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Population Genetics*

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Abstract

By the end of this section, you will be able to:

• Describe the di�erent types of variation in a population• Explain why only heritable variation can be acted upon by natural selection• Describe genetic drift and the bottleneck e�ect• Explain how each evolutionary force can in�uence the allele frequencies of a population

Individuals of a population often display di�erent phenotypes, or express di�erent alleles of a particulargene, referred to as polymorphisms. Populations with two or more variations of particular characteristics arecalled polymorphic. The distribution of phenotypes among individuals, known as the population variation,is in�uenced by a number of factors, including the population's genetic structure and the environment(Figure 1). Understanding the sources of a phenotypic variation in a population is important for determininghow a population will evolve in response to di�erent evolutionary pressures.

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Figure 1: The distribution of phenotypes in this litter of kittens illustrates population variation. (credit:Pieter Lanser)

1 Genetic Variance

Natural selection and some of the other evolutionary forces can only act on heritable traits, namely anorganism's genetic code. Because alleles are passed from parent to o�spring, those that confer bene�cialtraits or behaviors may be selected for, while deleterious alleles may be selected against. Acquired traits,for the most part, are not heritable. For example, if an athlete works out in the gym every day, buildingup muscle strength, the athlete's o�spring will not necessarily grow up to be a body builder. If there is agenetic basis for the ability to run fast, on the other hand, this may be passed to a child.



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Before Darwinian evolution became the prevailing theory of the �eld, French naturalist Jean-Baptiste Lamarck theorized that acquired traits could, in fact, be inherited; while this hypoth-esis has largely been unsupported, scientists have recently begun to realize that Lamarck was notcompletely wrong. Visit this site1 to learn more.

Heritability is the fraction of phenotype variation that can be attributed to genetic di�erences, or geneticvariance, among individuals in a population. The greater the hereditability of a population's phenotypicvariation, the more susceptible it is to the evolutionary forces that act on heritable variation.

The diversity of alleles and genotypes within a population is called genetic variance. When scientistsare involved in the breeding of a species, such as with animals in zoos and nature preserves, they try toincrease a population's genetic variance to preserve as much of the phenotypic diversity as they can. Thisalso helps reduce the risks associated with inbreeding, the mating of closely related individuals, which canhave the undesirable e�ect of bringing together deleterious recessive mutations that can cause abnormalitiesand susceptibility to disease. For example, a disease that is caused by a rare, recessive allele might exist

1http://openstaxcollege.org/l/epigenetic



in a population, but it will only manifest itself when an individual carries two copies of the allele. Becausethe allele is rare in a normal, healthy population with unrestricted habitat, the chance that two carrierswill mate is low, and even then, only 25 percent of their o�spring will inherit the disease allele from bothparents. While it is likely to happen at some point, it will not happen frequently enough for natural selectionto be able to swiftly eliminate the allele from the population, and as a result, the allele will be maintainedat low levels in the gene pool. However, if a family of carriers begins to interbreed with each other, this willdramatically increase the likelihood of two carriers mating and eventually producing diseased o�spring, aphenomenon known as inbreeding depression.

Changes in allele frequencies that are identi�ed in a population can shed light on how it is evolving. Inaddition to natural selection, there are other evolutionary forces that could be in play: genetic drift, gene�ow, mutation, nonrandom mating, and environmental variances.

2 Genetic Drift

The theory of natural selection stems from the observation that some individuals in a population are morelikely to survive longer and have more o�spring than others; thus, they will pass on more of their genes tothe next generation. A big, powerful male gorilla, for example, is much more likely than a smaller, weakerone to become the population's silverback, the pack's leader who mates far more than the other males of thegroup. The pack leader will father more o�spring, who share half of his genes, and are likely to also growbigger and stronger like their father. Over time, the genes for bigger size will increase in frequency in thepopulation, and the population will, as a result, grow larger on average. That is, this would occur if thisparticular selection pressure, or driving selective force, were the only one acting on the population. Inother examples, better camou�age or a stronger resistance to drought might pose a selection pressure.

