LECTURE NOTES

CHAPTER 21 LECTURE NOTES:

Natural selection works on individuals, but it is the population that evolves. Darwin understood this, but was unable

to determine its genetic basis.

1. The modern evolutionary synthesis integrated Darwinism and Mendelism: science

as a process

Shortly after the publication of The Origin of Species, most biologists were convinced that species evolved.

Darwin was less successful in convincing them that natural selection was the mechanism for evolution,

because little was known about inheritance. 9 An understanding about inheritance was necessary to explain:

How chance variations arise in populations. How these variations are precisely transmitted from

parents to offspring. Though Gregor Mendel was a contemporary of Darwin's, Mendel's principles of

inheritance went unnoticed until the early 1900's.

The Evolution of Populations 373

For Darwin, the raw material for natural selection was variation in quantitative characters that

vary along a continuum in a population.

• We now know that continuous variation is usually determined by many segregating loci

(polygenic inheritance).

• As did Mendel, geneticists in the early 1900's recognized only discrete characters

inherited on an either‑or basis. Thus, for them, there appeared to be no genetic basis for

the subtle variations that were central to Darwin's theory.

During the 1920's, genetic research focused on mutations, and a widely accepted alternative to

Darwin's theory was that evolution occurs in rapid leaps as a result of radical phenotypic changes

caused by mutations.

• This idea contrasted with Darwin's view of gradual evolution due to environmental

selection acting on continuous variation among individuals of a population.

• Another popular theory was orthogenesis, the idea that evolution has been a predictable

progression to more and more elite forms of life.

In the 193O's, the science of population genetics emerged, which:

• Emphasized genetic variation within populations and recognized the importance of

quantitative characters.

• Was an important turning point for evolutionary theory, because it reconciled Mendelian

genetics with Darwinian evolution.

In the 1940's, the genetic basis of variation and natural selection was worked out, and the

modern synthesis was formulated. This comprehensive theory:

Integrated discoveries from different fields (i.e. paleontology, taxonomy, biogeography,

and population genetics).

Was collectively developed by many scientists including:

‑=> Theodosius Dobzhansky ‑ geneticist

=> Ernst Mayr ‑ biogeographer and systematist

:=> George Gaylord Simpson ‑ paleontologist

=> G. Ledyard Stebbins ‑ botanist

Emphasized the following:

Importance of populations as units of evolution.

The central role of natural selection as the primary mechanism of evolutionary change.

=> Gradualism as the explanation of how large changes can result from an accumulation

of small changes occurring over long periods of time.

Most of Darwin's ideas persist in the modern synthesis although many evolutionists are

challenging some generalizations of the modem synthesis.

• This debate focuses on the rate of evolution and on the relative importance of evolutionary

mechanisms other than natural selection.

• These debates do not question the fact of evolution, only what mechanisms are most

important in the process.

Such disagreements indicate that the study of evolution is very lively and that it continues

to develop as a science.

374 The Evolution ofPopulations

frequencies

11. A population has a genetic structure defined by its gene pool's allele and genotype

Population = Localized group of organisms which belong to the same species.

Species = Groups of actually or potentially interbreeding natural populations, which are

reproductively isolated from other such groups.

Most species are not evenly distributed over a geographical range, but are concentrated in several

localized population centers.

• Each population center is isolated to some extent from other population centers with only

occasional gene flow among these groups.

• Obvious examples are isolated populations found on widely separated islands or in

unconnected lakes.

• Some populations are not separated by such sharp boundaries.

=> For example, a species with two population centers may be connected by an

intermediate sparsely populated range.

=> Even though these two populations are not absolutely isolated, individuals are more

likely to interbreed with others from their population center. Gene flow between the

two population centers is thus reduced by the intermediate range.

Gene pool = The total aggregate of genes in a population at any one time.

• Consists of all the alleles at all gene loci in all individuals of a population. Alleles from

this pool will be combined to produce the next generation.

• In a diploid species, an individual may be homozygous‑or heterozygous for a locus since

each locus is represented twice.

An allele is said to be fixed in the gene pool if all members of the population are

homozygous for that allele.

Normally there will be two or more alleles for a gene, each having a relative frequency in

the gene pool.

