# Populations

Genetics, populations, evolution and ecosystems (AQA A2 Biology) PART 2 of 4 TOPICS

 TOPICS: Inheritance  Populations  Evolution may lead to speciation  Populations in ecosystems

# Populations:

Gene pool is all the different genes and alleles found in a population at a given time.

Allele frequency is a proportion of organisms in a population with the same allele.

The Hardy-Weinberg principal provides a mathematical model, which predicts that allele frequencies will not change from generation to generation. The model assumes that a population is large, random mating occurs, no mutations therefore no selection and no migration. The Hardy-Weinberg equation/equilibrium is derived from the monohybrid genetic cross of two heterozygous parents with the genotype pq. Four offspring are made where one will be homozygous pp, one will be homozygous for q and two will be heterozygous pq. This is then turned into the following equation:

p = dominant allele

q = recessive allele

p2 = homozygous dominant

2pq = heterozygous

q2 = homozygous recessive

Also

NB: In exam questions the allele frequency may be given as a percentage of a population. It is your responsibility to change these percentages into decimals so that it can be used in the Hardy-Weinberg equilibrium as p and q are decimals. Allele frequencies are usually decimals.

EXAMPLE QUESTION ON HARDY-WEINBERG EQUILIBRIUM: Sea otters were close to extinction at the start of the 20th century. Following a ban on hunting sea otters, their population sizes started to increase. Scientists studied the frequency of alleles in one population of sea otters. The dominant allele, T, codes for an enzyme. The other allele, t, is recessive and does not produce a functional enzyme. In a population of sea otters, the allele frequency for the recessive allele, t, was found to be 0.2.

1. Use the Hardy-Weinberg equation to calculate the percentage of homozygous recessive sea otters in this population. Show your working.

NB: In this question the allele frequency for the recessive allele is already a decimal so no converting is needed.

As t is the recessive allele it is represented by the letter q in the Hardy-Weinberg equilibrium so q = 0.2

Therefore q2 = 0.22 so q2 = 0.04

1. Calculate the percentage of homozygous dominant sea otters in this population. Show your working.

Homozygous allele, T, is represented by the letter p.

To work out allele frequency of p you need to use the equation p + q = 1. q = 0.2

Therefore p = 1 – q so p = 1 – 0.2 = 0.8

Therefore p = 1 – q so p = 1 – 0.2 = 0.8

• Calculate the percentage of heterozygous sea otters in this population. Show your working

NB: Yes, the answer will be 32% but let us presume that we do not know the percentages of the homozygous recessive and dominant sea otters. We only the frequency of the homozygous recessive allele.

t is recessive therefore q = 0.2

To work out p you need to use the equation p + q = 1. q = 0.2

Therefore p = 1 – q so p = 1 – 0.2 = 0.8

Therefore p = 1 – q so p = 1 – 0.2 = 0.8

2pq is heterozygous therefore 2 x 0.2 x 0.8 = 0.32

# Evolution may lead to speciation

Genetics, populations, evolution and ecosystems (AQA A2 Biology) PART 3 of 4 TOPICS

 TOPICS: Inheritance  Populations  Evolution may lead to speciation  Populations in ecosystems

# Evolution may lead to speciation:

Individuals within a population of a species may show a wide variation in the phenotype. This is due to genetic and environmental factors. Meiosis and random fertilization of gametes during sexual reproduction produces further genetic variation:

• Meiosis: Independent segregation is when any combination of homologous chromosomes can align any way on the equator of the cell so that each daughter cell can inherit a different allele. Crossing over means that when two homologous pairs of chromosomes are aligned on the equator of the cell, the alleles may get swapped over from one chromosome to the other in the same pair.
• Random fertilization: This means daughter cells inherit new alleles.

Natural selection can occur when predation, disease or competition occurs. In each of these circumstances organisms with the best alleles survive and are more likely to reproduce than organisms with alleles that are at a disadvantage. The best alleles are passed on to offspring and the allele frequency increases. This is natural selection.

