3.7 Genetics, Populations, Evolution and Ecosystems Flashcards Preview

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Flashcards in 3.7 Genetics, Populations, Evolution and Ecosystems Deck (169)
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3.7.4 Populations in ecosystems - sampling methods

What is abundance?

The number of individuals of a species in a given space


What are the two main methods of studying abundance?

1. Frequency
- The likelihood of a particular species occurring in a quadrat
2. Percentage cover
- Estimate of the area within the quadrat that a particular plant species covers


What are the advantages and disadvantages of frequency?

 Often expressed as a percentage
 Useful where species is easier to count
 A quick idea of the species present

- Lacks information on the density
- No detailed information on distribution of species
- Can be quite time consuming


How do you calculate the percentage cover per quadrat?

Number of squares species covers/
Number of squares in total x100


What are the advantages and disadvantages of percentage cover?

+ Useful when a species is abundant or difficult to count
+ Data can be collected quickly
+ Gives a representation of density/ distribution

- Less useful where organisms occur in overlapping layers
- Can be less precise
- Often subjective


What are the two types of sampling?

1. Random Sampling
e.g. frame quadrats and point quadrats
2. Systematic Sampling
e.g. Transects


What do quadrats do?

Investigate the difference between two or more sites


What three key factors need to be considered when using quadrats?

1. The size of the quadrat used
- Depends on size of species being studied, size of area being studied, and whether the species are evenly distributed or clustered in small groups. If not evenly distributed, a large number of small quadrats need to be used
2. The number of sample quadrats to record within the study area
- Larger sample = more reliable results
- If there is a larger number of different species in sample, the greater the number of quadrats needed to produce valid results
3. The position of each quadrat within the study area
- Sampling MUST be random


How do you carry out random sampling (quadrats)?

Place two long tape measure at right angles along two sides of study area
Use coordinates by using random numbers from table/computer
Place a quadrat at the point of intersection
Record names and numbers of different species present
Repeat multiple times and calculate mean


Why must sampling be random?

To avoid sampling bias
Ensures data obtained is valid
Produce statistically significant results


What are the difficulties of using quadrats?

Difficult to identify and count every organism
Time consuming
May damage the habitat
Difficult to avoid introducing an element of personal bias
No indication of bias


What are point quadrats?

Horizontal bar supported by 2 legs
Pins are dropped through holes in top
Each species the pin touches is recorded


What are the advantages and disadvantages of using point quadrats?

+ Smaller organisms recorded
+ Small areas for specific detail

- Tedious
- Hard to use
- Time consuming


How could quadrats be used economically?

Construction sites (reduces environmental costs)
Optimum conditions for horticulture
Optimum conditions for agriculture


What is systematic sampling?

Studies the distribution of a species


When is systematic sampling used?

When gradual changes occur across a habitat
When transitions within communities occur
When zonation is present


What are the two types of transect?

Belt transects
Line transect


What is a line transect?

A string or tape
Organisms over which the line passes are recorded
Usually using a quadrat


What is a belt transect?

Strip or meter wide tape
Second placed parallel to first
Species between the two belts are recorded


When are transects used?

To show the trend of change in the number of two species in a given time
NOT a comparison of two areas


How do you carry out a transect?

Place transect (tape measure) down across a given area
Place quadrat down at equal intervals along the transect or at random intervals along transect
Carry out second transect in different part of the sample but still close by first sample
Calculate a mean across the 3 transects studied and display in a table
Plot a graph and describe the pattern shown


What is an alternative method to calculate population size?

Mark, release, re-capture


What is mark, release, re-capture?

A specific number of animals are caught, marked, and then released
After some time, a given number of animals are collected randomly
The number of marked animals counted
Calculate estimated population


How do you calculate estimated population?

Total no. in 1st sample x Total no. in 2nd /
Number of marked individuals recaptured x100


What assumptions must be made when using mark, release, re-capture?

1. Proportion of marked to unmarked in second sample is same as proportion of marked to unmarked in whole population
- Ensures results not due to different sample sizes
2. Marked individuals distribute themselves evenly
- Results are not valid in clusters
3. The population has a definite boundary
- Ensures you are studying the entire population
4. Markings are nontoxic and makes species no more conspicuous
- Ensures markings do not injure or reduce survival chances
5. Population is constant e.g. not breeding season
- ‘New’ individuals would not be marked
6. Markings cannot be rubbed off during investigation
- Ensures population size in 2nd sample is not reduced
- Ensures validity


What are the advantages of using mark, release, recapture?

+ Useful for measuring abundance when organisms can move, can hide or are difficult to find and identify


What are the disadvantages of using mark, release, recapture?

- Time consuming (need to leave for a time period)
- Risk of predation changing sample size
- Disturbance of habitat
- Trauma or injury to the organism (ethics)


3.7.1 Inheritance

What is a dominant allele?