III. The Hardy‑Weinberg Theorem describes a nonevolving population

NOTE: The Hardy‑Weinberg model is so much easier to teach if the students calculate gene

frequencies along with the instructor. This means that you must pause frequently to

allow plenty of time for students to actively process the information and practice the

calculations.

In the absence of other factors, the segregation and recombination of alleles during meiosis and

fertilization will not alter the overall genetic makeup of a population.

• The frequencies of alleles in the gene pool will remain constant unless acted upon by other

agents; this is known as the Hardy‑Weinberg Theorem.

• The Hardy‑Weinberg model describes the genetics of nonevolving populations. This

theorem can be tested with theoretical population models.

7he Evolution of Populations 375

To test the Hardy‑Weinberg theorem, imagine an isolated population of wildflowers with the

following characteristics: (See Campbell, Figure 21.3)

* It is a diploid species with both pink and white flowers.

* The population size is 500 plants: 480 plants have pink flowers, 20 plants have white

flowers.

Pink flower color is coded for by the dominant allele "A," white flower color is coded for

by the recessive allele "a."

* Of the 480 pink‑flowered plants, 320 are homozygous (AA) and 160 are heterozygous

(Aa). Since white color is recessive, all white flowered plants are homozygous aa.

There are 1000 genes for flower color in this population, since each of the 500 individuals

has two genes (this is a diploid species).

A total of 320 genes are present in the 160 heterozygotes (Aa): half are dominant (160 A)

and half are recessive (160 a).

* 800 of the 1000 total genes are dominant.

The frequency of the A allele is 80% or 0.8 (800/1000).

# of 4 of A alleles

Genotypes plants per individual

AA plants 320 X 2

.Aa plants 160 X I

* 200 of the 1000 total genes are recessive.

* The frequency of the aallele is 20% or 0.2 (200/1000).

Total #

A alleles

640

160

800

# of # of A alleles Total #

Genotype plants per individual A alleles

aa plants 20 X 2 40

Aa plants 160 X 1 160

200

Assuming that mating in the population is completely random (all male‑female mating

combinations have equal chances), the frequencies of A and a will remain the same in the next

generation.

• Each gamete will carry one gene for flower color, either A or a.

• Since mating is random, there is an 80% chance that any particular gamete will carry the A

allele and a 20% chance that any particular gamete will carry the allele.

376 The Evolution ofPopulations

The frequencies of the three possible genotypes of the next generation can be calculated using

the rule of multiplication: (See Campbell, Chapter 13)

The probability of two A alleles joining is 0.8 x 0.8 = 0.64; thus, 64% of the next

generation will be AA.

The probability of two a alleles joining is 0.2 x 0.2 = 0.04; thus, 4% of the next generation

will be aa.

Heterozygotes can be produced in two ways, depending upon whether the sperm or ovum

contains the dominant allele (Aa or aA). The probability of a heterozygote being produced

is thus (0.8 x 0.2) + (0.2 x 0.8) = 0. 16 + 0.16 = 0.32.

The frequencies of possible genotypes in the next generation are 64% AA, 32% Aa and 4% aa.

The frequency of the A allele in the new generation is 0.64 + (0.32/2) = 0.8, and the

frequency of the a allele is 0.04 + (0.32/2) = 0.2. Note that the alleles are present in the

gene pool of the new population at the same frequencies they were in the original gene

pool.

Continued sexual reproduction with segregation, recombination and random mating would

not alter the frequencies of these two alleles: the gene pool of this population would be in

a state of equilibrium referred to as Hardy‑Weinberg equilibrium.

If our original population had not been in equilibrium, only one generation would have

been necessary for equilibrium to become established.

From this theoretical wildflower population, a general formula, called the Hardy‑Weinberg

equation, can be derived to calculate allele and genotype frequencies.

The Hardy‑Weinberg equation can be used to consider loci with three or more alleles.

By way of example, consider the simplest case with only two alleles with one dominant to

the other.

In our wildflower population, let p represent allele A and q represent allele a, thus p = 0.8

and q = 0.2.

The sum of frequencies from all alleles must equal 100% of the genes for that locus in the

population: p + q = 1.

Where only two alleles exist, only the frequency of one must be known since the other can

be derived:

I ‑ p = q or I ‑q=p

When gametes fuse to form a zygote, the probability of producing the AA genotype is p 2 ; the

probability of producing aa is q 2 ; and the probability of producing an Aa heterozygote is 2pq

(remember heterozygotes may be formed in two ways).

