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Txtbook notes- Biological science.rtf

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McMaster University
Robert O' Brien

Chapter One Cell theory: all organisms are made of cells and all cells come from pre-existing cells - Robert Hooke examined structure of cork (tree bark tissue) under the microscope and he observed pore-like compartments that were invisible to the naked eye (these structures came to be called cells) -Anton van Leeuwenhoek inspected samples of pond water, and made the first observations of single-celled organisms (like Paramecium), he also observed and described structure of human blood cells and sperm cells -all organisms are made of cells -a cell: a highly organized compartment that is bounded by a thing, flexible structure called the plasma membrane and that contains concentrated chemicals in an aqueous solution -chemical reactions that sustain life take place inside cells -cells are capable of reproducing by dividing (making copies of themselves) -all cells come from pre-existing cells --> this challenged the spontaneous generation hypothesis (organisms arise spontaneously under certain conditions, cells arise spontaneously from non-living materials) --> all-cells-from-cells hypothesis (cells do not spring to life spontaneously but are produced only when preexisting cells grow and divide) - Louis Pasteur wanted to determine whether microorganisms could arise spontaneously in a nutrient broth or whether they would appear in a broth that was already exposed to a source of pre-existing cells --> created 2 treatment groups: 1) broth that is not exposed to a source of pre-existing cells (using a swan-necked flask) 2) broth that is exposed to a source of pre-existing cells (using a straight-necked flask) --> spontaneous generation hypothesis predicted that cells could appear in both broth treatments --> all-cells-from-cells hypothesis predicted that cells would appear only in treatments exposed to a source of pre-existing cells --> in conclusion, all cells come from pre-existing cells (as the spontaneous generation hypothesis was rejected...and cells only appeared the broth that was exposed to pre-existing cells) -cell theory goes to show that, if all cells come from pre-existing cells then all organisms must be related by common ancestry Theory of Evolution by Natural Selection: species have changed through time and all species are related to one another through common ancestry (eg/ bacteria, mushrooms, roses, humans are all a part of a family tree) - descent with modification: characteristics of species can be modified from generation to generation -Darwin and Wallace explained natural selection (how evolution occurs) Natural Selection -occurs when individuals within a population vary in characteristics that are heritable (traits can be passed onto offspring) -certain versions of these heritable traits help individuals survive better or reproduce more than do other versions (eg/ if tall wheat plants are easily blown by the wind, then in windy environments, shorter plants will tend to survive better and leave more offspring than tall plants will) -if certain heritable traits lead to increased success in producing offspring, then those traits become more common in the population over time (population's characteristics change as a result of natural selection acting on individuals -natural selection acts on individuals but evolutionary change affects only populations -evolution occurs when heritable variation leads to differential success in reproduction -fitness: the ability of an individual to produce offspring, individuals with high fitness produce many offspring -adaptation: trait that increases fitness of an individual in a particular environment -in summary, natural selection occurs when heritable variation in certain traits leads to improved success in reproduction. Because individuals with these traits produce many offspring with the same traits, the traits increase in frequency and evolution occurs (evolution is simply a change in the characteristics of a population over time) -SUMMARY: 1) individual organisms that make up a population vary in the traits they possess, such as their size and shape 2) some of the trait differences are heritable, meaning that they are passed on to offspring genetically. (eg/tall plants may tend to have tall offspring) 3) in each generation, many more offspring are produced than can possibly survive. thus, only some individuals in the population survive long enough to produce offspring; and among the individuals that produce offspring; some will produce more than others 4) the subset of individuals that survive best and produce the most offspring is not a random sample of the population- instead, individuals with certain heritable traits are more likely to survive and reproduce. Natural selection occurs when individuals with certain characteristics produce more offspring than do individuals without those characteristics. Artificial Selection -attempt to create broccoli, individuals of wild mustard species with large and compact flowering stalks were selected (the size and shape of flowering stalks in mustards is heritable), selected individuals were mated with one another and offspring turned out to have even large and more compact flowering repeating this process over many generations, a population with really large and compact flowering stalks was created (note: the size and shape of the flowering stalk in each individual plant did not change within its lifetime, the change occurred in the characteristics of the population overtime) - effect of trophy