Chapter 5: The Evolution of Populations

Question 1

What does the HW equlibirum tell us about a population?

Answer 1

It tells us that that population is not evolving

Question 2

What are the conditions that need to be met for the HW to be respected/for a population to be in HW?

Answer 2

1. No mutations
2. Random mating
3. No gene flow
4. No natural selection
5. Extremely large population

Question 3

What are the two ways to calculate allele frequencies in an HW problem?

Answer 3

1. If you have been giving the genotypic frequencies, you can take its square root
2. Dividing the number of one allele by the total number of alleles within that population

Question 4

When you want to find the number of individuals, which frequency should you use and why?

Answer 4

Always use the genotypic frequencies because individuals have two alleles for every gene. You multiply the number of alleles for one individual by the total number of individuals will give you the number of individuals that have those two alleles/that genotype

Question 5

What are mutations and why are they useful?

Answer 5

Mutations are a change in the DNA sequence of a gene. They are the source of genetic variability, which is the raw and basic material for natural selection

Question 6

Can environments cause mutations?

Answer 6

No. Mutations are random and cannot be associated with a specific environment

Question 7

What is an interesting thing about mutations?

Answer 7

Even though they occur randomly, the rates at which they occur is generally low

Question 8

What are point mutations and what kind of effect do they lead to ? Provide an example of a point mutation

Answer 8

Point mutations are the addition, deletion or substitution of a base. They can have a negative effect, a lethal effect, a neutral effect, or a positive effect. An example of point mutation is sickle cell disease. A simple substitution in the DNA base pairs, an A instead of a T, has a cascading effect on the formation of the sickle cell haemoglobin. The simple change in the amino acid will cause havoc in the formation of the red blood cells.
Due to the crescent shape of these cells, they do not travel well within the circulatory system. They can clog up capillaries in major organs of the body like the lungs or the brain ( can lead to stroke) and this can lead to many devastating effects on the human body and can eventually lead to the death of an individual

Question 9

When does a mutation have a positive effect?

Answer 9

A mutation only has a positive effect when it allows the individual to be better adapted to their environment

Question 10

Why are mutations weak evolutionary forces?

Answer 10

They are weak evolutionary forces because they do not have a large or important effect on the genotypic/allelic frequencies from generation to generation, especially in large populations because the small changes that will occur will be barely noticeable and will not have a major impact or drastically change the genetic structure of the population.

Question 11

What is an evolutionary force?

Answer 11

It is anything that leads to an evolutionary change in the phenotypes of a population from generation to generation

Question 12

How do mutations cause evolution?

Answer 12

Mutations can cause mutations by substituting one allele for another in the gene pool of a population. Basically, it changes the allele frequencies of a population

Question 13

What is random mating and how is it different from non random mating?

Answer 13

In random mating, also known as panmixia, there is no choice in a partner while non-random mating, also known as assortative mating, there is a choice in a partner based on the phenotypic traits.

Question 14

How does assortative mating modify the HW?

Answer 14

It means that some individuals will have a greater chance to mate than others, and therefore a greater chance to reproduce and pass on their genes. So, in the future generations, you would find that more of their genes are found than other genes and this modifies the HW

Question 15

How does random mating keep a population in HW?

Answer 15

Random mating means that every individual has an equal to chance to mate and reproduce and pass on their traits to the next generation.

Question 16

What effect does assortative mating have on allele frequencies? why?

Answer 16

It has no effect on the allele frequencies. It rather changes the genotypic frequencies by increasing the homozygosity or heterozygosity within a population. This is because the alleles being combined to form the genotypes are still the same in number and therefore do not change. It is just how they are combined that is changing

Question 17

What is positive assortative mating and what does it to do a population? What is an example of this

Answer 17

more frequent mating between similar individuals than expected chance. This increases homozygosity as individuals are similar.
Self-fertilization in Mendel’s pea plants is a perfect example of positive assortative mating

Question 18

What is negative assortative mating and what does it do to a population? Provide an example

Answer 18

more frequent mating between individuals that do not look alike than expected chance. This increases heterozygosity.
An example of this can be observed in flowers as they do not self fertilize. An example of negative assortive mating is how the anther of one flower, that produces the spores (sperms), releases those spores on the stigma of another flower that then reach the ovule, therefore fertilizing it.