Another way a population's allele and genotype frequencies can change is genetic drift (Figure 2), whichis simply the e�ect of chance. By chance, some individuals will have more o�spring than others�not due toan advantage conferred by some genetically-encoded trait, but just because one male happened to be in theright place at the right time (when the receptive female walked by) or because the other one happened tobe in the wrong place at the wrong time (when a fox was hunting).

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Figure 2: Genetic drift in a population can lead to the elimination of an allele from a population bychance. In this example, rabbits with the brown coat color allele (B) are dominant over rabbits withthe white coat color allele (b). In the �rst generation, the two alleles occur with equal frequency in thepopulation, resulting in p and q values of .5. Only half of the individuals reproduce, resulting in a secondgeneration with p and q values of .7 and .3, respectively. Only two individuals in the second generationreproduce, and by chance these individuals are homozygous dominant for brown coat color. As a result,in the third generation the recessive b allele is lost.



Do you think genetic drift would happen more quickly on an island or on the mainland?

Small populations are more susceptible to the forces of genetic drift. Large populations, on the other hand,are bu�ered against the e�ects of chance. If one individual of a population of 10 individuals happens to dieat a young age before it leaves any o�spring to the next generation, all of its genes�1/10 of the population'sgene pool�will be suddenly lost. In a population of 100, that's only 1 percent of the overall gene pool;therefore, it is much less impactful on the population's genetic structure.

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Go to this site2 to watch an animation of random sampling and genetic drift in action.

Genetic drift can also be magni�ed by natural events, such as a natural disaster that kills�at random�alarge portion of the population. Known as the bottleneck e�ect, it results in a large portion of the genomesuddenly being wiped out (Figure 3). In one fell swoop, the genetic structure of the survivors becomes thegenetic structure of the entire population, which may be very di�erent from the pre-disaster population.

2http://openstaxcollege.org/l/genetic_drift



Figure 3: A chance event or catastrophe can reduce the genetic variability within a population.

Another scenario in which populations might experience a strong in�uence of genetic drift is if someportion of the population leaves to start a new population in a new location or if a population gets dividedby a physical barrier of some kind. In this situation, those individuals are unlikely to be representative of theentire population, which results in the founder e�ect. The founder e�ect occurs when the genetic structurechanges to match that of the new population's founding fathers and mothers. The founder e�ect is believedto have been a key factor in the genetic history of the Afrikaner population of Dutch settlers in South Africa,as evidenced by mutations that are common in Afrikaners but rare in most other populations. This is likelydue to the fact that a higher-than-normal proportion of the founding colonists carried these mutations. As aresult, the population expresses unusually high incidences of Huntington's disease (HD) and Fanconi anemia

(FA), a genetic disorder known to cause blood marrow and congenital abnormalities�even cancer.3



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Watch this short video to learn more about the founder and bottleneck e�ects.

: Testing the Bottleneck E�ect

Question: How do natural disasters a�ect the genetic structure of a population?

Background: When much of a population is suddenly wiped out by an earthquake or hurricane,the individuals that survive the event are usually a random sampling of the original group. As aresult, the genetic makeup of the population can change dramatically. This phenomenon is knownas the bottleneck e�ect.

Hypothesis: Repeated natural disasters will yield di�erent population genetic structures; there-fore, each time this experiment is run, the results will vary.

Test the hypothesis: Count out the original population using di�erent colored beads. Forexample, red, blue, and yellow beads might represent red, blue, and yellow individuals. After



recording the number of each individual in the original population, place them all in a bottle with anarrow neck that will only allow a few beads out at a time. Then, pour 1/3 of the bottle's contentsinto a bowl. This represents the surviving individuals after a natural disaster kills a majority ofthe population. Count the number of the di�erent colored beads in the bowl, and record it. Then,place all of the beads back in the bottle and repeat the experiment four more times.

Analyze the data: Compare the �ve populations that resulted from the experiment. Do thepopulations all contain the same number of di�erent colored beads, or do they vary? Remember,these populations all came from the same exact parent population.

Form a conclusion: Most likely, the �ve resulting populations will di�er quite dramatically. Thisis because natural disasters are not selective�they kill and spare individuals at random. Now thinkabout how this might a�ect a real population. What happens when a hurricane hits the MississippiGulf Coast? How do the seabirds that live on the beach fare?

