18.3 - Natural Selection

Question 1

What are selection pressures

Answer 1

The environmental factors that limit the population of a species

Question 2

Describe what selection pressures are

Answer 2

• they include predation, disease, competition…
• Selection pressures vary from time to time and place to place.
• These selection pressures determine the frequency of all alleles within the gene pool.

Question 3

What is the gene pool

Answer 3

The total number of all the alleles of all the genes of all the individuals within a particular population at a given time

Question 4

What factors does the process of evolution by means of natural selection depend upon

Answer 4

1) organisms produce more offspring than can be supported by the available supply of food, light, space, ete.
2) there is genetic variety within the populations of all species.
3) a variety of phenotypes that selection operates against.

Question 5

What did Charles Darwin conclude about population sizes increasing/decreasing

Answer 5

• population rarely increase in size at fast rates
• high reproductive rates have evolved in many species to ensure a sufficiently large population survives to breed and reproduce the next generation
• in order to compensate for high death rates from predation, competition, etc…
• or
• Low reproductive rates with high degrees of parental care
• so that the lower death rate help maintain their population size

Question 6

Describe the link between over-production and natural selection

Answer 6

• where there’s too many offspring for the available resources, there’s competition amongst individuals (intraspecific competition) for the limited resources
• the greater the number of = the greater the competition = greater the struggle to survive
• individuals best suited to prevailing = more likely to survive than those less well adapted
• they’re more likely to breed = pass on their more favourable allele combinations to next generation = next generation has a different allelic frequency compared to the last one
• the new population will have the evolved combination of alleles that are better for survival
  ——> This selection process DEPENDS ON individuals of a population being genetically different from one another

Question 7

If an organism can survive in the conditions in which it lives, WHY doesn’t it always produce offspring that are identical to itself ?

Answer 7

• conditions change over time and having a wide range of genetically different (and therefore phenotypes)
  n the population means that some will have the combination of genes needed to survive in almost any new set of circumstances.
• Populations showing little individual genetic variation are often more vulnerable to new diseases and climate changes.
• It is also important that a species is capable of adapting to changes resulting from the evolution of other species.

Question 8

Why is it important that population are large and not identical

Answer 8

• 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 alleles that lead to a phenotype which is advantageous in the struggle for survival.
• These individuals will therefore be more likely to breed and pass their allele combinations on to future generations.
• Variation therefore provides the potential for a population to evolve and adapt to new circumstances.

Question 9

State four factors that lead to differential survival and reproduction

Answer 9

1) predation
2) competition for food/water/space
3) disease
4) natural disasters

Question 10

Sickle cell anaemia is a debilitating genetic disease that causes premature death but provides some resistance to the malarial parasite. Explain how selection might affect the distribution of the gene causing sickle cell anaemia in both malarial and non-malarial regions

Answer 10

• in Malarial regions, the Disadvantage of having the disease will be offset by the advantages of having resistance to malaria and so there will be little if any selection against the gene and its frequency will be relatively high
• in non-malarial regions there’s no advantage in having resistance to malaria and so individuals with sickle cell anaemia will be at a disadvantage; they will be selected against and the frequency of the gene will be low