chapter 23 the evolutions of populations section c...

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Section C: Genetic Variation, the Substrate forNatural Selection

1. Genetic variation occurs within and between populations2. Mutation and sexual recombination generate genetic variation3. Diploidy and balanced polymorphisms preserve variation

CHAPTER 23THE EVOLUTIONS OF

POPULATIONS

• The variation among individuals in a population is acombination of inheritable and non-heritable traits.

• Phenotype, the observable characteristics of an organism, isthe cumulative product of an inheritedgenotype and a multitude ofenvironmental influences.

• For example, these butterflies aregenetically identical at the loci forcoloration, but they emerge atdifferent seasons.

1. Genetic variation occurs within andbetween populations


Fig. 23.7

• Only the genetic component of variation can haveevolutionary consequences as a result of naturalselection.• This is because only inheritable traits pass from

generation to generation.


• Both quantitative and discrete characters contributeto variation within a population.

• Quantitative characters are those that vary along acontinuum within a population.• For example, plant height in our wildflower population

includes short and tall plants and everything in between.

• Quantitative variation is usually due to polygenicinheritance in which the additive effects of two or moregenes influence a single phenotypic character.

• Discrete characters, such as flower color, areusually determined by a single locus with differentalleles with distinct impacts on the phenotype.


• Polymorphism occurs when two or more discretecharacters are present and noticeable in apopulation.• The contrasting forms are called morphs, as in the red-

flowered and white-flowered morphs in our wildflowerpopulation or the butterflies in the previous slide.

• Human populations are polymorphic for a variety ofphysical (e.g., freckles) and biochemical (e.g., bloodtypes) characters.

• Polymorphism applies only to discrete characters,not quantitative characters, such as human height,which varies among people in a continuum.


• Population geneticists measure genetic variationboth at the level of whole genes and at themolecular level of DNA.

• Gene diversity measures the average percent ofgene loci that are heterozygous.• In the fruit fly (Drosophila), about 86% of their 13,000

gene loci are homozygous (fixed).

• About 14% (1,800 genes) are heterozygous.


• Nucleotide diversity measures the level ofdifference in nucleotide sequences (base pairdifferences) among individuals in a population.• In fruit flies, about 1% of the bases are different between

two individuals.

• Two individuals would differ at 1.8 million of the 180million nucleotides in the fruit fly genome.

• Humans have relatively little genetic variation.• Gene diversity is about 14% in humans.

• Nucleotide diversity is only 0.1%.

• You and your neighbor have the same nucleotide at999 out of every 1,000 nucleotide sites in your DNA.


• Geographic variation results from differences ingenetic structure either between populations orbetween subgroups of a single population thatinhabit different areas.• Often geographic variation results from natural selection

that modifies gene frequencies in response to differencesin local environmental factors.

• Alternatively, genetic drift can lead to chance variationsamong populations.

• Geographic variation can occur on a local scale, within apopulation, if the environment is patchy or if dispersal ofindividuals is limited, producing subpopulations.


• Geographic variation in the form of graded changein a trait along a geographic axis is called a cline.• Clines may represent intergrade zones where individuals

from neighboring, genetically different, populationsinterbreed.

• Alternatively, clines may reflect the influence of naturalselection on gradation in some environmental variable.

• For example, the average body size of many NorthAmerican species of birds and mammals increasesgradually with increasing latitude, perhaps conservingheat by decreasing the ratio of surface area to volume.


• Clines may reflect direct environmental effects onphenotype, but also genetic differences along thecline.

• For example, average size of yarrow plants (Anchillea),gradually decreases with increasing variation.

• Although the environmentaffects growth rate directlyto some extent withaltitude, common gardenexperiments havedemonstrated thatsome of the variationhas a genetic basis.


Fig. 23.8

• In contrast to clines, isolated populations typicallydemonstrate discrete differences.

• For example, populations ofhouse mice were first intro-duced to the island ofMadiera in the 15th century,but isolated populationsdeveloped that wereseparated by mountains.

• Some isolated populationshave evolved differencesin karyotypes probablythrough genetic drift.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 23.9

• New alleles originate only by mutation.• Mutations are changes in the nucleotide sequence of

DNA.

• Mutations of individual genes are rare and random.

• Mutations in somatic cells are lost when the individualdies.

• Only mutations in cell lines that produce gametes can bepassed along to offspring.

2. Mutation and sexual recombinationgenerate genetic variation


• Most point mutations, those affecting a single baseof DNA, are probably harmless.• Most eukaryotic DNA does not code for proteins and

mutations in these areas are likely to have little impacton phenotype.

• Even mutations in genes that code for proteins may leadto little effect because of redundancy in the genetic code.

• However, some single point mutations can have asignificant impact on phenotype.

• Sickle-cell disease is caused by a single pointmutation.


• Mutations that alter the structure of a proteinenough to impact its function are more likely to beharmful than beneficial.• A random change is unlikely to improve a genome that is

the product of thousands of generations of selection.• Rarely, a mutant allele may enable an organism to fit its

environment better and increase reproductive success.• This is especially likely if the environment is changing• These mutations may be beneficial now.

