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Bio 1001 lecture summaries 13- end of term

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Biology
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Biology 1001A
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Tom Haffie

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Biology notes for exam Lecture 12 1. Strategy to distinguish between a phenotype that resultsfrom codominance relative to incompletedominance ▯ Incomplete dominance occurswhen the effectsof recessive allelescan be detected to someextent in heterozygotes ▯ For example, in snapdragons, when a red flower and a white flower cross, they produce a pink flower ▯ Two red colour alleles are needed to make enough pigment to produce entirely redcolour.Whiteallelesmakenopigment whatsoever ▯ Of course, when crossed, the pink flowers can produce both red and white flowers ▯ In codominance, theeffectsof different allelesareequally detectable in heterozygotes ▯ The alleles have approximately equal effects in individuals ▯ Human blood types A and B are codominant ▯ You would not be able t o dist inguish bet ween codominance and incomplet e dominance by lookingat the inheritance patternsbecause they are the same 2. Characteristicsthat identify a pleiotropic allele ▯ In pleiotropy, two or more charactersare affected byasingle gene ▯ For example, sickle cell disease causes sickling of red blood cells, which also causes anaemia, fatigue, kidney failure, abdominal pain, etc. ▯ These wide-ranging effects are caused by the single sickle cell gene 3. Conditionsunder which Hardy Weinberg Equilibrium ispossible in a population. ▯ The Hardy-Weinberg principle is a null model that describes how evolution does not occur. It specifiesthe conditionsfor genetic equilibrium ▯ Genetic equilibrium, in which allele frequencies and genotype frequencies do not change in succeeding generation, is possible only if these conditions are met: 1. No mutationsare occurring. 2. The population isclosed to migration from other populations. 3. The population isinfinite in size 4. All genotypesin the population survive and reproduce equally well 5. Individualsin the population mate randomly with respect to genotype ▯ Under these condition, microevolutionnd ootoccur ▯ This model is used as a reference point to access condition in which microevolution doesoccur, by identifyingwhich conditionsare not met Lecture 12 lecture 1. General pathway of eukaryotic membrane protein production ▯ DNA in the nucleus is transcribed and the transcripts leave the nucleus ▯ Ribosomes t ake t hose t ranscript s t o t he ER and t hey are t ranslat ed on t he ER ▯ These translated proteins are packaged into vesicles that go to the Golgi complex, then into new vesicles that send those proteins to the cell membrane 2. General physiology of skin/hair pigmentation ▯ Pigment production is determined melanocytes which produce melanin packed intomelanosomeswhichget exportedintotheskin/hair cells ▯ There are two kinds of melanin: black and red, yellow melanin ▯ Brown is produced by a mixture of red and black melanin 3. Characteristicsof dominant alleles ▯ The dominant alleles do not inhibit the recessive alleles. They simply mask the effects of the recessive alleles and determine the phenotype ▯ An allele isn’t always dominant all the time (A dominant over O blood type but codominant with B blood type). It depends on the other allele present in the pair 4. Which allele in a heterozygote isdominant, given the biochemical mechanism of action of allele products? ▯ Dominance happens because of the interaction of the gene products ▯ Ex. If a R allele and a W allele are paired, t he W allele will be dominant because theblackmelaninissometimesproducedandpigmentstheskin/hair ▯ But if a W allele and a B allele pair, the B allele will be dominant because it producesblack melanin all the time which determinesthe pigmentation 5. Factorsthat affect how allele frequencieschange over time in a population ▯ If there isno natural/sexual selection (equal fitness), allele frequenciesdo not change ▯ Dominant alleles do not outcompete recessive alleles (or vice versa) in the absence of a difference in fitness ▯ Dominance in an allele by itself does not make it more evolutionarily fit ▯ In alarge population, in the absence of selection, the startingallele frequencies influencesfutureallelefrequencies ▯ Diploidy, dominance/recessive relationships, inheritan icetemrseloets sufficient to drive changes in allele frequency 6. Allele frequencies(p and q), given genotypic frequencies. ▯ Allele frequencies in a population can be anything, not just 50/50 2 2 ▯ If 60%of allelesin apopulation are p and 40%areq: pp =,pqq =q and pq = 2pq 7. Function of variousMC1Ralleles. ▯ MC1Risamembranereceptor whichproducesblackmelaninif cyclicAMP levelsarehigh.HighcyclicAMPmakesblackmelanin ▯ But under the influence of certain hormones, cyclic AMP levels can fall and the MC1Rreceptor producesredmelanininstead ▯ An allele (W allele) codesfor receptors that make brown and red melanin, dependingon cyclic AMPlevels ▯ Another allele (B allele) codes for receptors