Alleles which only need one gamete to show phenotypic trait.
The allele is always expressed in the phenotype e.g., BB or Bb


What is a recessive allele?

Alleles which need both gametes to show phenotypic trait.
The allele is not always expressed in the phenotype e.g., bb


What is an allele?

A version of a gene controlling characteristics but at the same site on homologous chromosome


What is a gene?

Specific section of DNA


What are homozygous chromosomes?

Two alleles are the same e.g., BB or bb


What are heterozygous chromosomes?

Two alleles are different e.g., Bb


What is a genotype?

The genetic information coding for the phenotype e.g., BbLl


What is a phenotype?

The physical characteristic expressed as a result of the genotype e.g., eye colour


What are the rules for inheritance?

Choose a letter to represent the trait you are investigating
Choose letters that are different lower case/capital
Capital = DOMINANT
Lower case = RECESSIVE
Label parents and their phenotype (parent 1 = brown eyes)
Label gametes the parents produce and put circle around them
Draw a punnet square
Label gender of each parent
Always write dominant allele first
Work out the cross
State the phenotypes
Calculate the ratio


What is a monohybrid cross?

A cross in which the alleles of any one gene are involved

- Inheritance of a single gene
- Used to determine the dominant relationship between two alleles
- The cross begins with the parental (P1 or P) generation


Example of monohybrid cross

Step 1 : Determine the gametes from each parent.

- Parent 1 = R r
- Parent 2 = R r

Step 2 : Create punnet square

R r
r Rr rr

Step 3 : Determine Phenotypic ratio
- 3:1


What is the basic law of genetics?

In diploid organisms, characteristics can be determined by alleles that occur in pairs
Only one of each pair of alleles can be present in a single gamete


What is pure breeding?

Pure breeding for a characteristic involves constantly breeding parent with desirable characteristic with another parent of desirable characteristic
- The organisms also become homologous
- All resulting offspring are known as first filial generation (F1)


What is a dihybrid cross?

Considers the inheritance of two characteristics at the same time
Shows how 2 characteristics, determined by 2 different genes located on different chromosomes are inherited e.g. shape and colour


Example of dihybrid cross

Seed colour = Yellow (Y, dominant) or green (y, recessive)
Seed shape = Round (R, dominant) or wrinkled (r, recessive)

Parent 1 = Round, yellow (RRYY)
Parent 2 = wrinkled, green (rryy)
ry ry
RY RYry RYry
RY RYry RYry

Phenotype of all F1 generation = round, yellow (genotype = RrYy)

Parent 1 from F1 generation = Round, yellow (RrYy) (gametes = RY, Ry, rY, ry)
Parent 2 from F1 generation = Round, yellow (RrYy) (gametes = RY, Ry, rY, ry)

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

Phenotypic ratio of F2 generation =

9 : 3 : 3 : 1
(round, yellow : round, green : wrinkled, yellow : wrinkled, green)


What is the theory of dihybrid inheritance?

The F1 generation produce 4 types of gamete.
They are able to do this as the gene for colour and gene for shape are on different chromosomes
During meiosis the chromosomes can arrange randomly at the equator
This means either shape allele can combine with either colour allele
Fertilisation is random so any of 4 gametes from one parent can mix with any of 4 from other parent


What is the law of independent assortment?

Each member of pair of alleles may combine randomly with either of another pair


What is codominance?

When both alleles are expressed in the phenotype
Neither allele is dominant nor recessive, they are both equally dominant


What are the rules for codominance?

We can’t use upper/lower case letters as this implies dominant/recessive
We use different letters instead e.g. R=Red, W=White
Letters should be superscript to the letter that represents the gene in the question e.g. Cᴿ


Example of codominance punnet square

Parent 1 = Red (CᴿCᴿ)
Parent 2 = White (CᵂCᵂ)

Cᴿ Cᴿ
Cᵂ CᴿCᵂ CᴿCᵂ
Cᵂ CᴿCᵂ CᴿCᵂ

Genotype of F1 generation = all CᴿCᵂ
Phenotype of F1 generation = all Pink as both colours are present

Parent 1 from F1 generation = Pink (CᴿCᵂ)
Parent 2 from F1 generation = Pink (CᴿCᵂ)

Cᴿ Cᵂ
Cᴿ CᴿCᴿ CᴿCᵂ
Cᵂ CᴿCᵂ CᵂCᵂ

Phenotypic ratio of F2 generation =
Pink : Red : White
2 : 1 : 1


What is the principle of multiple alleles?

Multiple alleles have more than two alternative forms of a single gene, located at the same loci of homologous chromosomes

Polygenic traits (e.g. eye colour) are determined by several genes at a different gene loci
Multiple alleles are involved in determination of a single trait by codominance


How is codominance and multiple alleles related to human blood groups?

