polymorphisms

Question 1

heterogeneity in environment ?

Answer 1

• different selection in different years or habitats
• ecocline = altitude, gradient
• topocline = climate, latitude

Question 2

heterozygot superiority; overdominance?

Answer 2

• balancing selection = 2 or more selction pressures that maintain polymorphisms
• eg. sickle cell anaemia - mutants have advantage against malaria as haemoglobin shape changes but reduced affinity for oxygen
• eg. cyanogenesis in white clover

Question 3

negative frequency dependant selection ?

Answer 3

• selection coefficients aren’t constant
• selection favours rare types (increase in frequency)
• eg. plants do better amongst plants of different type (increased fitness, S)
• self incompatibility; a new allele has initial advantage due to rarity

Question 4

super genes?

Answer 4

• co adapted linkage groups
• genes close together on same chromosome don’t segregate and rarely recombine
• eg. mimicry in butterflies
• eg. Cepea nemoralis (british land snail)
  dominance for background colour = brown>pink>yellow
  dominance for banding = unbanded>banded
• colour and banding genes are linked to form a coadapted linkage group

Question 5

cyanogenesis?

Answer 5

• balanced selection
• environmental differences in clines
• cyanogenic glycosidases; linamarin and lotaustralin
• glycosidase = AC/ac
• glycosides from valine and isoleucin
• glycosides = Li/li
• cyanogenic if AC/Li
• acyanogenic if AC/lili or acac/Li
• proportion of glycoside and glycosidase allele drop with altitude and low temps
• selection for cyanogenesis if grazing by molluscs or voles and high levels of turnover pop
• selection against if cold / tampling (self poisoning), stress, lack of nutrients