Polymorphism

Question 1

What is polymorphism?

Answer 1

Existence of 2 or more clearly different phenotypes in the same population or species
- must occur in the same habitat
- at the same time
Can apply to 
- biochemical
- morphological
- behavioural characteristics

Question 2

Give an example of a gastropod that shows polymorphism

Answer 2

Grove snail - Cepaea nemoralis

Question 3

What types of polymorphism are there?

Answer 3

Environmental and genetic

Question 4

Give some examples of environmental polymorphism

Answer 4

• Seasonal coat changes such as the arctic fox - genetically controlled but environmentally triggered (same genetic structure just switching on/off of genes)
• European Swallowtail butterfly (Papilio machaon) has either a brown or green pupa depending on where they pupate either brown stem or green leaves

Question 5

What are the 2 types of genetic polymorphism?

Answer 5

Transient - rare gene gains and preserves an overall advantage it will spread until the former normal allele is reduced to the status of a mutant, while this process is taking place (not before or after) it will generate a transient polymorphism
Balanced - stable system in which 2 or more forms remain indefinitely in one population, may be due to fine balance of environmental forced or may be inherent characteristic of the organism itself as in breeding systems

Question 6

What is the difference between polymorphism and a mutation?

Answer 6

Frequency of rare allele has to be higher than 1% to constitute as polymorphism, anything lower is a mutation

Question 7

Give examples of transient polymorphisms

Answer 7

Industrial melanism - evolution of over 100spp. of blackened forms of moth. Advantageous in smoke-blackened industrial regions e.g. Biston betularia (peppered moth)
Herbicide/pesticide resistance

Question 8

Give examples of balance polymorphisms

Answer 8

Cyanogenisis in Trifolium repens (white clover) - environmentally controlled warmer climates exhibit cyanogenesis while colder climates don't due to dangers associated with frost and cyanide. Controlled by 2 genes Ac and Li 
Grove snail (Cepaea nemoralis) - colour/banding darker/lighter colour usually depending on habitat

Question 9

What is the “rare male effect”?

Answer 9

Selection of dissimilar mates - females prefer to mate with males of a different morph rather than their own
- Scarlet Tiger moth (Callimorpha dominula) Common morph has several white dots, rare allele codes for fewer dots and heterozygote is intermediate

Question 10

What sexual selection mechanisms are there?

Answer 10

• apostasis - selection for phenotypes that will be less readily recognised by prey (advantage to rarer phenotype)
• heterozygote advantage
• diffusion cline - immigration may balance local selection
• sexual selection

Question 11

Give an example of male selection against diffusion

Answer 11

Arctic Skua (Stercorarius parasiticus) - melanic form of the bird is preferred by females but this is balanced by paler individuals reaching sexual maturity first and more importantly immigration from predominantly pale populations further North

Question 12

What is heterozygote advantage?

Answer 12

Individuals who are heterozygous at a particular locus have a greater fitness than do both kinds of homozygotes

Question 13

Give examples of heterozygote advantage

Answer 13

• resistance in the brown rat (Rattus norvegicus) - resistance to anti-coagulant posions in homozygotes and heterozygotes, homozygotes though need 10-20 times more vitamin K than the heterozygotes
• Sickle cell anaemia in humans

Question 14

Transient polymorphism studies

Answer 14

Ford 1953
Industrial melanism in Lepidoptera most striking form of transient polymorphism e.g. Biston betularia
Kilenyi 1972
Cyprideis torosa - noded and non-noded polymorphs
noded polymorphs more abundant in areas of low salinity
Used by palaeontologists to indicate salinity levels of water

Question 15

Study into balanced polymorphism in Grove snails

Answer 15

Surmacki et al., 2013

Capaea nemoralis morph influenced by avian predation and conspicuousness against background

Question 16

Study into Arctic Skua

Answer 16

Philips and Furness 1998

Question 17

Study into rare male effect

Answer 17

Hughes et al., 2013
Wild guppies (Poecilla reticulata)
Bright tail vs white tail - less common male chosen more often by female