Human blood groups have 3 associated with the immunoglobulin gene (I)
Gene I dictates the presence of certain antigens on the cell surface membrane of red blood cells

Allele Antigenproduced
Iᴬ A

Iᴮ B

Iᴼ Neither

There are three alleles but only two can be present in an individual because there are only 2 homologous chromosomes, so only 2 gene loci

Iᴬ and Iᴮ are codominant
Iᴼ is recessive to both

Blood group Possible genotypes
A IᴬIᴼ or IᴬIᴬ
B IᴮIᴼ or IᴮIᴮ
O IᴼIᴼ


How does immunology link to human blood group inheritance?

Blood group A = A antigens on red blood cells with anti-B antibodies in the plasma
Blood group B = B antigens with anti-A antibodies in plasma
Blood group O = no antigens, but both anti-A and anti-B antibodies in plasma
Blood group AB = has both A and B antigens, but no antibodies


How does receiving blood from the wrong ABO group be life threatening?

If someone with group B blood is given to group A blood, their anti-A antibodies will attack group A cells
This is why group A blood must never be given to someone with group B or O


Example of human blood group punnet square

Parent 1 = Group AB (IᴬIᴮ)
Parent 2 = Group O (IᴼIᴼ)

Iᴬ Iᴮ
Iᴼ IᴬIᴼ IᴮIᴼ
Iᴼ IᴬIᴼ IᴮIᴼ

Phenotypic ratio =
A : B
2 : 2
Does not produce offspring of same blood type as parents

Parent 1 = Group A (IᴬIᴼ)
Parent 2 = Group B (IᴮIᴼ)

Iᴬ Iᴼ
Iᴮ IᴬIᴮ IᴮIᴼ
Iᴼ IᴬIᴼ IᴼIᴼ

Phenotypic ratio =
AB : A : B : O
1 : 1 : 1 : 1
By crossing A and B heterozygotes you get offspring with one of each blood types


What is the difference between chromosomes and chromatids?

Before replication, one chromosome is composed of one DNA molecule
Following replication, each chromosome is composed of two identical DNA molecules (DNA replication increases the amount of DNA but does not increase number of chromosomes)
Two identical copies, each forming one half of replicated chromosome are called chromatids
During the later stages of cell division these chromatids separate longitudinally to become individual chromosomes


What is the human karyotype?

All chromosomes of human
46 chromosomes, 23 pairs
22 of 23 pairs have homologous partners which are identical
23rd pair are sex chromosomes (X and Y)

XY = male
XX = female


What is sex linkage?

Genes found on either X or Y are sex linked


What is the impact of X chromosome being longer than Y chromosome?

Some genes found on X do not have a homologous equivalent copy on Y

Recessive characteristics found on this portion will be more frequent in men because they have no homologous section on Y that could carry the dominant allele


What is haemophilia?

X-linked genetic disorder (defective gene on X)
Blood does not clot correctly – fatal if not treated.
Extremely rare in females because they have 2 X chromosomes and haemophiliac females died at puberty when menstruation begins.
Haemophilia is a recessive disorder.
The recessive allele codes for an alternative base sequence which results in faulty protein.
The healthy version of the protein allows clotting to occur.
The healthy protein is now produced by GH organisms


Punnet Square showing how haemophilia is inherited

H = clotting protein, h = non clotting protein

Parent 1 = Female Carrier (XᴴXʰ)
Parent 2 = Male Normal (XᴴY)

Xᴴ Xʰ
Xᴴ XᴴXᴴ XᴴXʰ

Phenotypic Ratio of F1 generation =

Healthy female : Carrier female : Healthy male : Haemophiliac male
1 : 1 : 1 : 1


What is autosomal linkage?

When 2 or more genes are carried on the same autosome

(all chromosomes that are not sex chromosomes are autosomes)
Any 2 genes found on the same chromosome are linked.
All linked genes stay together during meiosis.
This means they will pass into the gamete together meaning they will pass into offspring together.
They do not follow Mendel’s Law of Independent Assortment as each allele is not free to mix with either allele from another pair


What is epistasis?

Epistatsis describes a condition whereby one gene controls the expression of another gene.


