Crossing Over: During Prophase I of meiosis, non-sister chromatids of homologous chromosomes exchange segments of DNA. This process breaks old linkage groups and creates recombinant chromosomes with new allele combinations.
Independent Assortment: During Metaphase I, homologous pairs align randomly at the cell equator. The orientation of one pair is independent of others, resulting in possible chromosome combinations in gametes (where is the haploid number).
Random Fertilisation: This is a post-meiotic process where any one of the millions of genetically unique male gametes can fuse with a genetically unique female gamete. This exponentially increases the potential genetic diversity of the resulting zygote.
Genetic drift refers to random fluctuations in allele frequencies within a population due to chance events rather than natural selection.
In small populations, genetic drift has a disproportionately large impact because the loss of even a few individuals can significantly alter the gene pool or cause alleles to disappear entirely.
Unlike natural selection, which favors advantageous traits, genetic drift can cause harmful alleles to become fixed or beneficial alleles to be lost simply by 'bad luck' in sampling.
Over time, genetic drift leads to a reduction in genetic diversity within a population, making it more vulnerable to environmental changes or extinction.
| Feature | Genetic Variation | Environmental Variation |
|---|---|---|
| Cause | Mutations, Meiosis, Fertilisation | Diet, climate, light, accidents |
| Inheritance | Passed to offspring via gametes | Not inherited by offspring |
| Evolutionary Role | Provides raw material for selection | Does not directly drive evolution |
| Example | Blood groups, eye color | Scars, language, plant height |
Check the Population Size: If a question mentions a 'small' or 'isolated' population, always consider the impact of genetic drift as a primary driver of allele frequency change.
Explain the 'Why' of Inheritance: When asked why certain traits aren't passed on, specify that environmental factors do not alter the DNA base sequence of gametes, which is the only information transferred to the next generation.
Use Precise Terminology: Distinguish between 'new alleles' (caused only by mutation) and 'new combinations of alleles' (caused by meiosis and fertilisation).
Sanity Check: If an allele frequency changes rapidly in a massive population, it is likely due to a strong selection pressure rather than drift, as drift effects 'even out' in large groups.