3 Gene Flow

Another important evolutionary force is gene �ow: the �ow of alleles in and out of a population due to themigration of individuals or gametes (Figure 4). While some populations are fairly stable, others experiencemore �ux. Many plants, for example, send their pollen far and wide, by wind or by bird, to pollinate otherpopulations of the same species some distance away. Even a population that may initially appear to bestable, such as a pride of lions, can experience its fair share of immigration and emigration as developingmales leave their mothers to seek out a new pride with genetically unrelated females. This variable �ow ofindividuals in and out of the group not only changes the gene structure of the population, but it can alsointroduce new genetic variation to populations in di�erent geological locations and habitats.

Figure 4: Gene �ow can occur when an individual travels from one geographic location to another.



4 Mutation

Mutations are changes to an organism's DNA and are an important driver of diversity in populations.Species evolve because of the accumulation of mutations that occur over time. The appearance of newmutations is the most common way to introduce novel genotypic and phenotypic variance. Some mutationsare unfavorable or harmful and are quickly eliminated from the population by natural selection. Others arebene�cial and will spread through the population. Whether or not a mutation is bene�cial or harmful isdetermined by whether it helps an organism survive to sexual maturity and reproduce. Some mutations donot do anything and can linger, una�ected by natural selection, in the genome. Some can have a dramatice�ect on a gene and the resulting phenotype.

5 Nonrandom Mating

If individuals nonrandomly mate with their peers, the result can be a changing population. There are manyreasons nonrandom mating occurs. One reason is simple mate choice; for example, female peahens mayprefer peacocks with bigger, brighter tails. Traits that lead to more matings for an individual become selectedfor by natural selection. One common form of mate choice, called assortative mating, is an individual'spreference to mate with partners who are phenotypically similar to themselves.

Another cause of nonrandom mating is physical location. This is especially true in large populationsspread over large geographic distances where not all individuals will have equal access to one another. Somemight be miles apart through woods or over rough terrain, while others might live immediately nearby.

6 Environmental Variance

Genes are not the only players involved in determining population variation. Phenotypes are also in�uencedby other factors, such as the environment (Figure 5). A beachgoer is likely to have darker skin than a citydweller, for example, due to regular exposure to the sun, an environmental factor. Some major characteristics,such as sex, are determined by the environment for some species. For example, some turtles and other reptileshave temperature-dependent sex determination (TSD). TSD means that individuals develop into males iftheir eggs are incubated within a certain temperature range, or females at a di�erent temperature range.



Figure 5: The sex of the American alligator (Alligator mississippiensis) is determined by the temperatureat which the eggs are incubated. Eggs incubated at 30 ◦C produce females, and eggs incubated at 33 ◦Cproduce males. (credit: Steve Hillebrand, USFWS)

Geographic separation between populations can lead to di�erences in the phenotypic variation betweenthose populations. Such geographical variation is seen between most populations and can be signi�cant.One type of geographic variation, called a cline, can be seen as populations of a given species vary graduallyacross an ecological gradient. Species of warm-blooded animals, for example, tend to have larger bodies inthe cooler climates closer to the earth's poles, allowing them to better conserve heat. This is considered alatitudinal cline. Alternatively, �owering plants tend to bloom at di�erent times depending on where theyare along the slope of a mountain, known as an altitudinal cline.

If there is gene �ow between the populations, the individuals will likely show gradual di�erences inphenotype along the cline. Restricted gene �ow, on the other hand, can lead to abrupt di�erences, evenspeciation.

7 Section Summary

Both genetic and environmental factors can cause phenotypic variation in a population. Di�erent alleles canconfer di�erent phenotypes, and di�erent environments can also cause individuals to look or act di�erently.Only those di�erences encoded in an individual's genes, however, can be passed to its o�spring and, thus,be a target of natural selection. Natural selection works by selecting for alleles that confer bene�cial traitsor behaviors, while selecting against those for deleterious qualities. Genetic drift stems from the chance



occurrence that some individuals in the germ line have more o�spring than others. When individuals leaveor join the population, allele frequencies can change as a result of gene �ow. Mutations to an individual'sDNA may introduce new variation into a population. Allele frequencies can also be altered when individualsdo not randomly mate with others in the group.

8 Art Connections

Exercise 1 (Solution on p. 14.)

