selection and evolution

Question 1

define phenotypic variation

Answer 1

is the difference in phenotypes between organisms of the same species

Question 2

In some cases, phenotypic variation is explained by what?

Answer 2

explained by genetic factors:
For example, the four different blood groups observed in human populations are due to different individuals within the population having two of three possible alleles for the single ABO gene

explained by environmental factors:
For example, clones of plants with exactly the same genetic information (DNA) will grow to different heights when grown in different environmental conditions

can also be explained by a combination of genetic and environmental factors:
For example, the recessive allele that causes sickle cell anaemia has a high frequency in populations where malaria is prevalent due to heterozygous individuals being resistant to malaria

Question 3

What can the complete phenotype of an organism be determined by?

Answer 3

the expression of its genotype and the interaction of the environment on this:

Phenotype = Genotype + Environment

Question 4

define genetic variation

Answer 4

The small differences in DNA base sequences between individual organisms within a species population

Organisms of the same species will have very similar genotypes, but two individuals (even twins) will have differences between their DNA base sequences

Considering the size of genomes, these differences are small between individuals of the same species

Question 5

Genetic variation is transferred from….

Answer 5

one generation to the next and it generates phenotypic variation within a species population

Question 6

Genetic variation is caused by the following processes as they result in a new combination of alleles in a gamete or individual:

Answer 6

-Independent assortment of homologous chromosomes during metaphase I

• crossing over of non-sister chromatids during prophase I

-Random fusion of gametes during fertilization

Question 7

what do mutations result in?

Answer 7

generation of new alleles

The new allele may be advantageous, disadvantageous or have no apparent effect on phenotype (due to the fact that the genetic code is degenerate

New alleles are not always seen in the individual that they first occur in

They can remain hidden (not expressed) within a population for several generations before they contribute to phenotypic variation

Question 8

Genes can have varying effects on an organism’s phenotype. what are the effects?

Answer 8

The phenotype may be affected by a single gene or by several

The effect that the gene has on the phenotype may be large, small and/or additive

Question 9

Genes can have varying effects on an organism’s phenotype. what are the effects?

Answer 9

The phenotype may be affected by a single gene or by several

The effect that the gene has on the phenotype may be large, small and/or additive

Question 10

what can also have an impact on its phenotype?

Answer 10

the environment that an organism lives in.

Different environments around the globe experience very different conditions in terms of the:
Length of sunlight hours (which may be seasonal)
Supply of nutrients (food)
Availability of water
Temperature range
Oxygen levels

Changes in the factors above can affect how organisms grow and develop
For example, plants with a tall genotype growing in an environment that is depleted in minerals, sunlight and water will not be able to grow to their full potential size determined by genetics

Question 11

Variation in phenotype caused solely by environmental pressures or factors…

Answer 11

cannot be inhrited by an organisms offspring
- only alterations to the genetic componnt of gametes will ever be inhertied.

Question 12

define variation

Answer 12

refers to the differences that exist between at least two things (be it a level, amount, quantity or feature of something)

Question 13

define variation in relation to natural selection

Answer 13

refers to the differences that exist between individuals of a species
This may also be referred to as intraspecific variation

intraspecific- something that occurs/exists within a species

Question 14

Variation observed in the phenotypes of organisms can be due to..

Answer 14

Qualitative or quantitative differences.

Question 15

Features of Discontinuous variation

Answer 15

• distinct classes/categories exist
• characterisitcis cannot be measured over a wide range
• individiuals cannot have features that fall between categories
• (different size bars on bar graph)

see graphs on save my exams

Qualitative differences in the phenotypes of individuals within a population give rise to discontinuous variation

Qualitative differences fall into discrete and distinguishable categories, usually with no intermediates (a feature can’t fall in between categories)
For example, there are four possible ABO blood groups in humans; a person can only have one of them

It is easy to identify discontinuous variation when it is present in a table or graph due to the distinct categories that exist when data is plotted for particular characteristics

Question 16

Features of continuous variation

Answer 16

• no distinct classes/categories exist
• characteristics can be measured and fall within a range between 2 extremes
• (bell shaped curve graph)

occurs when there are quantitative differences in the phenotypes of individuals within a population for particular characteristics

Quantitative differences do not fall into discrete categories like in discontinuous variation
Instead for these features, a range of values exist between two extremes within which the phenotype will fall

For example, the mass or height of a human is an example of continuous variation
The lack of categories and the presence of a range of values can be used to identify continuous variation when it is presented in a table or graph

Question 17

Define discontinuous variation

Answer 17

refers to the differences between individuals of a species where the differences are qualitative (categoric)

