Module 6 Section 3: Evolution

Question 1

What is evolution

Answer 1

This is how the frequency of alleles in a population changes over time

Question 2

What is a gene pool

Answer 2

This is the complete range of alleles present in the population

Question 3

How is the rate at which an allele occurs displayed

Answer 3

The allele frequency is given as a percentage of the total population

Question 4

Process of natural selection

Answer 4

Individuals in a population vary because they have different alleles (caused by meiosis and mutations)
Predation, disease and competition (selection pressures) create a struggle for survival.
Because individuals vary, some are better adapted to the selection pressures than others.
Individuals that have an allele that increases their chance of survival (advantageous allele) are more likely to survive, reproduce and pass on the advantageous allele, than individuals with different alleles.
This means that a greater proportion of the next generation inherit the advantageous allele.
They, in turn, are more likely to survive, reproduce and pass on their genes.
So the frequency of the advantageous allele increases from generation to generatio
This process is called natural selection.

Question 5

What characteristics are selected by a stable environment

Answer 5

When the environment isn’t changing much, individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce.
This is called STABILISING SELECTION and it reduces the range of possible phenotypes.

Question 6

Example of how certain characteristics are chosen by a stable environment

Answer 6

Stabilising selection:
In any mammal population there’s a range of fur length.
In a stable climate, having fur at the extremes of this range reduces the chances of surviving as it’s harder to maintain the right body temperature.
Animals with alleles for average fur length are the most likely to survive, reproduce and pass on their alleles.
So these alleles increase in frequency.
The proportion of the population with average fur length increases and the range of fur lengths decreases

Question 7

What characteristics are selected by a changing environment

Answer 7

When there’s a change in the environment, individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce.
This is called DIRECTIONAL SELECTION.

Question 8

Example of how certain characteristics are chosen by a changing environment

Answer 8

If the environment becomes very cold, individual mammals with alleles for long fur length will find it easier to maintain the right body temperature than animals with short fur length.
So they’re more likely to survive, reproduce and pass on their alleles. Over time the frequency of alleles for long fur length increases

Question 9

How is genetic drift different compared to natural selection

Answer 9

This relies on chance instead of environmental factors to dictate which alleles are passed on

Question 10

Process of how does genetic drift work

Answer 10

Individuals within a population show variation in their genotypes (e.g. A and B).
By chance, the allele for one genotype (B) is passed on to the offspring more often than others.
So the number of individuals with the allele increases.
If by chance the same allele is passed on more often again and again, it can lead to evolution as the allele becomes more common in the population

Question 11

What processes is evolution influenced by in a natural environment

Answer 11

Natural selection and genetic drift work alongside each other to drive evolution
One process can drive evolution more than the other depending on the population size.

Question 12

How will natural selection and genetic drift work together in a smaller population

Answer 12

Evolution by genetic drift usually has a greater effect in smaller populations where chance has a greater influence and each allele makes up a greater proportion of the population (due to small gene pool) making it more likely to be passed on
If the environment changes for a small population, the lack of variety in the gene pool means that they will more likely go extinct than adapt as its less likely they will have an advantageous allele

Question 13

How will natural selection and genetic drift work together in a larger population

Answer 13

Genetic drift less prevalent in larger populations because the appearance of a new allele (from mutation) in a diverse gene pool will have less of an impact as it makes up a smaller portion of the gene pool
In larger populations any chance variations in allele frequency tend to even out across the whole population

Question 14

Example of genetic drift

Answer 14

Different Native American tribes show different blood group frequencies.
For example, Blackfoot Indians are mainly group A, but Navajos are mainly group O.
Blood group doesn’t affect survival or reproduction, so the differences aren’t due to evolution by natural selection.
In the past, human populations were much smaller and were often found in isolated group
By chance the allele for blood group O was passed on more often in the Navajo tribe, so over time this allele and blood group became more common.

Question 15

When may genetic drift have a greater effect

Answer 15

During a genetic bottleneck
During the founders effect

Question 16

What is a genetic bottleneck

Answer 16

This is an event (such as a natural disaster) that causes a big reduction in a population’s size, leading to a reduction in the gene pool and genetic diversity
New population may not be representative of the original population as rarer alleles may not be present in survivors

Question 17

Example of a genetic bottleneck

Answer 17

The mice in a large population are either black or grey.
The coat colour doesn’t affect their survival or reproduction.
A large flood hits the population and the only survivors are grey mice and one black mouse.
Grey becomes the most common colour due to genetic drift.

Question 18

What is the founders effect

Answer 18

The founder effect describes what happens when just a few organisms from a population start a new population and there are only a small number of different alleles in the initial gene pool

Question 19

How does the founders effect work

Answer 19

Individuals within a population show variation in their genotypes (e.g. alleles ABCD).
Some of these individuals start a new population.
By chance these individuals are mostly one particular genotype (A).
Without any further ‘gene flow’ (i.e. the introduction of new alleles from outside the population) the new population will grow with reduced genetic variation.
As the population is small, it’s more heavily influenced by genetic drift than a larger population

Question 20

When can the founders effect occur

Answer 20

Can occur as a result of migration leading to geographical separation
Or if a new colony is separated from original population for other reasons e.g. religion
E.g. the Amish have little genetic diversity so have a high incidence of certain genetic disorders

Question 21

What does the Hardy-Weinberg principle predict

Answer 21

Predicts that the frequencies of alleles in a population won’t change from one generation to the next.
But this prediction is only true under certain conditions

Question 22

Hardy-Weinberg principle conditions

Answer 22

Has to be a large population where there’s no:
Immigration
Emigration
Mutations
Natural selection
There also needs to be random mating where all possible genotypes can breed with all others

