Bio Anthropology Lecture 7/8 Notes
Bio Anthropology Lecture 7/8 Notes ANTH1013 001
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Date Created: 02/12/16
University of Arkansas ANTH 1013 – Intro to Biological Anthropology Delezene Bio Anthropology: Lecture 7 Notes (notes going towards exam 1) Genes in populations REVIEW: -Darwin predicted that a population’s phenotype can change over time -requires heritable variation among individuals in population -Darwin lacked a workable explanation for inheritance -because of Mendel: -traits are inherited in a particulate manner without blending -each member of a gene pair segregates during the production of gametes -different genes assort themselves independently during the production of gametes -modern cell biology: -Mendel’s laws can be concluded as the outcome of meiosis -meiosis shuffles variation into new combinations -modern genetics: -genes are made of DNA -genes are translated into products that control phenotype -mutations are constantly introducing new alleles into a population’s gene pool -early 1900s: evolutionary biologists merged Darwin’s and Mendel’s ideas to create a cohesive body of evolutionary theory known as: the NeoDarwinian synthesis or the evolutionary synthesis or modern evolutionary synthesis or modern synthesis -the focus is on understanding how the frequency of alleles change over time -when the population of a gene pool is considered, evolution can be defined as: a change in allele frequency over time **individuals don’t evolve, populations or species do University of Arkansas ANTH 1013 – Intro to Biological Anthropology Delezene Alleles in populations -phenotype frequency: the percentage of individuals in a population that exhibits a given phenotype -genotype frequency: the percentage of individuals in a population that exhibits a given genotype -allele frequency: the percentage of each allele in a population Evolution is phenomenon of the gene pool -gene pool: the collection of alleles present in a population -evolution: a change in allele frequency over time NeoDarwinism mechanisms of evolution -there are four central mechanisms that contribute to changing allele frequencies over time: 1) mutation: the creation of new alleles 2) natural selection: several categories including directional selection, stabilizing selection, and sexual selection 3) gene flow: the introduction of alleles from one population into another due to migration and mating 4) genetic drift: random fluctuation in allele frequency – two special cases: genetic bottlenecks & founders effect Mutation -create variation within the gene pool – making long term evolutionary change possible -plays a critical role in evolutionary change -new mutants are rare: initial frequency is 1/2n (n is the number of individuals in a population) -create new alleles – making new phenotypes possible -important because they’re the ultimate source of all heritable variation -the fate of these new alleles lies with natural selection, genetic drift, and gene flow -may lead to increase in fitness -example: sickle cell allele in western Africa -may have a low fitness -example: PKU allele University of Arkansas ANTH 1013 – Intro to Biological Anthropology Delezene Natural selection -fitness: the average ability of individuals of a given genotype to reproduce -gene pool: the collection of alleles present in a population Galapagos finches illustrate directional selection -the Grant’s observed the birds that survived had deeper bills and were larger in body size -directional selection: selection at one end of the range of phenotypes Stabilizing selection -stabilizing selection: selection for the average phenotype Gene flow -gene flow: the movement of alleles between populations -occurs among populations within a species, not between species Genetic drift -genetic drift: chance changes in allele frequency from one generation to the next -sampling through random mating -genetic drift is high in small populations -founder effect: loss of genetic variation when a new colony is established -genetic bottleneck: reduction in genetic diversity due to dramatic temporary decrease in size -causes the mean (average) of the population to fluctuate in unpredictable directions each generation Microevolution -evolution is a change in allele frequency over time -what if a population isn’t evolving? -the population is said to be in equilibrium Equilibrium: the absence of evolutionary change -to be in equilibrium: none of the evolutionary mechanisms are operating – otherwise allele frequencies would be changing -populations in equilibrium have special mathematical properties -Hardy-Weinberg equilibrium University of Arkansas ANTH 1013 – Intro to Biological Anthropology Delezene -two allele system: -allele frequencies: p + q = 1 -p = frequency of dominant allele -q = frequency of recessive allele -genotype frequencies: p^2 + 2pq +q^2 = 1 -p^2 = frequency of homozygous dominants -q^2 = frequency of homozygous recessives -2pq = frequency of heterozygotes (genotype frequencies that one expects if no evolution is occurring) Equilibrium is hard to achieve in natural populations -requirements for equilibrium (five ‘no’s’): -infinite population size (no genetic drift) -no mutation: no new alleles are being added to the gene pool -no gene flow: no exchange of alleles with other populations -no natural selection: alleles have equal fitness -no assortative mating (mating is random): all male/female mating has an equal probability Why do we care about H-W equilibrium? -gives a null hypothesis to test – outlines what to expect if no evolution is occurring -if the population isn’t in equilibrium, then the population is evolving -demonstrates that evolution occurs easily – no natural population will meet all five requirements SAMPLE QUESTIONS + ANSWERS • New genetic variation within a species is produced by… -mutation • Which of the following most accurately describes