Question 18

define continuous variation

Answer 18

is the differences between individuals of a species where the differences are quantitative (measurable)

Each type of variation can be explained by genetic and / or environmental factors

Question 19

Genetic basis of discontinuous variation

Answer 19

This type of variation occurs solely due to genetic factors

The environment has no direct effect

Phenotype = genotype

At the genetic level:
Different genes have different effects on the phenotype
Different alleles at a single gene locus have a large effect on the phenotype
Remember diploid organisms will inherit two alleles of each gene, these alleles can be the same or different

A good example of this is the F8 gene that codes for the blood-clotting protein Factor VIII

The different alleles at the F8 gene locus dictate whether or not normal Factor VIII is produced and whether the individual has the condition haemophilia

Question 20

Genetic basis of continuous variation

Answer 20

This type of variation is caused by an interaction between genetics and the environment

Phenotype = genotype + environment

At the genetic level:
Different alleles at a single locus have a small effect on the phenotype
Different genes can have the same effect on the phenotype and these add together to have an additive effect
If a large number of genes have a combined effect on the phenotype they are known as polygenes

Question 21

Genetic basis of continuous variation

Answer 21

This type of variation is caused by an interaction between genetics and the environment

Phenotype = genotype + environment

At the genetic level:
Different alleles at a single locus have a small effect on the phenotype
Different genes can have the same effect on the phenotype and these add together to have an additive effect
If a large number of genes have a combined effect on the phenotype they are known as polygenes

Question 22

The additive effect of genes

Answer 22

The height of a plant is controlled by two unlinked genes H / h and T / t
The two genes have an additive effect
The recessive alleles h and t contribute x cm to the plants height
The dominant alleles H and T contribute 2x cm to the plants height
The following genotypes will have the following phenotypes:
h h t t : x + x + x + x = 4x cm
H H T T : 2x + 2x + 2x + 2x = 8x cm
H h T t : 2x + x + 2x + x = 6x cm
H H T t : 2x + 2x + 2x + x = 7x cm
H h T T : 2x + x + 2x + 2x = 7x cm
h h T t : x + x + 2x + x = 5x cm
H h t t : 2x + x + x + x = 5x cm

Be careful when answering questions that involve polygenes or genes with an additive effect. It is not a given that each gene will have the same effect on the phenotype as in the example above so make sure to double check the information you have been given in the question.

Question 23

statistical test

t-test what is it used to comapare?

Answer 23

can be used to compare the means of two sets of data and determine whether they are significantly different or not
The formula for the t-test will be provided in the exam, but formulae for how to calculate the number of degrees of freedom is not provided in the exam and must be learnt

The sets of data must follow a rough normal distribution, be continuous and the standard deviations should be approximately equal
The standard deviation (s) must be calculated for each data set before the t-test can be carried out

A null hypothesis should also be given
This is a statement of what we would expect if there is no significant difference between two means, and that any differences seen are due to chance

Question 24

T-test

If there** is a statistically significant difference **between the means of two sets of data what do you conclude?

Answer 24

If there is a statistically significant difference between the means of two sets of data, then the observation is not down to chance and the null hypothesis can be rejected

look at save my exams for t-test formula and how to calculate

Question 25

Every individual within a species population has the potential to reproduce and have offspring which contribute to population growth what would occur if the offspring for every individual survived to adulthood and reproduced?

Answer 25

• then the population would experience exponential growth
  This type of growth only happens when there are no environmental factors or population checks acting on the population (for example, when there are plentiful resources and no disease)

In reality, there are several environmental factors that prevent every individual in a population making it to adulthood and reproducing

One well known but rare example of exponential growth in a population is the introduction of 24 European rabbits into Australia in the 1800s. The rabbits had an abundance of resources, little or no competition and no natural predators. This meant the population increased rapidly and they became a major pest

Question 26

Environmental factors

Answer 26

limit population sizes by reducing the rate of population growth whenever a population reaches a certain size

Question 27

Environmental factors can be..

Answer 27

biotic or abiotic

Biotic factors involve other living organisms
This includes things like predation, competition for resources and disease

Abiotic factors involve the nonliving parts of an environment
Examples of abiotic factors include light availability, water supply and soil pH

When biotic and abiotic factors come into play not all individuals within a population will survive
For example, if a food source is limited some animals within a population will not get enough to eat and will starve to death

For most populations in the wild, the number of offspring produced is much higher than the number of individuals that make it adulthood

Question 28

Population limitation by environmental factors
EG - For African lions living in the wild there are several environmental factors that limit their population growth rate:
what are the factors?