Those organisms with phenotypes providing a selective advantage are likely to produce offspring and pass on their favourable alleles to the next generation. The gene pool decreases because the numbers of different alleles are decreasing as a result of the favourable alleles increasing and the less favourable alleles decreasing. Natural selection can have different effects on the gene pool as to which alleles survive and pass on.

Stabilising selection is where organisms with phenotypes in the middle are more favourable than the two extremes:

EXAMPLE: Mass of babies at birth can be small, large or in the middle. The middle ones are more favourable. See what happens to the graph below:

Directional selection is where organisms at the higher phenotype are more likely to survive:

EXAMPLE: Cheetahs are the fastest animals on land. It is likely that this characteristic was inherited by directional selection as individuals that are the fastest are more likely to catch their prey. See what happens to the graph below:

Disruptive selection is a new type of selection that you will need to know for A2 as well as the above two which you have learnt about at AS. Disruptive selection is where the two extreme phenotypes are favoured more than the middle alleles:

EXAMPLE: Birds’ beaks can either be big or small depending on the appetite they have. Birds that have beaks that are in the middle cannot really eat anything as their beaks will always have a problem. See what happens to the graph below:

A change of allele’s frequency overtime is known as evolution where one method is natural selection as explained above. Another way is genetic drift which is as follows:

• Individuals within a population show variation in their genotypes e.g. genotype H and I
• By chance, the allele for one genotype (I) is passed on the offspring more than others
• So the number of individuals with this allele increases
• Change in the allele frequency in two isolated populations could lead to reproductive isolation and speciation (development of new species from existing species).

The diversity of life on Earth today is a result of the continuous speciation and evolutionary change over millions and millions of years. It is still continuing today.

Reproductive separation of the two populations can result in the accumulation of difference in their gene pools. New species arise when these genetic differences lead to an inability of members of the populations to interbreed and produce fertile offspring. In this way new species arise from existing species. There are two types of isolation in which this can happen:

• Allopatric speciation: This is geographical isolation where the population is split into two by a physical barrier made by a natural disaster etc. This causes the two populations to experience different climates and conditions in their new habitat. Different selection pressures are applied i.e. different alleles are favoured more than others in each population. This will cause different allele frequencies and different gene pools. Eventually the two groups become so different to each other that they no longer can breed with one another to produce fertile offspring. This means the two groups are no longer the same species.
• Sympatric speciation: This is when the population is reproductively isolated and does not geographical influences. There are two types of sympatric speciation:
• Pre-zygotic isolation mechanisms: These are things that stop two individuals coming together to mate. There are six types of pre-zygotic isolation mechanisms:
1. Temporal isolation: This is where organisms mate at different times of the year or at different time of the day.
2. Gametic isolation: The gametes of each population are different so that they cannot fuse.
3. Behavioural isolation: Difference in the mating rituals leads to different courtships or reproduction.
4. Ecological isolation: Different habitats mean less interaction.
5. Polyploidy: Some organisms may have more than two copies of chromosomes (called polyploid instead of diploid). This means that organisms that are polyploid cannot mate with organisms that are diploid. This is more common in plants.
6. Mechanical isolation: Different anatomy of the genitalia means cross-mating cannot occur.
• Post-zygotic isolation mechanisms: This when the zygote is prevented from further reproduction after it has formed. There are two types:
1. Inviable zygote: Two organisms are able to produce a zygote but it develops incorrectly. The offspring dies later.
2. Hybrid sterility: An offspring is produced but is sterile meaning that it cannot produce fertile offspring.

# Inheritance

Genetics, populations, evolution and ecosystems (AQA A2 Biology) PART 1 of 4 TOPICS

 TOPICS: Inheritance  Populations  Evolution may lead to speciation  Populations in ecosystems

# Inheritance:

Genotype is the genetic constitution of an organism. This is the alleles that are part of the genetic code for example TT, Tt or tt for height.

Phenotype is the expression of this genetic constitution and its interaction with the environment (the characteristic of the individual).

