Chapter 23: Population Geneitcs (Part 2, Week 2)

Question 1

T/F Natural Selection itself is not evolution, rather a key mechanism that causes evolution to happen.

Answer 1

True

Question 2

What does natural selection result in?

Answer 2

Adaptations

Question 3

What is the likelihood that an individual will contribute fertile offspring to the next generation?

Answer 3

Reproductive success - Natural selection favors these individuals.

Question 4

What two categories of traits are commonly attributed to reproductive success?

Answer 4

1. Certain characteristics allow organisms to survive to reproductive age.
2. Traits that involve reproduction itself like ability to find a mate and produce viable gametes and offspring.

Question 5

What are the four principles of natural selection that related to our knowledge of molecular genetics?

Answer 5

1. Allele variation arises random mutations which cause differences in DNA sequences, which can alter amino acid sequence and possibly can alter function of the protein.
2. The proteins that are encoded by alleles can enhance survival or reproductive capability.
3. Individuals with beneficial alleles are more likely to survive and contribute their alleles to the gene pool of the next generation
4. Over the course of many generations, allele frequencies of many different genes may change through natural selection, thereby altering the characteristics of the population. The net result is a population better adapted to its environment and more successful at reproduction.

Question 6

What is the relative likelihood that a genotype will contribute to the gene pool of the next generation as compared to other genotypes?

Answer 6

Fitness

This is a QUANTITATIVE measure of reproductive success, not necessarily a property correlated to physical fitness.

Ex. Let’s suppose the average reproductive successes of the three genotypes are

AA produces 5 offspring
Aa produces 4 offspring
aa produces 1 offspring

The genotype with the highest reproductive success is given a fitness value of 1.0. Fitness values are denoted by the variable w. The fitness values of the other genotypes are assigned values relative to this 1.0 value.

Fitness of AA: wAA = 1
Fitness of Aa: wAa = 4/5 = 0.8
Fitness of aa: waa = 1/5 = 0.2

Question 7

What is the average reproductive success of members of a population?

Answer 7

Mean fitness of the population

Natural selection can be viewed at the level of a population by using this.

Question 8

What is a pattern of natural selection that favors individuals at one extreme of a phenotypic distribution?

Answer 8

Directional selection

Individuals at one extreme of a phenotypic range have greater reproductive success in a particular environment

Question 9

What is a common reason for directional selection?

Answer 9

The population may be exposed to a prolonged change in its living environment. Then the relative fitness values may change to favor one genotype, which will promote the elimination of other genotypes.

(think finches migrating to a place with bigger seeds and those with the biggest beaks flourish)

Question 10

What is another way that directional selection may arise other than changes in its environment?

Answer 10

The introduction of a new allele into a population by mutation, and the new allele conferring a higher fitness in individuals who carry it. Over time, it may predominate in the population, prehaps even leading to a monomorphic (only one form & homozygous state) gene

Question 11

What is a pattern of natural selection that favors the survival of individuals with intermediate phenotypes?

Answer 11

Stabilizing selection

This tends to decrease genetic diversity. For birds, the ones that laid too few eggs didnt produce many offspring for future generations. The birds that produced vastly more eggs may not survive for two reasons; 1 - not enough resources for the new hatchlings or, 2 - the stress on the parents to maintain care.

The most successful parents are those that produce an intermediate sized nest.

Question 12

Why does stabilizing selection decrease genetic diversity?

Answer 12

Stabilizing selection eliminates alleles that result in phenotypes that deviate significantly from the average phenotype. For this reason, it tends to decrease genetic diversity.

Question 13

What is a pattern of natural selection that favors the survival of two or more different genotypes that produce different phenotypes?

Answer 13

Diversifying selection (aka disruptive selection)

Fitness values of the genotypes are higher in one environment than the other in a different environment and vice versa

This typically happens in an heterogeneous environment

Question 14

What is a type of natural selection that maintains genetic diversity in a population?

Answer 14

Balancing selection

Question 15

What is the phenomenon in which two or more alleles are kept in balance and maintained in a population over the course of many generations?

What is it a result of?

Answer 15

Balanced polymorphism; Balancing selection

Question 16

What is a phenomenon in which a heterozygote has a higher fitness than either corresponding homozygote?

Answer 16

Heterzygote advantage

Heterozygote advantage sometimes explains the persistence of alleles that are deletrious in a homozygous condition.