Question 19

Why cant flowers self fertilize?

Answer 19

They have specific mechanisms that do not allow their own pollen to fertilize them. There are ways that the pollen gets spread across such as through pollinators like insects and wind.

Question 20

What happens when positive assortative mating is paired with natural selection? why?

Answer 20

It will lead to a loss of genetic variability and an increase in homozygosity. Usually, if an allele is harmful but it is in a heterozygote individual, one allele, the dominant one, masks the expression of the other one, say the recessive one and that individual survives. But if in a homozygote recessive, the alleles of the genotype will be expressed and will affect the phenotype of the individual. This effect could negatively affect the survival and reproduction of the individual. Due to this, natural selection will wipe these individuals and essentially these alleles from the population since they are not fit to survive in the environment.

Question 21

What is an example of how an increase in homozygosity leads to the loss of genetic variation?

Answer 21

The example of the crazy mailman who used positive assortative mating to create a population of black guppies that were almost cloned and therefore had no genetic variability. A disease that normally kills less than 10% of fishes in aquariums affected one of the black guppies, and since the others were practically clones, they were all affected by the disease and died.

This created a population that no resilience to environmental change and in the end, the population went extinct.

Question 22

What is endogamic depresssion?

Answer 22

This refers to the when harmful alleles express themselves

Question 23

What is gene flow and when is its effect most powerful?

Answer 23

Gene flow is the migration of individuals between populations or the exchange of genes within populations It is most powerful when dealing with a small population

Question 24

How does gene flow affect allele frequencies?

Answer 24

Gene flow occurs when an individual from one population migrates to another population and brings its genotype with it, and if the genotype has new alleles then it will be equivalent to a mutation in that population because new alleles are being introduced.

It introduces new alleles in the gene pool of a population

Question 25

What is the ultimate goal of gene flow?

Answer 25

The ultimate result of gene flow is that if there is a lot of exchange between the individuals of both populations, then both populations will have the same genetic pool, the variability will be standardized betwenn both populations

Question 26

What is genetic drift and when does it have more of an effect?

Answer 26

Genetic drift is an evolutionary change that occurs within a population as a result of chance. It has a more pronounced effect on small populations as it would not have a noticeable effect if a population is large/ This si because chance will have a greater impact when populations are smaller becasue we are dealing with fewer individuals.

Question 27

What is the ultimate effect of genetic drift?

Answer 27

The ultimate effect of genetic drift is that over time only one allele will be preserved/fixed, so it will be the only one present within the population while the other alleles dissappear.

Question 28

Why does genetic drift have a less pronounced effect on large populations?

Answer 28

This is because in a large population, we are dealing with more individuals and the resulting effects of a chance will have a smaller impact and will only cause a small change in the allele frequencies found within that population/gene pool. Also, there will be a lot of fluctuations between the frequencies of both alleles as it will take a very very very long time before an allele will get fixed or lost.

Question 29

The probability for an allele to get fixed is associated with what?

Answer 29

The probability that an allele will get fixed, so meaning that it will attain the frequency of 1.0, is directly associated with the allelic frequency. The probability that an allele will get fixed, so meaning that it will attain the frequency of 1.0, is directly associated with the allelic frequency. This is possible only if no other evolutionary forces are acting on a population

Question 30

What is the bottleneck effect? provide an example

Answer 30

This occurs when a random event acts on population and only a few individuals of an originally large population survive. These individuals will not be a good representation of the original gene pool as some alleles will be overrepresented, underrepresented, or even absent. Now that the population is small, then it becomes highly susceptible to genetic drift. An example of this is with the cheetah. A bottleneck event happened that left only a few individuals. These few individuals that were left were similar genetically and therefore lack variability and resilience to changes in selective pressures.