When an allele of one gene affects/masks the expression of another in the phenotype


Example of epistasis punnet square

Gene A – distribution of black pigment (melanin)
A = Banded
A = uniform colour
Gene B – coat colour by determining whether A is expresses.
B = produces melanin
B = no melanin

Agouti Mouse = Grey/Brown Banded fur with melanin (A_B_)
Albino mouse = White with no melanin at all (A_bb or aabb)
Black mouse = Uniform black hairs with no banding but has melanin (aaB_)
Parent 1 = Agouti (AABB)
Parent 2 = Albino (aabb)

ab AaBb AaBb
ab AaBb AaBb

100% Agouti (AaBb) in F1 generation

Parent 1 = Agouti (AaBb)
Parent 2 = Agouti (AaBb)

AB Ab aB ab
Ab AABb AAbb AaBb Aabb
aB AaBB AaBb aaBB aaBb
ab AaBb Aabb aaBb aabb

Phenotypic ratio of F2 generation =
Agouti : white : black
9 : 4 : 3

The expression of gene B (melanin) affects the expression of gene A (bands)
If there is no melanin, then gene A cannot be expressed (there is no pigment to form bands)


How is epistasis linked to biochemical pathways?

Some genes act in sequence by coding for specific enzymes in a pathway
Dominant alleles code for the functional form of the enzyme
Recessive homozygotes would disrupt the pathway.
The presence of one non-functional gene will affect the other as failure to express one gene will result in no pigment being made


What is a X² test/ chi squared test?

Allows us to test the significance of differences between observed and expected results


When can chi squared be used?

When the data can be put into discrete categories e.g. phenotypes, no. of organisms in an area
When there is a large sample size
When there is raw data only (not percentages/rates)
When the portion of numbers expected in each category is known


Steps for chi squared

1. Formulate a null hypothesis
- There is no significant difference between ? and ?
2. Design a table for your data and calculations
3. Carry out calculations
4. Determine degrees of freedom
- (number of categories – 1)
5. Determine whether to reject or accept null hypothesis
- If chi value is BELOW critical value = no significant difference between observed and expected = any difference due to chance = ACCEPT null hypothesis
- If chi value is ABOVE/EQUAL TO critical value = there is significant difference between observed and expected = something other than chance is causing difference = REJECT null hypothesis


Example of chi squared question

A heterozygous long-winged fruit fly was bred with a heterozygous vestigial-winged fruit fly. They produced offspring of 145 long-winged and 55 vestigial winged

Show that the phenotypic ratio would be 3:1
Carry out chi squared test

1. Draw punnet square
L l
l Ll ll

Phenotypic ratio = 3:1

2. Carry out chi squared
Category O E O-E (O-E)² (O-E)²/E
Long-winged 145 150 -5 25 0.167
Vestigial-winged 55 50 5 25 0.5
X²= 0.667

Expected = 145 + 55 = 200
3 + 1 = 4
200/4 x 3 = 150
200/4 x 1 = 50

3. Calculate degrees of freedom
2-1 = 1
Critical value = 3.84

4. Conclusion
- X² value is below critical value so there is no significant difference between observed and expected. Any differences are due to chance so ACCEPT null hypothesis



What is ecology?

The study of inter-relationships between organisms and their environment


What are biotic factors?

Living factors in an environment

e.g Predation, competition, number of mates, disease, prey/predator numbers


What are abiotic factors?

Nonliving/physical factors in an environment

e.g. soil pH, water availability, space, rainfall, nutrients, light intensity


What is the biosphere?

The regions of the surface (land) and atmosphere (air) and Earth occupied by living organisms

- Shaped by interactions within organisms and the environment
- An interconnected network
- Involved in the study of ecology


What is an ecosystem?

All the interacting biotic and abiotic factors in an area


What is important in an ecosystem?

The flow of energy
The cycling of elements

- Important to ensure all species are provided with the nutrients needed to successfully survive and reproduce


What is the flow of energy in a pond ecosystem?

- Plants utilising the suns energy and converting some to ATP/chemical energy
Decay of material
- Releasing CO2 (element = carbon)
- Transfer of energy and elements
Nutrient cycles
- Nutrients contain elements (nitrates and phosphates) which are recycled in pond
Water cycle
- Cycling of water (hydrogen and oxygen) through the water cycle


What is a habitat?

A habitat is where a community (different organisms in the same place) is found


What is a microhabitat?

A habitat which is of small or limited extent and differs in character from surrounding habitat (own microclimate)


What is a population?

A group of interbreeding organisms of one species in a habitat


What is a community?

All of the populations of different species living and interacting in a particular place at the same time


What is an ecological niche?

How an organism fits into/its role within the environment

- Where it lives and what it does
- The biotic and abiotic factors required for an organisms survival


Why do no two organisms occupy the same ecological niche?

Different tolerance levels to environmental factors
Best adapted species survives and reproduces
At the expense of the weaker species (if conditions remain stable)
This reduces competition for food etc


What is the competitive exclusion principle?

Two species competing for the same limited resource cannot coexist at constant population values as one will always outcompete the other


What is population size?

The number of individuals within a population


What is exponential growth?