natural selection? -the nonrandom survival of heredity variants through many generations • When we consider the gene pool, biological evolution is defined as: -a change in allele frequency over time University of Arkansas ANTH 1013 – Intro to Biological Anthropology Delezene • Equation 1: p + q = 1 Equation 2: p^2 + 2pq + q^2 = 1 If a population is in the equilibrium, then which of the following will be true? -no change in allele frequencies will be observed between generations • The observed increase in average Galapagos finch beak depth during the mid-1970s drought is an example of… -directional selection • Which of the following is true of genetic drift? -it’s more pronounced in smaller populations Bio Anthropology: Lecture 8 Notes (notes going towards exam 1) Macroevolution -microevolution: change observed over a period of generations -the accumulation of micro-evolutionary change leads to longer term evolutionary trends and speciation -macroevolution and microevolution are the same thing, just viewed from a different temporal perspective -Darwin argued that organisms evolve through a process of descent with modification -evolving species also split occasionally into two or more species, resulting in a branching pattern of relationships -evolution has produced many species: the basic unit of evolution – group of interbreeding populations reproductively isolated from other groups -population: an identifiable group of individuals of the same species -subspecies: a collection of populations distinguished by one or more unique features Ernst Mayr’s biological species concept -a group of actually or potentially interbreeding populations that’s reproductively isolated from other groups -emphasizes the ability to reproduce since this is how alleles can be shared within a group of organisms University of Arkansas ANTH 1013 – Intro to Biological Anthropology Delezene Reproductive isolating mechanisms -pre-mating isolating mechanisms: -no initial attraction -physical incompatibility -post-mating isolating mechanisms: -sperm/egg incompatibility -zygote, embryo, or offspring isn’t viable (usable) -offspring sterility Allopatric speciation -geographic barriers lead to the fragmentation of species range – populations become isolated from each other -lack of gene flow leads to divergence, accumulation of reproductive isolating mechanisms, and eventually speciation Peripatric speciation -geographic barriers lead to the fragmentation of species range – populations become isolated from each other -peripheral isolate is much smaller than the main body of the species -the peripheral diverges from the ancestral state, but the main body doesn’t Speciation -this process takes a lot of time -if physical barriers disappear, populations can begin to exchange mates and establish a hybrid zone -lack of gene flow leads to divergence, accumulation of reproductive isolating mechanisms, but if speciation is incomplete, the populations will reestablish gene flow and set up a hybrid zone between the formerly geographically distinct populations Macroevolution and microevolution aren’t different evolutionary mechanisms -microevolution: small short term changes over just a few generations (easily observed) -macroevolution: larger changes over many generations – the logical extension of micro evolutionary processes -these two mechanisms are the same, just viewed from a different scale -directional selection: change in average phenotype -direction determined by environment University of Arkansas ANTH 1013 – Intro to Biological Anthropology Delezene -stabilizing selection: no change in average phenotype reduction in variance or range -genetic drift: change in average phenotype -direction of change is random Macro evolutionary patterns: TEMPO (rate of change) -tempo: rate at which the mean of a phenotypic character is changing over time -Darwin hypothesized that change was slow, constant, and gradual (gradualism) -although sometimes change can be really rapid (punctuated equilibrium) -sometimes the mean doesn’t change at all (stasis) Macro evolutionary patterns: speciation -cladogenesis: the splitting of lineages -the production of more than one daughter species -anagenesis: long term evolutionary change without the splitting of lineages -accumulation of lots of change without splitting -allopatric speciation causes cladogenesis -lack of gene flow leads to divergence, accumulation of reproductive isolating mechanisms and eventually speciation -a species may change significantly without splitting – it’s an evolving lineage -if so much change occurred that reproductive isolation would exist between early and late members of a lineage, would this produce a new species? Ernst Mayr’s biological species concept -a group of actually or potentially interbreeding populations that’s reproductively isolated from other such groups -emphasizes the ability to reproduce since this is how alleles can be shared within a group of organisms George Gaylord Simpson’s evolutionary species concept -a species is a lineage of ancestral-descendent populations of organisms that maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate -recognizes that species evolve University of Arkansas ANTH 1013 – Intro to Biological Anthropology Delezene -identify fossil species based on morphology not because of a guess about whether or not they could interbreed SAMPLE QUESTIONS + ANSWERS • Cladogenesis… -occurs when evolving species splits into two or more daughter species • Evolutionary stasis within a lineage is the result of long term… -stabilizing selection • According to the biological species concept, which of the following is true of species? -they are reproductively isolated from each other • When gene flow is cut off between geographically isolated populations, they may diverge genetically and form new species. This is an example of: -allopatric speciation
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