Bio Notes

.docx

School

Bishop Ryan Catholic Secondary School**We aren't endorsed by this school

Course

BIOLOGY 123

Subject

Biology

Date

Dec 17, 2024

Pages

Uploaded by MegaPuppy1724

BIO NOTES CHAPTER 20: EVOLUTIONCHAPTER 20.1: GENETIC VARIATION1. Genetic Variation Genetic Variation: Humans can notice differences in traits (like hairor eye color) easily. These differences are called phenotypic variation.However, despite these visible differences, humans are actually verygenetically similar to one another.Interesting Fact:On average, any two humans are 99.9% geneticallythe same, and only 0.1% different in their DNA. Meanwhile, fruit fliesare much more genetically different from one another compared tohumans. Even penguins (Adélie penguins) are two to three times moregenetically variable than humans, even though they may look quitesimilar.2. Population Genetics Population Genetics:This is the study of how traits (and thegenes responsible for them) vary across a population.What is a Species? A species is a group of individuals that canreproduce with one another, sharing their genes. Each specieshas a collection of genes called the gene pool, which includes allthe possible genes that exist in that species (for example, genesfor different hair colors or eye colors).

Populations: These are groups within a species that live in thesame area and can reproduce together. Population geneticsfocuses on understanding what causes these variations to existand how they change over time.3. Mutations and Recombination (Sources of Genetic Variation)Mutations:Mutations are changes in the DNA sequence that cancreate new genes. These new genes might produce new traitsthat weren’t there before.Recombination:This happens during reproduction (specificallymeiosis) when DNA from parents gets mixed. Recombinationhelps shuffle genes around, creating new combinations of traits inthe offspring.4.Types of Mutations Somatic Mutations:These are mutations that happen in bodycells (like skin cells) and are not passed on to the nextgeneration. For example, a mutation causing skin cancer won’tbe inherited by a person’s children.Germ-line Mutations: These occur in reproductive cells (spermor egg) and can be passed on to the next generation. This isimportant for evolution because these mutations can affect theoffspring.Mutations can also be classified based on their effects:Neutral Mutations: These don’t cause any noticeable changes.

Deleterious Mutations: These can harm the organism.Advantageous Mutations: These provide a benefit, like makingan organism better able to survive in its environment. Over time,these mutations can become more common in a population.5. Sources of Genetic Variation (Diagram Explanation)Mutation: As shown in the diagram, a mutation can change theDNA sequence, creating new alleles (different versions of a gene).Recombination:Recombination during reproduction furthershuffles the alleles, leading to even more variation amongoffspring.By combining these two processes (mutation and recombination),populations can develop new traits, leading to evolutionary changesover time.1. Why are germ-line mutations more important in evolution than somatic ones?Answer:Germ-line mutations occur in reproductive cells (likesperm and eggs), which means they can be passed on tooffspring. These mutations contribute to genetic variation in thenext generation, which is important for evolution. Evolutiondepends on changes that can be inherited by future generations.In contrast, somatic mutations occur in body cells and cannot bepassed on to offspring, so they don’t affect evolution.

2. Why is recombination critical to generating genetic variation?Answer: Recombination occurs during the formation of spermand eggs through meiosis. It shuffles segments of DNA betweenchromosomes, creating new combinations of alleles. Thisincreases genetic diversity among offspring, which is essential fornatural selection to act upon. Without recombination, offspringwould inherit exact copies of their parents' DNA, reducing theamount of variation in a population.CHAPTER 20.2 : MEASURING GENETIC VARIATION1. Measuring Genetic Variation (Page 416)Genetic Variation:Mutations—whether they are harmful(deleterious), helpful (advantageous), or have no effect (neutral)—are important because they are the main source of geneticvariation. The study of population genetics helps scientistsunderstand how genes change over time in nature.Allele Frequency:To understand genetic variation, we need tomeasure the frequency of different versions of genes (calledalleles) in a population.oAllele frequencyis the proportion of a certain allelecompared to the total number of alleles for that gene in apopulation. For example, in Mendel’s pea plants, the genefor pea color has two alleles: A (yellow) and a (green).Fixed Alleles:If only one allele is present in a population, theallele frequency is 100% and the population is said to be fixed

