Bio Notes Week 9
Bio Notes Week 9 BIO 121 - M001
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BIO 121 - M001
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This 6 page Class Notes was uploaded by Andrea Scota on Sunday November 1, 2015. The Class Notes belongs to BIO 121 - M001 at Syracuse University taught by Staff in Fall 2015. Since its upload, it has received 16 views. For similar materials see General Biology I in Biology at Syracuse University.
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Date Created: 11/01/15
Green- mentioned in class Bio Notes Week 9 TEXTBOOK CHAPTER 23 The Evolution of Populations The evolutionary impact of natural selection if only apparent in the changes in a population of organisms Microevolution: evolution at its smallest scale, a change in allele frequencies in a population over generation Genetic variation makes evolution possible (23.1) Genetic variation: differences among individuals in the composition if their genes or other DNA sequences (gene variability) o At the whole gene level, can be quantified as the average percent of loci that are heterozygous o Considerable genetic variation can also be measured at the molecular level of DBA but little results in phenotypic variation (nucleotide variability) Some phenotypic variation isn’t due to gene differences (environmental influences play a big role) Without genetic variation, evolution cannot occur Genetic variation originated when mutation, gene duplication, or other processes produce new alleles and genes o Genetic variations produced rapidly in organisms with short generation times o Sexual reproduction can also lead to genetic variation, mostly as a result from crossing over and fertilization Much of DNA in eukaryotic genomes does not encode proteins, point mutations in this noncoding regions usually results in natural variation, differences in DNA sequence that no not confer a selective advantage or disadvantage The Hardy-Weinberg equation can be used to test whether a population is evolving (23.2) Population: a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring, is limited by gene pool, the aggregate of all the alleles of a population Way to asses whether natural selection or other factors are causing evolution at a particular locus is to determine what genetic makeup would be if NOT evolving Hardy-Weinberg Equilibrium: population that is not evolving, genotype and allele frequencies stay the same. Must occur under specific conditions: o No mutations in population Causes change in frequencies New versions of genes ultimately come from mutations o Population is large If a population is small and something happens to it the there is a much larger impact on the population o Mating is random within population Having a specific allele doesn’t cause an organism to mate with a specific organism and vise versa o No gene flow occurs in population Ones that leave take the alleles that they have with them and people who come in bring in new alleles and influence frequencies o No natural selection occurs Favored trait is more likely to succeed through generations and frequency is usually shifted towards it In equation: o p= frequency of dominant allele o q= frequency of recessive allele o p^2= frequency of homozygous dominant o q^2= frequency of homozygous recessive o 2pq= frequency of heterozygous genotype p^2 + 2pq + q^2 = 1 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population (23.3) These three alter allele frequencies directly and cause most evolutionary change In natural selection, individuals that have certain inherited traits tend to survive and reproduce at higher rates that other individuals because of those traits o Adaptive evolution: consistently favoring some alleles over others, evolution that results in a better match between organisms and their environments In genetic drift, chance fluctuations in allele frequencies over generations tend to reduce genetic variation, it is a change in allele frequencies due to random events o Holds a gene pool together and prevents speciation o Significant in small populations o Causes allele frequency to change at random o Leads to loss of genetic variation within the populations o Can cause harmful alleles to become fixed in a population Gene flow: the transfer of alleles between populations, tends to reduce genetic differences between populations over time Bottleneck effect: if something drastic occurs to the population that drastically reduces it Founder effect: few individuals become isolated from a larger population and smaller group established a new population whose gene pool differs Natural selection is the only mechanism that consistently causes adaptive evolution (23.4) Evolution by natural selection is a blend of chance and “sorting”: o Chance in the creation of new genetic variations o Sorting as natural selection favors some alleles over others o The outcome is not random One organism has a greater relative fitness than another organism if it leaves more fertile descendants. Modes differ in effect of phenotype: o Directional selection: when conditions favor individuals exhibiting one extreme of phenotypic range shifting population frequency curve in one direction or another o Disruptive selection: occurs when conditions favor individuals at both extremes of phenotypic range over individuals with intermediate phenotypes o Stabilizing selection: acts against both extreme phenotypes and favors intermediate variants. Reduces variation and tends to maintain status quo for particular phenotypic character Unlike genetic drift and gene flow, natural selection consistently improves the match between organisms and their environments Sexual selection: individuals with certain inherited characteristics are more likely than other individuals to obtain mates, influences change in secondary sex characteristics that can give individuals advantages in mating o Intersexual selection: selection within the same sex, individuals of one sex compete directly for mates of opposite sex o Intersexual selection: “mate choice”, individuals of one sex (usually females) are choosy in selecting their mates from other sex Balancing selection: occurs when natural selection maintains two or more forms in a population, includes heterozygote advantage and frequency-dependent selection o Heterozygote advantage: if individuals who are heterozygous at a particular locus have greater fitness than do both kinds of homozygotes, tends to maintain two or more alleles at that locus o Frequency-dependent selection: fitness of phenotype depends on how common it is in the population Natural selection cannot fashion perfect organisms because: o Selection can act only on existing variations o Evolution is limited by historical constraints o Adaptations are often compromises TEXTBOOK CHAPTER 24 The Origin of Species Speciation: the process by which one species splits into 2 or more species, produces diversity of life o Forms a bridge between microevolution (changes over time in allele frequencies in population) to macroevolution (broad pattern of evolution above the species level) The biological species concept emphasizes reproductive isolation (24.1) Species: group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, but not with members of other species The biological species concept emphasizes reproductive isolation through prezygotic (before the zygote formation), and post zygotic (after zygote formation) barriers that separate gene pools Recall: gene flow is what holds gene pools of species together Formation is new species hinges in reproductive isolation, the existence of biological factors (barriers) that impede members of 2 species from and producing viable offspring o Limits formation of hybrids (offspring that result from interspecific mating) Biological species concept directs us to ways evolution can occur, but it has limitations o There is no way to examine the reproductive isolation of fossils or to organisms that only produce asexually in which we can use other species concepts Morphological species concept: emphasize the unity within species, distinguishes species but body shape and other structural features o Applied to sexual and asexual species Ecological species concept: defines species in terms of its ecological niche, the sum of how members of pieces interact with nonliving and living parts of their environment Phylogenetic species concept: defines a species as the smallest group of individuals that share a common ancestor, forming one branch of the tree of life o Applied to sexual and asexual species Speciation can take place with or without geographic separation (24.2) Focus on geographic setting in which gene flow is interrupted between populations of existing species Allopatric speciation: gene flow is interrupted when a population is divided into geographically isolated subpopulations, can be variance or dispersal o One or both populations may undergo evolutionary change during this period, resulting is establishment of prezygotic or post zygotic barriers o What determines how formidable geographic isolation needs to be for speciation to occur? – It depends on an organism’s ability to move about Species may originate from accident during division that results in an extra set of chromosomes called polyploidy o Autopolyploid: individual with more than 2 sets of chromosomes that are derived from a single species o Allopolyploid: 2 different species interbreed and produce a hybrid offspring. They are fertile when they are mating with each other but cannot interbreed with other parent species- thus a new species is formed Sympatric speciation: new species originated while remaining in same geographic area as parent species o Plant species (rarely animal) have evolved sympatric ally through polyploidy o Sympatric speciation can also result from sexual selection and habitat shifts Hybrid zones reveal factors that cause reproductive isolation (24.3) Many groups of organisms from hybrid zones, a region in which members of different species meet and mate, producing at least some offspring with mixed ancestry o Located typically where habitats of interbreeding species meet o Provide opportunities to investigate evolution of reproductive isolation Hybrid zones are stable, in that hybrids offspring continue to be produced over time Hybrids are often less fit than parents so natural selection strengthens prezygotic barriers, reducing formation of unfit hybrids in reinforcement (also called disruptive selection) In other hybrid zones barriers to reproduction may weaken over time resulting in fusion of species gene pools (reversing speciation process) Speciation can occur rapidly of slowly and can result from changes in few or many genes (24.4) New species can have formed rapidly once divergence begins, but it can take millions of years for that to happen The time interval between speciation events varied considerably, from a few thousand years to tens of millions of years New developments in genetics have enabled researchers to identify specific genes involved in some cases of speciation. Results show that speciation can be driven by few or many genes Punctuated equilibrium model: evolution occurs in spurts; species evolve relatively quickly and then remain unchanged for long periods of time Autopolyploid is shown to produce speciation most rapidly
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