Growth rate is proportional to time
As time increases the variable being recorded increases at an exponential rate

HOWEVER most population growth in nature does not follow this pattern


What pattern of growth do most populations take?

Period of slow growth
- Initially a small number of individuals to reproduce
Then period of rapid growth
- Increased number as each reproducing
- Population doubles so increasingly steep curve
Then period of stable state, no growth
- Rate of growth declines until roughly stable
- The population reaches its carrying capacity
- The number of organisms which an ecosystem can support without environmental degradation


What are limiting factors?

Environmental conditions that limit growth, abundance, or distribution of an organism or a population of organisms in an ecosystem


Which factors limit population growth?

- Mutation = new antigens, no memory cells
- New adaptations, introduction of new species, competition
- Transpiration, photosynthesis, osmotic lysis, drought tolerance (GM)
- Respiratory rate, haemoglobin, tertiary structures, affinity
- Intra and inter specific, food webs
- New sources, global warming, monocultures, deforestation
- Light intensity, photosynthesis, transpiration, food production
- Shade tolerance, competition
Waste accumulation
- Mutation for tolerance, pH changes = enzymes
- Enzymes, transpiration, all metabolic processes
- Enzymes, tissue fluid, mutations, O2 affinity, metabolic processes


Are all growth rates the same?

No the rate of population change will not be the same


Compare the growth rate of bacteria, humans and cats

- Quick/rapid growth
- Asexual reproduction
- Less complex
- Binary fission
- Slow growth/stabilises
- Sexual reproduction
- Long development
- Few offspring
- Middle term
- Long gestation
- Multiple births


When is it best to use logarithmic scales?

Rapid growth
Studying a long period of time


Why are logarithmic scales used?

Reduces wide-ranging quantities to tiny scopes
Clearer pattern shown


How do you calculate logs?

2 = Base, ³ = Exponent

The exponent of a number says how many times to use the number in a multiplication
(2³ = 2 x 2 x 2 = 8)

A logarithm asks what exponent produced this?
It gives you the exponent as the answer
2? = 8

23 = 8
Log2(8) = 3


Why has the human population doubled in last 50 years?

Health care
- Fewer people are dying/ aging population
Infrastructure and resources
- More houses/workplaces/stability
- More money for medical care, houses, food
Food availability (agricultural revolution)
- Better nutrition/growth
Better quality of life
- Better hygiene, fewer diseases
Industrial revolution


How to calculate population growth?

Population growth = (births + immigration) – (deaths + emigration)


How to calculate percentage population growth rate?

Population change during time period/
Population at start of period x100


What factors cause population variation globally?

Culture and religion
Social pressures
Politics (education and taxes)
Age of population
Life expectancy at birth
Food supply and range
Natural disasters and war
Water and sanitation


What is a population pyramid?

Where age and gender profiles are displayed graphically by stacked bars which represent the % age of males and females in each age group
They give information of future trends of populations e.g. how many females of child bearing age


What are the features of a stable population?

Birth and death rate are in balance
No change in population size
Typical of economically developed countries


What are the features of an increasing population?

Higher birth rate = wider base
Fewer old people = narrow apex
Typical of economically less developed countries


What are the features of a decreasing population?

Lower birth rate = narrower base
Lower mortality rate = more elderly people
Apex is wider
Applies to certain economically more developed countries (Japan)


What is demographic transition?

The pattern of growth displayed by human populations as countries develop economically


What are the four stage of demographic transition?

1. Small and stable growth
- High birth rate
- High death rate

2. Early expansion
- High birth rate
- Decreasing death rate

3. Late expansion
- Decreasing birth rate
- Low death rate

4. Large and stable
- Low birth rate
- Low death rate


What is a predator?

An organism that feeds on another organism (prey)


How have predators evolved to become better adapted for capturing their prey?

Better camouflage
Sense of smell/eyesight


What is prey?

An organism that is attacked/eaten by another organism


How have prey evolved to become better adapted for escaping predators?

Concealing behaviour
Protective features


A new species is introduced to an area. Explain how prey become adapted for escaping their predators

A random mutation occurs
Causing an advantageous allele e.g. camouflage
Those with advantageous allele survive and reproduce
At expense of less advantageous allele
Pass advantageous allele to subsequent generations
Increase in allele frequency


What is a stable community?

Where all species and environmental factors are in balance so that population sizes remain fairly constant


What is a predator prey cycle?

Predator and prey populations determine one another’s growth
The size of one population has a direct impact on the others
A cyclic relationship


Example of predator prey cycle

1. Predators eat prey. Prey population falls
2. Predator popualtion increases
3. Predator popualtion falls
4. Prey population increases
5. Predator population starts to increase again
6. Prey population decreases


Explain a predator prey cycle

Predator population increases as prey increase as there is more/enough food
All prey is eaten so prey decreases
Predators have less food so population decreases
Less prey is eaten so they increase
More food again for predators so they increase
Cycle continues


What is a cyclical relationship?