• For example, mutations that enable HIV to resistantiviral drugs are selected against under normalconditions, but are favorable under drug treatment.


• Chromosomal mutations, including rearrangementsof chromosomes, affect many genes and are likelyto disrupt proper development of an organism.• However, occasionally, these dislocations link genes

together such that the phenotype is improved.

• Duplications of chromosome segments, wholechromosomes, or sets of chromosomes are nearlyalways harmful.• However, when they are not harmful, the duplicates

provide an expanded genome.

• These extra genes can now mutate to take on newfunctions.


• Because microorganisms have very short generationtimes, mutation generates genetic variation rapidly.• In an AIDS patient, HIV generates 1010 new viruses per

day.• With its RNA genome, mutation rate is higher than DNA

genomes.• This combination of mutation and replication rate will

generate mutations in the HIV population at every site inthe HIV genome every day.• In the face of this high mutation rate, single-drug

treatments are unlikely to be effective for very longand the most effective treatments are multiple drug“cocktails.”• It is far less probable that mutations against all the

drugs will appear in individual viruses in a short time.


• In organisms with sexual reproduction, most of thegenetic differences among individuals are due tounique recombinations of the existing alleles fromthe population gene pool.• The ultimate origin of allelic variation is past mutations.

• Random segregation of homologous chromosomesand random union of gametes creates a uniqueassortment of alleles in each individual.

• Sexual reproduction recombines old alleles intofresh assortments every generation.


• The tendency for natural selection to reducevariation is countered by mechanisms that preserveor restore variation, including diploidy and balancedpolymorphisms.

• Diploidy in eukaryotes prevents the elimination ofrecessive alleles via selection because they do notimpact the phenotype in heterozygotes.• Even recessive alleles that are unfavorable can persist in

a population through their propagation by heterozygousindividuals.

3. Diploidy and balanced polymorphismpreserve variation


• Recessive alleles are only exposed to selection when twoparents carry the same recessive allele and these arecombined in one zygote.

• This happens only rarely when the frequency of therecessive allele is very low.

• Heterozygote protection maintains a huge pool of allelesthat may not be suitable under the present conditions butthat could be beneficial when the environment changes.


• Balanced polymorphism maintains geneticdiversity in a population via natural selection.

• One mechanism in balance polymorphism isheterozygote advantage.• In some situations individuals that are heterozygous at a

particular locus have greater survivorship andreproductive success than homozygotes.

• In these cases, multiple alleles will be maintained at thatlocus by natural selection.


• Heterozygous advantage maintains genetic diversityat the human locus for one chain of hemoglobin.• A recessive allele causes sickle-cell disease in

homozygous individuals.

• Homozygous dominant individuals are very vulnerableto malaria.

• Heterozygous individuals are resistant to malaria.


• The frequency of the sickle-cell allele is highest inareas where the malarial parasite is common.• The advantages of heterozygotes over homozygous

recessive individuals who suffer sickle-cell disease andhomozygous dominant individuals who suffer malariaare greatest here.

• The sickle-cell allelemay reach 20% ofthe gene pool, with 32%heterozygotes resistantto malaria and 4% withsickle-cell disease.


Fig. 23.10

• A second mechanism promoting balancedpolymorphisms is frequency-dependent selection.

• Frequency-dependent selection occurs when thereproductive success of any one morph declines ifthat phenotype becomes too common in thepopulation.• The relationships between parasites and their hosts often

demonstrate this type of relationship.


• Hosts often vary in their defense against parasitesand parasites in their ability to infect hosts.• Those parasites that are capable of infecting the most

common host type will increase in abundance.

• The rarer host types will increase as the geneticfrequencies in the parasite population shifts.

• These shifts in genetic frequencies among hosts andamong parasites maintain variation in both populations.


• Aspects of this teeter-totter of frequency-dependentselection can be seen in the host-parasite betweenclones of aquatic snails and a parasitic worm.• In these snails which reproduce asexually, the most

common snail clones suffer the higher infection ratesthan the leastcommon clone,suggestingfrequency-dependentselection.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 23.11

• Some genetic variations, neutral variation, havenegligible impact on reproductive success.• For example, the diversity of human fingerprints seems

to confer no selective advantage to some individuals overothers.

• Much of the protein and DNA variation detectable bymethods like electrophoresis may be neutral in theiradaptive qualities.

• The relative frequencies of neural variations willnot be affected by natural selection.

• Some neutral alleles will increase and others willdecrease by the chance effects of genetic drift.


• There is no consensus on how much geneticvariation can be classified as neutral or even if anyvariation can be considered truly neutral.• It is almost impossible to demonstrate that an allele

brings no benefit at all to an organism.• Also, variation may be neutral in one environment but

not in another.• Even if only a fraction of the extensive variation in a

gene pool significantly affects an organism, there is stillan enormous reservoir of raw material for naturalselection and adaptive evolution.


chapter 23 the evolutions of populations section c...

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