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BIOLOGY 101

CHAPTER 7b: Evolution of Populations:

Alleles Change

Evolution of Populations:Alleles Change

CONCEPTS:

• 7.4 Genetic Variation Makes Evolution Possible

• 7.5 Forces that can alter allele frequencies in a population

• 7.6 Natural selection is the only mechanism that consistently drives adaptive evolution

7.4 Genetic Variation Makes Evolution Possible

• What is it that evolves?

• A species is the minimal living unit that can evolve

• A species is a group of interbreeding organisms that are reproductively isolatedfrom other similar organisms



• Common Descent With Modification states that populations change over time, and

that these changes are inherited




that these changes are inherited and sometimes lead to the formation of new species

o Microevolution is defined as a change in allele frequencies in a population over

time.

o Macroevolution is when these changes lead to formation of a new species

• Natural Selection is the force that acts upon these changes (variation) and favors traits

that give a reproductive advantage



• In individuals, alleles can either be changed or redistributed

o Mutation is a permanent change to the nucleotide sequence of DNA

o Redistribution of alleles:

✓ This is when the number or location of alleles in an individual are changed

o In both situations, it is only when these occur in gametes that there is hereditary significance

➢ It is important to note that this is a change to the alleles of gametes, and not

the entire individual! Individuals cannot evolve, only their offspring



• There is only one force that can change alleles

o Mutation is a permanent change to the nucleotide sequence of DNA

o This can occur due to chemical agents, irradiation or errors in replication / repair of

DNA

o In multicellular organisms, only mutations in cell lines that form gametes can be

passed on to offspring

➢ Mutation can ultimately have a large effect on allele frequencies when it produces

new alleles that strongly influence fitness in a positive or negative way

What is it that causes allele frequencies to change in populations?



A point mutation is a change of a single base in a gene.

• Point mutations can have a significant impact on phenotype, as in the case of

sickle-cell disease.

• Most point mutations are harmless: - why?

1. Much of the DNA in eukaryotic genomes does not code for protein products.

2. Because the genetic code is redundant, some point mutations in genes that

code for proteins may not alter the protein’s amino acid composition. (aka

Silent Mutation)

3. Even if there is a change in an amino acid as a result of a point mutation, it

may not affect the protein’s shape and function.

• On rare occasions, a mutant allele may actually make its bearer better suited to

the environment, increasing its reproductive success. (improves adaptation)



A point mutation is a change of a single base in a

gene.

• Most point mutations (single base) are harmless:

- why?

2. Because the genetic code is redundant,

some point mutations in genes that code

for proteins may not alter the protein’s

amino acid composition. (aka Silent

Mutation)



Alterations in gene number or position

• Chromosomal mutations that delete, disrupt, or rearrange many loci at once are

usually harmful.

• In rare cases, chromosomal rearrangements may be beneficial.

For example, the translocation of part of one chromosome to a different

chromosome could link genes that act together for a positive effect.



Gene duplication is an important source of new genetic variation.

• Duplication may occur due to errors in meiosis or slippage during DNA replication

• Duplications of large chromosome segments are often harmful, but the duplication

of small pieces of DNA may not be.

✓ Example: mammalian ancestors carried a single gene for detecting odors that

has been duplicated many times.

o Modern humans have about 1,000 olfactory receptor genes and mice have

1,300.

▪ Dramatic increases in the number of olfactory genes benefited early mammals,

enabling them to detect faint odors and distinguish among smells.

➢ Because of mutations, 60% of these genes have been inactivated in humans.

➢ Mice, which rely more on their sense of smell, have lost only 20% of their olfactory

receptor genes



HOX Gene Mutations can have a dramatic affect on body plans and organization

• Hox proteins control the type of segment structures (e.g. legs, antennae, and wings;

or the different types of vertebrae in humans) that will form on a given segment.

o They control segmental identity, but do not form the actual segments themselves

• Hox genes produce Transcription Factors and can enhance the transcription of

some genes and repress others. They regulate genes that control large networks of

other genes that ultimately form the tissues, structures, and organs of each

segment.

✓ For example, variations in Hox gene expression are responsible for the formation of

different forelimbs in vertebrates.