One that is evolving (changes in population size) and is recurring in cycles (as one population changes it directly affects another)


What can cyclical relationships also be a result of?

Climate changes


What can the severity of a population crash be increased or decreased depending on?

1. Number of food sources relied upon
- If only one food source, and it is destroyed, the population will crash/die but if there are other options, survival is higher

2. Genetic diversity
- Increased probability that there is an advantageous allele in the population

3. Availability of mates
- Those with advantageous allele or those who survive can reproduce easily and increase population size rapidly. Lack of mates means population cannot increase

4. Gestation time
- Can lots of offspring be produced very quickly after a crash or decline to rebuild the population quickly?

5. Intensity of change
- Has the environment significantly changed i.e. drought or is it a period of prolonged dryness that can be survived?


How can climate change affect population size?

Environment changes
Population crash causes a selection pressure e.g. drought resistance
Population evolve to become better adapted
Population changes and increases
Competition increases so population decreases


How does competition arise?

When 2 or more individuals share any resources that are insufficient to satisfy their requirements


What factors do organisms compete for?



What is intraspecific competition?

Competition between members of SAME species


Why is intraspecific competition important?

The availability of resources determines the population size
The greater the availability, the larger the population size
More resources to grow and breed


Some oak trees have just started to grow. They are competing for light, water, and nutrient. Only some survive. Which will survive and why?

Mutation occurs
Mutation is advantageous so causes leaves to be larger or extended roots
Organisms better adapted to access requirements for photosynthesis
Organisms grow and survive to reproduce
Frequency of allele increases in subsequent generations
At the expense of the smaller leaves plants


What is interspecific competition?

Competition between members of DIFFERENT species


Why is interspecific competition important?

Populations of 2 different species initially occupy the same/similar niche
But one will normally have a competitive advantage over the other so
Best adapted survives and breed
Population increases at the expense of the weaker, less adapted species, resulting in
Complete removal of the weaker species


What are the problems in proving that competition is the factor affecting population sizes?

1. Many other factors involved (abiotic)
2. A causal link has to be established
3. Often a time lag (population change may be due to competition that took place many years earlier)
4. Data on population sizes are hard to obtain and not always reliable


What does the Student T Test tell you?

If there is a statistically significant difference between two means when:
- The sample is less than 25
- The data is normally distributed


What is the calculation or the student T Test?

t = (x1-x2) / Square root of (s1)2/n1 + (s2)2/n2

X1 = mean of first sample
X2 = mean of second sample
S1 = standard deviation of first sample
S2 = standard deviation of second sample
n1 = number of measurements in first sample
n2 = number of measurements in second sample


Example T test calculation question

The effect of 2 different temperatures on the enzyme urase were measured. Is there a significant difference between the 2 temperatures?

1. Create null hypothesis
- There is no significant difference between effect of enzyme urase in different temoeratures

2. Carry out calculation
36.5˚C 37.5˚C
Mean 48.6 54.3
N 5 6
SD 5.68 3.67
S² 32.26 13.47
S²/n 6.5 2.2

t = 48.6 – 54.3 / SQ6.5 + 2.2 = 5.7 / SQ8.7 = 1.9

3. Determine degrees of freedom
- n=5 for lower temp and n=6 for higher
- (5+6) – 2 = 9 degrees of freedom
- Critical value of t = 2.26

4. Conclusion
- Calculated value of t = 1.9 = lower than critical value
- Therefore there is more than 5% probability that differences in means are due to chance
- ACCEPT null hypothesis = there is no significant difference


Calculation for standard deviation

Sqaure root of sum of (x-x with line above)2 / n - 1


What is succession?

The change in species in an area over time


What are the characteristics of a pioneer species?

Reproduce asexually
- Does not rely on another species to increase its population size
Produce large quantities of wind dispersed seeds or spores
- Can be spread to remote areas and without the need for other organisms
Can tolerate/are adapted to survive in harsh conditions i.e. low light intensities, low/high pH
- Can survive and reproduce when no other organisms can
Short dormancy/fast germination
- Reprode and increase in number quickly
- Do not rely on food sources
Are nitrogen fixing
- Improves soil quality when there are few nutrients


What is the process of succession?

1. Pioneer species
- Colonises harsh environment
- Reproduce and increase population size
- Changes the environment (nitrogen fixing)
- Improve soil quality

2. Primary coloniser
- New species is better adapted e.g. lichens
- Outcompetes pioneers
- New community forms
- Continue to change the environment (abiotic factors)

3. Secondary coloniser
- Weathering of the rock creates sand and soil
- Lichens die and decompose
- Nutrients are recycled
- Nutrients support small plant species
- Quality and quantity of soil improves
- Rock erosion continues and plant species die and decompose
- The smaller plants again change and improve the abiotic factors
- Area can support larger species e.g. grass, shrubs, trees
- Continues until a CLIMAX COMMUNITY is reached (few if any new species are replaced)


What is biodiversity?