Question 31

What is the founder effect? provide an example

Answer 31

The founder effect occurs when individuals from an originally large population form their own colony and traits become more abundant in that colony than in the original population. A perfect example of this the example of myotonic dystrophy. In quebec, there are 189 cases of people with this condition caused by genetics. But in Europe, there are only 4. europe is the source population where individuals there migrated and moved to quebec, taking their genes with them (duh). It is possible that a few individuals or maybe just one person from this original population had the allele that caused myotonic dystrophy and spread that allele into the population in quebec because of inbreeding and since it is a small population, the allele now became more prevalent in Quebec than in europe.

Question 32

What are the negative sides of genetic drift?

Answer 32

Genetic drift can lead to the fixation of a harmful allele within a population and a loss of genetic variability. The latter means that the population lacks resilience and can go extinct

Question 33

What is one thing natural selection does that the other evolutionary forces don’t do?

Answer 33

It is the only evolutionary force that creates adaptations

Question 34

Does natural selection act upon variability caused by the environment?

Answer 34

No, natural selection only acts upon genetic variability, so differences that are only due to genetics

Question 35

When is a genotype said to have a high adaptive value?

Answer 35

When a particular genotype helps an organism survive and produce better in its environment at a particular time, so basically when it gives an organism an advanatge over the other genotypes,

Question 36

What is directional selection? provide an example

Answer 36

the selection that changes the mean value of the population. This is the most common type of selection. Example of this type of selection is in mosquitoes. When DDT, a poison for mosquitoes, was sprayed in a population of mosquitoes for the first time, it killed 95% of the mosquitoes. However, 5% of them survived because they had an allele that gave them resistance. Selection favoured this allele and it became more prevalent in the population compared to other alleles. In the graph, you can see that over time, the DDT only killed 20% of mosquitoes in that same population. This means that there was a directional selection that favoured mosquitoes with the resistance and helped them to be more abundant compared to mosquitoes that lacked the allele, so they were present in the greatest number. This kind of selection basically favours a new genotype that was previously not favoured but this does not mean that the other genotypes will be selected against or eliminated from the gene pool

Question 37

What is the disruptive selection? provide an example

Answer 37

This is rarer than directional selection. This is where selection occurs against the average value and favours the extreme, so the white and dark coloured mice from the image in your notes. In the example, a drought occurred and after the drought, the only kind of seeds that were found was either small or large. There were no medium-sized seeds found. So selection favoured the extremes, so birds that had small beaks and those that had large beaks, and acted against those of medium beaks.

Question 38

What is stabilizing selection? provide and example

Answer 38

This is where selection acts against the extremes and favours the mean. An example of this is in babies in 1930. It was determined that the best weight that would ensure a newborn lives was 8 pounds. So this means any baby lighter or heavier than this would have a very low chance of survival.

Question 39

Why is genetic variability is important?

Answer 39

Genetic variability means greater resilience within that population regarding possible environmental stress.

Organisms with greater genetic variability have a greater chance at survival while those with less/lower genetic variability are more susceptible to extinction.

Question 40

How does diploidy preserve genetic variability?

Answer 40

In diploids that are heterozygous, a considerable portion of the genetic variability is hidden by the dominant allele. This means that natural selection will not act upon because recessive alleles will not express themselves. Having them hidden allows us to keep them within the population instead of them being wiped out by natural selection because they are not favoured. This helps populations preserve genetic variability

Question 41

How does the heterozygote advantage preserve genetic variability?

Answer 41

This occurs when heterozygote individuals have greater fitness and have more offspring than homozygotes individuals. If these individuals are reproducing in greater numbers. then genetic variability is being preserved because the two kinds of alleles will remain in the population. Perfect example of this is with sickle cell, where heterozygotes for this trait have a greater resilience towards malaria than both kinds of homozygotes. So, selecting will favour these individuals and they will reproduce in greater umber, preserving the genetic variability.

Question 42

How does selection dependent on the inverse frequency preserve genetic variability?

Answer 42

The selection dependent on the inverse frequency means that the rarest phenotype will be selected and there will be a variation of both phenotypes as the frequencies of both phenotypes oscillates over time. See notebook for example of right mouthed and left mouhted fish

Question 43

How does the selection dependent on the positive frequency preserve genetic variability?

