7. Genetics, Popultions, Evolution & Ecosystems

Question 1

Define phenotype

Answer 1

The observable or biochemical characteristics of an organism.
It’s the result of the interaction between the expression of the genotype and the environment

Question 2

Define genotype

Answer 2

The genetic constitution of an organism. Describes all the alleles that an organism has

Question 3

What is monohybrid inheritance? give an example

Answer 3

the inheritance of a single gene
example: Gregor Mendel’s study of colour of pea plants

Question 4

What does the law of segregation state?

Answer 4

Only one of each pair of alleles can be present in a single gamete

Question 5

Why are actual results of genetic crosses rarely the same as the predicted results?
What should you do to have more representative results?

Answer 5

Statistical error
The larger the sample the more likely the actual results are to come near to matching the theoretical ones

Question 6

What is dihybrid inheritance?
Give an example

Answer 6

The way in which 2 characters, determined by 2 different genes located on different chromosomes are inherited
example: Gregor Mendel’s investigation of two characters of a pea plant (seed shape and seed colour)

Question 7

What does the law of independent assortment state?

Answer 7

Each member of a pair of alleles may combine randomly with either of the other pair

Question 8

What is codominance and when does it occur?

Answer 8

both alleles are expressed in the phenotype
occurs when both alleles are equally dominant

Question 9

How many pairs of chromosomes do humans have?

Answer 9

23

Question 10

What are the sex chromosomes in humans?

Answer 10

X and Y
females : XX
males : XY

Question 11

What is significant about the X chromosome being much longer than the Y chromosome ?

Answer 11

For most of the length of the X chromosome, there is no equivalent homologous portion of the Y chromosome.
Therefore the characteristics that are controlled by recessive alleles on this non-homologous portion of the X chromosome will appear more frequently in the male as there is no homologous portion on the Y chromosome that might have the dominant allele

Question 12

Give an example of an X linked disorder (disorder caused by a defective gene on the x chromosome)

Answer 12

Haemophilia (blood clots slowly, may be slow and persistent internal bleeding)
Potentially lethal if not treated
Almost entirely confined to males in part because haemophiliac females usually died with the onset of menstruation at puberty

Question 13

How is haemophilia caused ?

Answer 13

Recessive allele with an altered sequence of DNA nucleotide bases that therefore codes for a faulty protein with no function.

Question 14

Any 2 genes that occur on the same chromosome are said to be…

Answer 14

LINKED

Question 15

The remaining 22 chromosomes which are not sex chromosomes, are called…

Answer 15

AUTOSOMES

Question 16

What is autosomal linkage?

Answer 16

The name given to the situation where 2 or more genes are carried on the same autosome

Question 17

When does epistasis occur?

Answer 17

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

Question 18

Define gene pool

Answer 18

The sum of all the alleles of all the genes of all the individuals in a population at a given time

Question 19

Define population

Answer 19

A group of organisms of the same species that occupies a particular space at a particular time and that can potentially interbreed

Question 20

Define allelic frequency

Answer 20

The number of times an allele occurs within the gene pool

Question 21

What assumption does the Hardy-Weinberg principle make

Answer 21

That the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to the next

Question 22

The assumption made in the Hardy-Weinberg equation can be the case so long as which 5 conditions are met?

Answer 22

• No mutations arise
• The population is isolated
• There is no selection (all alleles are equally likely to be passed on)
• The population is large
• Mating within the population is random

Question 23

When is the Hardy-Weinberg principle particularly useful?

Answer 23

when studying gene frequencies

Question 24

What are the equations used in the Hardy-Weinberg principle when we are considering a gene that has 2 alleles (A and a)

Answer 24

Let the probability of A = p
Let the probability of a = q
p+q=1
As there are only 4 possible arrangements of the 2 alleles it follows that the probability of all four added together must also equal 1 ;
AA + Aa + aA + aa = 1
or expressed as a probability …
p2 + 2pq + q2 = 1

Question 25

Genetic variation arises as a result of what 3 factors?

Answer 25

• mutations
• meiosis
• random fertilisation of gametes

Question 26

Name environmental factors that influence variation

Answer 26

• temp
• rainfall
• sunlight
• soil pH
• food availability

Question 27

The environmental factors that limit the population of a species are called…

Answer 27

selection pressures

Question 28

What is included under the term ‘selection pressures’?