There may be many alleles of a single gene where they could be one of three of these:

• Dominant: An allele whose characteristic appears in the phenotype even when there is only one copy. These alleles are written as capital letters for example the capital t in the genotype used in the example for genotype for tall.
• Recessive: An allele whose characteristic appears in the phenotype if there are two copies unlike dominant. These alleles are written as lower case letters for example the lower case t in the genotype used in the example for genotype for tall.
• Codominant: Alleles that are both expressed in the phenotype and are represented by two capital letters where one letter being the allele is a superscript to the other capital letter being the gene for example the colour of a snapdragon where CR = Red flowers and CW = White flowers:
 Genotype Phenotype CRCR Homozygous Red flowers CRCW Heterozygous Pink flowers CWCW Homozygous White flowers

In a diploid organism (like us humans as we have two sets of chromosome), the alleles at a specific locus (location on the chromosome) can either be:

• Homozygous: An organism that has two copies of the same allele in the genotype for example TT or tt. The organism is said to be homozygote.
• Heterozygous: An organism that carries two different alleles in the genotype for example Tt. The organism is said to be heterozygote.

NB: The following genetic diagram needs to be known as you will be asked in the exam to predict the genotypes and phenotypes of the offspring. Monohybrid inheritance is the inheritance of a single characteristic controlled by a single gene. The images that say ‘monohybrid cross’ (the picture with alleles T) shows the layout that is recommended to do in the exam and these monohybrid crosses cover all the possible combinations between homozygous dominant, heterozygous dominant and homozygous recessive. These alleles are on autosomal genes carried on autosomes. Autosomes are chromosomes that are not sex chromosomes.

The monohybrid cross image with alleles CW is also part of the resource below. These alleles are part of the autosome.

Some genes have multiple alleles where more than two can code for the same gene but only two of these alleles can be expressed in the phenotype of the offspring as two parents are involved. This is an example of blood group which is also an image with the title ‘monohybrid cross – multiple alleles’ (the alleles with IB and IO). These alleles are part of the autosome.

Cystic fibrosis (CF) is caused by a recessive allele when a person who is homozygous recessive. Thick, sticky mucus is formed and stays on the lining of the lungs. These alleles are part of the autosome.

Albinism is an inherited condition where there is a lack of colour created by melanin in structures that have colour such as hair, iris and skin. Therefore albinos have red eyes, pinkish skin and pale yellow hair. It caused by a single recessive allele in the genotype. These alleles are part of the autosome.

Huntington’s disease, an incurable and fatal disease, is caused by a dominant allele in the genotype. These alleles are part of the autosome.

There is a 50-50 chance that a baby can be a boy or a girl. This can be proved by the Punnett square which is shown by the image called ’50-50 chance of being a boy or a girl’.

Sex linked characteristics are carried on the X of the sex chromosomes therefore the genes are not autosomal. An example of this is colour blindness which is the image called ‘colour blindness’. Boys are more likely to be colour blind than girls as boys only have one X chromosome. If this chromosome has the allele then he is colour blind. Girls have two X chromosomes and therefore the two are needed to have the allele for her to be colour blind.

Dihybrid inheritance is where two characteristics are adopted by two different alleles on different loci. An example follows:

Example: Two pea plants each with the genotype RRYY and rryy were crossed to create the first generation of offspring all with the genotype of RrYy. Two of the offspring were crossed to create the second generation of offspring. R is for round, r is for wrinkled, Y is for yellow and y is for green. NB: There has to be the two different types of alleles controlling a different characteristic in the same gamete as this is dihybrid inheritance – the inheritance of two characteristics. The other parent has the phenotype wrinkled and green with the genotype rryy so the gametes will be ry and ry. As the table below represents the offspring of the second generation, both parents have the genotype RrYy giving the gametes RY, Ry, rY and ry (the possible combinations of two different alleles controlling different characteristics:

 RY Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy

From the offspring above a phenotypic ratio can be concluded showing the two characteristics i.e. the phenotypic ratio for the offspring above is: 9 round and yellow seeds: 3 round and green seeds: 3 wrinkled and yellow seeds: 1 wrinkled and green seed.