hunting on bighorn sheep, bighorn sheep are hunted for trophies --> adult male ram's mating success is dependent on their body size and horn length and this same trait is what makes them attractive to trophy hunters as well --> it was found that reproductive success of these rams has decreased as they are not living long enough to produce many offspring --> the loss of rams has changed properties of population (overall weight and horn length has decreased) --> artificial selection against the larger rams has allowed smaller rams to breed and it's their trait that’s becoming more prevalent in populations --> trophy hunting has also left to the decrease in number of trophy animals available Speciation: natural selection has caused populations of one species to diverge and form new species -tree of life (family tree of organisms): diagram can explain genealogical relationship among species with a single, ancestral species at its base Linnaean Taxonomy -taxonomy: effort to name and classify organisms -two-part naming by Linnaeus --> genus (closely related group of Humans are in the genus "homo") --> species (individuals who regularly breed together or have characteristics that are distinct from those of other human species is "sapiens") -taxonomic levels: --> hierarchy of taxonomic groups (most specific grouping to least specific grouping): species, genus, family, order, class, phylum, kingdom --> essence of Linnaeus system is that lower-level taxa are nested within higher- level taxa -Linnaeus proposed that species could be organized into 2 kingdoms: plants and animals --> this is a problem because not all organisms fall neatly into those categories -there were also other proposed schemes in regards to kingdoms, such as the five- kingdom scheme (animalia, fungi, plantae, protista, moneira) -Eukaryotes: organisms with a nucleus, usually multicellular -Prokaryotes: organisms without a nucleus, usually unicellular - findings about eukaryotes and prokaryotes suggested that the most fundamental division in life was between these two factors Phylogeny -Woese attempted to understand relationships among organisms by analyzing chemical components -studied a molecule that was found in all organisms: mRNA --> made up of ribo-nucleotide sequences: A, U, C and G --> imp. in understanding relationship among organisms because ribo-nucleotide sequences a trait that can change over the course of evolution -if the theory of evolution was correct, then rRNA sequences should be very similar in closely related organisms but less in organisms that are less closely related. Groups that are closely related, should shared certain changes in rRNA that no other species have -researchers determined the sequence of ribo-nucleotides in the rRNA of a wide array of species, then they considered the differences and similarities in the sequences and implied relationships among species (goal: to produce a diagram that described the phylogeny of organisms in the study) -the rRNA and other genetic data implied in the tree of life had many findings: 1) fundamental division b/w organisms is not b/w plants and animals or eukaryotes and prokaryotes. In fact, the 3 major domains are: bacteria, Achaea (prokaryotes) and eukaryotes 2) genetic data reveals that some earlier defined kingdoms do not reflect how evolution actually occurred (eg/ fungi are more closely related to animals than plants) 3) bacteria and Achaea are actually very diverse Phylogenic tree: diagram that depicts evolutionary history, shows relationship among species --> branches that are close to one another represented species that are closely related --> branches that are far apart represent species that are more distantly related -originally they used rRNA sequences, but now scientists use a wide array of genes to depict relationships Giraffes and Long Necks 1) Food Competition Hypothesis - when food is scarce, giraffes with long necks can reach food that is unavailable to other species and to giraffes with shorter necks -therefore, giraffes with longer necks survive better and produce more offspring than shorter-necked giraffes...hence, the average neck length of the population increases with each generation -long necks are adaptations that increase the fitness of individual giraffes during competition for food (this natural selection has gone on for long enough that the population has become extremely long-necked) -this hypothesis makes several predictions including: neck length is variable among giraffes and that giraffes feed high in trees (especially when food is scarce) - studies show that neck length in giraffes is variable -however, studies indicate that giraffes do not feed high in trees but in fact feed with their necks bent (even when food is scarce) 2) Sexual Competition Hypothesis -males tend to fight amongst one another for the opportunity to mate with females - longer necked giraffes are able to strike harder blows during combat than shorter- necked giraffes -thus, longer necked males should win more fights producing more offspring than shorter necked males -then, if neck-length in giraffes is inherited then average neck length in the population should increase overtime -long necks are adaptations that increase the fitness of males during competition for females Chili Pepper- Why are they hot? -heat in fruits and seeds is due to capsaicin -hypothesis: the presence of capsaicin is an adaptation that protects chili fruits from being eaten by animals that destroy the seeds inside -when the fruit is eaten, the seeds inside have two fates: the seed is destroyed by the animals digestive system and they never sprout again or the seed