring of mixed ancestry o Allows researchers to investigate the evolution of reproductive isolation o Reproductive barriers could be reinforced overtime or weakened ▯ Reinforcement: strengthening reproductive barriers o When hybrids are less suited than parents o Type of natural selection ▯ Fusion is when so much gene flow occurs, reproductive barriers weaken and gene pools of the two species become increasingly alike ▯ Stable hybrid zones are when hybrids continue to be produced ▯ Punctuated equilibrium: described periods of apparent stasis punctuated by sudden change o Evolution occurs in spurts; species evolve relatively rapidly then remain unchanged for long periods ▯ New species can form rapidly once divergence begins although it takes millions of years ▯ *Reinforcement is a type of natural selection Chapter 29: Plant Diversit
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