The sum of these frequencies must equal 100%, thus:

p 2 +

Frequency

of AA

2pq + q 2

allele and a 20% chance that any particular gamete will carry the a allele.

376 The Evolution of Populations

The frequencies of the three possible genotypes of the next generation can be calculated using

the rule of multiplication: (See Campbell, Chapter 13)

The probability of two A alleles joining is 0.8 x 0.8 = 0.64; thus, 64% of the next

generation will be AA.

The probability of two a alleles joining is 0.2 x 0.2 = 0.04; thus, 4% of the next generation

will be aa.

Heterozygotes can be produced in two ways, depending upon whether the sperm or ovum

contains the dominant allele (Aa or aA). The probability of a heterozygote being produced

is thus (0.8 x 0.2) + (0.2 x 0.8) = 0. 16 + 0.16 = 0.32.

The frequencies of possible genotypes in the next generation are 64% AA, 32% Aa and 4% aa.

The frequency of the A allele in the new generation is 0.64 + (0.32/2) = 0.8, and the

frequency of the a allele is 0.04 + (0.32/2) = 0.2. Note that the alleles are present in the

gene pool of the new population at the same frequencies they were in the original gene

pool.

Continued sexual reproduction with segregation, recombination and random mating would

not alter the frequencies of these two alleles: the gene pool of this population would be in

a state of equilibrium referred to as Hardy‑Weinberg equilibrium.

If our original population had not been in equilibrium, only one generation would have

been necessary for equilibrium to become established.

From this theoretical wildflower population, a general formula, called the Hardy‑Weinberg

equation, can be derived to calculate allele and genotype frequencies.

The Hardy‑Weinberg equation can be used to consider loci with three or more alleles.

By way of example, consider the simplest case with only two alleles with one dominant to

the other.

In our wildflower population, let p represent allele A and q represent allele a, thus p = 0.8

and q = 0.2.

The sum of frequencies from all alleles must equal 100% of the genes for that locus in the

population: p + q = 1.

Where only two alleles exist, only the frequency of one must be known since the other can

be derived:

I ‑ p = q or I ‑q=p

When gametes fuse to form a zygote, the probability of producing the AA genotype is p 2 ; the

probability of producing aa is q 2 ; and the probability of producing an Aa heterozygote is 2pq

(remember heterozygotes may be formed in two ways).

The sum of these frequencies must equal 100%, thus:

p 2 +

Frequency

of AA

2pq + q 2

Frequency Frequency

of Aa ofaa

The Evolution of Populations 377

The Hardy‑Weinberg equation pen‑nits the calculation of allelic frequencies in a gene pool, if the

genotype frequencies are known. Conversely, the genotype can be calculated from known allelic

frequencies.

For example, the Hardy‑Weinberg equation can be used to calculate the frequency of inherited

diseases in humans (e.g. phenylketonuria):

• I of every 10,000 babies in the United States is born with phenylketonuria (PKU) which

can result in mental retardation if untreated.

• The allele for PKU is recessive, so babies with this disorder are homozygous recessive

• Thus q 2 = 0.0001, with q = 0.01 (the square root of 0.0001).

• The frequency of p can be determined since p = I ‑ q:

p = I ‑ 0.01 = 0.99

• The frequency of carriers (heterozygotes) in the population is 2pq.

2pq = 2(0.99)(0.01) = 0.0198

• Thus, about 2% of the U.S. population are carriers for PKU.

IV. Microevolution is a generation‑to‑generation change in a population's allele or

genotype frequencies: an overview

The Hardy‑Weinberg equilibrium is important to the study of evolution since it tells us what will

happen in a nonevolving population.

• This equilibrium model provides a base line from which evolutionary departures take

place.

• It provides a reference point with which to compare the frequencies of alleles and

genotypes of natural populations whose gene pools may be changing.

For Hardy‑Weinberg equilibrium to be maintained, five conditions must be met:

1. Very large population size.

2. Isolation ftom other populations. There is no migration of individuals into or out of the

population.

No mutations.

4. Random mating.

5, No natural selection. All genotypes are equal in survival and reproductive success.

Differential reproductive success can alter gene frequencies.

In real populations, several factors can upset Hardy‑Weinberg equilibrium and cause

microevolutionary change.