If there were 16 offspring, 9 of the offspring would have round and yellow seeds, 3 would have round and green seeds, another 3 would have wrinkled and yellow seeds and 1 would have wrinkled and green seeds as this is the expected value. Not all cases are like this where another set of 16 offspring either from the same parents or different parents of the same genotype as RrYy may have 10 that have round and yellow seeds, 2 that have round and green seeds, 4 that have wrinkled and yellow seeds and none of the offspring have wrinkled and green seeds instead of the classic 9:3:3:1 ratio. This is our observed value. So, how would we know if the difference of the expected values and observed values are due to chance? Solution: Chi-squared should be used.

0 = Observed value

E = Expected value

NB: You are not expected to work out chi squared in the exam however a demo will be given below following the same type of plant and crossing as the one above.

We have to come up with our null hypothesis which is: THERE IS NO SIGNIFICANT DIFFERENCE BETWEEN THE OBSERVED AND EXPECTED RESULTS.

It is best if you put your data in a table like the one below:

 O E O – E (O – E)2 (O – E)2/E Round and yellow seeds 10 9 1 1 1/9 Round and green seeds 2 3 -1 1 1/3 Wrinkled and yellow seeds 4 3 1 1 1/3 Wrinkled and green seeds 0 1 -1 1 1

As the chi squared formula has the funny symbol in front of the fraction which means sum of, all the values at the furthest right of the table above have to be added up to give chi-squared. Chi squared is therefore 16/9. This value should be referred to the table below:

 Probability, p Degrees of freedom 0.25 (25%) 0.20 (20%) 0.15 (15%) 0.10 (10%) 0.05 (5%) 0.02 (2%) 0.01 (1%) 1 1.32 1.64 2.07 2.71 3.84 5.41 6.63 2 2.77 3.22 3.79 4.61 5.99 7.82 9.21 3 4.11 4.64 5.32 6.25 7.81 9.84 11.34 4 5.39 5.99 6.74 7.78 9.49 11.67 13.28 5 6.63 7.29 8.12 9.24 11.07 13.39 15.09

NB: We should always use the column with the 0.05 or 5% probability highlighted in yellow as biologists always use this. The values in the table are known as critical values.

To know which degrees of freedom to use we must use the value that is 1 minus how many categories we have. So in this case as we have four categories (the different types of seeds that the offspring have), we subtract 1 from this and we get three which is our degrees of freedom. Therefore our critical value is 7.81 which is in bold and underlined. We compare our chi-squared value (16/9) to the critical value (7.81). Our chi-squared value is smaller than the critical value therefore we accept our null hypothesis saying also that there is a 5% or higher probability that the results are due to chance and there is no significant difference between observed and expected values. NB: If our chi-squared value was greater than the critical value then we reject our null hypothesis and also say that there is a 5% or lower probability that the results are not due to chance and there is a significant difference between observed and expected values.

Epistasis is where a gene interferes with another gene on a different locus. An example is as follows: Flowers can be either white, light blue or aqua blue. The alleles of the gene code for the enzyme used to catalyse the reaction between white and light blue and light blue and aqua blue. The reaction between white and light blue is controlled by an enzyme called Enzyme A, coded by the dominant allele A. The reaction between light blue and aqua blue is controlled by Enzyme B coded by the dominant allele B. An image with the title ‘Epistasis’ is on the resource for you to look at.

Linkage is where there are two alleles that code for a different characteristic on the same chromosome. Therefore variation is reduced. An example is sweet pea plants in the image named ‘linkage’.

Recombination is the reassortment of genes into different combinations from the parents. Offspring that have recombination are called recombinants and gives rise to different individuals in a natural population. Three things can give rise to recombination: crossing over, independent assortment/segregation and random fertilization.

6 POINTS ON HOW TO ANSWER ‘LOOKING FOR EVIDENCE’ QUESTIONS IN GENETICS

1. A TIP TO REMEMBER IS NEVER ASSUME THAT A GENETIC CROSS OR PEDIGREE DIAGRAM IS SEX-LINKED UNLESS IT TELLS YOU IN THE QUESTION.
2. Q1: WHAT IS THE EVIDENCE THAT AN ALLELE IS RECESSIVE?

