Quick questions on Mutation, gamete variation and the source of new alleles: HSC Biology Module 6

Meiosis produces haploid gametes from a diploid parent, reducing chromosome number by half. Three processes generate variation.

The gene pool is the total collection of all alleles in a population. Evolution, at its simplest, is a change in allele frequencies in that pool over time. Four processes change those frequencies: mutation, natural selection, gene flow and genetic drift. NESA asks you to define and distinguish the first, third and fourth of these (selection is dealt with separately), so be precise about what each one does and does not do.

During metaphase I, the homologous chromosome pairs line up at the spindle equator independently of one another. Either the maternal or the paternal chromosome of each pair can face either pole. With $n$ chromosome pairs, this produces $2^n$ possible combinations.

During prophase I, homologous chromosomes pair up (synapsis) and exchange segments at chiasmata. This recombines maternal and paternal alleles within each chromosome, generating combinations that were not present in either parent. Crossing over essentially makes the $2^n$ figure an underestimate; the actual number of unique gametes is astronomically larger.

Any one of the millions of possible sperm can fertilise any one of the possible eggs, multiplying the variation across the population.

Each daughter DNA molecule retains one original strand and one newly synthesised strand. This conserves the parental sequence.

After replication, mismatch repair proteins recognise base mismatches and excise the wrongly inserted base, reducing the error rate further to about 1 in $10^{10}$.

Meiosis can only shuffle what already exists.

A mutation is a heritable change in the DNA sequence. It is the only process that puts a brand-new allele into the gene pool. On its own it changes frequencies extremely slowly (new mutations are rare), but it is indispensable: without it, the other three processes would have nothing new to work with.

Gene flow (also called migration) is the transfer of alleles from one population to another, carried by individuals that move and then breed, or by gametes such as wind-blown pollen. It moves existing alleles around; it does not create new ones. Gene flow tends to introduce alleles a population lacked (raising diversity) and to make connected populations more genetically similar to each other.

Genetic drift is the random change in allele frequencies from one generation to the next, simply because of which individuals happen to survive and breed. Unlike selection, it is not based on fitness - it is chance. Its effect is strongest in small populations, where a few lucky or unlucky events sway a large fraction of the pool; in large populations chance effects average out.

Gene-pool questions are almost always data-based (a table, map or graph). State the trend you see, then name the mechanism and give cause and effect - do not describe the mechanisms in the abstract.

"Define" wants a one-line meaning; "distinguish" wants the contrasting feature of each; "explain" wants cause and effect; "evaluate" wants a judgement weighing the relative importance of the mechanisms (e.g. drift dominates in small populations, gene flow in connected ones).

Genetic drift and mutation are random; the direction of natural selection is not. Markers reward candidates who label each process random or non-random correctly.