D4.1 Natural selection

The question asks which process creates new alleles — not just new combinations of existing alleles. This is a key distinction in understanding sources of genetic variation.

Meiosis (crossing over and independent assortment) and fertilisation reshuffle existing alleles into new combinations. They do not change any DNA sequence — no new allele is created.

A mutation is a random change in the DNA sequence itself. It can convert one allele into a different form — one that did not previously exist in the gene pool. This is why mutation is called the ultimate source of variation.

Only random mutation in DNA sequences generates genuinely new alleles. All other options rearrange pre-existing genetic material without creating novel sequences.

The question asks which process creates new alleles — not new combinations. This distinction is critical to eliminating distractors.

Independent assortment randomly distributes existing maternal and paternal chromosomes into gametes. It creates new combinations of alleles already present in the individual but does not create new allele sequences.

Fertilisation combines alleles from two parents, increasing diversity of combinations in offspring. However, all alleles present were inherited — no new allele sequences are generated by the fusion of gametes itself.

Crossing over exchanges segments between non-sister chromatids, producing chromosomes with new allele combinations. The individual alleles themselves remain unchanged — crossing over does not produce a novel allele.

Random mutation in DNA sequences is the only option that actually changes the nucleotide sequence, producing an allele that was not previously present in the gene pool. This is the correct answer.

The dance does not help males survive — it is performed to impress females. Females are choosing which males to mate with based on dance quality. The outcome is greater mating success, not greater survival.

Sexual selection is selection that favours traits increasing mating success rather than survival. When one sex (here, females) evaluates and chooses among potential mates, this is intersexual selection (mate choice).

Males whose dance genes are inherited by offspring that also dance well continue to gain mating advantages. The alleles for elaborate dancing increase in frequency in the gene pool over generations — driven by female preference, not environmental survival pressure.

Sexual selection via intersexual mate choice is the correct classification. The driving force is female preference improving male reproductive success, not abiotic survival pressure.

The question asks which evolutionary mechanism best explains the elaboration of a trait that improves mating success but is not linked to survival.

Directional natural selection driven by abiotic factors (temperature, rainfall, etc.) favours survival traits. There is no mention of abiotic pressure in this scenario, and the dance is explicitly about attracting mates, not surviving environmental stress.

Stabilising selection would favour average dance complexity and reduce variation — but the scenario describes females preferring the most elaborate dancers, suggesting directional preference, not stabilising pressure on dance complexity.

Artificial selection requires human intervention in choosing which individuals reproduce. This scenario describes wild spiders with naturally evolved female preferences — no human selection is described.

Sexual selection via intersexual mate choice is correct. Females are the choosing sex, males compete for their attention through display, and the result is differential mating success — the defining features of intersexual sexual selection.