Evolution: Lecture 3

Question 1

What is an allele?

Answer 1

An allele is an individual variant of a gene at some locus.

ex. R or r

Question 2

What is a genotype?

Answer 2

A genotype is an individual’s combination of two alleles at a locus (for diploid species)

ex. Rr

Question 3

What is a population?

Answer 3

A localized (typically geographically clustered) group of interbreeding and interacting species.

• They are more likely to breed with their own population rather than those from other populations.

Question 4

Each species is made up of?

Answer 4

One or more populations that can interbreed when they meet.

Question 5

Populations evolve?

Answer 5

Populations are what evolve over generations, experiencing changes.

Question 6

What is a gene pool?

Answer 6

The gene pool is the total genetic variability in populations, including all alleles at all gene loci in all individuals.

Question 7

What are fixed alleles?

Answer 7

Fixed alleles occur when the whole population is homozygous at a locus for the same allele.

Question 8

What are polymorphic loci?

Answer 8

When there are 2 or more alleles in a population, each being present in the population at some frequency.

Question 9

Where are polymorphic loci found?

Answer 9

The standard sexual population contains mostly polymorphic loci.

Question 10

How many polymorphic loci do most populations have?

Answer 10

Most populations have thousands of polymorphic loci.

Question 11

Where do we see variation?

Answer 11

We see genetic variation at polymorphic loci, which is where natural selection comes in and acts on it.

Question 12

Source of Genetic Variation?

Answer 12

New alleles arise by mutation in existing alleles. A single mutation can result in a new allele.

• New allele creates variation

Question 13

Mutations occurring in a particular environment?

Answer 13

Most mutations don’t meaningfully affect fitness - neutral variation in respect to natural selection (called neutral alleles)

Some reduce fitness: harmful alleles
(a.k.a. ‘deleterious’ mutations/alleles)

A very few increase fitness: beneficial alleles

Question 14

Microevolution?

Answer 14

Microevolution is the change in the frequencies of alleles over generations

Question 15

Extreme evolutionary change?

Answer 15

At the extreme, ‘change’ can mean the fixation of an allele, or the loss (extinction) of an allele.

Question 16

Example of having 1 locus with 2 alleles?

Answer 16

Incomplete Dominance in a flower population.

Red: 320 RR
Pink: 160 RW
White: 20 WW

n = 500

Question 17

Genotypic frequency?

Answer 17

Number of observed / total number

Ex. RR - 320/500 = 0.64

Question 18

Allele frequencies?

Answer 18

Total R and r / total number times 2

Total R is number of homozygous individuals X 2 plus number of heterozygous individuals.

Question 19

Allele frequencies: p and q?

Answer 19

p + q = 1
p is one allele frequency
q is the other allele’s frequency

Question 20

p?

Answer 20

p = frequency of RR + frequency of RW/2

Question 21

q?

Answer 21

q = frequency of WW + frequency of RW/2

Question 22

H-W Principle uses?

Answer 22

Calculating genotype and phenotype frequencies and can be done by using census data of a population

Question 23

What is the Hardy-Weinberg Principle?

Answer 23

Describes expected relationships between allele and genotype frequencies in an idealized population when there is no evolution.

Question 24

When is H-W applicable?

Answer 24

Occurs under certain assumptions, including random mating.

Question 25

Model of random mating?

Answer 25

We can model random mating by taking two random alleles out of the population pool and sticking them together to create a combination of alleles.

Question 26

Hardy-Weinberg equation?

Answer 26

For two alleles, it is as follows: p^2 + 2pq + q^2 = 1 - p^2 and q^2 are the expected frequencies of the two homozygote genotypes - 2pq is the expected frequency of heterozygotes

Question 27

What does the Hardy-Weinberg equation let us do?

Answer 27

Lets us estimate the proportion of a population that are carrier of a recessive genetic disorder.

Question 28

Uses of the H-W principle?

Answer 28

We can estimate the allele and genotype frequencies. Example: finding the prevalence of carriers (heterozygotes) of recessive genetic disorders.

Question 29

Cystic Fibrosis - Allele and Genotype frequencies?

Answer 29

Cystic fibrosis is a single-locus recessive disorder, which affects 1 in 2500 people of European descent (i.e. frequency is 1/2500 = 0.0004)

Question 30

What is the estimated frequency of carriers (i.e. heterozygotes) in the European population? 1 in 2500 have the disorder, 0.0004.