for that allele. This means no other versions of the gene arepresent.2. Example of Allele Frequency in Pea Plants (Continuing Page 416)In a population of peas, if all peas are yellow and carry only the Aallele, the allele frequencyof Ais 100%, and the allelefrequencyof ais 0%. This means the population is fixed for theAallele.Genotype Frequency:Thegenotype frequencyis theproportion of individuals in a population that carry a particularcombination of alleles (genotype). For example, in another peaplant population, the genotypes might be:o50% aa (green peas),o25% Aa (yellow peas),oand 25% AA (yellow peas).Calculating Allele Frequencies in a Population:In apopulation of 100 pea plants, if 50 plants are aa, 25 are Aa, and25 are AA, we can calculate the frequency of the aallele bycounting the number of aalleles in the population.oEach aa plant has two aalleles, and each Aa plant has oneaallele.oTherefore, there are 125 aalleles (from 50 aa plants and 25Aa plants).If there are 200 total alleles (since each plant has two alleles), thefrequency of the aallele is:125200=62.5%This calculation helps us understand how common the aallele is in thispea population.3. Ways to Measure Genetic Variation (Page 416)Three Main Methods:

oObservable traits: Traits like flower color or pea color canbe measured, but this method is limited because manytraits are influenced by multiple genes.oGel electrophoresis: This method helps scientists studydifferences in proteins that reflect underlying geneticvariation. DNA segments are separated based on size andcharge, providing a visual of which alleles are present.oDNA sequencing: This is the most accurate methodbecause it reveals the exact sequence of DNA, showingeven small changes that other methods might miss.4. DNA Sequencing (Page 417)Why DNA Sequencing is Important:Protein gelelectrophoresis is limited because it only detects mutations thatchange the protein structure. Silent mutations (those that don'tchange the protein but change the DNA sequence) cannot bedetected with electrophoresis. DNA sequencing allows scientiststo detect all genetic differences, not just those that affectproteins.Calculating Allele Frequencies with DNA Sequencing:DNAsequencing helps calculate allele frequencies by counting theoccurrences of different alleles in a population. For example, if 50individuals are sampled and each has two alleles, the totalnumber of alleles is 100. By counting the occurrences of aspecific allele, scientists can calculate its frequency.5. Polymorphisms and Allele Frequencies (Page 418)Polymorphisms:These are positions in the DNA where there isvariation between individuals. For example, at a certain positionin a gene, some people might have an "A" while others mighthave a "G." These differences are called polymorphisms.Example Calculation:If 70 sequences in a population have an"A" at a specific DNA position and 30 sequences have a "G," theallele frequency of "A" is 0.7 (70 out of 100 sequences), and thefrequency of "G" is 0.3.

6. Observable Traits in Genetics (Page 417)Phenotypes and Genotypes:In the past, geneticists relied onobservable traits (like the color of a ladybug or blood type) tostudy genetic variation. For example, blood type in humans isdetermined by three alleles: A, B, and O. People with AA or AOgenotypes have blood type A, people with BB or BO genotypeshave blood type B, and people with OO genotypes have bloodtype O.7. Gel Electrophoresis (Page 417)Gel Electrophoresis Explained:Gel electrophoresis separatesproteins based on their size and electrical charge. Proteins thathave different charges or sizes will move differently through thegel when an electric current is applied. This helps scientistsidentify different alleles by visualizing distinct bands in the gel.SELF ASSESMENT QUES . What does it mean to say that an allele is "fixed" in a population?Answer:An allele is said to be "fixed" in a population whenevery individual in that population has the same allele. Thismeans there is no variation for that gene because all individualscarry the exact same version of the allele. Essentially, thepopulation is 100% for that particular allele, and no other allelesexist for that gene in that population.4. Why, before the advent of molecular tools in the 1960s, was genetic variation so hard to measure?Answer:Before molecular tools, scientists had to rely onobservable traits (phenotypes) to infer genetic variation.However, many traits are influenced by multiple genes and theenvironment, making it difficult to directly link phenotypes to