that are insensitive to hormonal stimulation and produce black melanin all the time ▯ In heterozygotes, Band W allelesare both present ▯ The B allele is on all the time, while the W allele is sensitive to hormonal stimulation. The B allele masks the effects of the W allele by coding for black melanin even though cyclic AMPlevelsare low, creatingblack skin/hair ▯ The R allele is off all the time and produces red melanin ▯ If either Ballelesor W allelespair with the Rallele, it’seffect will be masked by production of black melanin from theBor W membrane receptors Lecture 13 1. Meaning of deme, population, allele frequency, genotype frequency ▯ Deme: a local population of organisms of one species that interbreed with one another and share a distinct gene pool. Demes can be differentiated from one another on the basis of specific gene frequencies ▯ Population: a group of potentially interbreeding organisms ▯ Allele frequency: the proportion of different alleles within a population ▯ Genotype frequency: the frequency of genetic constitutions within a population 2. Allele frequenciesin a population, given the genotype frequencies ▯ Two alleles, p and q. Three genotypes, pp, pq, and qq ▯ Frequency of genotypes: f(p 2,fpq=)=2pq, f(qq) =q2 ▯ Allele frequencies can be derived from these genotype frequencies by doing the reverseoperation(squareroots,division,etc.) 3. Genotype frequenciesin the next generation, given the allele frequenciesand assuming Hardy-Weinberg equilibrium ▯ If the population isat geneticequilibrium of the locusp and q, the predicted frequencyof genotypeppisthesquareof theallelefrequency,p 2 ▯ For genotype pq, it is twice the product of the allele frequencies of p and q, 2pq ▯ For genotype qq, it is the square of the allele frequency, q 4. Assumptionsof Hardy-Weinberg equilibrium ▯ In apopulation of randomly matingindividuals, allele frequenciesare conserved and in equilibrium unless external forces act upon them ▯ Assumptions underlying this principle: 1. Parentsrepresent a random sample of gene frequenciesin a population 2. Genessegregate normally into gametes(heterozygotesfor any gene pair produce their two kindsof gametesin equal frequencies) 3. Parentsare equally fertile 4. The gametesare equally viable (have equal chance of becoming a zygote) 5. The population isvery large 6. Mating between parentsisrandom 7. Gene frequenciesare the same in both male and female parents 8. All genotypeshave equal reproductive ability ▯ In summary: no evolutionary forces=allele frequency equilibrium ▯ The ideal Hardy-Weinberg situation: genes on separate chromosomes, at least twoalleles,alargeeffectivepopulationsize,andnoinbreeding Lecture 13 lecture 1. Conditionsnecessary for Hardy-Weinbergequilibrium ▯ In alarge, random-matingpopulation where mutationsare rare enough to be ignored,intheabsenceof immigrationandemigrationandif thereisno 2 2 selection, allele frequencies will be conserved (p + q= 1) ▯ So large populat ion, random mat ing, no gene flow, and no select ion are t he conditions necessary for Hardy-Weinberg equilibrium 2. Whether a population isin HWE, given observed genotype or phenotype frequencies ▯ Use allele frequencies to calculate genotype frequencies: 2 2 ▯ Expect ed HWE frequenciA 1 1:=pf(And f (1 2) =2pq and f(2 2) =q ▯ Frequency of p = 2f(pp) + f(pq)/ (total alleles) ▯ Frequency of q = 2f(qq) + f(pg)/ (total alleles) ▯ Compare act ual observed frequency of p and q compared t o predict ed HWE frequencies(ex.compareactualf(A A ) with predicted f(AA ) =p) 1 1 1 1 ▯ If they are not equal, one or more assumptionsviolated. The population may be evolving 3. Effect of selection on changesin allele frequency ▯ Select ion against / select ion for a genot ype can cause t he genot ypic frequencies in a population to change asone genotypeisweeded out by predatorsor lack of reproductivesuccess,etc.Thisbringsthepopulationout of HWE 4. Relative vs. absolute fitness ▯ Relat ive fit ness (w) is t he fit ness of a genot ype relat ive to ot her genot ypes in a population (better than average, worse, etc). Letsone predict genotypechanges ▯ Absolute fitness(W) is the number of offspring/survival rates/lifespan/etc. produced by a certain genotype 5. How to calculate relative fitness ▯ To calculate relative fitness, use w =max W ▯ The fittest genotype = 1, others between 0 and 1 6. How to quantify strength of selection ▯ The greater the difference in w between other genotypes, the stronger the selection and the faster the evolution of the group 7. Relationship between dominance/recessivenessof allelesand response to selection. ▯ When a dominant allele is selected against, it tends to disappear from the population quickly asit isweeded out ▯ When it is selected for, it spreads quickly but may never reach a frequency of 1 ▯ When a recessive allele is selected against, it can remain at low frequenciesin the population indefinitely asit ismasked by the dominant allelein regardsto phenotype ▯ Carriers of t he recessive allele have good fit ness and live t o pass on t he allele ▯ This explains why most human genetic disorders are caused by recessive alleles ▯ When recessive alleles