The evolutionary origin of four-legged amphibians, including frogs and toads, has been debated for a long time.
Of the living fish, the elusive coelacanth and the lungfish are regarded as the closest relatives of these amphibians.
Blood samples were collected from two coelacanths that had been caught recently near Comoros.
The amino acid sequences of the $\alpha$ and $\beta$ chains of coelacanth and lungfish haemoglobin were compared with the established sequences of adult amphibians and their aquatic larvae (tadpoles). Organisms with more amino acid matches in a polypeptide chain have a more recent common ancestor than organisms with fewer matches.
The comparisons with three amphibian species, $\textit{Xenopus laevis}$ ($Xl$), $\textit{X. tropicala}$ ($Xt$) and $\textit{Rana catesbeiana}$ ($Rc$) are shown in Table 2.1.
(a(i))[4]
Explain whether the evidence in Table 2.1 supports the suggestion that coelacanths and amphibians share a more recent common ancestor than lungfish and amphibians do.
(a(ii))[2]
Suggest a reason why the haemoglobin of adult amphibians and tadpoles of the same species has different amino acid sequences.
(b(i))[3]
Explain how an environmental factor, for example the low oxygen concentration in deep water, can function as a stabilising force in natural selection.
(b(ii))[3]
Explain how an environmental factor, for example the low oxygen concentration in deep water, can act as an evolutionary force within natural selection.
(c)[3]
Explain the part played by isolating mechanisms in the evolution of new species.
Worked solution & mark scheme
This 15-mark question has a full step-by-step worked solution and mark scheme. One marking point: “The coelacanth $\alpha$ chain shows a greater percentage of matches” …