Answer 43

The selection dependent on the positive frequency is when they phenotype that is more abundant is always favoured. So you have the following species of butterflies and there are 6 kinds of them. Say you can find each kind in a different location such as 6 different mountains. Because of Mullerian mimicry, these butterflies will send a strong message to their predators to not hunt them because they are toxic. So this means that the butterflies that do not look like this are the ones that will be hunted for. So selection will be favouring the following butterflies which happen to be the most abundant on their respective mountains. So when these butterflies survive and live, it preserves genetic variability because the different kinds of butterflies survived and so the species retains its genetic variability.

Question 44

How does neutral variation preserve genetic variability?

Answer 44

Neutral variation is another way genetic variability is preserved in populations. Here, a good portion of our genome is not affected by natural selection. These genes may be pseudogenes, so genes that were active but have been inactive for millions of years. SO there may be variability in those genes but it is not affected by natural selection.

Question 45

How does the external mechanism preserve genetic variability?

Answer 45

this is where several selective pressures act on a population. These selective pressures use the genetic variability present in the population to in order to act on it. The different selective pressures lead to a resulting adaptation that is a compromise between these selective forces

Question 46

How do temporal changes in selective pressures preserve genetic variability?

Answer 46

There are temporal changes in selection pressure becasue we live on a planet that is dynamic in terms of its geology and climate. Species with more genetic varaibility are prone to survive and when the selective pressure changes through time, it preserves this genetic variability.

Question 47

How do habitat mosaics preserve genetic variability?

Answer 47

hen the variability in the habitat of a species maintaisn the genetic variability found within that species.

Question 48

How does nonassortative mating preserve genetic variability?

Answer 48

This kind of mating increases heterozygosity in a population and therefore increases/maintains genetic variability.

Question 49

What is sexual selection?

Answer 49

a form of evolution in which individuals that possess some specific hereditary trait are more likely to find partners.

Question 50

What is intersexual selection? Give an example

Answer 50

when individuals chose a sexual partner to mate with based on traits that indicate the quality of the genetic baggage of the other sex. in widow birds, female birds will choose male birds with longer tails because the longer tails indicate to the females that the males are good quality for the females to invest their gametes.

Question 51

What is intrasexual selection? Give an example

Answer 51

Intrasexual selection refers to the competetion of members of the same sex for access to mates. A good example of this is when the male sea lions fought in the video because one of the male sea lions was trying to mate with the female sea lions on the island. They fought until one of them gave up and returned to the sea

Question 52

What is a penis bone and how does it benefit sea lions?

Answer 52

In the sea lions, the females come to shore one month ahead to give birth and have their young. They have three weeks where they feed their young and during this time, they do not eat and grow very hungry and are only receptive to the male for a few hours before they return to the sea . But before they go back, the male sea lion has to mate with several of the females. Male sea lions having a penis bone allows them to do this fairly quickly and swiftly and so this bone is actually advantageous to him

Question 53

Why is the penis bone present in primates?

Answer 53

These organisms have polygamist relationships, meaning they mate with more than one female/several females. Having a penis bone means that the intermission time of the penis within the female is longer, the males can stay there for a long time. The idea is that if a male is inseminating a female for a long time then other males will not have access to the female

Question 54

What is one possible hypothesis for why males do not have a penis bone?

Answer 54

MAles have evolved a monogamist relationship and this implies two things: there is no attempt at mating with many females so there is no need for a penis bone and the penetration time does not need to be very long. So the usefulness of the penis bone is not there and humans may have lost this because of their different mating strategy

Question 55

What is another possible hypothesis for why males do not have a penis bone?

Answer 55

another possible hypothesis has to do with intersexual selection. Maybe a male’s capacity to have a full erection may be an indication of his genetic health. SO females choose males according to their capacity to have an erection. Having a penis bone will send a false message to the female, so there will be a selection against the presence of this structure., meaning if they become obsolete, the energy associated with this structure may be oriented towards something else.