Answer 28

1. Competition for food
There is a limited supply of prey: other lions and carnivores will also be hunting the same prey. If a lion is not able to hunt and feed then they will die from starvation

2. Competition for a reproductive mate
Female lions will often outnumber male lions in a population. This means the males compete with each other to mate with the females. When one male is in a contest with another male one (or both) could be injured or killed. Whoever loses the contest won’t be able to mate with the females in a pride and so won’t pass on his genes to any offspring

3. Supply of water
African habitats can be very arid during the dry season. The water sources that the lions drink from can be miles apart. If a lake or source of water dries up then they can die due to dehydration

4. Temperature
The extreme heat experienced in the lion’s African habitat can cause them to overheat and die. It can also prevent them from hunting for long periods during the day, meaning they are less likely to get the food they need to survive
The combined effect of all these environmental factors leads to a decrease in population growth as fewer individuals survive to adulthood and reproduce

Question 29

Variation exists within a species population. What does this mean?

Answer 29

This means that some individuals within the population possess different phenotypes (due to genetic variation in the alleles they possess; remember members of the same species will have the same genes)

Question 30

Environmental factors affect the chance of survival of an organism. So what do they act as?

Answer 30

selection pressure

Question 31

Selection pressures increase the chance of individuals with a specific phenotype….

Answer 31

surviving and reproducing over others

Question 32

The individuals with the favoured phenotypes are described as…

Answer 32

having a higher fitness

Question 33

define fitness of an organism

Answer 33

the ability to survive and reproduce (pass on its alleles to offspring)

Organisms with higher fitness posses adaptations that make them better suited to their environment

Question 34

When selection pressures act over several generations of a species what do they have an effect on?

Answer 34

the frequency of alleles in a population through natural selection

Question 35

What is the process of natural selection?

Answer 35

Natural selection is the process by which individuals with a fitter phenotype are more likely to survive and pass on their alleles to their offspring so that the advantageous alleles increase in frequency over time and generations

EG:
Rabbits have natural predators like foxes which act as a selection pressure
Rabbits with a white coat do not camouflage as well as rabbits with brown fur, meaning predators are more likely to see white rabbits when hunting
As a result, rabbits with white fur are less likely to survive than rabbits with brown fur
The rabbits with brown fur therefore have a selection advantage, so they are more likely to survive to reproductive age and be able to pass on their alleles to their offspring
Over many generations, the frequency of alleles for brown fur will increase and the frequency of alleles for white fur will decrease

Question 36

define selection pressures

Answer 36

Environmental factors that affect the chance of survival of an organism

• Orgnaisms with 1 phenotype are more likely to survive and reproduce than those with a diff phenotype
• can have different effects on the allele frequencies of a population through natural selection

For example, there could be high competition for food between lions if there is not plentiful prey available; this environmental factor ‘selects’ for faster, more powerful lions that are better hunters

Question 37

What are the 3 types of selection?

Answer 37

Stabilising
Disruptive
Directional

Question 38

Stabilising selection

Answer 38

Stabilising selection is natural selection that keeps allele frequencies relatively constant over generations

This means things stay as they are unless there is a change in the environment

A classic example of stabilising selection can be seen in human birth weights
Very-low and very-high birth weights are selected against leading to the maintenance of the intermediate birth weights
- bell shaped curve

see one note graphs/save my exam graphs

Question 39

Directional selection

Answer 39

Directional selection is natural selection that produces a gradual change in allele frequencies over several generations

This usually happens when there is a change in environment / selection pressures or a new allele has appeared in the population that is advantageous

For example: A recent finding has shown that climate change is having an effect on fish size in certain habitats
The increase in temperature is selecting for a smaller body size and against a larger body size
Warmer seas cause fish metabolism to speed up and so increases their need for oxygen; oxygen levels are lower in warmer seas
Larger fish have greater metabolic needs than smaller fish, and so they feel the effect of increased temperatures more strongly
Organisms are sensitive to changes in temperature primarily because of the effect that temperature can have on enzyme activity
Fish with a smaller body size are therefore fitter and better adapted to living in seas experiencing increased temperatures
Fish body size is determined by both genetic and environmental factors
Fish of a smaller size are more likely to reproduce and pass on their alleles to offspring
Over generations, this leads to an increase in the frequency of alleles that produce a small body size and a decrease in the frequency of alleles that produce a larger body size

Question 40

Disruptive selection

Answer 40

Disruptive selection is natural selection that maintains high frequencies of two different sets of alleles

In other words, individuals with intermediate phenotypes or alleles are selected against

Disruptive selection causes polymorphism: the continued existence of two or more distinct phenotypes in species
This can occur in an environment that shows variation

For example, birds that live on the Galapagos Islands use their beaks to forage for different sized seeds
The size of the bird’s beaks are either small or large with the intermediate medium-sized beak selected against
The reason for this is that the different types of seed available are more efficiently foraged by a shorter or longer beak

Question 41

What does evolution involve?