378 The Evolution ofPopulations

Microevolution = Small scale evolutionary change represented by a generational shift in a

population's relative allelic frequencies.

Microevolution can be caused by genetic drift, gene flow, mutation, nonrandom mating,

and natural selection; each of these conditions is a deviation from the criteria for Hardy‑

Weinberg equilibrium.

Of these five possible agents for microevolution, only natural selection generally leads to

an accumulation of favorable adaptations in a population.

The other four are nonadaptive and are usually called non‑Darwinian changes.

V. Genetic drift can cause evolution via chance fluctuation in a small population's gene

pool: a closer look

Genetic drift = Changes in the gene pool of a small population due to chance.

If a population is small, stochastic events have a greater impact on gene frequencies.

Chance events may cause the frequencies of alleles to drift randomly from generation to

generation, since the existing gene pool may not be accurately represented in the next

generation.

For example, assume our theoretical wildflower population contains only 25 plants, and the

genotypes for flower color occur in the following numbers: 16 AA, 8 Aa and I aa. In this case, a

chance event could easily change the frequencies of the two alleles for flower color.

A rock slide or passing herbivore which destroys three AA plants would immediately

change the frequencies of the alleles from A = 0.8 and a = 0.2, to A = 0.77 and a = 0.23.

Although this change does not seem very drastic, the frcquencies of the two alleles were

changed by a chance event.

The larger the population, the less important is the effect of genetic drift.

Even though natural populations are not infinitely large (in which case genetic drift could

be completely eliminated as a cause of microevolution), most are so large that the effect of

genetic drift is negligible.

However, some populations are small enough that genetic drift can play a major role in

microevolution, especially when the population has less than 100 individuals.

Two situations which result in populations small enough for genetic drift to be important are the

bottleneck effect and thefounder effect.

A. Bottleneck Effect

The size of a population may be reduced drastically by such natural disasters as volcanic

eruptions, earthquakes, fires, floods, etc. which kill organisms nonselectively.

• The small surviving population is unlikely to represent the genetic makeup of the

original population.

• Genetic drift which results from drastic reduction in population size is referred to as

the bottleneck effect.

I The Evolution ofPopulations 379

By chance some individuals survive. In the small remaining population, some

alleles may be overrepresented, some under‑represented, and some alleles may be

totally absent.

Genetic drift which has occurred may continue to affect the population for many

generations, until it is large enough for random drift to be insignificant.

The bottleneck effect reduces overall genetic variability in a population since some alleles

may be entirely absent.

• For example, a population of northern elephant seals was reduced to just 20

individuals by hunters in the 1890's.

Since this time, these animals have been protected and the population has

increased to about 30,000 animals.

Researchers have found that no variation exists in the 24 loci examined from the

present population. A single allele has been fixed at each of the 24 loci due to

genetic drift by the bottleneck effect.

This contrasts sharply with the large amount of genetic variation found in

southern elephant seal populations which did not undergo the bottleneck effect.

• A lack of genetic variation in South African cheetahs may also have resulted from

genetic drift, since the large population was severely reduced during the last ice age

and again by hunting to near extinction.

B. Founder Effect

When a few individuals colonize a new habitat, genetic drift is also likely to occur.

Genetic drift in a new colony is called thefounder effect.

The smaller the founding population, the less likely its gene pool will be

representative of the original population's genetic makeup.

The most extreme example would be when a single seed or pregnant female moves

into a new habitat.

• If the new colony survives, random drift will continue to affect allele frequencies

until the population reaches a large enough size for its influence to be negligible.

• No doubt, the founder effect was instrumental in the evolutionary divergence of the

Galapagos finches.

The founder effect probably resulted in the high frequency of retinitis pigmentosa (a

progressive form of blindness that affects humans homozygous for this recessive allele) in

the human population of Tristan da Cunha, a group of small Atlantic islands.

• This area was colonized by 15 people in 1814, and one must have been a carrier.

• The frequency of this allele is much higher on this island than in the populations

from which the colonists came.

Although inherited diseases are obvious examples of the founder effect, this form of

genetic drift can alter the frequencies of any alleles in the gene pool.

380 The Evolution of Populations

V1. Gene flow can cause evolution by transferring alleles between populations: a closer

look

Gene flow = The migration of fertile individuals, or the transfer of gametes, between

populations.