Answer 28

• predation
• competition
• disease

Question 29

The process of evolution by means of natural selection is dependent on which factors?

Answer 29

• organisms produce more offspring than can be supported by the available resources
• there is genetic variety within the populations of all species
• a variety of phenotypes that selection operates against

Question 30

What is the link between over-production of offspring and natural selection?

Answer 30

Where there are too many offspring for the available resources, there is intraspecific competition amongst individuals for the limited number of resources available.
Those who die however is not totally random; the individuals who are best suited to prevailing conditions are more likely to survive and reproduce passing on more favourable allele combinations resulting overtime with a population better adapted to prevailing conditions

Question 31

Describe and explain the role of variation in natural selection

Answer 31

The larger a population is and the more genetically varied the individuals within it, the greater the chance that one or more individuals will have the combination of phenotypes that is advantageous for survival.
These individuals will therefore be more likely to breed and pass on their allele combinations to future generations.
Therefore, variation provides the potential for a population to evolve and adapt to new circumstances

Question 32

What are the 3 main types of selection that affect the characteristics of a population ?

Answer 32

• Stabilising selection
• Directional selection
• Disruptive selection

Question 33

How does stabilising selection affect the characteristics of a population ?

Answer 33

Preserves the average phenotype of a population by favouring average individuals.
Selection is against the extreme phenotypes

Question 34

How does directional selection affect the characteristics of a population

Answer 34

Changes the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population
Selection for one extreme phenotype

Question 35

How does disruptive selection affect the characteristics of a population ?

Answer 35

Favours individuals with extreme phenotypes rather than those with phenotypes around the mean of the population
Most important for bringing about evolutionary change

Question 36

When does stabilising selection often occur

Answer 36

Where the environmental conditions are constant over long periods of time
Graph changes to a narrower normal distribution curve

Question 37

When does disruptive selection occur

Answer 37

When an environmental factor takes 2 or more distinct forms
Graph changes from normal distribution curve to an M shape

Question 38

When does directional selection occur

Answer 38

If the environmental conditions change so will the optimum value for survival. As a result there is a selection pressure favouring the combination of alleles that result in the mean shift to the left or the right of its original value .
Graph changes to either the left or right

Question 39

Give an example of directional selection

Answer 39

Selection in peppered moths
Birds favoured individuals that lie at one extreme of different colour types. The melanic form is selected for in industrial areas as it is less visible against the soot covered buildings whilst the natural form is selected for in natural areas

Question 40

What is evolution by natural selection in short?

Answer 40

A change in the allelic frequencies within a population

Question 41

Define speciation

Answer 41

The evolution of new species from existing ones

Question 42

What is by far the most important way in which new species are formed

Answer 42

Reproductive separation followed by a genetic change due to natural selection

Question 43

What is adaptive radiation

Answer 43

The way that the different phenotypes that each combination of allele produces will be subject to selection pressure that will lead to each population becoming adapted to its local environment.
As a result of these genetic differences, it may be that even if the population were no longer physically separated from one another, they would be unable to interbreed successfully

Question 44

Why are the effects of genetic drift greater in smaller populations

Answer 44

Less genetic diversity as there a smaller variety of alleles.
As a result, pop. changes relatively rapidly (more likely to develop into separate species) as a mutation to one of these alleles that is selectively favoured will more quickly affect the whole population.
In large populations the effect of a mutant allele is diluted due to its frequency being far less in the much larger gene pool. (Development into a new species is slower)

Question 45

What are the two forms of speciation

Answer 45

Allopatric and Sympatric

Question 46

What is allopatric speciation

Answer 46

Speciation that occurs when biological populations of the same species become isolated due to geographical changes.
This geographical separation may prevent them from interbreeding .
If environmental conditions either side of the barrier vary, then natural selection will influence the 2 populations differently and each will evolve adaptations local to their communities

Question 47

What is sympatric speciation

Answer 47

Describes the form of speciation that results within a population in the same area leading them to become reproductively isolated

Question 48

What does carrying capacity mean?