Answer 41

involves changes in allele frequencies over time

• can be caused by natural selection
  Selection pressures (caused by the environment) increase the likelihood that certain individuals with specific alleles survive to reproductive age, enabling them to pass on their alleles to their offspring
  -it is also possible for allele frequencies to change as a result of chance; this can occur due to a process known as genetic drift

Question 42

define genetic drift

Answer 42

the gradual change in allele frequencies in a small population, where some alleles are lost or favoured just by chance and not by natural selection.

When a population is very small chance can affect which alleles get passed on to the next generation
Meiosis results in haploid gametes, meaning that a fertilisation event only passes on half of the alleles of an individual; the half that gets passed on is the result of random fertilisation, and the other half of the alleles may be lost to the next generation
Over time some alleles can be lost or passed on purely by chance; this is genetic drift

Question 43

What are the processes that can cause changes in allele frequencies due to chance ?

Answer 43

The founder effect
The bottleneck effect

Question 44

When a new allele arises in a population or a change in the environment occurs what type of selection occurs?

Answer 44

Directional selection
- prduces gradual change in allele frq over several generations

There is always phenotypic variation within a population

There is a selection pressure in the environment, e.g. the presence of a predator

Some individuals in a population may have a phenotype that aids their survival in the presence of a selection pressure

The phenotype is produced by particular alleles
Individuals with the favoured phenotype are fitter and so more likely to reproduce and pass on the advantageous alleles to their offspring

Those who do not possess the advantageous allele or phenotype are less likely to survive and pass on their alleles to their offspring

So over time and several generations the frequency of the advantageous allele increases and the frequency of other alleles decreases

Question 45

What type of population is genetic drift more likely to affect the allele freq of?

Answer 45

small population

E.g. if a coin is tossed 10 times it is reasonably likely that heads will not come up at all, whereas if a coin is tossed 100 times and heads didn’t come up at all you would think you had a dodgy coin!

In a similar way the chances of a certain allele simply being lost by chance as a result of random fertilisation is much greater if only 10 pairs of birds are breeding than if there were 100 pairs of birds breeding

Question 46

EG of genetic drift in plants

Answer 46

In a small population of five plants growing near a playground with a rubber floor, three of the plants have blue flowers and two of the plants have pink flowers

By chance most of the seeds from the pink flowered plants end up on the rubber floor of the playground while all the seeds from the blue flowered plants land on fertile soil where they are able to germinate and grow

Note that the seeds from the pink flower do not fall on the impermeable surface because of any disadvantageous allele in the plant’s genome, but purely by chance, e.g. because of a gust of wind or a passing animal

If this happens by chance over several generations the allele for the pink flowers may be lost from this population

Question 47

define the founder effect

Answer 47

the reduction in a gene pool compared with the main populations of a species resulting from only 2 or 3 individuals (with onlt a selection of the alleles in the gene pool) starting off a new population

The founder effect occurs when a small number of individuals from a large parent population start a new population
The founder effect can come about as the result of chance
E.g. a chance event such as a storm may separate a small group of individuals from the main population
As the new population is made up of only a few individuals from the original population only some of the total alleles from the parent population will be present
In other words, not all of the gene pool is present in the smaller population
Because the population that results from the founder effect is very small it is more susceptible to the effects of genetic drift

EG:
The founder effect in lizards
Anole lizards inhabit most Caribbean Islands and they can travel from one island to another via floating debris or vegetation
A small number of lizards may be separated from the main population on a larger island and carried away to a smaller island by a chance event such as a large ocean wave or a storm
The lizards arriving at a new island may only carry a small selection of alleles between them, with many more alleles present in the lizard population on the original island
E.g. the lizards on the original island could display a range of scale colours from white to yellow and the two individual lizards that arrived on the island may have white scales
This means that the whole population that grows on that island might only have individuals with white scales
In comparison the original island population has a mixture of white and yellow scaled individuals.
If the yellow allele were recessive and present as a single copy in the original two lizards that arrived on the island, the chance of it being lost as a result of genetic drift is increased due to the small size of the gene pool

