Genetic Information and Variation - Evolution and Natural Selection

Question 1

What leads to genetic diversity?

Answer 1

DNA determines the considerable variety of proteins that make up each organism. Therefore genetic similarities and differences between organisms may be defined in terms of variation in DNA. Hence it is differences in DNA that lead to the vast genetic diversity we find on Earth.

Question 2

What are genes and alleles?

Answer 2

A section of DNA that codes for one polypeptide is called a gene. All members of the same species have the same genes. But they may differ in alleles, which are different versions of the same gene. As such, an allele is also a length of DNA on one chromosome of a homologous pair.

Question 3

What is genetic diversity?

Answer 3

Genetic diversity is described as the total number of different alleles in a population.

Question 4

What is a population?

Answer 4

A population is a group of individuals of the same species that live in the same place and can be interbreed (breed with one another).

Question 5

What is a species?

Answer 5

A species consists of one or more populations.

Question 6

What are the benefits of genetic diversity?

Answer 6

The greater the number of different alleles that all members of a species possess, the greater the genetic diversity of that species. Genetic diversity is reduced when a species has fewer different alleles.

The greater the genetic diversity, the more likely that some individuals in a population will survive an environmental change. This is because of a wider range of alleles and therefore a wider range of characteristics. This gives a greater probability that some individuals will possess a characteristic that suits it to the new environmental conditions. Genetic diversity is a factor that enables natural selection to occur.

Question 7

How does natural selection lead to the evolution of populations?

Answer 7

Not all alleles of a population are equally likely to be passed to the next generation. This is because only certain individuals are reproductively successful and so pass on their alleles.

Question 8

How do differences between the reproductive success of individuals affect allele frequency in populations?

Answer 8

• Within any population of a species there will be a gene pool containing a wide variety of alleles.
• Random mutation of alleles within this gene pool may result in a new allele of a gene which in most cases will be harmful.
• However, in certain environments, the new allele of a gene might give its possessor an advantage over other individuals in the population.
• These individuals will be better adapted and therefore more likely to survive in their competition with others.
• These individuals are more likely to obtain the available resources and so grow more rapidly and live longer. As a result, they will have a better chance of breeding successfully and producing more offspring.
• Only those individuals that reproduce successfully will pass on their alleles to the next generation.
• Therefore it is the new allele that gave the parents an advantage in the competition for survival that is most likely to be passed on to the next generation.
• As these new individuals also have the new, ‘advantageous’ allele, they in turn are more likely to survive, and so reproduce successfully.
• Over many generations, the number of individuals with the new, ‘advantageous’ allele will increase at the expense of the individuals with the ‘less advantageous’ alleles.
• Over time, the frequency of the new, ‘advantageous’ allele in the population increased while that of the ‘non-advantageous’ ones decreases.

Question 9

What does being an advantageous allele depend on?

Answer 9

environmental conditions

Question 10

What is a gene pool?

Answer 10

The total number of alleles and their frequencies is called the population’s gene pool. The frequency and variety of alleles determines the amount of genetic variation in a species. If the frequencies of alleles in the gene pool changes, then the species has changed.

Question 11

What is evolution?

Answer 11

a change in a species

Question 12

What are the three causes of genetic variation?

Answer 12

1. sexual reproduction and meiosis
2. natural selection (survival of the fittest)
3. a random mutation of a gene
4. genetic bottleneck (an infrequent event that has a big change on the population)
5. founder effect (small group of individuals migrate and start a new isolated gene pool)
6. artificial selection (selective breeding)

Question 13

What is the niche of a species?

Answer 13

The niche of a species is its role within the environment. Species which share the same niche compete with each other. The idea that better adapted species survive is the basis of natural selection.

Question 14

Is fertilisation random?

Answer 14

not completely because not all gametes can fertilise

Question 15

What is selection?

Answer 15

Selection is the process by which organisms that are better adapted to their environment tend to survive and breed, while those that are less well adapted tend not to.

Question 16

What is selection based on?

Answer 16

Every organism is subjected to a process of selection, based on its suitability for surviving the conditions that exist at the time.

Different environmental conditions favour different characteristics in the population. Depending on which characteristics are favoured, selection will produce a number of different results.

Question 17

What are the two types of selection?

Answer 17

• Directional Selection
  Selection may favour individuals that vary in one direction from the mean of the population. This is called directional selection and changes the characteristics of the population.
• Stabilising Selection
  Selection may favour average individuals. This is called stabilising selection and preserves the characteristics of a population.

Question 18

What genes are most characteristics influenced by?

Answer 18

Most characteristics are influenced by more than one gene (polygenes). These types of characteristics are more influenced by the environment than ones determined by a single gene. The effect of the environment on polygenes produces individuals in a population that vary about the mean. When you plot this variation on a graph, you get a normal distribution curve.

Question 19

What is directional selection?