Natural populations may gain or lose alleles by gene flow, since they do not have gene

pools which are closed systems required for Hardy‑Weinberg equilibrium.

Gene flow tends to reduce between‑population differences which have accumulated by

natural selection or genetic drift.

An example of gene flow would be if our theoretical wildflower population was to begin

receiving wind blown pollen from an all white‑flower population in a neighboring field.

This new pollen could greatly increase the frequency of the white flower allele, thus also

altering the frequency of the red flower allele.

Extensive gene flow can eventually group neighboring populations into a single

population.

V11.Mutations can cause evolution by substituting one allele for another in a gene pool:

a closer look

A new mutation which is transmitted in gametes immediately changes the gene pool of a

population by substituting one allele for another.

In our theoretical wildflower population, if a mutation in a white flowered plant caused that plant

to begin producing gametes which carried a red flower allele, the frequency of the white flower

allele is reduced and the frequency of the red flower allele is increased.

Mutation itself has little quantitative effect on large populations in a single generation, since

mutation at any given locus is very rare.

Mutation rates of one mutation per 10 5 to 106 gametes are typical, but vary depending on

the species and locus.

An 5 allele with a 0.50 frequency in the gene pool that mutates to another allele at a rate of

10‑ mutations per generation would take 2000 generations to reduce the frequency of the

original allele from 0.50 to 0.49.

The gene pool is effected even less, since most mutations are reversible.

If a new mutation increases in frequency, it is because individuals carrying this allele are

producing a larger percentage of offspring in the population due to genetic drift or natural

selection, not because mutation is producing the allele in abundance.

Mutation is important to evolution since it is the original source of genetic variation, which is the

raw material for natural selection.

Evolution ofPopulations 3 8 1

Nonrandom mating can cause evolution by shifting the frequencies of genotypes in a

gene pool: a closer look

Nonrandorn mating increases the number of homozygous loci in a population, but does not in

itself alter frequencies of alleles in a population's gene pool. There are two kinds of nonrandom

mating: inbreeding and assortative mating.

A. Inbreeding

Individuals of a population usually mate with close neighbors rather than with more distant

members of a population, especially if the members of the population do not disperse

widely.

• This violates the Hardy‑Weinberg criteria that an individual must choose its mate at

random from the population.

• Since neighboring individuals of a large population tend to be closely related,

inbreeding is promoted.

• Self‑fertilization, which is common in plants, is the most extreme example of

inbreeding.

2 2

Inbreeding results in relative genotypic frequencies (p , 2pq, q ) that deviate from the

frequencies predicted for Hardy‑Weinberg equilibrium, but does not in itself alter

frequencies of alleles (p and q) in the gene pool.

Self‑fertilization in our theoretical wildflower population would increase the frequencies

of homozygous individuals and reduce the frequency of heterozygotes.

• Selfing of AA and aa individuals would produce homozygous plants.

• Selfing of Aa plants would produce half homozygotes and half heterozygotes.

• Each new generation would see the proportion of heterozygotes decrease, while the

proportions of homozygous dominant and homozygous recessive plants would

increase.

Inbreeding without selfing would also result in a reduction of heterozygotes,

although it would take much longer.

One effect of inbreeding is that the frequency of homozygous recessive phenotypes

increases.

An interesting thing to note is that even if the phenotypic and genotypic ratios change

values of p and q do not change in these situations, only the way they are combine(

smaller proportion of recessive alletes are masked by the heterozygous state.

B. Assortative Mating

Assortative mating is another type of nonrandom mating which results when individuals

mate with partners that are like themselves in certain phenotypic characters. For example:

Snow geese occur in a blue variety and a white variety, with the blue color allele

being dominant. Birds prefer to mate with those of their own color, blue with blue

and white with white; this results in a lower frequency of heterozygotes than

predicted by Hardy‑Weinberg.

Blister beetles (Lyt1a magister) in the Sonoran Desert usually mate with a same‑size

individual.

382 The Evolution ofPopulations

IX. Natural selection can cause evolution via differential reproductive success among

varying members of a population: a closer look

The Hardy‑Weinberg equilibrium condition that all individuals in a population have equal ability

to produce viable, fertile offspring is probably never met.

• In any sexually reproducing population, variation among individuals exists and some

variants leave more offspring than others.