Question 48

Bottleneck effect

Answer 48

a period when the number of species fall to a very low level, resulting in the loss of a large number of alleles and therefore a reduction in the gene pool of the species

The bottleneck effect is similar to the Founder effect
It occurs when a previously large population suffers a dramatic fall in numbers
A major environmental event can greatly reduce the number of individuals in a population which in turn reduces the genetic diversity in the population as alleles are lost
The surviving individuals end up breeding and reproducing with close relatives

Example of the bottleneck effect
A clear example of a genetic bottleneck can be seen in cheetahs today
Roughly 10,000 years ago there was a large and genetically diverse cheetah population
Most of the population was suddenly killed off when the climate changed drastically at the end of the Ice Age
As a result the surviving cheetahs were isolated in small populations and lots of inbreeding occurred
This meant that the cheetah population today has a lack of genetic variation
This is problematic for conservation as genetic variation within a species increases the likelihood that the species is able to respond in the event of any environmental changes
Remember the environment exerts a selection pressure on organisms

Question 49

Processes that cause Allele changes summary

Answer 49

Natural selection -> selection pressures produce a gradual change in allele freq over sevel generations

Founder effect -> change in allele freq occur in a diff direction for the newly isolated small pop comparison to the larger parent pop due to chance

Genetic drift -> gradual change in allel freq in a small pop due to chance and not natural selection

Bottleneck effect -> reduction in the gene pool of a pop due to a dramatic decrease in pop size

Question 50

Pneumonic for processes that cause Allele changes

Answer 50

Never - natural selec
Fight - founder effect
Giant - genetic drift
Beasts - bottleneck effect

Question 51

define gene pool

Answer 51

the complete range of DNA base sequences in all organisms in a species or population

• is the collection of genes within an interbreeding population
• can be thought of as the sum of all the alleles at all of the loci within the genes of a population of a single species or a population

Question 52

When humans experience a pathogenic bacterial infection they are often prescribed what by a healthcare professional?

Answer 52

Antibiotics

Question 53

What are antibiotics?

Answer 53

chemical substances that inhibit or kill bacterial cells with little or no harm to human tissue

Question 54

what are antibiotics derived from?

Answer 54

naturally occurring substances that are harmful to prokaryotic cells (structurally or physiologically) but usually do not affect eukaryotic cells

Question 55

What is the aim of antibiotic use?

Answer 55

is to aid the body’s immune system in fighting a bacterial infection

Penicillin is a well-known example; it was the first antibiotic to be discovered in 1928 by Sir Alexander Fleming

Antibiotics are either described as being bactericidal (they kill) or bacteriostatic (they inhibit growth processes), they target prokaryotic features but can affect both pathogenic and mutualistic bacteria living on or in the body

However, like in all species, there exists genetic diversity within populations, and the same applies to disease-causing bacteria

Individual bacterial cells may possess alleles that confer resistance to the effects of the antibiotic

These alleles are generated through random mutation and are not caused by antibiotic use, but antibiotic use exerts selection pressures that can result in the increase in their frequency

Question 56

what happens when someone takes the antibiotic penicilin to treat a bacterial infection?

Answer 56

• bacteria that are sensitive to penicilin die - this is the entire population of the disease-causing bacteria.
• by chance there may be 1+ individual bacteria with an allele giving resistance to penicilin.
• This allele arises by random mutation
• one eg: Staphylococcus, where some individual bacteria have an allele that codes for the production of an enzyme, penicillinase, which inactivates penicilin.
• As bacteria have only a single loop of DNA, they have only 1 copy of each gene, so the mutant allele will have an immediate effect on the phenotype of any bacterium possessing it.

When an antibioitc is present:
- Individuals with the allele for antibiotic resistance have a massive selective advantage so they are more likely to survive, reproduce and pass their genome (including resistance alleles) on
Those without alleles are less likely to survive and reproduce
Over several generations, the entire population of bacteria may be antibiotic-resistant

Antibiotic resistance is an important example of natural selection

Question 57

Staphaylococus

Answer 57

There are known populations of the bacterium Staphylococcus that possess alleles which make them resistant to the effects of penicillin

These are known as resistant strains

Due to the rapid reproduction rate of bacteria (generations of 20-30 minutes for some species in optimal conditions) a single resistant bacterium can produce 10 000 million resistant descendants within a day

Question 58

The future of antibiotic resistance

Answer 58

Antibiotic-resistant strains are a major problem in human medicine

New resistant strains are constantly emerging due to the overuse of antibiotics

By using antibiotics frequently, humans exert a selective pressure on the bacteria, which supports the evolution of antibiotic resistance

Scientists are trying hard to find new antibiotics that bacteria have not yet been exposed to, but this process is expensive and time-consuming

Some strains of bacteria can be resistant to multiple antibiotics and they create infections and diseases which are very difficult to treat

When antibiotics were discovered, scientists thought they would be able to eradicate bacterial infections, but less than a century later a future is being imagined where many bacterial infections cannot be treated with current medicines

Question 59

What does The Hardy-Weinberg principle state?