Question 17

[INFO] Heterozygote advantage in Humans

• Hs allele of human B-globin gene.
• Homozygous HsHs is sickle cell disease.
• HsHs homozygote has a lower fitness than a homozygote with two copies of the more common B-globin allele, HaHa.
• Heterozygotes, HaHs, do not typically show symptoms of the disease, but they have an increased resistance to malaria.
• Compared with HaHa homozygotes, heterozygotes have the highest fitness because they have a 10-15% chance of surviving if infected by the malarial parasite Plasmodium falciparum.

Flip for Conclusion Fact!

Answer 17

Therefore, the Hs allele is maintained in populations where malaria is prevalent, such as the Democratic Republic of Congo, even though the allele is detrimental in the homozygous state.

This balanaced polymorphism results in a higher mean fitness of the population. In areas where malaria is endemic, a population composed of all HaHa individuals would have a lower mean fitness.

Question 18

What is a second way that natural selection produces balanced polymorphism?

The fitness of a genotype decreases when its frequency becomes higher.

Answer 18

Negative Frequency-dependent selection.

In other words, common individuals are less likely to reproduce, whereas rare individuals are more likely to reproduce.

It is thought this selection is thought to maintain ploymorphisms among species that are preyed upon by predators. Research has shown that certain predators form a mental search image for their prey, which is usually based on the common type of prey in an area. A prey that exhibits a rare polymorphism that affects its appearence is less likely to be recognized by the predator. The rare organisms are subjected to a lower rate of predation.

Question 19

[23.3 Start]

What is a type of natural selection that is directed at certain traits of sexually reproducing species that make it more likely for individuals to find or choose a mate and/or engage in successful reproduction?

Answer 19

Sexual selection.

Darwin originally described sexual selection as “the advantage that certain individuals have over others of the same sex and species solely with respect to reproduction.”

Question 20

What does sexual selection result in?

Answer 20

The prevalence of traits, called secondary sex characteristics, that favor reproductive success.

AND

Result in sexual dimophism-a significant difference between the morphologies of the two sexes within a species.

Question 21

What are the two ways sexual selection operates and explain briefly what they are?

Answer 21

Intrasexual selection is where one sex, usually males, compete for an opportunity to mate with the opposite sex. Exampls include horns on sheep, antlers on moose, and enlarged claw of male fiddler crabs. Usually, the larger of the two win to mate.

Intersexual selection, also called mate choice, members of one sex, usually females, choose their mates from individuals of the other sex on the basic of certain desirable characteristics. This type of sexual selection often results in showy characteristics in males like male peacocks showing their colored feathers and prowess to females.

Elks are a combination of both. Males fight and females choose the strongest of bulls.

Sexual selection can explain the existence of traits that could decrease an individual’s chances of survival but increase the chances of reproducing (guppies and their bright colors but higher risk to predation; females choosing brightly colored guppies)

Question 22

What is a less obvious example of intersexual selection which has an advantage of inhibiting inbreeding and promotion of genetic diversity by favoring interbreeding among genetically unrelated individuals?

Answer 22

Female reproductive system influences the relative success of sperm by selecting sperm that are genetically unrelated to the female. This can happen in animals and plants.

Question 23

T/F Sexual selection is governed by the same processes involved in the evolution of traits that are not directly related to sex and can occur by directional, stabilizing, diversifying, or balancing selection.

Answer 23

True

For example, the evolution of the large and brightly colored tail of the male peacock is the result of directional selection.

Question 24

[23.4 Start]

What is the random change in a population’s allele frequencies from one generation to the next that is attributable to chance? It occurs more quickly in small populations.

Answer 24

Genetic Drift (also called random genetic drift)

Question 25

What does the term genetic drift derived from in observations?

Answer 25

That allele frequencies may “drift” randomly from generation to generation as a matter of chance.

Question 26

T/F Changes in allele frequencies due to genetic drift happen because of the fitness of the individuals that carry those alleles.

Answer 26

False. It happens regardless.

For example, an individual with a high fitness value may, by chance, not encounter a member of the opposite sex. Likewise, random chance can influence which alleles happen to be carried in the gametes that with fuse with eah other in a successful fertilization.

Question 27

T/F Over the longrun, genetic drift favors either the elimination (frequency of 0%) or the fixation (frequency of 100%) of an allele in a population.

Answer 27

True

However, the number of generations it takes for an allele to be lost or fixed greatly depends on the population size.

In nature, genetic drift may rapidly alter the allele frequencies whne the size of a population dramtically decreases.

Question 28

What are two ways in nature that genetic drift may rapidly alter the allele frequencies when the size of the population dramatically decreases?

Answer 28

Bottleneck and Founder effect.

Question 29

What can dramatically reduce the size of a population?

Answer 29

Earthquakes, floods, drought, and human destruction of habitat.