✓ Hox Gene mutations can explain the transformation of one limb type to another



• Rearrangement and Distribution of Existing Alleles

o Sexual Reproduction: In organisms that reproduce sexually, most of the genetic

variation in a population results from the unique combination of alleles that each

individual receives from its parents.

o Three mechanisms contribute to this shuffling:

1. crossing over

2. independent assortment of chromosomes, and

3. Fertilization (union of 2 gametes)


7.5 Forces that can alter allele frequencies in a population

• Forces that Cause Allele Frequencies to Change in populations

o Gene Drift is when a catastrophe leads to the separation of a large population into

smaller fragments (generally resulting in a dramatic reduction in the number of alleles

within a population)

o Gene Flow is when populations merge (generally introducing new alleles to a

population)

Gene flow tends to increase the number of available alleles

o Natural Selection acts upon different (existing) alleles, improving the match

between organisms and their environment.

➢ Natural selection is not a direct change to alleles or allele frequencies, but acts

upon changes to alleles or allele frequencies!



• Forces that Cause Allele Frequencies to Change

o Genetic drift, resulting in smaller fragmented populations may occur as a result of

two situations: the founder effect or the bottleneck effect

o The founder effect occurs when a new population is started by a small number of

individuals who do not represent the gene pool of the larger source population.

✓ At an extreme, a population could be started by a single pregnant female or a single seed with only a tiny fraction of the genetic variation of the source

population.

o The founder effect is due to a separation from a larger population






o The founder effect occurs when a new population is started by a small number of

individuals who do not represent the gene pool of the larger source population.

✓ At an extreme, a population could be started by a single pregnant female or a single seed with only a tiny fraction of the genetic variation of the source

population.

o The founder effect is due to a separation from a larger population – this often results

in inbreeding and the appearance of adverse recessive alleles






o The bottleneck effect occurs when the numbers of individuals in a large population

are drastically reduced by a disaster (death)

✓ By chance, some alleles may be overrepresented and others underrepresented among the survivors. Some alleles may be eliminated altogether.

✓ The bottleneck effect and founder effect both act to reduce the number of

common alleles in a population




o Gene flow is the transfer of alleles among populations due to the migration of fertile

individuals or gametes. (new alleles are generally introduced)

✓ For example, if a nearby wildflower population consisted entirely of white

flowers, its pollen could be carried into our hypothetical population.

➢ The result would be an increase the frequency of new alleles in the

hypothetical population in the next generation.

o Gene flow tends to reduce differences between populations.

✓ If extensive enough, gene flow can combine neighboring populations into a

single population with a common gene pool.




o Natural Selection is based on differential survival and reproductive success

✓ Individuals in a population vary in their heritable traits.

✓ Individuals with variations better suited to the environment tend to produce

more offspring

➢ As a result of selection, alleles are passed on to the next generation in

frequencies different from those in the present population.



Why are evolutionary changes nearly always positive changes?

➢ We will see that the reason behind this is Natural Selection




o Natural Selection is based on differential survival and reproductive success

By consistently favoring some alleles over others, natural selection can cause

adaptive evolution

(this is evolution that results in a better match between organisms and their

environment)


By consistently favoring some alleles over others, natural selection can cause

adaptive evolution


7.6 Natural selection is the only mechanism that consistently causes adaptive

evolution


o Natural selection is the only evolutionary mechanism that consistently leads to

adaptive evolution.

✓ Genetic drift can cause the frequency of a slightly beneficial allele to

increase, but it also can cause the frequency of such an allele to decrease.

✓ Gene flow may introduce alleles that are advantageous or ones that are disadvantageous.

✓ Mutation may change alleles in a way that may be either advantageous or

disadvantageous



evolution

• Evolution by natural selection is a blend of chance and “sorting”: chance in the

creation of new genetic variations (as in mutation) and sorting as natural selection

favors some alleles over others.

o Because of this favoring process, the outcome of natural selection is not random



evolution

Three modes of Natural Selection:

• Natural selection can alter the frequency distribution of heritable traits in three

ways, depending on which phenotypes in a population are favored.

o Directional Selection

o Disruptive Selection

o Stabilizing



evolution


Directional selection is most common during periods of environmental change or when

members of a population migrate to a new habitat with different environmental conditions.


