Evolutions Of Populations (Part 2)

Question 1

Evolution

Answer 1

A change in allele frequencies in the gene pool of a population

Question 2

Four mechanisms of evolution in populations

Answer 2

1. Natural Selection
2. Genetic Drift
3. Gene Flow
4. Mutation

Question 3

Mechanism of Genetic Drift

Answer 3

Random increase or decrease in allele frequencies due to chance events.

Question 4

Genetic drift effect on genetic variation

Answer 4

Tends to decrease genetic variation.

Question 5

Genetic drift effect on average fitness

Answer 5

Changes are random with respect to fitness. Usually decreases fitness.

Question 6

What is genetic drift?

Answer 6

Genetic drift is any change to allele frequencies in a population that is due to chance. It causes allele frequencies to drift up and down randomly over time due to sampling error.

Question 7

genetic drift results in what?

Answer 7

• Genetic drift can occur by any process or event that involves sampling error where observed results do not fit expected results based on probabilities.

Question 8

What can result in random changes in allele frequencies in a population?

Answer 8

1. Random fertilization in every generation in every population of species that reproduce sexually.
2. Accidents that remove individuals at random occur in every population in every generation.
3. A small population can be separated from a parent population.

Question 9

3 important points illustrated here about genetic drift due sampling in sexual reproduction:

Answer 9

1. Genetic drift changes are random with respect to fitness as they are cause by chance.
2. Over time, genetic drift can reduce genetic variation, cause harmful alleles to be fixed, cause beneficial alleles to be lost.
3. The effect of genetic drift is more pronounced in small populations. The smaller the population, the larger the sampling error.

Question 10

Random samples of populations leads to genetic drift

Answer 10

Two examples:
1. Founder Events
2. Population Bottlenecks

Both of these take a small random (with respect to fitness) sample from a larger population and can lead to genetic drift.

Question 11

The small new populations they create are

Answer 11

Subject to continued genetic drift because of their small size.

Question 12

What is a founder event

Answer 12

Founder Event: group of individuals from a large population emigrates to a new geographic area and establishes a new population.

Question 13

Founder effect

Answer 13

Founder Effect: change in allele frequencies that occurs when this new population is established.

Immigrants establish new population: New population is likely to have different allele frequencies than the source population, by chance

Question 14

Once breeding takes place (founder effect)

Answer 14

• Drift continues to act via sampling errors that occur during fertilization.
• Small population more sensitive to effects.

Question 15

Population Bottleneck

Answer 15

sudden reduction in size of a large population due to disease outbreaks, natural catastrophes such as floods or fires or storms, or other sudden destructive events.

Question 16

Genetic bottleneck

Answer 16

• sudden reduction in the number of alleles in a population. Usually follow a population bottlenecks.
• Bottlenecked population is likely to have different allele frequencies than original population, by chance
• high mortality strikes individuals at random

Question 17

Once breeding takes place (bottleneck)

Answer 17

Drift continues to act via sampling errors that occur during fertilization. Small population more sensitive to effects.

Question 18

Gene Flow Mechanism

Answer 18

Addition or loss of alleles by movement of individuals in or out of the population.

Question 19

Gene flow Effect on Genetic Variation

Answer 19

May increase or decrease genetic variation.

Question 20

Gene flow effect on average fitness

Answer 20

Changes are random with respect to fitness. May increase or decrease average fitness.

Question 21

Gene Flow mechanism

Answer 21

• Gene flow is the movement of alleles from one population to another.
• It occurs when individuals leave one population, join another, and breed.

Question 22

Gene flow consequences

Answer 22

• It can increase or decrease genetic variation in a population.
• Reciprocal gene flow equalizes allele frequencies between populations.

Question 23

Time 1 and Time 2 (gene flow)

Answer 23

At time 1, populations differ in allele frequencies
At time 2, Gene flow causes allele frequencies in the two populations to be more alike

Question 24

Reducing genetic variation

Answer 24

• Many mechanisms in the evolution of populations tend to reduce genetic variation.
• Population with reduced genetic variation may not be able to adapt if the environment changes rapidly and run the risk of extinction.

Question 25

Restoring Genetic Variation

Answer 25

• Most forms of natural selection favour certain alleles and lead to a decrease in overall genetic variation.
• Genetic drift tends to decrease genetic variation over time, as alleles are randomly lost or fixed.
• Gene flow may increases genetic variation in a recipient population if new alleles arrive with immigrating individuals. But gene flow may decrease genetic variation in the source population if alleles leave with emigrating individuals.

Question 26

Mutation mechanism

Answer 26

Production on new alleles

Question 27

Mutation effect on genetic variation

Answer 27

Increases genetic variation.

Question 28

Mutation effect on average fitness

Answer 28

Random with respect to fitness. May increase or decrease average fitness.

Question 29

Consequences of mutations

Answer 29

Small-scale mutations in gene sequences can result in:
A. Deleterious alleles
B. Neutral alleles
C. Beneficial alleles

Question 30

Deleterious alleles

Answer 30

These alleles decrease fitness and can be eliminated by natural selection (called purifying selection).

Question 31

Neutral alleles

Answer 31

These alleles have no effect on fitness and are under no selection pressure.

Question 32

Beneficial alleles

Answer 32

These alleles are rare but will increase fitness and be favoured by natural selection.

Question 33

Does mutation occur often enough to make it an important mechanism of evolution?

Answer 33

No. Mutation is not an important mechanism by itself.

Question 34

As an evolutionary mechanism

Answer 34

In eukaryotes, mutation is slow compared with natural selection, genetic drift, and gene flow.

Question 35

Based on the highest mutation rates recorded at individual genes in humans, it would take how long for mutation to produce a change in allele frequency of 1%!

Answer 35

4000 years

Question 36

Mutation can be a significant force in

Answer 36

bacteria and archaea, which have short generation times.

Question 37

Take home messages on mutation

Answer 37

1. Mutation is the ultimate source of genetic variation. Crossing over and independent assortment shuffle existing alleles into new combinations, but only mutation creates new alleles.
2. If mutation did not occur, evolution would eventually stop. Many evolutionary processes tend to eliminate alleles. Without mutation, there would be no variation in alleles, no variation in traits to fuel evolution.
3. Mutation alone is usually rare and inconsequential in changing allele frequencies at a particular gene. However, when our many genes across the genome are considered and combined with natural selection, it becomes an important evolutionary mechanism.

Question 38

Genetic drift is a concern for conservation biology

Answer 38

• Many populations are being reduced in size by habitat destruction/fragmentation and other human activities.
• Small population size subject to continued genetic drift.
• Populations become less fit and are subject to extinction.