Evolution and speciation

Evolution requires heritable variation: a change must affect genetic information in cells that contribute to reproduction and then be passed to descendants. Acquired traits, such as stronger muscles from exercise, usually affect somatic cells and do not alter the DNA of gametes. They therefore are not directly inherited. Option A states the essential condition; B and D support an incorrect idea, while C says acquired traits always change chromosomes, which is false.

Genetic drift is a random change in allele frequencies, especially noticeable in small populations; chance events can make a trait common or rare without regard to usefulness. Natural selection is non-random with respect to reproductive success: heritable traits that improve survival or reproduction tend to spread. Therefore A is correct. B wrongly assigns drift to environmental choice, while C and D deny established distinctions.

Sedimentary rock layers generally accumulate over time, so, when the layers have not been disturbed, deeper layers are usually older than layers above them. Comparing fossil positions therefore helps establish a relative sequence of organisms and changes, although it does not by itself give an exact numerical age. A is correct; B, C, and D misunderstand fossil preservation and stratigraphy.

Fossilisation is uncommon, and the record is biased by which organisms had hard parts, were buried quickly, and were preserved in suitable conditions. Many organisms and transitional stages may therefore be missing. Nevertheless, fossils reveal past forms, their distribution, and sequences in rock layers, providing evidence for change through time. A is correct; B, C, and D deny their evidential value or misstate what they show.

Geographical isolation separates populations by barriers such as distance or habitat. Reduced contact means less interbreeding and less exchange of genes between the groups. Mutations, natural selection, and genetic drift can then produce different trait frequencies in each environment. Over many generations, these differences may contribute to reproductive isolation and speciation. Option A correctly describes this first step; the other options are biologically unsupported extremes.

Natural selection changes the frequency of heritable traits according to environmental advantage, while genetic drift changes allele frequencies by chance, especially in small populations. If two isolated groups experience different environments and different random events, their genetic compositions can shift in different directions. Continued divergence may eventually contribute to speciation. Thus option A is correct; immediate identity, no change, and chromosome loss do not follow from these mechanisms.

Natural selection usually acts on small heritable variations that affect survival or reproduction by different amounts. One generation normally produces only limited change, and a population must pass through many generations for advantageous differences to become common. If populations remain separated and continue to accumulate differences, reproductive isolation may eventually develop, leading to speciation. Thus option A is correct; the other choices wrongly describe evolution as instant or unrelated to time.

Analogous organs perform similar functions but differ in their basic structural plans and evolutionary origins. Bird wings and insect wings both enable flight, yet they are built differently and did not arise from the same ancestral wing structure. Their similarity can result from convergent evolution, in which unrelated groups face similar demands. Consequently, similar function alone is insufficient evidence of common ancestry, so option A is correct.

Different environments can impose different selection pressures. If colour variation is heritable, one colour may improve camouflage or survival in one environment, while another colour is favoured elsewhere. Over many generations, the populations may become increasingly different. If geographical or reproductive isolation prevents gene flow for a long time, these differences can contribute to speciation. Thus option A is correct. Digestion, chromosome destruction, and becoming identical do not explain population divergence.

Geographical isolation occurs when physical barriers such as mountains, rivers, or distance separate members of one population. Because the groups meet and interbreed less often, gene flow decreases. Different mutations, selection pressures, and random genetic changes can then accumulate in each group. Over many generations, reproductive differences may arise and produce new species. Therefore, option A is correct; the other choices are not consequences of geographical isolation.

Speciation generally requires restricted gene flow and the accumulation of differences between populations. When separated groups interbreed again, alleles move between them, increasing gene flow. This mixing can reduce genetic divergence and weaken reproductive separation, so speciation may slow or be prevented. It does not guarantee immediate completion, destroy genes, or strengthen geographical isolation; therefore option A is correct.

Geographical barriers such as mountains, rivers, or large distances can prevent members of one population from mating with members of another. Gene flow therefore decreases, while mutation, recombination, drift, and natural selection may cause the separated groups to diverge. If reproductive isolation eventually develops, speciation can occur. Thus option A is correct; isolation does not stop mutations, make organisms identical, or instantly create fossils.