The range and variety of genes, species and habitats within a particular region

1. Species Diversity
2. Genetic diversity
3. Ecosystem Diversity


What is preservation?

Maintaining individuals, populations and ecosystems in their current state without the exploitation of natural resources


What is conservation?

Allowing ecosystems to evolve naturally, without resources being overused


What are the two forms of conservation?

- Whole ecosystems and landscapes within their habitat monitoring succession

- Individual endangered species in zoos outside natural habitat


Why do we manage succession?

Allows ecosystems to evolve naturally
No detrimental effects on any stage of succession
Enables maximum diversity and ecosystems


Explain the stage of succession that should be of greatest focus to conservationists?

Mid succession because:
- Not dominated by one species
- Not a very hostile environement
- Can support multiple species, more habitats/food sources = higher biodiversity

NOT climax community because:
- Dominated by one species
- High competitive element
- Competition exclusion principle
- Low biodiversity


How can we manage succession?

1. Burning heathlands
- Some areas are burnt to prevent succession
- Prevents dominance/climax community
- Older forms are less palatable and less nutritious
- New heather and other new species can grow
- Biodiversity is increased

2. Control of moorland for rearing grouse
- Grazing by sheep and/or periodic burning
- Maintains low growing plant populations
- Provide plentiful food sources for grouse
- Increases number of grouse
- Happy hunters


What is the equation for the biodiversity index?

N(N-1) /
d= sum of n(n-1)

N = total number of organisms of all species
n = total number of organisms of each species

- The higher the value of d = the greater the species diversity = greater the stability (greater genetic diversity)


What is selection?

The process by which organisms that are better adapted to their environment survive and reproduce at the expense of those less well adapted


Why does phenotypic variation occur?

Genetic factors
Environmental factors


Why does genetic variation occur?

Sexual reproduction
Main factor = mutations


What are normal distribution curves?

Most traits are normally distributed
Most common result (mode) is near the average (mean) and the middle value (median)
When selection occurs, the normal distribution curve may change shape


What is directional selection?

Changes in the environment change the mean
Exteme is selcted for
e.g. antibiotic resistance
Distribution curve shifts to left or right


What is stabilising selection?

Selection against both extremes
Phenotypic variation decreases
Environment has remained stable
e.g. birth weight
Distribution curve gets taller


Differences between the reproductive success of individuals affects the allele frequency in populations. Explain how

A random mutation of alleles occurs within the gene pool resulting in an advantageous allele
These individuals are better adapted to the specifi environment
They survive and reproduce at the expense of those less well adapted
They pass advantageous allele to offspring who survive and reproduce
The allele frequency increases
Change in allele frequency in population = evolution


What are selection pressures?

Environmental factors that limit the population of a species

- Determine the frequency of all alleles within the gene pool
- Vary from time to time and place to place


What are examples of selection pressures?

Natural disasters


What effect do selection pressures have?

Stabilising selection:
- Selection pressure against both extremes
- Mean remains the same

Directional selection:
- Selection pressure for an extreme
- Mean changes


What is exponential growth?

The rate increases in proportion to the growing total number or size

- All species have the potential to grow exponentially


What factors are required for evolution/population change to occur?

Evolution only occurs by natural selection if:
- Organisms reproduce
- Have sufficient resources
- Genetic variation within the population is present
- Phenotypic variation within the population is present
= greater diversity = adapt to change


What characteristics would help a population to grow exponentially?

- A suitable popualtion size (minimise intraspecific competition)
- A high reproductive success


What factors/characteristics make exponential growth unlikely?

- High death dates from predation
- Low parental investment
- Lack of resources


Why will death rates within a population not be completely random/ exponential growth unlikely to occur?

1. Organism will be better adapted to environment i.e. camouflaged
2. Organism will be suited to prevailing conditions i.e. catch food
3. Organism will be able to resist disease


What is disruptive selection?

Least common form of selection
Opposite to stabilising selection
Favours the extremes at the expense of the intermediate phenotypes
Can lead to speciation
Distribution curve increases at both sides of mean (extremes) but decreases at mean


When will disruptive selection occur?

When an environmental factor i.e. temperature takes two or more distinct forms (summer = warm, winter = cold)


Why is disruptive selection important/what impact will it have?