are selected for, it takes a while to spread, but given enough time,beneficialrecessiveallelesmaycompletelyoutcompetetheirdominantpartner 8. Effect of heterozygote advantage on genetic variation ▯ Heterozygote advantage increases the number of heterozygotes in a population but it doesnot weed out all of the homozygousgenotypes ▯ This is because heterozygotes require equal proportions of homozygotes to maintain a high frequency. Both alleleswill be maintained in equal frequency ▯ This is a situation where there is selection (population out of HWE) but no evolution 9. Why the amount of genetic variation in a population isimportant ▯ Genetic variation is the raw material for evolution to occur ▯ If the population lacksgeneticvariation, it cannot adapt to achangingenvironment ▯ Inbreedingdepression: the offspringocflose relativestend to have low fitness because deleteriousrecessive allelesare more likely to combine in the offspring 10. Different typesof selection (stabilizing, directional) and their effect on genetic variation ▯ The vast majority of traits exhibit more continuous, quantitative variation ▯ St abilizing select ion: individuals wit h ext reme phenot ypes (really big body size, reallysmall bodysize,etc.)areselectedagainst inapopulation,reducingthe amount of variation of the trait (standard deviation from the mean). ▯ Directional selection: an extreme phenotype (ex. long tail length, high running speed) isselectedfor,shiftingthemeanof thepopulationinthedirectionof that extreme ▯ Disruptive selection: extreme phenotypes on either side of the distribution are selected for with intermediates selected against (small beaks vs. big beaks, with intermediatebeaksselectedagainst),splittingthedistributionintotwo. Lecture 14 1. Difference between Batesian and Mullerian mimicry ▯ Batesian mimicry is a mechanism based on frequency-dependent selection ▯ In Batesian mimicry, palatable speciesthat mimicdistasteful modelsare protected against predators ▯ In Mullerian mimicry, mimicry between different speciesbenefitsboth because predatorslearn asingle warningpattern that appliesto all thesepotential but distasteful prey 2. How the population frequency of a mimic phenotype may affect itsfitness ▯ In general, in Batesian mimicry, the more frequent the mimicand the less frequent themodel,thegreaterchancethat predatorswillattackthemimic ▯ Conversely, t he less frequent t he mimic compared t o the model, t he great er t he chance the mimic will be protected ▯ In Mullerian mimicry, when rare, conspicuouswarningpatternson unpalatable individualsofferslittleprotectionbecausepredatorshavefew chancestolearn theirdistastefulness ▯ Distinctive patterns offer greater protection when they are at higher densities 3. Why the same phenotype may be selected against in one environment but have a selective advantage in a different environment ▯ A population sufficiently widespread enough may maintain a variety of genotypes, each of which is superior in a particular habitat ▯ For example, there can exist both light coloured and dark coloured (melanic) moths. Each ismore fit in one environment than another ▯ Melanicmothsaremorefit insootyareaswhichallowthemtocamouflagebetter. Light moths, on the other hand, have an advantage in areas without soot because thelightcolouredlichensontreesofferbettercamouflageforthelightmoths ▯ Both phenotypes are selected against in one environment but are advantaged in another ▯ Melanicformsexist inhighfrequenciesinurbanareas.Almost zeroinruralareas 4. Meaning of genetic load and genetic death ▯ Genetic load: the extent to which a population departs from an optimal genetic constitution ▯ Genetic death: the loss of some individuals through any means that reduces reproductiveability ▯ Most,if not all,populationscarrygeneticloads ▯ Environment s change wit h t ime and t he advant ages of different genot ypes vary accordingly. A population with a relatively high genetic load, may however, find an environment where the previously detrimental alleles may benefit survival Lecture 14 lecture 1. Effect of varioustypesof selection on amount of variation in a population ▯ St abilizing select ion causes less variat ion about t he mean of a populat ion ▯ Not all populations experience directional selection over the same time in the same way, so selection pressures can vary and maintain genetic variation ▯ Select ive environment s may change and cause disrupt ive select ion (ex. drought ) ▯ Negative frequency-dependent selection maintains both alleles as they switch fromraretocommontorareandbackagain,balancingtheiraveragefitness 2. Examplesof stabilizing, directional, disruptive ▯ St abilizing select ion removes ext remes from t he populat ion like low birt h weight babiesor very large sized birds(most widespread form of selection) ▯ Directional selection favours one extreme phenotype, like extremely long tail lengthsinwidow birdsor extremelyfast runningspeedincheetahs ▯ Disruptive selection (least common) selects against intermediate phenotypes, increasingthefrequencyof phenotypesat bothextremes(ex.selectionfavours bigbeaksor small beaksin finchesand selectsagainst intermediate