A selection pressure is any environmental factor that affects the chance of an organism surviving and reproducing. Examples include predation, disease, competition for food, mates or light, and abiotic factors such as temperature or the presence of a toxin such as an antibiotic or pesticide.Selection pressures act on phenotypes, but the effect is on alleles. Natural selection works like this:1. There is variation in a population, caused by different alleles.
2. A selection pressure acts. Individuals with alleles that give an advantageous phenotype are more likely to survive.
3. These individuals reproduce and pass the advantageous alleles on to their offspring.
4. Over many generations the frequency of the advantageous allele in the gene pool increases, and the frequency of the less favourable allele falls.For example, when a population of bacteria is exposed to an antibiotic, the antibiotic is the selection pressure. Bacteria with an allele for resistance survive and divide, so the resistance allele becomes more common.Natural selection is not random: it consistently favours the alleles best suited to the current environment. This is what makes it different from genetic drift. If the selection pressure changes, a different phenotype may become advantageous.Selection pressures break the Hardy-Weinberg assumption that all genotypes have an equal chance of surviving and reproducing. Depending on which phenotypes are favoured, selection can be stabilising, directional or disruptive.
2. A selection pressure acts. Individuals with alleles that give an advantageous phenotype are more likely to survive.
3. These individuals reproduce and pass the advantageous alleles on to their offspring.
4. Over many generations the frequency of the advantageous allele in the gene pool increases, and the frequency of the less favourable allele falls.For example, when a population of bacteria is exposed to an antibiotic, the antibiotic is the selection pressure. Bacteria with an allele for resistance survive and divide, so the resistance allele becomes more common.Natural selection is not random: it consistently favours the alleles best suited to the current environment. This is what makes it different from genetic drift. If the selection pressure changes, a different phenotype may become advantageous.Selection pressures break the Hardy-Weinberg assumption that all genotypes have an equal chance of surviving and reproducing. Depending on which phenotypes are favoured, selection can be stabilising, directional or disruptive.