genotypes. As a result, it was challenging to measure geneticvariation accurately since they couldn’t look directly at the DNA.5. Has the application of DNA sequencing to population genetics revealed more or less genetic variation in naturalpopulations than previous methods such as protein gel electrophoresis?Answer:The application of DNA sequencing has revealed moregenetic variation in natural populations. This is because DNAsequencing allows scientists to detect all forms of geneticdifferences, including those that don’t affect proteins (whichcouldn’t be detected by methods like protein gel electrophoresis).With DNA sequencing, researchers can identify even silentmutations that were previously missed.6. In a survey of DNA sequences from a population of 500 mice, we find one polymorphic nucleotide position, which is either G or T. We find 800 G's in our sample. What is thefrequency of the T allele?Answer:oWe are given 500 mice, meaning there are 1000 alleles(since each mouse has two alleles for this gene).oThere are 800 G alleles, so the remaining alleles must be T.oTo find the frequency of the T allele:𝑁𝑢𝑚𝑏𝑒𝑟𝑜𝑓𝑇𝑎𝑙𝑙𝑒𝑙𝑒𝑠=1000−800=200Therefore, the frequency of the T allele is:Frequency of T =1000200=0.2.

CHAPTER 20.3 : EVOLUTION AND THE HARDY WEINBERG1. What is Evolution? (Page 418)Definition:Evolution, at the genetic level, is defined as a changein the frequency of alleles or genotypes in a population over time.For example, if 200 copies of an allele exist in the firstgeneration, and 300 copies exist in the second generation, thisindicates that evolution has occurred.Important Point:Evolution happens in populations, notindividuals. The genes in individuals do not change over time, butthe collective gene pool of the population does. This meansevolution acts on the population, not the individual.Can Evolution Happen Without Allele Frequency Changes?Yes, evolution can occur without changing allele frequencies. Forexample, the genotype combinations (like AA, Aa, and aa) canchange over time even if the allele frequencies remain the same.2. The Hardy-Weinberg Equilibrium (Page 418-419)What is it?The Hardy-Weinberg equilibrium describes asituation where the allele and genotype frequencies in apopulation do not change from generation to generation. Thismeans the population is not evolving.Why is this important?It serves as a null hypothesis forscientists. If a population is in Hardy-Weinberg equilibrium, thereare no evolutionary forces (like natural selection or genetic drift)acting on it. If the population is not in equilibrium, scientists knowthat evolution is occurring.Conditions Required for Hardy-Weinberg Equilibrium:oNo Selection:No natural selection is acting on thepopulation. This means that all individuals have an equalchance of surviving and reproducing, regardless of theirgenotype.

oLarge Population Size:The population must be large toprevent random changes in allele frequencies. In smallpopulations, random events (genetic drift) can significantlyimpact allele frequencies.oNo Migration:No individuals can move into or out of thepopulation. If they do, they could introduce new alleles,altering the allele frequencies.oNo Mutation:There must be no new mutations thatintroduce new alleles into the population.oRandom Mating:Individuals must mate randomly,meaning they don’t select mates based on their genotypes.Nonrandom mating can alter genotype frequencies withoutchanging allele frequencies.3. Hardy-Weinberg Formulas (Page 419)Allele Frequency Calculation:If we know the frequency of oneallele (A), we can calculate the frequency of the other allele (a).p+q=1where pis the frequency of the A allele and qis the frequency of the aallele. If pis 0.7 (70%), then qmust be 0.3 (30%).Genotype Frequencies:Once we know the allele frequencies,we can calculate the expected genotype frequencies in thepopulation using the Hardy-Weinberg equation:oFrequency of AA = p2oFrequency of Aa = 2pqoFrequency of aa = q2For example, if p(A) = 0.7 and q(a) = 0.3, then:oAA frequency = 0.72=0.49 or 49%oAa frequency = 2(0.7)(0.3)=0.42 or 42%oaa frequency = 0.32=0.09 or 9%

4. Graph Explanation (Page 419, Figure 20.4)Hardy-Weinberg Graph:The graph in Figure 20.4shows therelationship between allele and genotype frequencies. Forexample, if the frequency of allele A is 1 (meaning only A allelesare present), the frequency of the AA genotype is also 1, and thefrequencies of Aa and aa genotypes are 0.Inverse Calculation:You can also work backward usinggenotype frequencies to calculate allele frequencies. Forinstance, if 49% of the population has genotype AA, you cancalculate the allele frequency of A.5. What Happens When a Population is NOT in Hardy-Weinberg Equilibrium? (Page 419-420)Evolution is Occurring:If a population is not in Hardy-Weinbergequilibrium, it means evolution is happening. Scientists candetect this by comparing the actual genotype frequencies in thepopulation to those predicted by Hardy-Weinberg equilibrium. Ifthey are different, evolutionary forces like natural selection,genetic drift, mutation, or migration are likely at work.CHAPTER 20.4 : NATURAL SELECTIONOverview of Natural Selection:Natural selection leads to changes in allele frequencies across generations.Alleles that positively affect survival and reproduction increase in frequency.Harmful mutations tend to be eliminated through natural selection, whilebeneficial mutations lead to adaptations over time.Natural Selection Brings About Adaptations:Adaptationsare traits that allow organisms to better survive in theirenvironment.

