Investigating Selection

Question 1

What are the types of natural selection and what do they do?

Answer 1

• Stabilising selection
• Directional selection

They affect allele frequency in different ways

Question 2

What is directional selection?

Answer 2

Individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce

Question 3

What could be the cause of directional selection?

Answer 3

Response to environmental change

Question 4

What is the example used in exams for directional selection?

Answer 4

Bacteria evolving into antibiotic resistance

Question 5

Diagram of Directional Selection before and after

Answer 5

Question 6

Explain this diagram:

Answer 6

• Some individuals in population have alleles giving resistance to antibiotic
• Population exposed to antibiotic, killing bacteria without resistant allele
• Resistant bacteria survive and reproduce without competition , passing on allele to give antibiotic resistance to offspring
• After some time, most organisms in population carry antibiotic resistance allele

Question 7

What is stabilizing selection?

Answer 7

• Individuals with alleles for characteristics towards the range are more likely to survive and reproduce

Question 8

When does stabilising selection occur?

Answer 8

When the environment isn’t changing

Reduces range of possible characteristics

Individuals with extreme characteristics are less likely to survive and reproduce

Question 9

Diagram of Stabilising Selection

Answer 9

Question 10

Explain this graph:

Answer 10

• Humans have a range of birth weights
• Very small babies are less likely to survive - partly it is hard to maintain their body temperature - Less SA: VOL raito
• Giving birth to large babies are difficult - less likely to survive
• Conditions are most favourable for middle-sized babies - weight of human babies tend to shift towards middle of range
• Mean remains same

Question 11

Describe what the data shows:

Answer 11

Over the first two years the average fur length is about 21 mm

However, average length gradually increases from 21mm to 24mm

Shows directional selection

Question 12

Suggest a possible cause for this data:

Answer 12

Rabbits with longer fur are more likely to survive the cold conditions than short-furred rabits

More likely to reproduce and when they do pass on allele for longer fur

Over time, the allele for the longest fur becomes more common and population of average fur length of rabbit increases

Question 13

Method of effects of antibiotics using agar plates

Answer 13

• Sterile pipette to evenly distribute the bacteria from broth to agar plate - lawn spreading
• Use sterile forceps to place paper discs to place paper discs soaked with different antibiotic spaced apart
• Lightly tape lid and incubate at 25 degree for 48 hours - form a lawn

Question 14

What is an agar plate?

Answer 14

Petri dish containing agar jelly

Question 15

Why should you let it incubate at 25 degrees for 48 hours?

Answer 15

Let the bacteria grow to form a ‘lawn’

Question 16

What does the size of inhibition zone tell you?

Answer 16

How well the antibiotic works

Larger the zone , more bacteria inhibited from growing

Question 17

What to use as a control?

Answer 17

Negative control disc only soaked in sterile water

Question 18

Why should you not incubate above 25 degrees?

Answer 18

Not 37 degrees

could encourage growth of pathogens - microorganisms

Question 19

Do you repeat test?

Answer 19

Yes

Question 20

How to repeat practically?

Answer 20

Use different compounds/concentration

Question 21

What happens if the zone of inhibition is not perfectly round?

Answer 21

• If not round use mean diameter

Question 22

How does antibiotic cause zone inhibition?

Answer 22

Antimicrobial compound diffuses out of the disc and inhibits the growth of bacterium

Question 23

What are aseptic techniques?

Answer 23

• Lab processes/procedures and performed in sterile conditions to prevent contamination that can affect the growth of microorganism you are working in

Question 24

Why is it important to carry out aseptic techniques?

Answer 24

Avoid contamination with disease-causing microbes that make you ill

Question 25

Examples of aseptic techniques

Answer 25

• Regularly disinfect the work area/bench
• Discard contaminated utensils
• Work near a Bunsen flame
• Minmise time spent with lid off
• Birefly flame neck of the glass containor of broth/bottles
• Replace lids
• Don’t put bottles on bench
• Flame the incoluating loop after transferring microorganisms

Question 26

Why should you regularly disinfect work surfaces?

Answer 26

To minimize contamination

Question 27

Why should contaminated utensils place carefully?

Answer 27

Don’t put utensils on work surface

They should put in a beaker of disinfectant

Question 28

How do you sterilise equipment

Answer 28

• Glass can be sterilised before and after in autoclave (steam at high pressure)
• Pre-sterilised plastic instruments used once and discarded

Question 29

Why should you work next to a Bunsen burner?

Answer 29

Hot air rises so any microbes in air drawn away from culture

Question 30

Why should you minimise the time spent with the lid off the agar plate ?

Answer 30

Reduce the chance of airborne microorganisms contaminating the culture

Question 31

Why should you briefly flame glass container of broth/bottles?

Answer 31

Just after it is opened/before its closed

Cause air to move out of container , prevent unwanted organisms from falling in

Question 32

Should you wash hands before and after handling culture?

Answer 32

Yes

Question 33

A group of scientists monitored the colour of oyester shells on beach changed over time.

Graph shows the colour of oyster shells in scientists inital sample and in their final sample

Oysters were mainly found in the sand which was mid-brown colour

a) What type of selection , explain

Answer 33

Example of stabilising selection

inital :shows a fairly wide range of shell colours from light to dark

Over time, the average colour shifted towards the middle of range

More oyesters have a mid-range coloured shell in final than inital

Question 34

A group of scientists monitored the colour of oyester shells on beach changed over time.

Graph shows the colour of oyster shells in scientists inital sample and in their final sample

Oysters were mainly found in the sand which was mid-brown colour

a) Suggest the changes shown in the graph might have taken place

Answer 34

Oyesters at extreme of light and dark are less likely to survive because they are easily seen by predators against the sand

Means more mid-range coloured oysters have an advantage to survive and reproduce

Advantegous allele for this are passed onto next generation leading to increase in population

Question 35

What is selection?

Answer 35

Selection is the process by which organisms that are better adapted to their environment survive and breed,

while those less well adapted fail to do so

Question 36

Distinguish between stabilizing and directional selection

Answer 36

• DS: Favours phenotype at one extreme of a population
• SS: Selects phenotypes around the mean of population
• DS: Change the characteristics of poulation
• SS: Preserves characteristic of population
• Graph change

Question 37

Three examples of antimicrobial substances

Answer 37

1. Antibiotics
2. Antiseptics
3. Disinfectants

Question 38

What is present in a bacterial broth?

Answer 38

Distilled water

Bacterial culture

Nutrients

Question 39

What is zone of inhibition?

Answer 39

A clear patch in lawn of bacteria where bacteria can’t grow

Question 40

How to sterilise a insulated loop before and after use

Answer 40

Pass through Bunsen burner flame for 5 seconds