• Natural selection is this differential success in reproduction.

Due to selection, alleles are passed on to the next generation in disproportionate numbers relative

to their frequencies in the present generation.

• If in our theoretical wildflower population, white flowers are more visible to herbivores

than pink flowers, plants with pink flowers (both AA and Aa) would leave more offspring

on the average.

• Genetic equilibrium would be disturbed and the frequency of allele A would increase and

the frequency of the a allele would decrease.

Natural selection is the only agent of microevolution which is adaptive, since it accumulates and

maintains favorable genotypes.

Environmental change would result in selection favoring genotypes present in the

population which can survive the new conditions.

Variability in the population makes it possible for natural selection to occur.

X. Genetic variation is the substrate for natural selection

Members of a population may vary in subtle or obvious ways. It is the genetic basis of this

variation that makes natural selection possible.

A. How Extensive Is Genetic Variation Within and Between Populations?

Darwin considered the slight differences between individuals of a population as raw

material for natural selection.

While we are more conscious of the variation among humans, an equal if not greater

amount of variation exists among the many plant and animals species.

Phenotypic variation is a product of inherited genotype and numerous environmental

influences.

Only the genetic or inheritable component of variation can have adaptive impact as a

result of natural selection.

The Evolution ofPopulations 3 83

Polygenic characters which vary quantitatively within a population are responsible for

much of the inheritable variation.

For example, the height of the individuals in our theoretical wildflower population

may vary from very short to very tall with all sorts of intermediate heights.

Discrete characters which are determined by only one locus vary categorically, such as

flower color in our wildflowers, without intermediates.

In our wildflower population, the red and white flowers would be referred to as

different morphs (contrasting forms of a Mendelian character).

A population is referred to as polymorphic for a character if two or more morphs are

present in noticeable frequencies. (See Campbell, Figure 21.8)

Polymorphism is found in human populations not only in physical characters (e.g.

presence or absence of freckles) but also in biochemical characters (e.g. ABO blood

group).

Darwin did not realize the extent of genetic variation in populations, since much of the

genetic variation can only be determined with biochemical methods.

Electrophoresis has been used to determine genetic variation among individuals of a

population. This technique allows researchers to identify variations in protein

products of specific gene loci.

Electrophoretic studies show that in Drosophila populations the gene pool has two or

more alleles for about 30% of the loci examined, and each fly is heterozygous at

about 12% of its loci.

Thus, there are about 700 ‑ 1200 heterozygous loci in each fly. Any two flies in a

Drosophila population will differ in genotype at about 25% of their loci.

Electrophoretic studies also show comparable variation in the human population.

Note that electrophoresis underestimates genetic variation:

Proteins produced by different alleles may vary in amino acid composition and still

have the same overall charge, which makes them indistinguishable by

electrophoresis.

Also, DNA variation not expressed as protein is not detected by electrophoresis.

Geographical variation in allele frequencies exists among populations of most species.

Natural selection can contribute to geographical variation, since at least some

environmental factors are different between two locales. For example, one

population of our wildflowers may have a higher frequency of white flowers because

of the prevalence in that area of pollinators that prefer white flowers.

Genetic drift may cause chance variations among different populations.

Also, subpopulations may appear within a population due to localized inbreeding

resulting from a "patchy" environment.

3 84 The Evolution ofPopulations

Clin‑e = One type of geographical variation that is a graded change in some trait along a

geographic transect.

• Clines may result from a gradation in some environmental variable.

• It may be a graded region of overlap where individuals of neighboring populations

interbreed.

For example, the average body size of many North American mammal species

gradually increases with increasing latitude. It is presumed that the reduced surface

area to volume ratio associated with larger size helps animals in cold environments

conserve body heat.

Studies of geographical variation confirm that genetic variation affects spatial

differences of phenotypes in some clines. For example, yarrow plants are shorter at

higher elevations, and some of this phenotypic variation has a genetic basis. (See

Campbell, Figure 21.9)

B. How Is Genetic Variation Generated?

Genetic variation results from mutation and sexual recombination.

1. Mutation

Mutations produce new alleles. They are rare and random events which usually occur

in somatic cells and are thus not inheritable.

• Only mutations that occur in cell lines which will produce gametes can be

passed to the next generation.

• Geneticists estimate that only an average of one or two mutations occur in each

human gamete‑producing cell line.