oPre-Darwinian biologists viewed these adaptations as evidence of adivine Creator’s work.oExamples include desert plants adapted to low water environmentsand fast-swimming fish streamlined for efficient movement in water.Charles Darwin’s "On the Origin of Species"(1859) changed this view:oDarwin argued that species are not fixed but evolve over time.oHe proposed natural selectionas the mechanism behind adaptation.oNatural selection explains how organisms evolve to better fit theirenvironments.Darwin and Wallace’s Contributions:Darwin and Alfred Russel Wallaceindependently developed the theory ofevolution by natural selection.oDarwin took 20 years to collect evidence and only published his theoryafter receiving a letter from Wallace, who had independently reachedsimilar conclusions while collecting specimens in Indonesia.Key Observations Underlying Natural Selection:1.Variation Among Individuals:Within a species, individuals differ from eachother.2.Heritability of Traits:Some of these differences are passed on to the nextgeneration (offspring resemble their parents).3.Competition for Resources:Individuals compete for resources like food,shelter, and mates. Not all individuals survive to reproduce.4.Differential Reproductive Success:Those individuals with traits that helpthem survive and reproduce are more likely to pass on their genes to thenext generation. Over time, these advantageous traits increase in thepopulation.Types of Selection:Positive Selection: Increases the frequency of advantageous alleles.Eventually, the population can become "fixed" for this allele, meaning 100%of individuals have it.oExample: If an allele gives a fish better camouflage, that fish is morelikely to survive and reproduce, passing the allele to the nextgeneration.Negative Selection: Decreases the frequency of deleterious alleles,especially lethal mutations.oExample: A lethal mutation might cause organisms to die beforereproducing, so those alleles gradually disappear from the population.

Balancing Selection:Balancing Selection: Maintains two or more alleles in a population atintermediate frequencies, rather than eliminating one or fixing the other.oExample: A species may have different survival advantages in wetversus dry environments, keeping two different alleles present in thepopulation.Fitness:Fitness: Describes how well an individual survives and reproduces in aparticular environment.oFitness depends on the environment (e.g., a gazelle’s fitness is high ifit can run fast to escape predators).oRelative Fitness: The fitness of one individual compared to anotherwithin the same population.oHigher fitness means leaving more offspring that survive to adulthood.Types of Natural Selection:1.Stabilizing Selection: Acts against extreme traits and favors intermediatetraits.a.Example: Human birth weight—babies with very low or very high birthweights have higher mortality rates, so intermediate birth weights arefavored.2.Directional Selection: Favors one extreme phenotype, causing a shift in thepopulation’s traits in one direction.a.Example: Darwin’s finches—after a drought, birds with larger beakswere better at eating tough seeds and had a survival advantage. Overtime, the average beak size in the population increased.3.Disruptive Selection: Favors both extremes at the expense of intermediatetraits.a.Example: In some populations, individuals with very large or very smallbeaks may be favored, while medium-sized beaks are notadvantageous.

Heterozygote Advantage:Heterozygote Advantage: A type of balancing selection whereheterozygous individuals (Aa) have higher fitness than either homozygousdominant (AA) or homozygous recessive (aa).oExample: Sickle-cell anemia in regions affected by malaria.Individuals with one sickle-cell allele (AS) are resistant to malariaand don’t suffer from severe sickle-cell disease.Individuals with two sickle-cell alleles (SS) have sickle-celldisease and experience health complications.Individuals with no sickle-cell alleles (AA) are vulnerable tomalaria.As a result, natural selection maintains both the A and S allelesin the population.Evolution of Human Traits and Sickle-Cell Anemia:Malaria and Sickle-Cell Anemia:oIn regions with malaria, natural selection has led to an advantage forheterozygotes (AS), maintaining both the A and S alleles.oIn areas without malaria, the S allele is gradually being eliminatedbecause it no longer offers a survival benefit.oThis example shows how environmental pressures shape allelefrequencies over time.Directional Selection in Darwin’s Finches:Directional Selection Example:oThe 1977 drought in the Galápagos Islands killed many seed-producingplants, leaving only plants that produced large seeds.oFinches with larger beaks had an advantage because they could eatthe tough seeds, so their survival rate was higher.oAfter the drought, finches with larger beaks made up a greaterproportion of the population, illustrating how natural selection drivesevolutionary change.oHomozygous dominant (AA) frequency = p2p^2p2oHeterozygous (Aa) frequency = 2pq2pq2pqoHomozygous recessive (aa) frequency = q2q^2q2 Example: If apopulation has an allele frequency of p=0.6p = 0.6p=0.6 and q=0.4q =0.4q=0.4, then:

