Selection and speciation (BIOL4)

Question 1

Features of allele frequency

Answer 1

• Freq. within population changes when allele codes for characteristics affects organism’s survival
• Individual w/ allele that inc. survival more likely to survive, reproduce, pass on genes than others w/ diff genes
• Thus greater prop. of next gen. inherits beneficial allele Freq. of beneficial allele inc. from gen. to gen.
• This is natural selection

Question 2

What are the different types of natural selection?

Answer 2

Stabilising selection

Directional selection

Question 3

Features of stabilising selection

Answer 3

• Individuals w/ alleles for characteristics towards mid-range likelier to survive/reproduce
• Occurs when environment isn’t changing - reduces range of possible phenotypes

Question 4

Features of directional selection

Answer 4

• Individuals w/ alleles for characteristic of an extreme type (far right) likelier to survive/reproduce
• Response to environmental change
• Example - faster likely to survive, freq. allele for fast inc., population becomes faster

Question 5

Define speciation

Answer 5

Development of a new species from evolution

Question 6

Features of speciation

Answer 6

• Caused by geographical isolation
• Populations isolated by extrinsic barrier and population becomes reproductively isolated
• Populations geographically isolated experience diff conditions; experience diff selective pressures
• Different alleles are beneficial in different populations
  
  • Directional selection, changes allele freq.
• Allele freq. also result of mutations, independently
• Changes in allele freq. lead to different gene pools and phenotypes, such that if barrier b/d, population can’t interbreed

Question 7

Define allele frequency

Answer 7

How often allele occurs in population

Question 8

Hardy-Weinberg principle predicts:

What feature…

What happens to it…

Providing that there’s…

Answer 8

• What feature:
  
  • frequency of alleles
• What happens to it:
  
  • will stay constant from one generation to the next
• Providing that there’s…
  
  • no mutation
  • no selection
  • large population/no migration
  • mating random

Question 9

How is the Hardy-Weinberg principle used to predict allele frequency?

Answer 9

P + q = 1

p = freq of dominant allele

q = freq of recessive allele

Example - R (dominant), r (recessive) –> R freq. = 0.4 –> r freq. = 1 - 0.4 = 0.6

Question 10

How is the Hardy-Weinberg principle used to predict genotype frequency?

Answer 10

p2 + 2pq + q2 = 1

Freq of individuals genotypes must add up to 1

p2 = freq of homozygous dominant genotype

2pq = freq of heterozygous genotype

q2 = freq of homozygous recessive genotype

Example - 3 possible genotypes = RR, Rr and rr p2 = RR = 0.34 2pq = Rr = 0.27 —> rr (q2) =1 - 0.34 - 0.27 = 0.37

Question 11

How is the Hardy-Weinberg principle used to predict the percentage of population with a genotype?

Answer 11

Example - frequency of CF (ff) is 1/2000

Frequency of cystic fibrosis (ff): ff = q2 = 1 / 2000 = 0.0005 —-> f = q = √0.0005 = 0.022

Frequency of dominant allele: F = p = 1 -0.022 = 0.978

Frequency of genotype Ff (carriers): Ff = 2pq = 2 x 0.978 x 0.022 = 0.043 = 4.3% are carriers

Question 12

How does Hardy-Weinberg Principle show that external factors are affecting allele frequency?

Answer 12

Example - frequency of CF 50 years later is 1/3000

Calculate q = ff= q2= 1/3000 = 0.000333 q = √0.00033 = 0.018

Thus, principe doesn’t apply, so there must have been some factors affecting allele frequency e.g migration, mutations