size beaks due to availability of food duringdroughts) 3. Reasonswhy directional selection doesnot remove all genetic variation from a population ▯ Natural selection can’t forecast the future so variation isn’t there in case the environment might change, but if it does change, selection forces also change ▯ Select ion pressure varies t emporally (over t ime) and across habit at s ▯ Adaptations (traits the increase bearer’s relative fitnnesisrnreent-specific ▯ Recessive alleles may “ hide” in a gene pool, increasing t he variat ion 4. Characteristics, and examples, of frequency dependent selection ▯ Somet ime t he fit ness of a part icular phenot ype will depend on it s relat ive frequencyinapopulation(howrareor howcommon) ▯ Predators form search images of prey which creates a negative frequency- dependent selection (advantage of rarity). It isbetter to be the rarer form in a prey speciesbecausepredatorspreferentially hunt the most common form ▯ Of course, eventually, due to selective forces (higher fitness among rarer forms, lower fitnessamongcommonforms)therarityof theformsswitchandit isno longer adaptivetobeof thepreviouslyrareformasit isnow themost common ▯ As well, there is rare male mating advantage. Ex. In drosophila, females will mate preferentially with maleswho are different ▯ This is balancing selection and both alleles will be maintained in a population ▯ Positive-frequency selection gives a selective advantage to the most common forminapopulation.Ex.warningcoloursteachapredatornot toeat acertain colour of frog. But this only works if that colour is the most common in a population becauseotherwise predatorsdon’t learn to avoid that colour ▯ Advantage to the most common form will quickly cause that allele to replace theotherallelesinapopulation 5. Reasonswhy all living thingsare not perfectly adapted to their environment ▯ The environment changes constantly (adaptation is at least one generation behind environmental change) ▯ Select ion doesn’t always choose t he most perfect allele because it may not exist ▯ Select ion is const rained by available genet ic variat ion ▯ Select ion is not t he only evolut ionary force operat ing so alleles may randomly disappear or reappear ▯ Traits often represent a compromise between competing demands(trade-offs). Ex. t he t rait t hat prot ect s against HIV makes one suscept ible t o West Nile virus ▯ Limited by dominance relationships (can’t always weed the recessive alleles out) 6. Effect of genetic drift on allele frequencieswithin a population, particularly in the case of bottlenecksetc ▯ Genetic drift occurs whenever population size is less than infinite ▯ It israndom, unpredictable changesin allele frequency due to samplingerror. Allele frequencies change not to adapt, but just due to random luck ▯ The smaller a population, the more heavily influenced by genetic drift ▯ Bottlenecks cause a population to become small (ex. cheetahs) which reduces the amount of genetic variation. Cheetahs are virtually genetically identical to each other due to geneticdrift (all other allelesarelost, one allelecomesto fixation) ▯ Founder events are when a population starts with only a few members (ex. polydactyly in PennsylvaniaAmish). The Amish were founded by only a few hundred people and asluck would have it, a few of these individualscarried copies of alleles associated with polydactyly 7. Effect of genetic drift on variationsbetween populations ▯ Drift opposes selection and the outcome depends on the strength of the selection and the population size ▯ Genetic drift reduces variation within a population and among population ▯ If 100 populationsstarted with equal allele frequencies, in some of those populationsone allelewill go to fixation and in other the other will ▯ This is not due to selection, it is random and due to genetic drift ▯ So genet ic drift reduces variat ion wit hin a populat ion but increases it among different populations 8. Mechanism that explain why mutation isNOTdirected toward the needsof the organism. ▯ Mutationsarealwaysoccurring,not just becauseselectiveforceschange ▯ The reason a mutation increases in frequency is due to selection ▯ Ex. an HIV virion doesn’t cause t he AZT resist ance mut at ion, it occurs nat urally thengoestofixationduetoselectionforcesifpresent 9. General fitnesseffectsof mutations ▯ Most mutationshaveneutralor nearlyneutraleffectsonfitness(becausemost DNA is non-coding) ▯ Of those that affect fitness, most (but not all) are harmful 10. Why most mutationsthat affect fitnessare harmful ▯ Analogy: if you just randomly move switch things around in a car or a laptop, it iseasier tobreaksomethingthantomakeit workbetter ▯ One switch in a base pair causes sickle-cell anemia 11. Effect of gene flow on allele frequencies ▯ Gene flow can introduce new alleles to a population ▯ Often opposes selection (selection-migration balance) ▯ Phenotypes with high fitness in one environment may migrate to an environment where they have low fitness ▯ Ex. dark coloured rock pocket mice vs. pale coloured rock pocket mice ▯ These mice do not stay in the environment where they are best adapted ▯ Prevents local