Question 23

What can the Hardy-Weinberg equations be used for

Answer 23

They can be used to estimate the frequency of particular alleles and genotypes within populations
If the allele frequencies do change between generations in a large population then immigration, emigration, natural selection or mutations have happened

Question 24

How to use Hardy-Weinberg equation to predict allele frequency

Answer 24

p+q = 1
p: frequency of dominant allele
q: frequency of recessive allele
Total frequency of all possible alleles for a characteristic in a population is 1.0 so p and q add to 1.0

Question 25

How to use Hardy-Weinberg equation to predict genotype frequency

Answer 25

Total frequency of all possible genotypes for one characteristic in a population is 1.0 so frequencies of all groups add to 1.0

Question 26

Use both Hardy-Weinberg equations

Answer 26

Question 27

What is a species

Answer 27

A species is a group of organisms, with a similar morphology and physiology, which can interbreed with one another to produce fertile offspring

Question 28

What is speciation

Answer 28

The development of a new species

Question 29

How does a species evolve into two separate species

Answer 29

For a species to evolve into two new species, it must be split into two genetically isolated populations
This happens when:
A particular set of circumstances reduces gene flow between two population of a species
Over time genetic differences between the populations (caused by genetic drift or natural selection) start to accumulate
Once sufficient physiological/ behavioural changes occur, the populations may become reproductively isolated

Question 30

How are alleles coding for a characteristic usually set out and what are the different types

Answer 30

Populations are usually polymorphic where they display multiple distinct phenotype for most characteristics
The alleles coding for the most common, normal characteristic is the wild type gene
Other forms of the allele, resulting from mutations, are called mutants

Question 31

What is artificial selection and describe the process

Answer 31

Artificial selection is where the selection pressure applied to the organism is changed from being natural to breeding for a specific desired phenotypic traits which are desirable to farmers (e.g. for plants and animals)
Individuals with the desired characteristics are selected and interbred
Offspring from this cross showing the best examples of the desired traits are then selected to breed
The process is repeated over many generations resulting in changes to the frequency of alleles within the population and eventually speciation

Question 32

How can artificial selection have negative impacts

Answer 32

Artificial selection means that only organisms with similar traits and similar alleles are bred together.
This leads to a reduction in the number of alleles in the gene pool.

A reduced gene pool could cause problems in the future
E.g. less chance of the alleles that could offer resistance to a new disease that becomes present in the population
Could mean that potentially useful alleles are accidentally lost from the population when other alleles are being selected for.

Question 33

What is inbreeding and what are the results of inbreeding

Answer 33

This is the breeding of closely related individuals and can be a result of artificial selection
It limits the gene pool and decreases genetic diversity
This can decrease the chances of a population of inbred organisms evolving and adapting to changes in their environment

Question 34

Why is inbreeding bad

Answer 34

Many genetic disorders are caused by recessive alleles which aren’t uncommon in a population
People that are closely related are more likely to have the same recessive alleles which can increase the chances of the offspring being homozygous recessive and therefore be affected by the genetic disorder
It also reduces the ability for species to adapt to change
This will slowly reduce the ability of these organisms to survive and reproduce which limits the chances of them producing an offspring

Question 35

What are the function of gene banks

Answer 35

These keep samples of seeds from both wild type and domesticated varieties which make them an important genetic resource
They can also store other biological samples such as sperm and egg which are frozen

Question 36

What is the benefit of gene banks

Answer 36

The alleles stored are used to increase genetic diversity which limits the amount of inbreeding and instead allows for outbreeding

Question 37

What is outbreeding

Answer 37

Outbreeding can be achieved by breeding unrelated or distantly related varieties
This reduces the occurrence of homozygous recessives and increases the potential to adapt to environmental changes

Question 38

How does allopatric speciation occur

Answer 38

Geographic isolation occurs where a physical barrier divides a population of a species from the main population (e.g. floods, volcanic eruptions, earthquakes)
Conditions on either side of the barrier will be different which means different characteristics will be more common due to natural selection
Eventually individuals from the different populations will have changed so much that they won’t be able to breed to produce fertile offspring
So they’ve become a different species

Question 39

Role of natural selection in allopatric speciation

Answer 39

Because different characteristics will be advantageous on each side, the allele frequencies will change in each population
e.g. if one allele is more advantageous on one side of the barrier, the frequency of that allele on that side will increase.

Mutations take place independently in each population, also changing allele frequencies.

Changes in allele frequencies will lead to changes in phenotype frequencies
e.g. advantageous characteristics (phenotypes) will become more common on that side

Question 40

Which type of speciation is more common

Answer 40

Allopatric speciation is more common than sympatric speciation

Question 41

Diagram of allopatric speciation

Answer 41

Question 42

Why does reproductive isolation occur

Answer 42

Reproductive isolation occurs because the changes in the alleles and phenotypes of the two populations prevent them from successfully breeding together

Question 43

Changes that can result in reproductive isolation

Answer 43

Seasonal changes: individuals from the same population develop different flowering or mating seasons, or become sexually active at different times of the year.
Mechanical changes: changes in genitalia prevent successful mating.
Behavioural changes: a group of individuals develop courtship rituals that aren’t
attractive to the main population

Question 44

How does sympatric speciation occur

Answer 44

This is speciation without geographical isolation
This is where random mutations occur within a population to result in the changes that cause reproductive isolation

E.g. mutations from two species of plants breeding to make hybrid offspring with a different number of chromosomes which reproductively isolates it (if mutation isn’t fatal)
This prevents members of that population breeding with other members of the species
New species can occur if mutated offspring reproduce asexually

Question 45

Answer 45

Question 46

Answer 46