Answer 59

if certain conditions are met then the allele frequencies of a gene within a population will not change from one generation to the next

can be useful when building models and making predictions, but the assumptions listed are very rarely, if ever, all present in nature

Question 60

What are the seven conditions or assumptions that must be met for the Hardy-Weinberg principle to hold true?

Answer 60

• Organisms are diploid
• Organisms reproduce by sexual reproduction only
• There is no overlap between generations, i.e. parents do not mate with offspring
• Mating is random
• The population is large
• There is no migration, mutation, or selection
  This would mean no individuals entering the population (immigration) or leaving (emigration)
  Selection refers to both natural and artificial selection
• Allele frequencies are equal in both sexes
• The Hardy-Weinberg principle can be useful when building models and making predictions, but the assumptions listed are very rarely, if ever, all present in nature

Question 61

Whhat does the Hardy-Weinberg equation allow?

Answer 61

for the calculation of allele and genotype frequencies within populations
It also allows for predictions to be made about how these frequencies will change in future generations

Question 62

What is the process of artificial selection/selective breeding?

Answer 62

is the process by which humans choose organisms with desirable traits and selectively breed them together to enhance the expression of these desirable traits over time and many generations

Humans have been selectively breeding organisms for thousands of years, long before scientists understood the genetics behind it

Knowledge of the alleles that contribute to the expression of the desired traits are not required as individuals are selected by their phenotypes, and not their genotypes

As the genetics is not always understood, breeders can accidentally enhance other traits that are genetically linked to the desirable trait
These other traits can sometimes negatively affect the organism’s health

Examples of artificial selection include:
Increased milk yield from cattle
Faster racehorses
Disease-resistant crops

There are always biological limitations to how extreme a trait can become in an organism

Question 63

Principles of selective breeding

Answer 63

1. The population shows phenotypic variation - there are individuals with different phenotypes / traits
2. Breeder selects an individual with the desired phenotype
3. Another individual with the desired phenotype is selected. The two selected individuals should not be closely related to each other
4. The two selected individuals are bred together
5. The offspring produced reach maturity and are then tested for the desirable trait. Those that display the desired phenotype to the greatest degree are selected for further breeding
6. The process continues for many generations: the best individuals from the offspring are chosen for breeding until all offspring display the desirable trait

Artificial selection in racing horses
Selective breeding has been a major part of the horseracing industry for many years. Breeders have found that horses tend to have one of the three following phenotypes:
Good at sprinting short distances
Good endurance over long distances
All-rounder

If a breeder wanted to breed a horse for a sprinting event they are likely to do the following:
Select the fastest sprinting female horse they have
Select the fastest sprinting male horse they have
Breed the two selected horses
Allow their offspring to reach maturity and test their sprinting speeds to find the fastest horse (male or female)
The breeder could then use this horse for racing, or they could continue the process of selective breeding by breeding this horse with another horse that is fast or descended from fast-sprinters
Over several generations, it would be hoped that the offspring are all fast-sprinters (but remember there are biological limitations to this)

Question 64

What is the aim of most selective breeding?

Answer 64

increasing the yield of a sellable product
It is not done with the organism’s survival in mind, and unlike natural selection, it can lead to organisms that are poorly adapted to their environments

Question 65

Unless the genetic mechanism behind a trait is fully understood, is highly likely that other traits could also be accidentally enhanced
Some examples of selective breeding in agriculture and livestock include:

Answer 65

• Disease-resistance in wheat and rice varieties
• Hybridization in maize
• Milk yield in cattle

Question 66

Disease-resistance in wheat & rice

Answer 66

• Wheat plants have been selectively bred for hundreds of years as a crop
• Wheat crops can be badly affected by fungal diseases: Fusarium is a fungus that causes “head blight” in wheat plants
• Fungal diseases are highly problematic for farmers as they destroy the wheat plant and reduce crop yield
• By using selective breeding to introduce a fungus-resistant allele from another species of wheat, the hybrid wheat plants are not susceptible to infection, and so yield increases
• Introducing the allele into the crop population can take many generations and collaboration with researchers and plant breeders
• Rice is another crop that has been subject to large amounts of selective breeding
• Rice plants are prone to different bacterial and fungal diseases
  Examples include “bacterial blight” and “rice blast” caused by the Magnaporthe fungus
• These diseases all reduce the yield of the crop as they damage infected plants
• Scientists are currently working hard to create varieties of rice plants that are resistant to several bacterial and fungal diseases