population from becoming perfectly adapted to their environments 12. Characteristicsof adaptive vs. non-adaptive mechanismsaffecting allele frequency ▯ Select ion (several kinds) is t he only form of adapt ive evolut ion ▯ Select ion is t he only t hing t hat increases t he fit ness of a populat ion by removing harmful allelesand increasingthe number of beneficial alleles ▯ Genetic drift, mutation, gene flow are non-adaptive mechanisms ▯ They are random and often oppose selection but cause a population to go out of HWE and evolve (adaptively or non-adaptively) 13. How variousevolutionary forcesreinforce or oppose one another ▯ Most mutationsareharmfulandsoopposeselection,but theyalsoprovidethe rawmaterial for geneticvariationandadaptiveevolution ▯ Genetic drift reduces variation within a population but increases it between populations. Gene flow may then balance the allele frequenciesamongthese twopopulations Lecture 15 1. How Darwin'stheory of evolution differed from that proposed by Lamarck ▯ Lamarck proposed a metaphysical perfecting principle caused organisms to become better suited to their environments ▯ Two mechanisms fostered evolutionary change ▯ The principle of use and disuse stated that body structures grow in proportion to how much they are used and unused structuresget weaker and shrink ▯ The inheritance of acquired characteristics stated that changes an animal aquired duringitslifetime are inherited by offspring ▯ Darwin proposed instead that variations in hereditary traits enable some individualstosurviveandreproducewhilethosethat lackedfavourabletraits would die, leavingfew, if any, offspring ▯ If the next generation wasthen subject to the same process, these traitswould be even more common. He called thisprocessnatural selection ▯ Darwin provided purely physical rather than spiritual explanations about the originsof biological diversity ▯ He recognized that evolutionary changed occurred in groups rather than individuals 2. Meaning of catastrophism, gradualism, uniformitarianism ▯ Cat ast rophism: a t heory t hat reasons abrupt changes bet ween geological st rat a marked dramatic shiftsin ancient environments ▯ One group of animals wiped out (by, say, a flood) and somewhat different species repopulated the area until the next catastrophe ▯ Gradualism: the view that Earth changed slowly over its history under the influenceof continuousprocessesactingover longperiodsof time ▯ Uniformitarianism: the processes that shaped Earth’s surface over long periods of time are thesame asthe processesobserved today (ex. volcaniceruptions, earthquakes, erosion, and glacial movement) 3. Difference between relative vs. absolute agesof rock formationsand the fossilsthey contain ▯ Relat ive ages: sediment s found in any one place form different st rat a which are arranged with the youngest layers on top. Because each stratum was formed at a specific time, the sequence of fossils from highest (oldest) to lowest (youngest) strata reveal their relative ages ▯ Geologists use the sequence of strata to establish the geologic time scale ▯ Absolute ages: radiometric dating involves the use of isotopes and sometimes allows actual ages to be associated with different rock strata 4. Principle behind radiometric dating of rock strata ▯ Radiomet ric dat ing exploit s t he fact t hat isot opes decay at st eady rat es so rock can be dated when the amounts of isotopes can be measured and the rates of decay are known ▯ This approach is limited by the half-life of the isotope ▯ Fossils that still contain organic matter can be dated by measuring the amount of the unstable isotope4C relat ive Ct o 14 ▯ Living organisms maintain the leC veils tf eir bodies but as soon as t hey die, no further replacement occursandC begins it s st eady radioact ive decay ▯ Scient ist s use t he raC14t t n ofin a fossil t o det ermine it s age 5. Why most living thingsnever form fossils ▯ The soft remains of organisms are usually consumed by scavengers or decomposed by bacteria ▯ Fossils rarely form in habitats where sediments do not accumulate or where soils are acidic ▯ The absence of skeletons and hard parts makes some organisms (ex. jellyfish) lesslikelytobefossilizedthanothers(ex.trilobites) ▯ Manyfossilsaredeformedbypressurefromoverlayingrockor fromerosion Lecture 15 lecture 1. Typesof non-random mating ▯ Inbreedingvs. inbreedingavoidance ▯ Some species, like insect , will mat e only wit h close relat ives ▯ Some species, like humans, will almost never even consider mat ing wit h a close relat ive ▯ Assortative vs. disassortative mating ▯ Like mates with like. Ex. white snow geese mate preferentially with white snow geese and blue snow geese mate preferentially with blue snow geese ▯ Opposites attract. Ex. in white-throated sparrows, there are differences in plumage(white stripesvs. tan stripes) and individualswill only mate with other individualswhodo nothave the same plumage asthey do ▯ Plumage is linked to behaviour (white striped is more aggressive, male or female) 2. Effect of non-random mating on HWEand on evolution ▯ Random mat ing is a requirement for HWE ▯ If apopulationsbeginsmatingassortatively for