Regressive evolution is a population change over time in which some phenotypic characteristics are lost. It is thought to arise through either genetic drift or natural selection.
One example of regressive evolution is the disappearance of eyes in one form of the Mexican cavefish, Astyanax mexicanus. These eyeless cavefish live in caves with complete darkness.
Three theories are offered to explain how the cavefish lost their eyes.
Theory A
In a cave with total darkness and scarce energy sources, eyes give no benefit. Having eyes may even be a disadvantage because they may require energy.
Theory B
A mutation has occurred in a single gene. This mutation has two effects:
• no eye development
• an increase in the number of chemoreceptors on the skin.
Theory C
Several mutations happened in the genes responsible for eye development over time. By chance, these mutations became more frequent in small isolated populations. In the end this produced a population of eyeless cavefish.
(a(i))[1]
State one theory, A, B or C, that identifies genetic drift as the reason for eye loss.
(a(ii))[4]
State and explain which theory or theories are based on natural selection as the reason for eye loss.
(b(i))[2]
Suggest how DNA evidence could be used to decide whether the eyeless A. mexicanus and the A. mexicanus with eyes are the same species or different species.
(b(ii))[3]
Suggest why mtDNA is chosen rather than nuclear DNA when comparing how closely related populations are.
Worked solution & mark scheme
This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: “C” …