Most important for causing evolutionary changes
Could result in two distinct species


Give two examples of disruptive selection

1. Fur length
- In colder temperatures in winter there is long fur
- In warmer temperatures in summer there is short fur
- The result is two different species, each active in each season
- Those active in summer will breed with others active in summer, producing short haired offspring
- Those active in winter will breed with other active in winter, producing long haired offspring

2. Coho Salmon
- Large males and small males have selective advantages
- Small males sneak up to females and reproduce and pass on alleles
- Large males are fierce competitors so attract females and reproduce passing on alleles
- Intermediate size is selected against as has no advantage


What is speciation?

The evolution of new species from existing species
Enables evolutionary change and diversity


What is speciation due to?

Reproductive isolation
Genetic differences (mutations)
Geographical isolation (differences in two populations gene pools)


What are the two forms of speciation?

1. Allopatric speciation
- More common
- Different locations
- Geographic seperation

2. Sympatric speciation
- Less common
- Same location
- Reproductive isolation


How does allopatric speciation occur?

- Each species will have its own range, within which there will be a number of other popualtions within that species
- Populations can still interact and interbreed, unless a physical barrier occurs (i.e. lake, desert, mountain) preventing migration between populations of the same species
- The environments of each new range may be different
- Different mutations occur in each range
- Different selection pressures
- Selection favours different/advantageous characteristics
- No gene flow/mixing of alleles can occur
- Changes in allele frequency
- Unable to interbreed if barrier is removed


What is adaptive radiation?

The diversification of a species into forms adapted to different ecological niches


Example of sympatric speciation

Apple maggot fly
- Only lay eggs inside Hawthorns Fruits
- Apple trees introduced
- Some flies lay eggs in apples
- Mates are found by searching site where they were raised
- Raised in apples, mate with flies raised in apples
- Mutations may result in two new species


What is genetic drift?

Genetic change due to chance

In a small poualtion there will be:
- A small variety of alleles (lower genetic diversity)
- Not equal chance of each allele being passed on
- Or if a mutation occurs one allele/mutation is passed on very quickly
- Increase in allele frequency
- Speciation is more likely

By chance fewer descendants of one phenotype are left behind


What is the hardy Weinberg principle?

A mathematical equation used to predict the fequencies of alleles in a popualtion

P + q = 1

P = dominant allele
Q = recessive allele
100% of alleles in gene pool will be dominant and recessive


What is the Hardy Weinberg formula?

P2 + 2pq + q2 = 1

P2 = frequency/number of individuals that are homozygous dominant (AA)
2pq = frequency/number of individuals that are heterozygous (Aa or aA)
Q2 = frequency/number of individuals that are homozygous recessive (aa)
1 = All alleles present must add up to 1 or 100%


Example of Hardy Weinberg calculation

A characteristic is result of recessive allele. We know that one person in 25,000 display characteristic. Use HW equation to find freqency of carriers (heterozygotes)

1. Find frequency of homozygous recessives
= 1/25000
2. Therefore q2 = 1/25000 or 0.00004
3. Q = square root 0.00004
4. Frequency of recessive allele (q) = 0.00063
5. Frequency of all alleles = p + q = 1
6. P = 1 – 0.00063 = 0.9937
7. Frequency of dominant allele (p) = 0.9937
8. Frequency of heterozygous genotype = 2pq
9. 2 x (0.9937 x 0.00063) = 0.0125
10. Frequency of heterozygotes/carriers = 0.0125
11. This means 1.25% of population are heterozygotes (carriers for characteristic)


What are the assumptions of Hardy Weinberg principle?

1. We must assume that proportion of dominat and recessive alleles of ANY gene in a popualtion remains the same from one generation to the next
2. No natural selection so new evolution so no new alleles
3. No mutation so no new alleles
4. No migration so no new gene pools
5. Large population
6. Randomly mating so no selective breeding so no alleles are favoured


What is the correlation coefficient?

Tells you if there is a statistically significant correlation between two measured variables x and y and if the correlation is negative or positive


What is the calculation for correlation coefficient?

See calculation


Example of correlation coefficient

1. Formulate null hypothesis
- There is no significant correlation between arm length and height

2. Calculate results
X Y x² y² xy
1 167 90 27889 8100 15030
2 152 81 23104 6561 12312
3 193 107 37249 11449 20651
4 184 102 33856 10404 18768
5 177 98 31329 9604 17346
6 151 83 22801 6889 12533
x = 1024 y = 561 x² = 176228 y² = 53007 xy = 96640

96640 – (1024 x 561) / 6
(176228 – ((1024)² / 6)) (53007 – ((561)² / 6)

= 0.995

Critical value = 0.75
REJECT null hypothesis so there is a significant correlation between arm length and height, not due to chance

Negative r value = negative correlation
Positive r value = positive correlation
R will always be number between -1 and +1
If value for r exceeds critical value, reject null hypothesis