• Directional selection shifts the

frequency curve for a phenotypic

character in one direction by

favoring individuals who deviate from

the average.

• For example, a greater number of dark mice survived in an environment

with dark rocks


• For example, an environment with

scattered black and white rocks

would favor both black and white

mice

o Intermediate (gray) mice would not

be favored

• Gray (intermediate) mice would not be selected


evolution


Disruptive selection is when environmental conditions favor individuals at both extremes of

the phenotypic range over those with intermediate phenotypes.

• Stabilizing selection reduces variation

and maintains the status quo for a

trait.

• An example would be a granite

environment that is gray and favors

gray mice.

• Both White and Black mice would

stand-out and would not be selected



evolution


Stabilizing selection favors intermediate variants and acts against extreme phenotypes.

Reproductive Isolation Leads to Speciation


that these changes are inherited and sometimes lead to the formation of new species

o Microevolution is defined as a change in allele frequencies in a population over

time.

o Macroevolution is when these changes lead to formation of a new species

• Natural Selection is the force that acts upon these changes (variation) and favors traits

that give a reproductive advantage. This can lead to adaptive evolution

Speciation: Reproductive IsolationAlleles Change


• What is it that evolves?

• A species is the minimal living unit that can evolve

• A species is a group of interbreeding organisms that are reproductively isolatedfrom other similar organisms



• The strongest and most common starting point for speciation is Allopatric

Speciation

• Allopatric Speciation is due to Extrinsic Reproductive Isolation

o Extrinsic forces are forces outside, or not in control of the individual

o These forces are physical barriers that prevent mating

o The effect of extrinsic forces can often result in Geographic Isolation

• Geographic Isolation is responsible for allopatric speciation and is the result of

physical barriers to mating (mountain ranges, oceans, continental separation,

etc.)

➢ There is no stronger barrier to mating than physical separation!



➢ Allopatric speciation is the result of Extrinsic reproductive isolation

➢ This means that geographic isolation is the primary force behind

speciation

• Allopatric speciation often leads to Sympatric Speciation

➢ Sympatric speciation is due to Intrinsic Reproductive Isolating

Mechanisms

➢ Intrinsic Mechanisms are those due to physiological, anatomical or

behavioral difference that prevent successful mating


Intrinsic Reproductive Isolating Mechanisms

✓ Prezygotic Barriers (mating does not occur)

➢ Habitat Isolation: Due to the selection of habitat

➢ Behavioral Isolation: Low sexual attraction – depends on

courtship behaviors

➢ Temporal Isolation: Due to seasonal differences (mating

seasons)

✓ Barriers to Mating (sperm does not fertilize an egg)

➢ Mechanical Isolation: Incompatible genitalia

➢ Gametic Isolation: Sperm cannot physically fertilize the egg

✓ Postzygotic Barriers (sperm fertilizes egg)

➢ Hybrid Inviability: Hybrids do not survive to reproduce

➢ Hybrid Infertility: Hybrids form, but are sterile (Mule)



evolution

Natural selection cannot fashion perfect organisms.

• There are at least four reasons natural selection cannot produce perfection

1. Selection can act only on existing variations.

o Natural selection favors only the fittest phenotypes among those currently in

the population, which may not be the ideal traits.

o New advantageous alleles do not arise on demand.



evolution



2. Evolution is limited by historical constraints.

o Evolution does not scrap ancestral features and build new complex

structures or behavior from scratch.

o Evolution acts upon existing features and adapts them to new situations.

o For example, birds might benefit from having wings plus four legs. However,

birds descended from reptiles that had only two pairs of limbs. Co-opting

the forelimbs for flight left only two hind limbs for movement on the ground



evolution



3. Adaptations are often compromises.

o Each organism must do many different things.

o Because the flippers of a seal must allow it to walk on land and also swim

efficiently, their design is a compromise between these environments.

o Human limbs are flexible and allow versatile movements but are prone to injuries, such as sprains, torn ligaments, and dislocations.

• Better structural reinforcement of human limbs would compromise their

agility.



evolution



4. Chance, natural selection, and the environment interact.

o Chance events affect the subsequent evolutionary history of populations.

o For example, the founders of new populations may not necessarily be the

individuals best suited to the new environment, but rather those individuals that were carried there by chance.


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