Answer 30

q2 is frequency of disorder genotype = 0.0004 q (frequency of disorder allele) = √0.0004 = 0.02 p (frequency of normal allele) = 1 - q p = 1 - 0.02 = 0.98 Frequency of heterozygote genotype = 2pq = 2 x 0.98 x 0.02 = 0.0392 So an estimated ~4% (1 in 25) are carriers

Question 31

Other uses of Hardy-Weinberg principle?

Answer 31

Populations with genotype frequencies that conform to the equation are said to be in Hardy-Weinberg equilibrium at that locus. We can look at population genotype frequencies to see if they are actually in Hardy-Weinberg equilibrium.

Question 32

Is this population in Hardy-Weinberg equilibrium? 1000 individuals: 400 RR 200 RW 400 WW

Answer 32

ACTUAL frequencies of genotypes: {RR} = 400/1000 = 0.4 {RW} = 200/1000 = 0.2 {WW} = 400/1000 = 0.4 ACTUAL frequencies of alleles: p = {RR} + {RW}/2 = 0.4 + 0.1 = 0.5 q = 1 - p = 0.5 Hardy-Weinberg equation: p2 + 2pq + q2 = 1 EXPECTED genotype frequencies (when p = 0.5 & q = 0.5): {RR} = p2 = 0.52 = 0.25 {RW} = 2pq = 2 x 0.5 x 0.5 = 0.5 {WW} = q2 = 0.52 = 0.25

Question 33

What are the assumptions of the Hardy-Weinberg principle?

Answer 33

1) No net mutations 2) Random mating 3) No natural selection 4) Very large (infinite) population size 5) No migration

Question 34

What happens if there is a violation of one of these assumptions?

Answer 34

Violation of any of these assumptions usually signals evolutionary change.

Question 35

What are the three causes of microevolution?

Answer 35

Natural selection Gene flow Genetic drift

Question 36

Natural selection violates?

Answer 36

No natural selection

Question 37

Gene flow violates?

Answer 37

No migration

Question 38

Genetic drift violates?

Answer 38

Very large (infinite) population size

Question 39

Natural selection is the only?

Answer 39

Of these causes of microevolution, only natural selection results in adaptive evolution. That is, it is the only thing that causes greater suitability for the environment.

Question 40

What is gene flow?

Answer 40

Migration, where an individual moves from one population to another (population of the same species) or interbreeds with a member of another population.

Question 41

Gene flow is also?

Answer 41

Dispersal of gametes (e.g. pollen) or migration * Gene flow from population with different allele frequencies causes a change in allele frequencies, causing evolutionary change * Gene flow can introduce new alleles to a population

Question 42

Assumption: very large population?

Answer 42

Frequencies of alleles should not change if it is a large population.

Question 43

What is Random Genetic Drift?

Answer 43

A fluctuation from one generation to the next in the frequency of alleles within one population.

Question 44

Random Genetic Drift?

Answer 44

‘Sampling error’: random changes in allele frequencies over generations * Can lead to fixation (or extinction) of alleles in the population with the absence of natural selection

Question 45

Rate of drift depends on?

Answer 45

The rate of drift strongly depends on the size of the population. It can be ignored in very large populations but can be detected in smaller populations in just a few generations. - The time taken to have an allele go extinct or become fixed.

Question 46

Genetic drift is random:

Answer 46

A neutral allele at frequency 0.5 is equally likely to eventually be fixed or to go extinct, as it is equally far away from both. [In theory, chance of eventual fixation of a neutral allele is the same as its frequency]

Question 47

What is a genetic bottleneck?

Answer 47

Breeding population is very small for a time – Genetic drift powerful: allele frequencies change, many alleles fixed or go extinct.

Question 48

Genetic bottleneck diversity?

Answer 48

Lower genetic diversity overall, even if population later expands in numbers as it won't recover well

Question 49

Can rare alleles frequencies change?

Answer 49

Some rare alleles can increase in frequency = high frequencies of harmful alleles possible (even fixation of slightly harmful alleles)

Question 50

Founder effect in progress?

Answer 50

A few individuals found a new population from the ancestral population. A new population grows, and the gene pool of new population reflects the small sample of alleles present in the founders population.

Question 51

What is the founder effect?

Answer 51

Some previously rare alleles end up being much more common in the new population e.g. High prevalence of particular genetic diseases in isolated human populations

Question 52

Greater Prairie Chickens of Illinois?

Answer 52

Went through a genetic bottleneck effect. - There was a lower genetic variability than larger populations - They had much reduced reproductive success: fitness lowered by accumulated harmful alleles.

Question 53

Endangered Populations/species?

Answer 53

- Genetic diversity (gene pool) can be increased by adding individuals from other populations - Captive breeding programs manage matings to preserve remaining genetic diversity