Answer 19

If the environmental conditions change, the phenotypes that are best suited to the new conditions are most likely to survive. Some individuals, which fall to either the left or right of the mean, will possess a phenotype more suited to the new conditions. These individuals will be more likely to survive and breed. They will therefore contribute more offspring (and the alleles these offspring possess) to the next generation than other individuals. Over time, the mean will then move in the direction of these individuals.

Question 20

How did bacteria develop antibiotic resistance?

Answer 20

Shortly after the discovery of antibiotics it became apparent that the effectiveness of some antibiotics at killing bacteria had developed resistance to antibiotics such as penicillin. The resistance was not due to the development of tolerance to the antibiotic, but rather a chance mutation within the bacteria.

Question 21

How is resistance to penicillin an example of directional selection?

Answer 21

• A spontaneous mutation occurred in the allele of a gene in a bacterium that enabled it to make a new protein. The new protein was an enzyme that broke down the antibiotic penicillin before it was able to kill the bacterium. The enzyme was given the name penicillinase.
• The bacterium happened by chance to be in a situation where penicillin was being used to treat an individual. In these circumstances, the mutation gave the bacterium an advantage in being able to use penicillinase to break down the antibiotic and so survive while the rest of the population of bacteria were killed by it.
• The bacterium that survived was able to divide by binary fission to build up a small population of penicillin-resistant bacteria.
• Members of this small penicillin-resistant bacteria increased at the expense of the non-resistant population. Consequently the frequency of the allele that enabled the production of penicillinase increased in the population.
• The population’s normal distribution curve shifted in the direction of a population having greater resistance to penicillin.

Question 22

What causes resistance?

Answer 22

Remember that resistance is the result of a chance mutation. These are rare events. If they were common events it is unlikely that any antibiotics would still be effective in treating disease.

New mutations that give bacteria resistance to antibiotics arise randomly all the time. However, the more we use antibiotics, the greater the chance that the mutant bacterium will gain an advantage over the normal variety.

Question 23

Why do bacteria mutate?

Answer 23

Bacteria do not mutate because of the presence of antibiotics. Mutations occur randomly and are very rare. However as there are so many bacteria around, the total number of mutations is large. Many of these mutations will be of no advantage to a bacterium. Indeed most will be harmful, in which case the bacterium will probably die. Very occasionally a mutation will be advantageous. Even then it depends upon the circumstances.

Question 24

What does directional selection result in?

Answer 24

Directional selection therefore results in phenotypes at one extreme of the population being selected for and those at the other extreme being selected against.

Question 25

What is stabilising selection?

Answer 25

If environmental conditions remain stable, it is the individuals with phenotypes closest to the mean that are favoured. These individuals are more likely to pass their alleles on to the next generation. Those individuals with phenotypes at the extremes are less likely to pass on their alleles. Stabilising selection therefore tends to eliminate the phenotypes at the extremes.

Question 26

How are human birth weights an example of stabilising selection?

Answer 26

• Stabilising selection results in phenotypes around the mean of the population being selected for and those at both extremes being selected against.
• Body mass of babies at birth is within a relatively narrow range. The likely explanation for this can be found by looking at infant mortality. There is a much greater risk of infant death when the birth weight is outside the mean range.
• This illustrates stabilising selection because the mortality rate is greater at the two extremes. The infants with the highest and lowest birth masses are more likely to die (are being selected against) while those around the mean are less likely to die (are being selected for/favoured).
• The population’s characteristics are being preserved rather than changed.

Question 27

What happens if the environmental change is great enough?

Answer 27

There may be no phenotype suited to the new conditions, in which case the population will die out.

Question 28

What does stabilising selection result in?

Answer 28

Stabilising selection results in phenotypes around the mean of the population being selected for and those at both extremes being selected against.

Question 29

What does natural selection result in?

Answer 29

Natural selection results in species that are better adapted to the environment they live in.

Question 30

What adaptations can give you a survival advantage?

Answer 30

• Anatomical: (physical adaptations, either external or internal), such as shorter ears and thicker fur in arctic foxes compared to foxes in warmer climates, or the length of the Loop of Henle, which is longer in desert animals to produce concentrated urine and minimise water loss.
• Physiological: (processes inside an organism’s body) for example oxidising of fat rather than carbohydrate in kangaroo rats to produce additional water in a dry desert environment.
• Behavioural: (changes in behaviour) such as the autumn migration of swallows from the UK to Africa to avoid food shortages in the UK winter.

Question 31

What is interspecific and intraspecific variation?

Answer 31

Interspecific: variation between individuals of different species.

Intraspecific: variation between individuals of the same species.

Question 32

Why must you take the full course of antibiotics?

Answer 32

Bacteria have variations in antibiotic resistance. Some are easier to kill, while others take longer. If you don’t finish the course and kill them all, the more resistant bacteria will reproduce and it will be even harder to destroy them.