A3.1 Diversity of organisms

The biological species concept defines a species as a group of organisms that can potentially interbreed to produce fertile offspring. Applying this concept requires the ability to observe or test reproductive compatibility.

The biological species concept cannot be applied to fossil organisms because there is no way to test whether extinct plants from the fossil record could have interbred — reproductive behaviour leaves no trace in fossils.

Wind-pollinated sexual plants can be tested for interbreeding compatibility in principle. Mountain-separated populations are alive and could theoretically be brought together to test fertility. Colour variation is a morphological difference but does not prevent a breeding experiment.

Only fossil organisms with no living relatives present an absolute barrier to applying the biological species concept, because interbreeding cannot be observed or experimentally tested.

The question asks which scenario makes the biological species concept most difficult to apply. This relates to situations where reproductive compatibility cannot be tested.

Wind-pollinated sexually reproducing plants can still be cross-pollinated under controlled conditions to test for fertile offspring — the concept can be applied. This option is eliminated.

Geographic separation prevents natural interbreeding but the plants are living — samples could be brought together to test reproductive compatibility. This is a limitation of gene flow, not of applying the concept.

Petal colour variation is a morphological trait and does not prevent application of the biological species concept — breeding experiments can still be conducted to test fertility of offspring.

Fossil organisms cannot be tested for interbreeding at all. This is a well-established limitation of the biological species concept, making it the correct answer.

In humans, XX = female and XY = male. The karyogram shows XX sex chromosomes, so this individual is female.

A normal human diploid number is 46 chromosomes (23 pairs). This individual has 47 chromosomes, with three copies of chromosome 21 — this is trisomy 21, also known as Down syndrome.

The individual is female (XX) with Down syndrome (trisomy 21). This matches the option 'Female with Down syndrome'.

The question requires interpreting two features of the karyogram: the sex chromosome pair and the chromosome number/arrangement to detect any abnormality.

'Male with Down syndrome' and 'Male with Turner syndrome' both require an XY or abnormal sex chromosome configuration. The karyogram shows XX, ruling out all male classifications.

A normal female karyotype would show exactly 46 chromosomes including XX. This individual has 47 chromosomes with trisomy 21 — clearly not normal. This option is eliminated.

XX confirms female sex; 47 chromosomes with three copies of chromosome 21 confirms Down syndrome (trisomy 21). The correct answer is 'Female with Down syndrome'.