Point mutation = Mutation affecting a single base in DNA.

• Much of the DNA in eukaryotes does not code for proteins, and it is uncertain

how a point mutation in these regions affect an organism.

• Point mutations in structural genes may cause little effect, partly due to the

redundancy of the genetic code.

Mutations that alter a protein enough to affect the function are more often harmful than

beneficial, since organisms are evolved products shaped by selection and a chance

change is unlikely to improve the genome.

• Occasionally, a mutant allele is beneficial, which is more probable when

environmental conditions are changing.

• The mutation which allowed house flies to be resistant to DDT was present in

the population and under normal conditions resulted in reduced growth rate. It

became beneficial to the house fly population only after a new environmental

factor (DDT) was introduced and tipped the balance in favor of the mutant

alleles.

7he Evolution ofPopulations 3 85

Chromosomal mutations usually affect many gene loci and tend to disrupt an

organism's development.

On rare occasions, chromosomal rearrangement may be beneficial. These

instances (usually by translocation) may produce a cluster of genes with

cooperative functions when inherited together.

Duplication of chromosome segments is nearly always deleterious.

• If the repeated segment does not severely disrupt genetic balance, it may persist

for several generations and provide an expanded genome with extra loci.

These extra loci may take on new functions by mutation while the original genes

continue to function.

• Shuffling of exons within the genome (single locus or between loci) may also

produce new genes.

Mutation can produce adequate genetic variation in bacteria and other microorganisms

which have short generation times.

Some bacteria reproduce asexually by dividing every 20 minutes, and a single

cell can produce a billion descendants in only 10 hours.

With this type of reproduction, a beneficial mutation can increase in frequency

in a bacteria] population very rapidly.

A bacterial cell with a mutant allele which makes it antibiotic resistant could

produce an extremely large population of clones in a short period, while other

cells without that allele are eliminated.

Although bacteria reproduce primarily by asexual means, most increase genetic

variation by occasionally exchanging and recombining genes through processes

such as conjugation, transduction and transformation.

2. Recombination

The contribution of mutations to genetic variation is negligible.

• Mutations are so infrequent at a single locus that they have little effect on

genetic variation in a large gene pool.

• Although mutations produce new alleles, nearly all genetic variation in a

population results from new combinations of alleles produced by sexual

recombination.

Gametes from each individual vary extensively due to crossing over and random

segregation during meiosis.

• Thus, each zygote produced by a mating pair possesses a unique genetic

makeup.

• Sexual reproduction produces new combinations of old alleles each generation.

Plants and animals depend almost entirely on sexual recombination for genetic

variation which makes adaptation possible.

386 The Evolution ofPopulations

C. How Is Genetic Variation Preserved?

Natural selection tends to produce genetic uniformity in a population by eliminating

unfavorable genotypes. This tendency is opposed by several mechanisms that preserve or

restore variation.

1. Diploidy

Diploidy hides much genetic variation from selection by the presence of recessive

alleles in heterozygotes.

Since recessive alleles are not expressed in heterozygotes, less favorable or

harmful alleles may persist in a population.

This variation is only exposed to selection when two heterozygotes mate and

produce offspring homozygous for the recessive allele.

If a recessive allele has a frequency of 0.01 and its dominant counterpart 0.99,

then 99% of the recessive allele copies will be protected in heterozygotes. Only

1% of the recessive alleles will be present in homozygotes and exposed to

selection.

The more rare the recessive allele, the greater its protection by heterozygosity.

That is, a greater proportion are hidden in heterozygotes by the dominant allele.

This type of protection maintains a large pool of alleles which may be beneficial

if conditions change.

2. Balanced Polymorphism

Selection may also preserve variation at some gene loci.

Balanced polymorphism = The ability of natural selection to maintain diversity in a

population.

One mechanism by which selection preserves variation is heterozygote advantage.

Natural selection will maintain two or more alleles at a locus if heterozygous

individuals have a greater reproductive success than any type of homozygote.

An example is the recessive allele that causes sickle‑cell anemia in

hornozygotes. The locus involved codes for one chain of hemoglobin.

Homozygotes for this recessive allele develop sickle‑cell anemia which is often

fatal.