acertain trait, but all genotypes have the same fitness(2 alleles, 3 genotypes, intermediate dominance, and each genotype mates assortatively) this perturbs HWEbut doesn’t cause evolution ▯ Homozygoteswill produce only homozygotes, but heterozygotes will produce some homozygotes and some heterozygotes ▯ This reduces the amount of heterozygotes in a population ▯ If apopulation ismatingnon-randomly assortatively, it changesthe genotype frequenciesbut not theallelefrequencies(allelefrequenciesstaythesame) 3. Characteristicsof a scientific theory ▯ A coherent set of testable hypothesis that attempt to explain facts about the natural world ▯ “Truth”isanassertionforwhichthereissomuchevidence,itwouldbe perverse to deny it ▯ Theories graduate to facthood after repeated testing fails to falsify them ▯ A theory must be able to be tested and to be proven false (falsifiability) 4. Componentsof the theory of evolution ▯ Evolut ion happens: changes in allele frequencies in a populat ion, bet ween generat ions ▯ Most evolutionisgradual ▯ Speciat ion happens ▯ All life is related through common ancestry ▯ Muchof evolutionarychangeiscausedbyselection ▯ Evolut ion occurs in populat ions, not wit hin individuals 5. Evidence for "descent with modification" ▯ Evidence for common descent in t he form of homologies and int ermediat es in thefossilrecord ▯ Lots of evidence for change within a species (ex. HIV) 6. Examplesof homology and why they support the idea of evolution ▯ Homologies can be structural, developmental, molecular… ▯ Any similarity between two species, not explainable by shared function/environment,that reflectssharedancestry ▯ Molecular homologies(ex.thegeneticcode) ▯ Morphologicalhomologies(samearrayof bonesarrangedinthesamemanner among invertebrate species that use their limbs for vastly different purposes) ▯ Embryonic homologies show similarit ies in early embryonic development t hat make no sense unlessthere isshared ancestry behind it ▯ Ex. humans form t ails in early development which are t hen reabsorbed 7. Examplesof vestigial traitsand why they support the idea of evolution ▯ Vestigial traits only make sense in the context of evolution ▯ Cave salamanders have rudiment ary eye buds t hough t hey live complet ely in thedark.Thistraitonlymakessenseifitisaremainderofevolutionfroma common ancestor that did use its eyes ▯ A dandelion has anthers and pollen but it reproduces asexually. This must mean thedandelionhasonlyrecentlyevolvedtobeasexualfromadifferent,sexually reproducingspeciesof flower ▯ Vestigial genes also provide evidence for evolution. A functional gene codes for protein. But throughout the human genome, there are pseudo-genesthat look very similar to functioning genesbut do not code for anything ▯ Genes can be duplicated and take on a new function, but they can also be duplicated and lose itsfunctionor become disabled 8. Role of fossil record asevidence for evolution ▯ Descent: transitional forms “link” related groups (from fossil record) ▯ Modification:fossilevidenceof change Lecture 16 1. Relationship between sexual reproduction and genetic variation ▯ Recombinat ion or genet ic exchange: sect ions of chromosomes can exchange genetic material (crossing over) thereby forming different arrays of nucleotides ▯ Recombinat ion can produce different combinat ions of genes along a chromosome ▯ Millionsof different kindsof gametescanbeproducedfromrecombinationalone ▯ The production of genetic variation is one of the hypothesis for the persistence of sexual reproduction ▯ Sex in a populat ion get s rid of delet erious alleles fast er and makes it easier t o combine beneficial alleles as they arise ▯ The advantages of genetic variation is that it allows a population to persist and adapt in a changing environment (ex. to resist parasites) 2. Different modesof genetic sex determination ▯ Particular genes are located on those chromosomes associated with sex determination (sex chromosomes) ▯ For many organisms, sex determination is associated with chromosomal differencesbetween the two sexes(XXvs. XYin humans, XX:XOin nematodes, ZZ:ZW in snakes and birds) ▯ Sex can also be affect ed by aut osomal genes ▯ In Drosophila, the sex of an individual isdetermined by the ratio of X chromosomes to sets of autosomes (A) ▯ MaleshaveanX/Aratioof o.5whilefemaleshavea1ratio ▯ In other species, the potential for becomingmale of female existsat the timeof fertilization,nomatterwhat sexchromosomesarepresent 3. Different modesof environmental sex determination ▯ Penis fencing in flatworms turns whichever worm is pierced with the other worm’spenisinto thefemale ▯ Temperature dependent sex determination in alligator and other reptiles sets thesexoftheorganismbythetemperatureatwhichtheeggsdevelop ▯ Fish can change sex as adults, possibly due to hormones ▯ Hormones can trigger a chicken to develop an ovary, a testis, or an “ovotestis” 4. Meaning of haplodiploidy ▯ This is a mode of sex determination in which males develop from unfertilized, haploid eggsand femalesdevelop from fertilized, diploid eggs 5. Meaning of hermaphrodite. Whether hermaphroditism isgenerally rare or common in plants ▯ Hermaphrodite: having both female and male function ▯ Some 90% of seed plant s produce bot h male and female gamet es ▯ Only a minority of the five percent of plant species with separate male and femaleplantshavesexchromosomes Lecture 16 lecture 1. Relationshipsamong sexual reproduction, meiosisand genetic variability ▯ Recombinat ion in meiosis creat es genet ic diversity (new combinat ions of alleles) ▯ Crossing over, independent assort ment generat es genet ic diversit y ▯ Offspring are distinct from either parent and (usually) siblings 2. Mechanismsof asexual reproduction ▯ Binary fission: the mother cell divides and gives rise to two genetically identical daughter cells ▯ Plants can send out runners and create clones (vegetative propogation) ▯ Pseudo-sex creates genetic recombination without reproduction ▯ Bacteria exchange genetic information without binary fission ▯ Facultatively sexual organisms can reproduce sexually or asexually ▯ Obligately asexual animals (like the Amazon molly) reproduce entirely asexually but may not be able to reproduce without some courtship behaviou ▯ The Amazon molly must expose her eggs to sperm so they develop properly, even though no actual fertilization takes place 3. Examplesand predictionsof size-advantage model of sex change ▯ The relationship between body size and fitness is different between males and femalesinsomespecies ▯ There may be some advantage to being large for females or for male ▯ If the fitnessfunction issteeper for say, femalesthan males, all organismsborn will start out maleand switch to femalewhen theyreach athreshold bodysize ▯ Or vice versa (female to male if bigger is better for males) ▯ Protandry: male to female sex change. Bigger females can produce more eggs ▯ Dominance can result in better fitness for bigger males ▯ This maximizes reproductive fitness 4. Distribution of sexual reproduction among all life forms, and particularly among animals ▯ Sexually reproducing organisms may be dioecious or monoecious ▯ In dioeciousorganisms, male and female functionsare housed in different individuals(eachindividual iseither maleor femalewithseparategametes) ▯ In monoeciousorganism, male and female functionscan be found in the same individual (hermaphrodites).Most plantsaremonoecious ▯ Sequent ial monoecy means an organism can swit ch from male t o female ▯ Most things,except plantsandanimals,reproducemostlyasexually ▯ First life forms almost certainly reproduced asexually ▯ But among animals, the vast majority of species reproduce sexually th ▯ Less than 1/ 1o0f 1%of animalsare obligately asexual 5. Costsof sexual reproduction ▯ Cost of finding a mat e, cost of court ship and mat ing (vulnerabilit y t o predat ors) 6. Cost of meiosis ▯ If you reproduce sexually, you only passon half your allelesto offspring ▯ Genome is diluted ▯ Cloning allows you t o pass on all your genes, not just half 7. Cost of sons ▯ Malesarean“evolutionarydeadend” ▯ Malesdonot allowonetoproducethemaximumthepopulationor the production of grandchildren ▯ If one comparesasexually reproducingorganismswith sexually reproducing organisms, because it takestwo individuals(male and female) to produce offspringinstead of one female just cloningherself, the amount of potential grandchildren is halved (because a female produces half daughter and half sons) 8. "Muller'sRatchet" mutational load explanation for advantage of sexual reproduction ▯ Muller’sratchet:asexuallineagesaccumulateharmfulmutations ▯ There’s more ways of coming up with a harmful mutation than a helpful one so organismsthat reproduce asexually accumulate harmful mutationsover time ▯ There’s no way for asexual lineages to get rid of harmful mutations ▯ Sex breaks t his rat chet by cont inually reforming genot ypes 9. "Ruby in the Rubbish" hypothesisexplanation for advantage of sexual reproduction ▯ Sex cont inually creat es genot ypes wit h fewer (and more) harmful mut at ions thanparentalgenotypes ▯ It doesnot increase the average fitnessbut it increasesthe amount of variation infitnessamongoffspring.Thisallowsselectiontoweedout harmful mutations and increase the spread of beneficial mutations 10. Combination of beneficial mutationsfor advantage of sexual reproduction ▯ Some small minorit y of mut at ions are advant ageous ▯ Sex can speed up t he rat e at which beneficial mut at ions occur in t he same individual. ▯ It’svery unlikelythat one individual will independently acquire three beneficial mutationsat once, but sexual reproduction can combine these mutations ▯ If alarge population issexually reproducing, some individualswill havetwo out of three mutationsand in the next, three out of three, and these individualswill have great fitnessand spread quickly ▯ If in an asexually reproducingpopulation three beneficial mutationsarise, there’snowaytocombinethem,theindividualswitheachmutationwillbe competing against one another ▯ The speed of evolution is faster in sexually reproducing organisms 11. Relationship between extinction rate and sexual reproduction ▯ Sexual reproduct ion speeds up t he rat e of evolut ion by discarding harmful mutationsand combiningbeneficial mutations, thusdecreasingthe