What to look for: LOOK FOR PARENTS WHO ARE UNAFFECTED AND HAVE A CHILD WHO IS AFFECTED.

Explanation: PARENTS MUST BE HETEROZYGOUS BECAUSE THEY ARE UNAFFECTED AND ALSO THEY PASS ON THEIR RECESSIVE ALLELE TO THEIR CHILD.

1. Q2: WHAT IS THE EVIDENCE THAT AN ALLELE IS DOMINANT?

What to look for: LOOK FOR TWO PARENTS WHO ARE AFFECTED AND HAVE A CHILD THAT IS NOT AFFECTED.

Explanation: PARENTS MUST HETEROZYGOUS BECAUSE THEY ARE AFFECTED AND ALSO THEY PASS ON THEIR RECESSIVE ALLELE TO THEIR CHILD.

1. Q3: WHAT IS THE EVIDENCE THAT A SEX-LINKED ALLELE IS RECESSIVE?

What to look for: LOOK FOR A MOTHER WITHOUT THE CONDITION AND A SON WITH THE CONDTION.

Explanation: MUM MUST BE HETEROZYGOUS FOR HER TO NOT HAVE THE CONDITION.

1. Q4: WHAT IS THE VEIDENCE THAT THE RECSSIVE ALLELE IS NOT SEX-LINKED?

What to look for: LOOK FOR UNAFFECTED FATHER AND AFFECTED DAUGHTER.

Explanation: DAUGHTER MUST HAVE TWO RECESSIVE ALLES BUT COULD NOT INHERIT THE RECESSIVE ALLELE FROM DAD BECAUSE HE IS UNAFFECTED.

1. Q5: WHAT IS THE EVIDENCE THAT A DOMINANT ALLELE IS NOT SEX-LINKED?

What to look for: LOOK FOR AN AFFECTED FATHER AND UNAFFECTED DAUGHTER.

Explanation: IF FATHER’S X CHROMOSOME CARRIES THE DOMINANT ALLELE THE DAUGHETR WOULD BE AFFECTED WHICH IS NOT THE CASE.

# Populations in ecosystems

Genetics, populations, evolution and ecosystems (AQA A2 Biology) PART 4 of 4 TOPICS

 TOPICS: Inheritance  Populations  Evolution may lead to speciation  Populations in ecosystems

# Populations in ecosystems:

Populations of different species form a community within a habitat. The community together with non-living features (known as abiotic conditions) which include temperature and water form a community. Ecosystems can vary in size from small e.g. a garden of a property to large ecosystems e.g. rainforest.

Within a habitat each organism has a different niche (the role of a species within a habitat). The niche a species can occupy includes:

• Its biotic interactions (living features): What it eats and what it is eaten by.

EXAMPLE: Sea otters have to smash clams and shellfish open with rocks.

• Its abiotic interactions: The oxygen that the organism inspires and the CO2 it expires.

EXAMPLE: Otters have webbed paws which means they can walk both on land and swim.

An ecosystem supports a certain size of a population of a species which is called the carrying capacity. This population size can vary as a result of:

• The effects of abiotic conditions: If temperature, light, water or other abiotic factors fall it means that the reproduction and growth rate will be low.

EXAMPLE: When a temperature of a mammal’s environment is ideal for the metabolic reactions to occur, it means that they do not have to use much energy to maintain a constant body temperature therefore the energy can be used for growth and reproduction therefore increasing the size of the population.

• The effects of biotic conditions: There are two types of competitions and predation that you need to know:
• Interspecific competition: This is competition between different species for the same resources such as food and space. As the food in this case would not be available totally to one species, the availability for food to both species will be lower causing them to have less energy for growth and reproduction therefore making both the populations lower for both species. In some cases one species may be better adapted than the other making the species that is stronger more likely to survive and reproduce passing on the advantageous alleles to the offspring causing the allele frequency to increase.
• Intraspecific competition: This is competition within a species for the same resources:

1 = The population of a species increases when there are plenty of resources available for each individual.