Question 67

Inbreeding & hybridization in maize

Answer 67

• Maize (also known as corn) is a staple crop in many countries around the world; it is grown to feed both livestock and people
• In the past, maize plants have been heavily inbred (bred with plants with similar genotypes to their own)
• This has resulted in small and weaker maize plants that have less vigour
• This is inbreeding depression which:
  Increases the chance of harmful recessive alleles combining in an individual and being expressed in the phenotype
  Increases homozygosity in individuals (paired alleles at loci are identical)
  Leads to decreased growth and survivability
• A farmer can prevent inbreeding depression by outbreeding
• This involves breeding individuals that are not closely related
• Outbreeding produces taller and healthier maize plants
• It decreases the chance of harmful recessive alleles combining in an individual and being expressed in the phenotype
• Increases heterozygosity (paired alleles at loci are different)
• Leads to increased growth and survivability (known as hybrid vigour)
• Crops of these plants have a greater yield
• Uniformity is important when growing a crop:
  If outbreeding is carried out completely randomly, it can produce too much variation between plants within one field
  A farmer needs the plants to ripen at the same time and be of a similar height; the more variation there is, the less likely this is
• In order to achieve heterozygosity and uniformity, farmers buy sets of homozygous seeds from specialised companies and cross them to produce an F1 generation
• Different hybrids of maize are constantly being created and tested for desirables traits such as: resistance to pests / disease, higher yields and good growth in poor conditions

Question 68

Improving milk yield in cattle

Answer 68

• Milk is a global food source, rich in calcium and protein (essential for growth)
• Over many years and generations farmers have selected female cows that have the highest milk yield and crossed them with male bulls related to high yield females
• Over time this selective breeding has resulted in cows with greater milk yields, which has been of great economical benefit to farmers
• The selective breeding of cows for increased milk yield is a good example of how artificial selection (controlled by humans) does not take into account an organism’s survival
• Selective breeding usually focuses on only one, or a handful of, characteristics, often to the extreme. Little thought is given to other traits important to an organism’s health
  In cows it has been observed that selectively bred individuals are much more prone to ailments such as mastitis (inflammation of the udder), milk fever and lameness compared to those that were allowed to breed at random

Question 69

define species

Answer 69

a group of organisms with similar morphological and physiological features that are able to interbreed and produce fertile offspring that are reproductively isolated from one another.

Question 70

The gene pool (or allele frequencies) in a species population can change over time due to processes such as:

Answer 70

Natural selection
Genetic drift
The founder effect

Question 71

When the gene pool within a species population changes sufficiently over time…

Answer 71

the characteristics of the species will also change
The change can become so great that a new species forms
This is evolution

Question 72

Evolution

Answer 72

is the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation

Question 73

For a population to have evolved into a separate species it must be what?

Answer 73

genetically and reproductively isolated from the pre-existing species population

Question 74

Reproductive isolation can occur due to ?

Answer 74

mutations that lead to the incompatibility of gametes or sex organs, or differences in breeding behaviour.
When two populations are reproductively isolated, they can also be said to be genetically isolated from each other, meaning that they do not exchange genes with each other in the production of offspring

Question 75

Define speciation

Answer 75

Changes in the allele frequencies of isolated populations are not shared so they evolve independently of each other; this can lead to the formation of two groups that are no longer successfully able to interbreed and that are said to be separate species

the formation of new species in this way is known as speciation

The evolution of a new species can take a very long time and many generations

For organisms with a short generation time (such as bacteria), evolution of new species can be observed far more quickly

Question 76

Evidence of Evolutionary Relationships in DNA

Answer 76

DNA found in the nucleus, mitochondria and chloroplasts of cells can be sequenced and used to show evolutionary relationships between species

The differences between the nucleotide sequences (DNA) of different species can provide a lot of information:
The more similar the sequence the more closely related the species are
Two groups of organisms with very similar DNA will have separated into separate species more recently than two groups with less similarity in their DNA sequences

DNA sequence analysis and comparison can also be used to create family trees that show the evolutionary relationships between species

Question 77

Evidence of Evolutionary Relationships in DNA

Answer 77

DNA found in the nucleus, mitochondria and chloroplasts of cells can be sequenced and used to show evolutionary relationships between species