can travel down (undamaged) through the animal and is re-planted in a new location (seeds become dispersed) -direct dispersal hypothesis: natural selection should favor fruits that taste bad to animals species that act as seed predators, but these same fruits should not deter species that act as seed dispersers -research question: does capsaicin deter seed predators? - researchers used cactus mice (that destroy chili seeds) and curve-billed thrashers (that disperse seeds effectively) - cactus mice and curve-billed thrashers were offered three kinds of fruit: hackberries, fruits from a strain of chilies that can't synthesize capsaicin and pungent chilies that have lots of capsaicin -according to direct dispersal hypothesis, seed dispersers will eat pungent chilies readily but seed predators won't (if the direct dispersal hypothesis is wrong, however, there shouldn't be any difference in what various animals eat) --> ** the latter possibility is called a null hypothesis (specifies what should be observes when the hypothesis being tested doesn't hold) -the prediction was (if the direct dispersal hypothesis is correct)that both the mice and the thrasher will eat the hackberry but only the thrashers will eat pungent chilies -the prediction of the null hypothesis was that there would be no difference between what the thrashers and mice consumed -the conclusion should that capsaicin deters cactus mice but not the thrashers and we know this because the cactus mice didn't eat chilies that contain capsaicin and the thrashers ate all types of fruits equally (this confirms the direct dispersal hypothesis) -the presence of capsaicin in chilies is an adaptation that keeps seeds from being destroyed by mice and in habitats that contain mice...the production of capsaicin increases the fitness of individual chili plants -this experiment tells a lot about how experiments should be carried out --> there should always be a control group (eg/ use of hackberries...they ensure that mice (for example) didn't just not eat the berries because they were simply not hungry) --> experimental conditions should be controlled (controlling all variables such as the feeding choice setup, time interval..etc) ..this is crucial because it eliminated alternative explanations for results (all variables should be controlled except one- the fruits being presented) --> repeating test is essential Chapter Twenty-Four History -Special Creation Theory: all species are created independently by God, species are incapable of change and thus, have been unchanged since the moment of their creation (Plato) -Aristotle: created the great chain of being (ladder of life)...species were organized into a sequence based on increased size and complexity with the humans at the top -Lamarck: was first to propose that species do change overtime, his initial ideas were based off of the great chain of being --> claimed that simple organisms originate at the base of the chain by spontaneous generation and evolve by moving up the chain overtime --> thus, Lamarckian’s version of evolution is progressive in the sense of always producing larger, more complex and "better" species (basically, he turned the ladder of life into an escalator) Evidence for Change Through Time -Fossil records: fossils are organized into relative ages based on the layers of sedimentary rocks (formed a geologic timescale) --> the discovery of radioactivity allowed researchers to use radiometric techniques in order to assigned absolute ages to relative ages in the geological time scale -Extinction: --> some fossil records were found of species that no longer exist today --> this led to the idea that there are some species who are extinct --> there's been striking resemblances between the fossils found and the living species in the same geographic region (law of succession: the general observation that extinct species in the fossil record were succeeded in the same region by similar species)...shows that extinct and living forms are somewhat related further representing an ancestor-descendant relationship -Transitional Forms: fossil species with traits are intermediate between those of older and younger species --> some fossils show that there has been a gradual change over time from land dwelling mammals that had limbs to ocean-dwelling mammals that had reduced limbs or no limbs --> eg/ transitional forms document the changes that occurred as whales evolved from terrestrial mammals to the aquatic mammals of today -Vestigial Traits: a reduces or incompletely developed structure that has no function or reduced function but it is clearly similar to functioning organs or structures in closely related species --> eg/ human goose bumps --> this is inconsistent with special creation theory (which maintains that species were perfectly designed by the supernatural being and all characteristics of species are static) --> vestigial traits are evidence that species change overtime -Current Examples: there are many current examples where species are changing --> eg/ bacteria is becoming resistant to certain drugs Evidence that Species are Related -Geographic Relationships: the presence of similar species in the same geographic area is still considered strong evidence that species are related by common ancestry..specifically that their common ancestor lived in the same region --> mockingbirds between neighboring islands were similar because they descended from the same common ancestor --> mockingbirds are a part of a phylogeny --> instead of being created independently, mockingbird populations that colonized different islands had changed through time and formed new species -Homology: similarity that exists in species descended from a common ancestor --> remarkable similarity between the anatomy of organisms --> eg/ human hair and dog fur are homologous features (because they share a common ancestor that was a mammal and also had hair) 1) genetic homology: similarity in DNA sequences of different species --> eg/ eyeless gene in fruit flies and Aniridia genes in humans, the genes are very similar in DNA sequence, this is interesting because the eye structure is very different in both species (this observation can be explained because fruit flies and humans descended from a common ancestor that had a similar gene to the eyeless and Aniridia....the structure of the organ diverged as insects and mammals evolved but the same gene remained responsible for where the eyes are located) 2) developmental homology: similarity in embryonic traits --> general resemblance among the embryos of vertebrates...early in development, gill pouches and tails form in chicks, humans and cats..but later in development, gill pouches are lost in all 3 species and tails are lost in humans (note that in fish gill pouches stay intact all the way to adulthood) ...this observation is explained by the hypothesis that gill pouches and tails exist in chicks, humans and cats because they existed in fishlike species that was a common ancestor of today's fish, birds and mammals...(embryonic gill pouches are a vestigial trait in chicks, humans and cars and embryonic tails are vestigial traits in humans) 3) structural homology: similarities in adult morphology --> eg/ limbs of vertebrates...limbs with different functions have the same underlying structures (even though their function varies, all vertebrate limbs are modifications of the same # and arrangement of bones) ...structural homologies as a product of descent with modification, since all mammals descend from a common ancestor and if that ancestor had a limb with the same basic plan then it would be logical to observe that descendants had modified forms of the same design Moth Example -moths are active at night and rest on tree trunks/branches during the day, where they are hunted by birds -in an environment where trees with light-coloured bark are common, birds can find and eat dark-winged individuals much more readily than they can find and eat light- winged individuals -because predation by birds causes natural selection on wing colour, there is differential success: in this environment, light-winged individuals survive better than dark-winged individuals -since only certain types of moths can survive, due to natural selection...a change in allele frequencies occurs -evolution is defined as a change in allele frequencies in a population over time Bacteria becoming Resistant to Antibiotics -how and why did the evolution of drug resistance occur? -large population of TB bacteria in the lungs make the patient sick -drug therapy begins killing most TB bacteria, the patient seems cured and drug therapy ends. however, a few of the original bacteria has a point mutation that made them resistant to the drug treatment -the mutant cells proliferate resulting in another major infection of the lungs- the patient becomes sick again -a second round of drug therapy begins but is ineffective on the drug-resistant bacteria- the patient dies -this sequence of events shows that evolution by natural selection has occurred 1) variation in the population existed- due to mutation, both resistant and nonresistant strains of TB were present prior to administration of the drug 2) the variation was heritable 3) there was variation in reproductive success because some TB bacteria cells survived better and produced more offspring than other TB bacteria cells. (a small # of drug resistant bacteria were able to survive and keep reproducing after the onset of the first drug treatment) 4)selection did occur- a nonrandom subset of the population produced more offspring (the TB bacteria present early in the infection was different from the TB bacteria population present at the end) --> could have occurred only if the cells with the drug-resistant allele had higher reproductive success when rifampin was present than did cells with the normal allele --> the TB bacteria with the mutant gene had higher fitness (the mutant allele produces a protein that is an adaptation when the cell's environment contains antibiotic) -SUMMARY: evolution by natural selection has occurred. the TB bacteria population evolved because the mutant allele increased in frequency. the individuals, however, did not evolve however. When natural selection occurred, the individual cells did not change through time, they simply survived or died or produced more or fewer offspring. **natural selection acts on individuals bcz individuals experience differential success (allele frequencies change in populations, not in individuals) Hunting and Fishing- Selection for Undesirable Traits -when ppl hunt/fish, they may cause strong directional selection by harvesting the largest, oldest or fastest-growing individual in the population -if mortality due to hunting/fishing exceeds mortality due to natural causes, the result may be undesirable changes in the harvested population (as we are selection against the very traits we value) -decline in # of cods -fishers targeted the largest and oldest fish (therefore, cod that were likely to mature at a large size or greater age were likely to be caught before they were able to reproduce) - cod that matured early at a smaller size had greater reproductive success -this resulted in strong directional selection for early-maturing, small cod (exactly what