oHomozygous dominant frequency (p2p^2p2) =0.62=0.360.6^2 =0.360.62=0.36 (36%)oHomozygous recessive frequency (q2q^2q2) =0.42=0.160.4^2 =0.160.42=0.16 (16%)oHeterozygous frequency (2pq2pq2pq) = 2×0.6×0.4=0.482 \times 0.6 \times 0.4 =0.482×0.6×0.4=0.48 (48%)CHAPTER 20.5: NON-ADAPTIVE MECHANISMS OF EVOLUTION Nonadaptive Mechanismsare evolutionary processes that change allelefrequencies, but they don't necessarily help the organism adapt to its environment.1. Genetic DriftDefinition: Genetic drift is the random change in allele (gene variant)frequencies from one generation to the next. It happens purely by chance,and notbecause some alleles are more beneficial.Key Idea: It is most impactful in small populations. In a small group,random events (like one individual having more offspring) can significantlychange which alleles are passed on. In large populations, random eventsare less impactful.Effect: Over time, genetic drift can cause certain alleles to become verycommon or even disappear completely.Examples of Genetic Drift:oPopulation Bottleneck: This happens when a large population isdrastically reduced in size (e.g., by a natural disaster), leaving only afew survivors. These survivors may not represent the originalpopulation's genetic diversity. As they reproduce, the gene pool of thepopulation changes.Example: If most of a tortoise population dies from a volcaniceruption, the small group of survivors will pass on their genes,which may have different frequencies than the originalpopulation.oFounder Effect: When a few individuals from a large population starta new population in a new area. These few individuals only carry asmall portion of the genetic diversity from the original population.

Example: If a small group of birds flies to a new island, thegenes they carry will form the new population, and these genesmay be different from the original larger population.2. Migration (Gene Flow)Definition: Migration is when individuals move from one population toanother and bring their alleles with them. When individuals from onepopulation breed with another, they change the allele frequencies in the newpopulation.Effect: Migration tends toreduce genetic differencesbetweenpopulations. Over time, populations that exchange individuals will becomemore genetically similar.Example: If white rabbits from one island migrate to an island with blackrabbits, their genes will mix, and both populations will eventually have bothwhite and black rabbits.3. MutationDefinition: A mutation is a change in the DNA sequence that creates a newallele. Mutations are random and can introduce new genetic material into apopulation.Key Role: Mutations are essential because they create new geneticvariations. Without mutations, there would be no new genes for naturalselection to act on.Effect: Although mutations are rare, they are the source of all geneticdiversity. Most mutations don’t have a significant effect immediately, butthey can accumulate over time and introduce important variations inpopulations.4. Nonrandom MatingDefinition: Nonrandom mating happens when individuals do not materandomly but choose their mates based on specific traits. This can affect thefrequency of genotypes (combinations of alleles) in the population but doesnot change the overall allele frequencies.Example: If animals choose their mates based on size, certain genotypesmight become more common (like animals that inherit genes for larger size),but the actual alleles for size in the population don’t change.Effect: Nonrandom mating can increase the number of individuals withparticular gene combinations (genotypes), but it doesn’t add or removealleles from the population.

