4: GENETICS - NATURAL SELECTION

Question 1

Why does natural selection occur?

Answer 1

• when a mutation results in a beneficial allele for a characteristic that increases the chance of an organism surviving, its frequency in the population increases
• the process is known as natural selection

Question 2

Why does natural selection not always occur when a new allele is produced by a mutation?

Answer 2

some mutations result in alleles that can be harmful which usually means that the mutated allele quickly dies out

Question 3

Describe the process of natural selection (5 marks)

Answer 3

• random mutation result in new alleles in population
• variation within the population
• differential reproductive success in population / individuals with an advantageous allele which increases chance of survival are more
  likely to survive, reproduce and pass on their genes
• advantageous allele inherited by next generation
• increase in frequency of the beneficial allele .

Question 4

What is evolution?

Answer 4

the gradual change in species over time

Question 5

What is an adaptation?

Answer 5

a feature that can help an organism to survive in its environment

Question 6

What are the types of adaptation?

Answer 6

• behavioural adaptations
• physiological adaptations
• anatomical (structural) adaptations

Question 7

What are behavioural adaptations?

Answer 7

ways in which an organism acts that increase its chance of survival

Question 8

Give an example of a behavioural adaptation

Answer 8

possums “play dead” if they’re being threatened by a predator to escape attack

Question 9

What are physiological adaptations?

Answer 9

processes in an organisms body that increase its chance of survival

Question 10

Give an example of a physiological adaptation

Answer 10

• brown bears hibernate
• they lower their metabolic rate over winter which conserves energy so they don’t have to look for food in months when its scarce

Question 11

What are anatomical adaptations?

Answer 11

structural features of an organism that increase its chance of survival

Question 12

Give an example of an anatomical adaptation

Answer 12

otters gave a streamlined shape, making it easier to catch prey and escape predators as they can glide through water easily

Question 13

What are the 2 types of selection?

Answer 13

• directional selection
• stabilising selection

Question 14

What is directional selection?

Answer 14

• when individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce
• could be in response to an environmental change

Question 15

What is stabilising selection?

Answer 15

• when individuals with alleles for characteristics in the middle of the range are more likely to survive and reproduce
• decreases range of possible characteristics

Question 16

Compare and contrast directional selection and stabilizing selection (5 marks)

Answer 16

• both type of natural selection resulting in increase in frequency of beneficial allele / individuals in population well adapted to their environment
• directional in response to environmental change whereas stabilising is not
• variation/ range of alleles same in directional whereas variation/ range of alleles decreases in stabilising
• mean changes/ shifts in directional whereas mean stays the same in stabilising
• directional selection against an extreme, favours other extreme whereas stabilising selection is against both extremes

Question 17

Explain why bacteria evolve bacterial resistance as an example if directional selection

Answer 17

• some individuals have alleles that give them resistance to an antibiotic
• the population is exposed to the antibiotic, killing the bacteria without the resistance allele
• resistant bacteria survive and reproduce without competition, passing on the resistance allele to their offspring
• eventually most organisms in the population will carry the antibiotic resistance allele

Question 18

Explain why human birth weight is usually a medium weight as an example of stabilising selection

Answer 18

• very small babies are less likely to survive as they have a high SA:V ratio, which makes it hard to maintain body temp, putting pressure on their respiratory and cardiac systems which can be fatal
• very large babies are also less likely to survive as it can be difficult for them to fit through the mother’s pelvis which can lead to complications for both mother and child
• conditions are most favourable for medium sized babies so birth weight tends to shift to the middle of the range