fishers do not want) -these changes were a genetic response to strong directional selection caused by intense fishing pressure -this has negative affects bcz small adult cod tend to suffer from natural mortality, have a shorter lifespan and produce fewer offspring than large cod (which were targeted by fishers)...this marks population growth and recovery of cod stocks less likely even if fishing pressure is reduced/eliminated Galapagoes Finches- Why beak size/shape and body size are changing? -Peter and Rosemary Grant have studied the changes in beak shape, beak size and body size in finches native to Galapagos Islands -these traits vary among individuals and beak morphology and body size are heritable= therefore, there is heritable variation -medium ground finches eat seeds by cracking the seeds with their beaks -due to drought, plants were unable to produce seeds and a majority of the finch population disappeared (these individuals died of starvation) -individuals with large and deep beaks were more likely to crack the types of seeds available, efficiently and survive- they also found that survivors tended to have much deeper beaks than did the birds that died - natural selection led to an increase in average beak depth in the population (in only 1 generation, natural selection had led to a measurable change in the characteristics of the population. Alleles that led to the development of deep beaks must have increased in frequency. Large, deep beaks were an adaptation for cracking large fruits and seeds. -later on, the environment changed again with abundant rainfall and this led to finches feeding on primarily soft, small seeds (that were being mostly produced) --> this resulted in the characteristics of the population changing again (continued evolution in response to continuous changes in the environment) -Changes in Bmp4 Expression Change Beak Depth and Width -difficult to know which exact genes and alleles are involved in the evolution of finch beaks therefore scientists have used the work on topics of genetic control of limb development in chickens and other vertebrates -study of beak development in an array of Galapagos finch species (looked for variation in the pattern of expression of cell-cell signals that had already been identified as important in the development of chickens) -did situ hybridizations showing where cell-cell signals called Bmp4 is expressed -strong correlation between the amount of Bmp4 expression when beaks are developing in young Galapagos finches and the width and depth of adult beaks -when researchers experimentally increased Bmp4 expression in chickens= beaks got wider and deeper (this means, that during drought...selection for the increase of Bmp4 expression took place) -lower Bmp4 expression= shallow beak - higher Bmp4 expression= deep beak Selection Acts on Individuals but Evolutionary Change Occurs in Populations -eg/ in the case of Galapagos finches: during the drought, the beaks of individual finches did not become deeper, instead average beak depth of the population increased overtime because deep-beaked individuals produced more offspring than shallow-beaked individuals did -natural selection acts on individuals but evolutionary change occurred in the characteristics of the population -Natural Selection by Evolution by the Inheritance of Acquired Characteristics (Lamarck) --> Lamarck proposed that individuals change in response to challenges posed by the environment and that changed traits are then passed onto the offspring --> HOWEVER, Darwin realized that individuals do not change when they are selection- they simply produce more offspring than other individuals do -in some cases, individuals do change in response to changes in the environment... --> eg/ wood frogs in northern North America are exposed to extremely cold temperatures where ice forms on their skin, but as a response...their bodies naturally produce an antifreeze (molecules that protect their tissues from being damaged by ice crystals), therefore these individuals are changing in response to a change in temperature -acclimation: describes changes in an individuals phenotype that occurs in response to changes in environmental conditions (phenotypic changes due to acclimation are not passed onto offspring, therefore they cannot cause evolution) Misconceptions About Natural Selection -Evolution isn't Goal-Directed/Progressive -adaptations don't occur because organisms want/need them -although organisms that appear later seem to be morphologically complex or advanced -evolution is not necessarily progressive (eg/ tapeworms lost their digestive tracts...they just absorb nutrients from the environment through their plasma membrane) -Animals Do Not Do Things for the Good of the Species -individuals w/self-sacrificing alleles die and do not produce offspring -individuals with selfish, cheater alleles survive and produce offspring -selfish alleles increase in frequency while self-sacrificing alleles decrease in frequency -there has been no recorded instance in nature where an individual engaged in purely self-sacrificing behaviour, where they received no fitness benefit in return (eg/ common misconception is that Lemmings drown themselves in high populations where there is a threat of starvation and extinction...but this is not true) -Not all Traits are Adaptive -certain vestigial traits such as human tailbone, goose bumps and appendix don't increase the fitness of individuals with those traits -eg/ changes in DNA sequences: mutation may change a base in the third position of a codon without changing the amino acid sequence of the protein encoded by that gene (these