Question

How do organisms as a species build up a resistance to toxic things through natural selection? Would you say its more of a natural by chance resistance to the toxin due to genetic variation or more prolonged exposure = resistance due to the toxin now being considered a common place substance

Answer 1

Short answer: by natural selection acting on heritable variation. Which source of that variation matters depends on the organism and the situation — often both pre‑existing (standing) variation and new changes matter. Exposure doesn’t “create” useful mutations in a Lamarckian way, it simply selects the individuals that already (or newly) have them.

Key points

- What selection does: if a toxin kills or reduces reproduction, any heritable genetic change that reduces harm will increase in frequency. That is natural selection.

- Where resistant variants come from:
- Standing genetic variation — alleles already present in the population at low frequency. If exposure is sudden, these are often the fastest source of resistance because they’re already available.
- New mutations — if no useful alleles exist, new beneficial mutations can arise and be selected. This is slower in species with long generation times and small populations.
- Gene duplication/amplification or regulatory mutations — can increase expression of detox pathways.
- Horizontal gene transfer (HGT) — very important in bacteria: resistance genes on plasmids can move between strains/species.
- Phenotypic plasticity/epigenetic changes — can give temporary tolerance and buy time for genetic adaptation to spread.

- Which is more important depends on biological and ecological factors:
- Large populations with lots of genetic diversity (and short generation times) often adapt from standing variation or rapid mutation supply. Microbes additionally use HGT.
- Small populations or novel chemicals not previously encountered are more likely to require new mutations, which takes longer.
- The strength, duration and frequency of exposure determine the selection pressure. Frequent/prolonged exposure strongly favors resistant variants and can drive rapid increases in their frequency.

- Examples:
- Bacteria: antibiotic resistance commonly spreads by mutation plus plasmid-borne genes (HGT).
- Insects: single-point “kdr” mutations in the sodium channel confer resistance to DDT/pyrethroids; metabolic resistance through overexpressed detox enzymes (P450s, esterases) often evolves via gene amplification or regulatory changes.
- Plants: glyphosate resistance in Amaranthus palmeri via amplification of the EPSPS gene.
- Heavy‑metal tolerant plants and industrial melanism in moths are classic examples of selection acting on pre‑existing variation or new mutations.

- Costs and reversibility: resistance mutations can carry fitness costs in toxin-free environments, so if exposure is removed the resistant allele may decline again. That’s why managing exposure (e.g., rotating pesticides, prudent antibiotic use) can slow resistance spread.

Bottom line: natural selection works on whatever heritable variation exists. Often resistance comes fastest from pre‑existing variation in large, diverse populations, but new mutations and mechanisms like HGT are equally crucial, especially for microbes. Prolonged exposure increases the strength of selection (and thus the chance resistant types become common), but it does not itself directly produce purposeful mutations.