likelihood of extinction ▯ Almost all the obligately asexually reproducing animals are of recent evolutionary origins, This suggests that asexually reproducing species go extinct fairlyquickly(orwewouldhaveancient linesof asexuallyreproducinganimals) ▯ Sex reduces ext inct ion rat e 12. Doessex for the good of the speciesexplain itspersistence? ▯ Traits almost never spread throughout a population at the expense of the individual ▯ Things almost never spread simply because they benefit the species or there is some long-term benefit ▯ This is because selection only works on the individual ▯ The costs of sex means that, all thing being equal, an asexually reproducing organism should quickly outcompete all the sexually reproducingorganisms Lecture 17 1. Meaning of monogamy, polygamy, polygyny, polyandry, promiscuity, leks ▯ Monogamy:thesituationinwhichamaleandafemaleformapair bondfor a matingseason or for the individual’sreproductive lives ▯ Polygamy: males and females have more than one active pair bond ▯ Polygyny: one male has active pair bonds with more than one female ▯ Polyandry: one female has active pair bonds with more than one male ▯ Promiscuity: when males and females have no pair bonds beyond the time it takestomate ▯ Leks: congregation of displaying males where females come only to mate 2. Conditionsfavouringthe evolution of monogamousversuspolygynousmating systems ▯ If youngrequire agreat deal of care both parentscan provide, monogamy prevails ▯ In some birds, both malesand femalescan bringfood to the nest ▯ Monogamyoccursinmammalspeciesinwhichmalesindirectlyfeedtheyoung by bringingfood to themother while she producesmilk ▯ If maleshave high-quality territories, the femaleslivingthere may be able to raiseyoungontheirown ▯ As such, males tend to be polygynous and serve more as sperm donor and protector than active parent to hisyoung ▯ Polygyny is prevalent among mammals because the females make a much larger investment inraisingyoungthandomales 3. Handicap explanation for why femalesprefer maleswith extravagant ornaments ▯ These features are signals of male quality (like health, efficiency in harvesting resources,age)andif theyreflect themale’sgeneticmakeup,heislikelyto fertilizeafemale’seggswithspermcontainingsuccessfulalleles ▯ As well, large, showy males may hold large territories. Females who choose thesemalescangainaccesstotheseterritories ▯ The handicap hypothesis states that females select males who are more successful – the ones with ornate structures. These structures impede locomotionandmayattract predators,sofemalesmatewiththeornatemales who surviveddespitecarrying such a handicap 4. Meaning of sexual dimorphism, intersexual selection, intrasexual selection ▯ Sexual dimorphism: differences in size or appearance of males and female ▯ Intersexual selection: selection based on the interaction between malesand females.Malesproduceornatestructuresbecausefemalesfindthemattractive ▯ Intrasexual selection: selection based on the interactionsbetween member of thesamesex.Malesusetheirlargebodysize,antlers,ortuskstointimidate, injure,or kill rival males. Lecture 17 lecture 1. “Lottery ticket hypothesis” and “red queen hypothesis” to describe the relationship between environmental stability and benefitsof sexual vs. asexual reproduction ▯ Lottery ticket hypothesis: sex in unpredictable environments benefits the individual.Asexual reproductionisbeneficial inextremelystableenvironments ▯ If afemale hassurvived to reproductive age, she isadapted verywell to the current environment but she may not be well adapted to future environments ▯ If the environment isunpredictable, sexual reproduction producesdiversity in offspring. You are maximizingthe chancethat at least some offspringwill survive ▯ Red queen hypot hesis: sex is favoured when your environment is cont inually evolving (natural enemies). If parasites are an important selective force, these species are continually evolving better ways to kill you ▯ Thus, it benefits the individual to produce a variety of offspring genotypes to better “arm” them against the parasites ▯ Ex. fresh wat er snails are facult at ively sexual and when t here is a great er number of parasites, the snail tend to reproduce sexually more than asexually 2. Long-term vs. short term advantagesto sexual vs. asexual reproduction ▯ The long-term advantage is that sexual reproduction speeds up evolution by weedingout harmful mutationsand combiningbeneficial mutations ▯ The short-term advantage of sexual reproduction is production of offspring that are genetically diverse. In a changing environment, this increases the chance thatatleastsomeoffspringwillsurviveandgoontoreproduce ▯ Reduced ext inct ion risk may be just a consequence of sex, not an explanat ion 3. Why sex placesdifferent selective forceson malesvs. females ▯ Sexual dimorphism and t rait s t hat reduce survival are explained by sexual select ion ▯ Malesmust competetogainaccesstofemalesor viceversa. ▯ Generations of choosy females have driven the evolution of sexual dimorphism inbirdsof paradise 4. Relationship between
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