changes are said to be silent), these neutral changes in DNA sequences re not adaptive -adaptations are constrained by genetic constraints, fitness trade-offs and historical constraints Genetic Constraints eg/ in the case of Galapagos finches, although wider beaks were not the best possible beak shape for individuals living in a dry environment, the beaks evolved anyways due to genetic correlation --> selection on alleles for one trait (increased beak depth) caused a correlated increase in another trait (beak width) -lack of genetic variation also pose genetic constraints Fitness Trade-Offs -fitness trade-offs: compromise b/w traits in terms of how those traits perform in the environment -eg/ when food is scarce, large individuals are more prone to starvation..even if large size is advantageous in an environment, there is always counteracting selection that prevents individuals from getting bigger Historical Constraints -why? all traits have evolved from previously existing traits -eg/ bones that transmit sounds from outer ear to inner ear are not the best solution of transmitting sounds but they are the best solution, given the historical constraints Chapter Twenty-Five -natural selection is not the only process that causes evolution. there are four mechanism that shift allele frequencies in populations: natural selection, genetic drift, gene flow, mutation... --> natural selection: increases frequency of certain alleles (the ones that contribute to the success in survival and reproduction) --> genetic drift: causes allele frequencies to change randomly (in some cases, drift may even cause alleles that decrease fitness to increase in frequency) --> gene flow: occurs when individuals leave one population, join another and breed. Allele frequencies may change when gene flow occurs because individuals introduce alleles to their new population and departing individuals remove alleles from their old population --> mutation: modifies allele frequencies by continuously introducing new alleles. These alleles created by mutation may be beneficial or detrimental or have no affect on fitness. -natural selection is the only mechanism that can result in adaption --> mutation, gene flow and genetic drift do not favour certain alleles over others Hardy-Weinberg Principle -analyzing the consequences of mating among all of the individuals in a population --> they are imagining that all of the gametes produced in each generation go into a single group called the gene pool and then they combine at random to form offspring -to determine what genotypes will be present in the next generation and in what frequency, H&W simply calculate what happened when 2 gametes were plucked at random out of the gene pool, many times and each of these gamete pairs was then combined to form offspring. These calculations would predict the genotype of the offspring that would be produced, as well as the frequency of each genotype -p + q =1 (p: A1, q: A2) -A1A1: p2 -A1A2: 2pq -A2A2: q2 -if the frequencies of alleles A1 and A2 in a population given by p and q, then the frequencies of genotypes A1A1, A1A2 and A2A2 will be given p2, 2pq, q2 for generation after generation -when alleles are transmitted according to the rules of Mendellian inheritance, their frequencies do not change overtime. For evolution to occur, some other factor (s) must come into play -Assumptions: 1) no natural selection: the model assumes that all members of the parental generation survived and contributed equal numbers of gametes to the gene pool, no matter what their genotype is 2) no genetic drift or random allele frequency changes 3) no gene flow: no new alleles were added or lost 4) no mutation 5) random mating with respect to the gene in question -H&W as a null hypothesis --> in addressing questions like whether natural selection is acting on a particular gene, nonrandom mating is occurring or about whether another evolutionary mechanism is at work...the Hardy-Weinberg principle functions as a null hypothesis --> given a set of allele frequencies, it predicts what the genotype frequencies will be when natural selection, mutation, genetic drift and gene flow are not affecting the gene and when mating is random with respect to that gene --> if biologists observe genetic frequencies that fo not conform to the Hardy- Weinberg prediction, it means that something interesting else is going on: either nonrandom mating is occurring or allele frequencies are changing due to another reason Are MN Blood Types in Hardy-Weinberg Equilibrium? -most human populations have 2 alleles for the MN blood group --> individuals are either MM, MN, NN -we can test if this locus is in HWE by: 1) estimate genotype frequencies by observation (to estimate the frequency of each genotype in the population, geneticists obtain data from a large # of individuals then divide the # of individuals w/each genotype by the total # of individuals in the sample) 2) calculate allele frequencies from the observed genotype frequencies 3) use the allele frequencies to calculate the expected genotype frequencies, assuming HWE 4) compare the observed and expected genotype frequencies -for every population surveyed, the MN locus are in Hardy-Weinberg proportions --> therefore, biologists conclude that the assumptions of the H&W model are valid for this locus --> the results imply that when these data were collected, the M and N alleles in these populations were not being affected by the 4 evolutionary mechanisms and that mating was random with respect to this gene-meaning that humans were not choosing mates on the basis of their MN genotype Are HLA Genes in Humans in HWE? -HLA-A and