Heterozygotes are resistant to malaria. Heterozygotes thus have an advantage in

tropical areas where malaria is prevalent, since homozygotes for the dominant

allele are susceptible to malaria and homozygous recessive individuals are

incapacitated by the sickle‑cell condition.

In some African tribes from areas where malaria is common, 20% of the

hemoglobin loci in the gene pool is occupied by the recessive allele.

The Evolution ofPopulations 3 87

,Other examples of heterozygote advantage are found in crop plants (e.g. corn) where

inbred lines become homozygous at more loci and show stunted growth and sensitivity

to diseases.

• Crossbreeding different inbred varieties often produces hybrids which are more

vigorous than the parent stocks.

• This hybrid vigor is probably due to:

1. Segregation of harmful recessives that were homozygous in the inbred

varieties.

2. Heterozygote advantage at many loci in the hybrids.

Balanced polymorphism can also result from patchy environments where different

phenotypes are favored in different subregions of a populations geographic range.

Frequency‑dependent selection also causes balanced polymorphism.

• In this situation the reproductive success of any one morph declines if that

phenotype becomes too common in the population.

• For example, in Papilio dardanus, an African swallowtail butterfly, males have

similar coloration but females occur in several morphs.

The female morphs resemble other butterfly species which are noxious to bird

predators. Papilio females are not noxious, but birds avoid them because they

look like distasteful species.

This type of protective coloration (Batesian mimicry) would be less effective if

all the females looked like the same noxious species, because birds would

encounter good‑tasting mimics as often as noxious butterflies and would not

associate a particular color pattern with bad taste.

D. Does All Genetic Variation Affect Survival and Reproductive Success?

Some genetic variations found in populations confer no selective advantage or

disadvantage. They have little or no impact on reproductive success. This type of

variation is called neutral variation.

Much of the protein variation found by electrophoresis is adaptively neutral.

For example, 99 known mutations affect 71 of 146 amino acids in the beta

hemoglobin chain in humans. Some, like the sickle‑cell anemia allele, affect the

reproductive potential of an individual, while others have no obvious effect.

The neutral theory of molecular evolution states than many variant alleles at a locus

may confer no selective advantage or disadvantage.

Natural selection would not affect the relative frequencies of neutral variations.

Frequency of some neutral alleles will increase in the gene pool and others will

decrease due the chance effects ofgenetic drift.

Variation in DNA which does not code for proteins may also be nonadaptive.

Most eukaryotes contain large amounts of DNA in their genomes which have no

known function. Such noncoding DNA can be found in varying amounts in closely

related species.

3 8 8 The Evolution ofPopulations

• Some scientists speculate that noncoding DNA has resulted from the inherent

capacity of DNA to replicate itself and has expanded to the tolerance limits of the

each species. The entire genome could exist as a consequence of being self

replicating rather than by providing an adaptive advantage to the organism.

• Transposons might fit this definition of "selfish DNA," although the degree of

influence these sequences have on the evolution of genomes is not known.

Evolutionary biologists continue to debate how much variation, or even whether variation,

is neutral.

• It is easy to show that an allele is deleterious to an organism.

It is not easily shown that an allele provides no benefits, since such benefits may

occur in immeasurable ways.

• Also, a variation may appear to be neutral under one set of environmental conditions

and not neutral under other conditions.

We cannot know how much genetic variation is neutral, but if even a small portion of a

population's genetic variation significantly affects the organisms, there is still a

tremendous amount of raw material for natural selection and adaptive evolution.

X1. Natural selection is the mechanism of adaptive evolution

Adaptive evolution results from a combination of both:

Chance events that produce new genetic variation (e.g. mutation and sexual

recombination).

Natural selection that favors propagation of some variations over others.

Fitness

Darwinian fitness is measured by the relative contribution an individual makes to the gene

pool of the next generation.

• It is not a measure of physical and direct confrontation, but of the success of an

organism in producing progeny.

• Organisms may produce more progeny because they are more efficient feeders,

attract more pollinators (as in our wildflowers), avoid predators, etc.

Survival does not guarantee reproductive success, since a sterile organism may outlive

fertile members of the population.

• A long life span may increase fitness if the organism reproduces over a longer period

of time (thus leaving more offspring) than other members of the population.

• Even if all members of a population have the same life span, those that mature early

and thus have a longer reproductive time span, have increased their fitness.

• Every aspect of survival and fecundity are components of fitness.