Figure 2 Do you think genetic drift would happen more quickly on an island or on the mainland?

9 Review Questions


When male lions reach sexual maturity, they leave their group in search of a new pride. This canalter the allele frequencies of the population through which of the following mechanisms?

a. natural selectionb. genetic driftc. gene �owd. random mating


Which of the following evolutionary forces can introduce new genetic variation into a population?

a. natural selection and genetic driftb. mutation and gene �owc. natural selection and nonrandom matingd. mutation and genetic drift


What is assortative mating?

a. when individuals mate with those who are similar to themselvesb. when individuals mate with those who are dissimilar to themselvesc. when individuals mate with those who are the most �t in the populationd. when individuals mate with those who are least �t in the population


When closely related individuals mate with each other, or inbreed, the o�spring are often not as�t as the o�spring of two unrelated individuals. Why?

a. Close relatives are genetically incompatible.b. The DNA of close relatives reacts negatively in the o�spring.c. Inbreeding can bring together rare, deleterious mutations that lead to harmful phenotypes.d. Inbreeding causes normally silent alleles to be expressed.


What is a cline?

a. the slope of a mountain where a population livesb. the degree to which a mutation helps an individual survivec. the number of individuals in the populationd. gradual geographic variation across an ecological gradient



10 Free Response


Describe a situation in which a population would undergo the bottleneck e�ect and explain whatimpact that would have on the population's gene pool.


Describe natural selection and give an example of natural selection at work in a population.


Explain what a cline is and provide examples.



Solutions to Exercises in this Module

to Exercise (p. 12)Figure 2 Genetic drift is likely to occur more rapidly on an island where smaller populations are expectedto occur.to Exercise (p. 12)Cto Exercise (p. 12)Bto Exercise (p. 12)Ato Exercise (p. 12)Cto Exercise (p. 12)Dto Exercise (p. 13)A hurricane kills a large percentage of a population of sand-dwelling crustaceans�only a few individualssurvive. The alleles carried by those surviving individuals would represent the entire population's gene pool.If those surviving individuals are not representative of the original population, the post-hurricane gene poolwill di�er from the original gene pool.to Exercise (p. 13)The theory of natural selection stems from the observation that some individuals in a population survivelonger and have more o�spring than others: thus, more of their genes are passed to the next generation.For example, a big, powerful male gorilla is much more likely than a smaller, weaker one to become thepopulation's silverback: the pack's leader who mates far more than the other males of the group. Therefore,the pack leader will father more o�spring who share half of his genes and are likely to grow bigger andstronger like their father. Over time, the genes for bigger size will increase in frequency in the population,and the average body size, as a result, grow larger on average.to Exercise (p. 13)A cline is a type of geographic variation that is seen in populations of a given species that vary graduallyacross an ecological gradient. For example, warm-blooded animals tend to have larger bodies in the coolerclimates closer to the earth's poles, allowing them to better conserve heat. This is considered a latitudinalcline. Flowering plants tend to bloom at di�erent times depending on where they are along the slope of amountain. This is known as an altitudinal cline.

Glossary

De�nition 5: assortative matingwhen individuals tend to mate with those who are phenotypically similar to themselves

De�nition 5: bottleneck e�ectmagni�cation of genetic drift as a result of natural events or catastrophes

De�nition 5: clinegradual geographic variation across an ecological gradient

De�nition 5: gene �ow�ow of alleles in and out of a population due to the migration of individuals or gametes

De�nition 5: genetic drifte�ect of chance on a population's gene pool

De�nition 5: genetic variancediversity of alleles and genotypes in a population



De�nition 5: geographical variationdi�erences in the phenotypic variation between populations that are separated geographically

De�nition 5: heritabilityfraction of population variation that can be attributed to its genetic variance

De�nition 5: inbreedingmating of closely related individuals

De�nition 5: inbreeding depressionincrease in abnormalities and disease in inbreeding populations

De�nition 5: nonrandom matingchanges in a population's gene pool due to mate choice or other forces that cause individuals tomate with certain phenotypes more than others

De�nition 5: population variationdistribution of phenotypes in a population

De�nition 5: selective pressureenvironmental factor that causes one phenotype to be better than another


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