Answer 48

The certain size population of a species that an ecosystem can support

Question 49

Define community

Answer 49

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

Question 50

What is a habitat

Answer 50

The place where an organism normally lives
Characterised by physical conditions & other types of organisms present

Question 51

What is a niche

Answer 51

Describes how an organism fits into the environment (what it’s role is and how it is adapted for that role)

Question 52

What is the competitive exclusion principle

Answer 52

States that if 2 competing species coexist they must have differentiated niches
If there is no difference between niches of competing organisms, one species will be eliminated or excluded

Question 53

When is it not possible to plot a pop. growth curve

Answer 53

Where a population grows rapidly over a short period of time (eg. microorganisms)
In these cases a logarithmic scale can be used to represent the data

Question 54

Why can no population grow indefinitely

Answer 54

Limiting factors such as food availability, light, water, shelter, disease and predators

Question 55

What are the abiotic factors that influence the size of a population

Answer 55

Temperature
Light
pH
Water & humidity

Question 56

What are the two types of competition and what differs in short between the 2?

Answer 56

Intraspecific competition: when individuals of the same species compete for resources
Interspecific competition: when individuals of different species compete for resources

Question 57

Explain the effect of predator-prey relationship on population size.

Answer 57

• predators eat their prey, reducing the pop. of prey
• predators are in greater competition due to fewer prey available
• predator population is reduced as some are unable to obtain enough prey for survival & reproduction
• with fewer predators left, fewer prey are eaten and so more survive & reproduce
• prey pop. increases
• in turn predator pop. increases

Question 58

Give reasons for cyclic fluctuations in populations

Answer 58

Predator prey relationships
Disease
Climactic factors

Question 59

Give 2 sampling techniques used in the study of habitats

Answer 59

• random sampling using frame quadrats or point quadrats
• systematic sampling along a belt transect

Question 60

What is a point quadrat ?

Answer 60

Consists of a horizontal bar supported by 2 legs .
At set intervals along the bar there are 10 holes through which a pin may be dropped.
Each species that touches the pin is recorded

Question 61

What is a frame quadrat

Answer 61

Consists of a square frame divided by string or wire into equally sized subdivisions.
Placed in different locations within the area being studied
The abundance of each species within the quadrat is recorded

Question 62

What are the 3 factors that should be considered when using quadrats

Answer 62

• size of the quadrat to use : depends on size of organisms counted and their distribution in the area
• the number of sample quadrats to record within the study : larger number = more reliable results
• the position of each quadrat within the study area : random sampling must be used to produce statistically significant results

Question 63

Give a method for random sampling to avoid bias and ensure data obtained is reliable

Answer 63

1. Lay out two long tape measures at right angles along 2 sides of the study area
2. Obtain a series of coordinates by using random numbers generated by a computer
3. Place a quadrat at the intersection of each pair of coordinates and record the species within it

Question 64

Give the calculation to determine the size of a population using mark-release techniques

Answer 64

. total number of individuals in 1st sample x total in 2nd sample
Estimated pop. size = —————————————————————————————-
number of marked individuals recaptured

Question 65

What are the common features that emerge during any succession

Answer 65

• the abiotic environment becomes less hostile
• a greater number & variety of habitats and niches that in turn produce
  -increased biodiversity
  -more complex food webs
  -increased biomass

Question 66

What is a climax community

Answer 66

Communities that are in a stable equilibrium with the prevailing climate

Question 67

What is the role of a pioneer species

Answer 67

Colonisation of an inhospitable environment.

Question 68

What are some of the possible features a pioneer species may have to suit them to colonisation ?

Answer 68

Asexual reproduction
Ability to photosynthesise
Tolerance to extreme conditions
Ability to fix nitrogen from atmosphere
Rapid germination of seeds on arrival
Production of vast quantities of wind-dispersed seeds

Question 69

When does secondary succession occur

Answer 69

The process by which an ecosystem returns to its climax community when land that has already sustained life is suddenly altered.
Usually occurs much more rapidly

Question 70

What is conservation

Answer 70

The management of the earths natural resources by humans in such a way that maximum use of them can be made in the future

Question 71

What are the main reasons for conservation

Answer 71

• Personal
• ethical
• economic
• cultural and aesthetic

Question 72

When should frequency/ percentage cover be used individually ?

Answer 72

Frequency- useful where a species if hard to count, gives quick ideas of the species present and their general distribution within an area.
Does not provide info on density and detailed distribution of the species
Percentage cover - useful where a species is particularly abundant/ difficult to count. Data can be collected rapidly however is less useful where organisms occurs in several overlapping layers