Sigma Learn

2(a)(i)Statistical Tests BiologyEasy2 marks

The Galápagos Islands are a group of islands in the Pacific Ocean that are approximately 900 km from Ecuador in South America. Charles Darwin developed his theory of evolution by natural selection after visiting the Galápagos Islands in the 1830s.

Finches are small birds that are commonly found on the Galápagos Islands.

Some biologists studied one species of finch, Geospiza conirostris, living on Genovesa Island in the Galápagos. Genovesa Island is a small island with no mountains.

Figure 2.1 shows G. conirostris.

Figure 2.1, a photograph of Geospiza conirostris, a small dark finch with a short stout conical bill, perched side-on among thorny bare branches

The biologists studied several breeding pairs of G. conirostris. Adult male G. conirostris sing a song to attract females for mating. Female G. conirostris do not sing. The biologists noticed that adult male G. conirostris could be separated into two groups based on their song type. Adult male G. conirostris sing either song type A or song type B. At the start of the investigation, the biologists caught the birds in a mist net.

Figure 2.2 shows a bird caught in a mist net. The mist net does not harm the bird.

Figure 2.2, a photograph of a G. conirostris tangled in a fine mist net with its wings spread, with leader lines labelling the bill of the bird and the mist net

For each male G. conirostris caught in the mist net, the biologists:

attached a coloured ring to the leg of the bird so that it could be easily identified in the future
measured the mass of the bird
measured the depth, width and length of the bill of the bird
released the bird back into the ecosystem
observed the song type and other behaviours of the bird over a period of four months.

Table 2.1 shows the data from the investigation.

song type of male G. conirostris	number of birds	mean mass / g	mean bill depth / mm	mean bill width / mm	mean bill length / mm
song type A	11	25.85	10.79	9.99	15.47
song type B	13	24.96	11.02	10.23	14.55

Statistical analysis showed that there was no significant difference between the mean mass, the mean bill depth and mean bill width of male G. conirostris with song type A and male G. conirostris with song type B. The biologists then carried out a t-test to compare the mean bill lengths of male G. conirostris with song type A and male G. conirostris with song type B.

State two conditions of the data that allowed the biologists to carry out a t-test.

Answer

0 words

2(a)(ii)Statistical Tests BiologyEasy1 mark

State the null hypothesis for the t‑test that the biologists carried out.

2(a)(iii)Statistical Tests BiologyEasy1 mark

Calculate the number of degrees of freedom (v) for the t‑test that the biologists carried out.

2(a)(iv)Statistical Tests BiologyMedium3 marks

The biologists calculated the value of

t

as 3.768.

Table 2.2 shows the probability table for the

t

-test.

Critical values

degrees of freedom (v)	$p = 0.50$	$p = 0.10$	$p = 0.05$
20	0.687	1.725	2.086
21	0.686	1.721	2.080
22	0.686	1.717	2.074
23	0.685	1.714	2.069
24	0.685	1.711	2.064
25	0.684	1.708	2.060
26	0.684	1.706	2.056

Use the calculated value of

t

and Table 2.2 to explain what can be concluded about the mean bill lengths of male G. conirostris with song type A and male G. conirostris with song type B.

Answer

0 words

2(b)Experimental DesignMedium2 marks

G. conirostris feed on a range of food sources, including:

the leaves, flowers and fruits of the prickly pear cactus
the seeds of the palo santo tree.

Initial observations suggested that male G. conirostris with song type A feed on different food sources than male G. conirostris with song type B.

Suggest a method that the biologists could use to measure the proportion of time spent feeding on each food source by male G. conirostris with song type A and male G. conirostris with song type B.

Answer

0 words

2(c)Evolution SpeciationHard4 marks

The biologists concluded that G. conirostris on Genovesa Island may be in the early stages of sympatric speciation.

Outline how this investigation:

supports the conclusion that G. conirostris may be in the early stages of sympatric speciation
could be extended to increase confidence in the conclusion.

Answer

0 words

May/June 2026 Paper 51

Questions