2 = Eventually resources start to run out therefore competition is on the rise and the population starts to decline as the weaker ones are out-competed.

3 = The population starts to increase after it has been at its lowest point as there is less competition and resources start to become available to everyone.

• Predation: Predator and prey populations are linked to each other. As one changes it causes the other to change as shown below:

NB: The population of the predators is always below the prey because of the energy levels in the food pyramid. Prey have more energy than predators. This graph is based on the fact that the predator only eats the prey shown in the graph and nothing else.

1 = The prey population increases causing the predator population to increase too as there is more food available for the predators.

2 = This reduces the amount of prey for the predators therefore the population of the prey decreases causing the predator population to decrease.

The size of a population can be estimated using:

• randomly placed quadrats or quadrats placed along a belt transect for slow-moving or non-motile (do not move) organisms:

EXAMPLE FOR RANDOMLY PLACED QUADRATS: If a quadrat with dimensions 0.5m by 0.5m was used then it would have an area of 0.25m2. Percentage cover is then worked out by counting the squares which is half filled or over with the species you are investigating. There are usually 100 squares in a quadrat making the number of squares the percentage cover. If there are not then you will need to get the number of squares that have the species divided by the total number of squares in the quadrat and multiply it by 100.

EXAMPLE FOR QUADRATS PLACED ALONG A BELT TRANSECT: In belt transects, quadrats are placed next to each other along the transect to work out species frequency or percentage cover. The quadrats do not have to be placed next to each other but can be placed at intervals which is known as an interrupted belt transect.

• the mark-release-recapture method for motile (moving) organisms:

EXAMPLE: A group of scientists wants to find the total population of the Violet Turaco, a native bird of Ghana.

To do this capture the birds in the most appropriate, safe, ethical and harmless way. Mark them also in the most appropriate, safe, ethical and harmless way e.g. put a tag on them that will not affect their behaviour and roaming, record the number and then release them back into their habitat. Wait for a week or for how long you want making sure that you do not wait too short or too long and then return to the same population to count how many birds you recapture with a mark you left on them and also other birds of the same species without the mark. (You should have three sets of data which are number caught the first time and marked, number caught the second time just with the same mark you left on them and the total number caught the second time with a mark you left and without the mark). Once your data is obtained you use the equation below to work out the total size of the population:

Where T is the total size of the population of a species, N1 is the number caught the first time and marked, N2 is the total number caught with a mark and without the mark and mN2 is the number caught the second time with just the mark you left on them.

When using the mark-release-recapture method there are a few assumptions which include that the marked sample has had enough time and opportunity to mix back in with the population hence why in the method you should wait not too long and not too short,; the markings do not affect the species chances of survival but the marking should still be on the individuals when you return the second time hence why the markings should be appropriate, safe, ethical and harmless to the species, and; there should be no changes in the population size due to births, deaths and migration during the time of study hence why you should not wait too long for the second capture.

Ecosystems are dynamic systems meaning they are constantly changing.

Succession is the process where an ecosystem changes overtime. NB: There are two types of succession being primary and secondary but only primary succession needs to be known. Primary succession starts with a species colonising the new land where these species are known as pioneer species – an example of pioneer species is lichen which you may have heard in GCSE. Lichen is an organism which is in between a plant and a fungus. This succession takes place on newly formed or land without soil where the abiotic conditions are harsh. Pioneer species start to grow as they are adapted to hostile abiotic conditions and eventually change them to become less hostile by dying and getting decomposed to make basic soil. This soil helps to retain water therefore changing the abiotic conditions. The change in the conditions causes new species such as grass and small plants with adaptations being similar to the conditions to grow. These new species then die and are decomposed making the abiotic conditions much more friendly as more organic matter (humus) is added making the soil more richer in minerals. The soil can now retain even more water helping larger species such shrubs and hedges to be grown. In some cases the new species causes the abiotic condition s to be less suitable to the pioneer species. An example of this would be that sand sedges stabilise in the sand by the use of rhizomes (an underground stem). This is not convenient for marram grass as they need constant reburial by sand in order to grow healthy therefore these start to die. Shrubs begin to grow and become the dominant species where diversity increases. The final stage comes; large trees start to grow and a climax community is established where the ecosystem is supporting the largest and most complex community of animals and plants. This will not change as it is in a steady state.