The differences between the nucleotide sequences (DNA) of different species can provide a lot of information:
The more similar the sequence the more closely related the species are
Two groups of organisms with very similar DNA will have separated into separate species more recently than two groups with less similarity in their DNA sequences

DNA sequence analysis and comparison can also be used to create family trees that show the evolutionary relationships between species

Question 78

DNA Analysis and Comparison

Answer 78

DNA is extracted from the nuclei of cells taken from an organism
DNA can be extracted from blood or skin samples from living organisms or from fossils
The extracted DNA is processed, analysed and the base sequence is obtained
The base sequence is compared to that of other organisms to determine evolutionary relationships

The more similarities there are in the DNA base sequence, the more closely related (in that the less distant the species separation) members of different species are

In 2005, the chimpanzee genome was sequenced, and when compared to the human genome it was discovered that humans and chimpanzees share almost 99% of their DNA sequences, making them our closest living relatives

In 2012, the sequencing of the bonobo genome also revealed that humans and bonobos also share 98% of their genome (with slight differences to the differences seen in chimpanzees)

Question 79

Mitochondrial DNA

Answer 79

When analysing DNA from the mitochondria is is important to remember that:
- A zygote only contains the mitochondria of the egg and none from the sperm so only maternal mitochondrial DNA is present in a zygote
- There is no crossing over that occurs in mtDNA so the base sequence can only change by mutation

• The lack of crossing over in mtDNA has allowed scientists to research the origins of species, genetic drift and migration events
• It has even been possible to estimate how long ago the first human lived and where
• Mitochondrial Eve is thought to have lived in Africa ~200,000 years ago
• The estimation of this date relies on the molecular clock theory which assumes there is a constant rate of mutation over time
• The greater the number of differences there are between nucleotide sequences, the longer ago the common ancestor of both species existed
• The molecular clock is calibrated by using fossils and carbon dating
• A fossil of a known species is carbon-dated to estimate how long ago that organism lived
• This mtDNA of this species is then used as a baseline for comparison with the mtDNA of other species

Although for your exams you should say that only maternal mitochondrial DNA can be passed on or inherited by the zygote, recent research suggests that paternal mDNA may also be present in zygotes

Question 80

What does evolution cause?

Answer 80

speciation: the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation

Genetic isolation between the new population and the pre-existing species population is necessary for speciation

Question 81

What are two different situations when speciation can take place?

Answer 81

• Two groups of a species are separated by a geographic barrier
• Two groups of species are reproductively isolated but still living in the same area (experiencing similar environmental selection pressures)

Question 82

Allopatric Speciation

Answer 82

• occurs as a result of geographical isolation
• It is the most common type of speciation
• A species population splits into one or more groups which then become separated from each other by geographical barriers
• The barrier could be natural like a body of water, or a mountain range
• It can also be man-made (like a motorway)
• This separation creates two populations of the same species who are isolated from each other, and as a result, no genetic exchange can occur between them
• If there is sufficient selection pressure or genetic drift acting to change the gene pools within both populations then eventually these populations will diverge and form separate species
• The changes in the alleles/genes of each population will affect the phenotypes present in both populations
  Over time, the two populations may begin to differ physiologically, behaviourally and morphologically (structurally)

Example of Allopatric Speciation in Trees

Imagine there is a population of trees that are all one species
A new mountain range forms that divides the population into two
The natural barrier prevents the two groups from interbreeding, so there is no gene flow between them
The two populations experience different selection pressures and genetic drift
Over thousands of years the divided populations form two distinct species that can no longer interbreed

Question 83

Sympatric Speciation

Answer 83

• takes place with no geographical barrier
• A group of the same species could be living in the same place but in order for speciation to take place there must exist two populations within that group and no gene flow occurs between them
• Something has to happen that splits or separates the population:
• Ecological separation: Populations are separated because they live in different environments within the same area
  For example, soil pH can differ greatly in different areas. Soil pH has a major effect on plant growth and flowering

-Behavioural separation: Populations are separated because they have different behaviours
For example differences in feeding, communication or social behaviour

Example of Sympatric Speciation in Fish

A species of fish lives in a lake
Some individuals within the population feed on the bottom while others remain higher up in the open water
The different feeding behaviours separates the population into different environments
Behavioural separation leads to ecological separation
The separated groups experience different selection pressures
Long jaws are advantageous for bottom-feeding whereas shorter jaws are advantageous for mid-water feeding
Over time natural selection causes the populations to diverge and evolve different courtship displays
They can no longer interbreed; they are separate species