5. InbreedingDefinition: Inbreeding occurs when individuals that are closely related matewith each other. This increases the number of homozygousindividuals(those with two identical alleles for a gene).Key Effect: Inbreeding increases the likelihood of inbreeding depression,which happens when harmful, recessive traits become more common. Thesetraits can lead to problems with health or survival.Example: In the 1990s, inbreeding depression almost led to the extinction ofthe Florida panther. Inbreeding was causing many recessive, harmful traitsto show up, reducing the panther's fitness. Conservation efforts introducednew panthers from a different population to increase genetic diversity andreduce inbreeding problems.Self-Assessment Questions1.Why does genetic drift have a larger effect in small populations?a.Answer: Genetic drift has a larger effect in small populations becauseeach individual's genetic contribution makes a bigger difference. Insmall populations, random events, like one individual producing moreoffspring, can have a larger impact on the overall gene pool, causingcertain alleles to become more common or disappear entirely. In largepopulations, the effects of random events are smaller, and allelefrequencies tend to remain more stable over time.2.What happens during a population bottleneck, and how does it affectgenetic diversity?a.Answer: A population bottleneck occurs when a large population issuddenly reduced to a small size, often due to a natural disaster orother events. This reduces genetic diversity because only a fewindividuals survive to reproduce. The alleles carried by the survivorsmay not represent the original population’s gene pool, so futuregenerations will have less genetic variation.3.How does migration (gene flow) affect populations?a.Answer: Migration can reduce genetic differences betweenpopulations by introducing new alleles from one population into

another. Over time, migration causes populations to become moresimilar genetically. This movement of individuals (and their genes)between populations can increase genetic diversity in the populationreceiving the migrants.CHAPTER 20.6: MOLECULAR EVOLUTION What is Molecular Evolution?Molecular evolutionis the process of change in the DNA sequence oforganisms over time. The genetic differences that arise from this processlead to variations among species.Imagine starting with two populations that are genetically identical, but youplace them on two separate islands (geographically isolated). Over time, thepopulations accumulate mutations that differ from one another.Key Concept:These mutations happen independently in each population becausethey are isolated. Over many generations, these differences add up, and thepopulations become genetically distinct. This is an example of molecular evolution.How Does This Process Work?Parent Population Splits: When the parent population is split and isolated,the two "daughter" populations do not share mutations with each other. Thisisolation allows for mutationsto occur independently in each population.Over time, these mutations may become fixed (permanent) in eachpopulation, creating differences. The more time passes, the more differencesaccumulate.Molecular ClocksThe concept of a molecular clockis used to estimate the time since twospecies diverged (when they split into different species).How does it work?

oA molecular clock measures the rate of mutationin DNA sequencesover time.oThe idea is that the longer two species have been separated, the moregenetic differences accumulate between them.oFor example, imagine two species split 6 million years ago. By lookingat how many mutations have occurred in each species' DNA over thattime, we can estimate how long they have been evolving separately.Rates of Molecular EvolutionThe rate at which mutations accumulate can vary from one species toanother and from one gene to another. This is influenced by a process callednegative selection.Negative Selection: This process removes harmful mutations, slowing downthe rate of molecular evolution in genes where mutations could disrupt thefunction of important proteins.oExample:The histone geneevolves very slowly because histones(proteins that help organize DNA) are vital for survival. Any mutationthat changes their structure is removed because it could be harmful.oOn the other hand, genes with less critical functions may evolve faster,accumulating more mutations over time.Molecular Clock ExampleOne study compared Old World monkeysand New World monkeystoestimate when they split from a common ancestor.oUsing molecular clocks, researchers found that the divergencehappened about 30 million years ago.oBy comparing the number of differences between humans andchimpanzees, scientists estimated that humans and chimpanzeesdiverged around 6 million years ago. This result is consistent with fossilevidence.Self-Assessment Questions1.How is a molecular clock used to determine the time of divergenceof two species?

a.Answer from Screenshot: A molecular clock is a region of DNA orprotein that has a known rate of accumulation of mutations over time.The more differences we observe when comparing these sequencesfrom two different species, the longer it has been since the speciesdiverged from each other. This relative timescale of divergence can befurther clarified using chronological information from the fossil record.2. Why do we expect a protein’s rate of molecular evolution to becorrelated with the protein’s function?a.Answer from Screenshot: The molecular clocks of different proteins“tick” at different rates, meaning that they evolve at different rates.The primary determinant of the rate of evolution of a particular proteinis the strength of negative selection acting against mutations causingamino acid changes. If a protein provides an absolutely critical function(for example, a histone protein, around which DNA is wrapped inchromatin), negative selection is very strong, eliminating virtually allamino acid-changing mutations because any change disrupts thefunction of the histone. Another protein, with a less fundamentalfunction, may, in contrast, be able to accommodate the occasionalamino acid-changing mutation that does not disrupt the protein’sfunction. Negative selection is not as pervasive in the second proteinas it is in the histone.CONCEPT SUMMARY