NB: Only a few details of secondary succession need to be known. Secondary succession takes place on land which has soil and the pioneer species are bigger.

Conservation, the protection and management of ecosystems, helps maintain the habitats by preventing succession in order to keep it at its current stage. Maintaining the number of habitats saves the animals and plants that live in the area. There are a couple of ways to carry out conservation:

• Grazing: The animals grazing have a similar effect to mowing a garden. The animals eat newly growing shoots of shrubs preventing them from establishing themselves to keep the vegetation low.
• Managed fires: These fires are lit for secondary succession. The first species to grow (pioneer species) are conserved. Larger species will establish themselves again and are removed when the next fire is lit.

Some conservation plans do not just protect habitats but may protect species too:

• Seedbanks: These are stores of a vast amount of seeds from a variety of species. If a population of a certain species of plants become extinct conservationists can bring them back by planting the stored seeds.
• Fishing quotas: These are limits as to how much of a certain type of fish can be caught. This can also be done by having nets which have large holes so that young offspring can be left in the water for breeding can catching fish in a certain cycle allowing time for the population to grow.
• Protected areas: These are areas such as national parks and natural reserves where urban development industrialisation and farming are restricted to protect the wildlife and their habitats.
• Endangered species: These are species that have a low population due to overhunting, natural disaster or disease. These organisms are bred in captivity such as zoos to raise the numbers and are then returned to the wild.

However not everyone agrees with every conservation plan and measure that is carried out as there often conflicts between human needs and conservationists. Therefore careful management is needed to find the balance between the two so that there is sustainability of natural resources. An example of this would be the Maasai Mara in Kenya where the Maasai people overgraze their cattle to earn a living therefore conservationists need to step in to protect the natural reserves causing conflict.

Evaluating evidence and data about conservation issues:

NB: You need to know this for the exam.

EXAMPLE: Otters make a comeback. In the 1950s and 1960s there was a dramatic decline in the otter population. Removal of vegetation from riverbanks wiped them out in almost all of the Midlands and South-East England. In the 1990s the government, through conservation organisations such as local wildlife trusts, carried out a number of ambitious projects designed to restore otters to all rivers where they existed before the 1950s. An experiment was carried out over 15 years to see what would happen to the population of otters if the vegetation on the riverbanks increased. A controlled group was also used with the same species of otter living in the same habitat but under the normal conditions i.e. the vegetation was left and not changed. The graphs shows the results:

NB: You may be asked to interpret and analyse the data.

• Describe the data:
• For the first five years the population of otters decreased from 50% to 25% with a steep gradient. After vegetation started to grow in the fifth year, the population of otters started to increase but with a shallow/small gradient.
• The control group experienced a steady decline in the percentage of otters for the whole of the 15 years where there was a drop from 50% to 20%
• Draw up conclusions:
• The results of both the groups together show that an increase in vegetation has caused an increase in the population of otters. This suggests that in the 1950s and 1960s the otters lost food, habitats and materials which helped them to survive and reproduce.
• Evaluate the method:
• The effects of other variables such as changing temperatures were removed so that the validity of our results increases.
• The study area and sample was large to further increase the validity of the results.
• Random sampling removed biasness – the data is more likely to be an accurate estimate of the whole area.

Some experiments show conflicting evidence. If the above experiment was to be carried out in another area but of the same land with a smaller sample size and no control group, it would not be enough to say that vegetation is the cause of the decline o the otters. This is because the control group provides evidence that under normal condition the otters are declining and that increasing the vegetation has an affect on the otter numbers. With no control group it may show that another factor is involved in increasing the population. This experiment may be that there was no control group and the experiment’s results showed that vegetation is not the answer to increase the population as the population continued to decrease after vegetation started to increase.