HLA-B genes recognize slightly different disease-causing organisms and further destroy the invading bacteria and viruses to help the human immune system --> based on this, the idea is that those who are heterozygous at one or both of these genes may have a strong fitness advantage- heterozygous people have a variety of HLA proteins so that their immune systems can recognize and destroy more types of bacteria and viruses (they should also be healthier and have more offspring than homozygous) -to test the hypothesis, researchers used their data on observed genotype frequencies to determine the frequency of each allele present - when they used these allele frequencies to calculate the expected number of each genotype according to the HW principle, they found that in the observed and expected values, there were more heterozygote’s than homozygote’s (this did not meet the expectations of the HW conditions) -this meant that one of the HW assumptions were being violated: 1) mating may not be random with respect to the HLA genotype (people subconsciously mate with HLA genotypes unlike their own, and thus produce an excess of heterozygous offspring) 2) heterozygous individuals may have higher fitness -over all, HW-principle serves as a null hypothesis when researchers test whether nonrandom mating or evolution if occurring at a particular gene Balancing Selection/Heterozygote Advantage -when balancing selection occurs, heterozygous individuals have higher fitness than homozygous individuals do -the consequence of this pattern is that genetic variation is maintained in populations --> Genetic Variation: refers to the # and relative frequency of alleles that are present in a particular gene --> lack of genetic variation in a population is usually a bad thing --> if genetic variation is low and the environment changes, it is unlikely that any alleles present will have high fitness under new conditions (as a result, the average fitness of a population will decline and the population may even face extinction when environmental changes are extreme) Types of Natural Selection (And their Effects on Genetic Variation) 1) Directional Selection -average phenotype of populations change in one direction (eg/ changes in beak shape and body size of medium ground finches) -tends to reduce the genetic diversity of populations -if directional selection continues overtime, the favoured alleles reach a frequency of 1.0 (fixed) while disadvantageous traits reach an frequency of 0.0 (lost) --> when disadvantageous alleles decline in frequency, purifying selection is said to occur -one cause of directional selection on a trait is counterbalanced by a difference factor that causes selection in the opposite direction 2) Stabilizing Selection -when selection reduces both extremes in a population -there is no change in the average value of a trait overtime - genetic variation in a population is reduced 3) Disruptive Selection - eliminates phenotypes near the average value and favours extreme phenotypes - opposite of stabilizing selection - over all genetic variation in a population is maintained - disruptive selection is important because it sometimes plays a part in speciation, or the formation of new species - when disruptive selection occurs, only individuals with extreme phenotypes experience high reproductive success Genetic Drift - any change in allele frequencies in a population that is due to chance -undirected -when drift occurs, allele frequencies occur due to blind luck (sampling error) -genetic drift is random with respect to fitness --> allele frequency changes it produces are not adaptive -genetic drift is most pronounced in small populations (the smaller the sample, the larger the sampling error) -overtime, genetic drift can lead to the random loss or fixation of alleles -therefore, genetic drift tends to decrease genetic diversity overtime, as alleles are randomly lost or fixed - genetic drift occurs any time allele frequencies change due to change -drift violates the assumptions of the HW-principle and occurs during many different types of events, including random fusion of gametes at fertilization, founder events and populations bottlenecks -Experimental Studies of Genetic Drift --> fruit flies that contained a genetic marker (specific allele that causes a distinctive phenotype) --> the genetic marker in this case was a fruit flies' bristles which can either be bent or straight depending on a single gene --> experiment in a lab setting where fruit flies were place in 96 cages, the 96 populations were bred through 16 generations (frequency of normal and forked alleles was 0.5 in the starting 96 populations) --> during the course of the experiment, no migration from one population to another occurred (conditions were designed so that the only evolutionary process operating on these populations was genetic drift) --> conclusion: in 29 populations, the forked alleles had been fixed and the normal allele had been lost. in 41 populations, all individuals had normal alleles and forked alleles had been lost due to change --> overall, the study showed that in 73% of these populations, genetic drift reduces allelic diversity at this gene to 0. --> therefore, genetic drift decreased genetic variation within populations and increased genetic differences between populations -Genetic Drift in Natural Populations 1) Founder Effects (how to founder effects cause drift): -when a group of individuals immigrates to a new geographic area and establishes a new population, a founder event has occurred - if the group is small enough, the allele frequen
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