These occurences may elminate most members of the population without regard to their genetic composition.

The population is said to have passed through a bottleneck.

Question 30

What is a change in allele frequencies due to genetic drift in a population that has been dramatically reduced in size; this effect can reduce the genetic diversity of the population?

Answer 30

Bottleneck Effect

Question 31

What can be a result of the bottleneck effect?

Answer 31

Some alleles may be over-represented whereas others may even be eliminated.

The surviving population often has allele frequencies that differ from those of the original population that was much larger.

Secondly, genetic drift acts more quickly to reduce genetic variation when the population size is small.

The population can recover but it will have less genetic variation than the original one.

Question 32

How does the bottleneck effect undermine the efforts of conservation biologists who are trying to save species nearing extinction?

Answer 32

The bottleneck effect tends to decrease genetic diversity. This may eliminate adaptations that promote survival and reproductive success. Therefore, the bottleneck effect makes it more difficult for a population to survive.

Real life example is the northern elephant seal.

Question 33

What is the genetic drift that occurs when a small group of individuals separates from a larger population and establishes a colony in a new location?

Answer 33

Founder effect

Both effects revolve around a smaller population size but founder moves to a remote location.

Question 34

What are the two important consequences of the founder effect?

Answer 34

1. First, the founding population, which is relatively small, is expected to have less genetic variation.
2. Second, as a matter of chance, the allele frequencies in the founding population may differ markedly from those of the original population.

Example, 1960s Amish of Lancaster County, Pennsylvania were studied. 8,000 people descended from three couples that immigrated to the U.S. in 1770.

Among this population of 8,000, a genetic disease known as Ellis-van Creveld syndrome (a recessive form of dwarfism) was found at a frequency of 0.07 or 7%. Very rare disease otherwise.

Question 35

What are changes in genes and proteins that result from genetic drift and do not have an effect on reproductive success?

Answer 35

Neutral variation

Question 36

What is the idea that much of the modern variation in gene sequences is explained by neutral variation rather than adaptive variation?

Answer 36

non-Darwinian evolution (aka “survival of the luckiest”)

Kimura agreed with Darwin that natural selection is responsible for adaptive changes in a species during evolution. The main idea is that much of the variation in DNA sequences is explained by neutral variation rather than adaptive variation.

Question 37

When it comes to codon sequence mutations, what base usually results in neutral effects?

Answer 37

The third base.

In contrast, random mutations at the first and second base are more likely to be harmful than beneficial and tend to be eliminated from a population.

Question 38

T/F Migration between two different populations can alter allele frequencies.

Answer 38

True.

EXAMPLE TIME:

Two populations of a particular species of deer that are separated by a mountain range running north and south.

On rare occasions, a few deer from the western population may travel through a narrow pass between the mountains and become members of the eastern population.

If the two populations are different with regard to genetic variation, this migration will alter the frequencies of certain alleles in the eastern population. Or vice versa.

This is a transfer of alleles into or out of a population, called gene flow, occurs whenever fertile individuals move between populations with different allele frequencies.

Question 39

What is the phenomenon that occurs when individuals choose their mates based on their genotypes or phenotypes?

Answer 39

Nonrandom mating

This happens in human populations and others.

Question 40

What is assortive mating?

Answer 40

It occurs when individuals with similar phenotypes are more likely to mate.

Similar phentotypes and genotypes tends to increase homozygotes while differential attraction increases heterozygosity.

Question 41

What is another form of nonrandom mating that involves the choice of mates based on their genetic history rather than their phenotypes?

This is typically a mate that is a part of the same genetic lineage.

Answer 41

Inbreeding

This can sometimes occur in human societies and is more likely to take place in nature when population size becomes very small.

Question 42

Why is inbreeding risky?

Answer 42

Because the offspring can become homozygous for many different deletrious or recessive genes. The same allele of a gene can be inherited twice from a common ancestor.

Inbreeding does not favor any particular allele–it does not favor c over C–but it does increase the likelihood that an individual will be homozygous for any given gene.

Rare recessive diseases in humans are more frequent when inbreeding occurs.

Question 43

T/F In natural populations, inbreeding lowers the mean fitness of the population if homozygous offspring have lower fitness values.

Answer 43

True

Can be a serious problem if the population has become smaller due to human destruction of habitat. As the population shrinks, inbreeding becomes more likely because individuals have fewer potential mates which to choose.

Question 44

What is the phenomenon whereby inbreeding produces homozygotes that are less fit, thereby decreasing the reproductive success of a population?

This can be an issue with endangered species.

Answer 44

Inbreeding depression