Here's why:
* Evolution is a change in the allele frequencies within a population over time. Alleles are different versions of a gene.
* A single organism cannot evolve. Evolution requires the passing of genetic traits to offspring. An individual's genes don't change within their lifetime, only the frequency of alleles in the overall population can change.
* A population is a group of individuals of the same species living in the same area and interbreeding. This allows for the exchange of genetic material and for natural selection to act upon the population.
For example, imagine a population of bacteria. Some bacteria might have a mutation that makes them resistant to a certain antibiotic. If that antibiotic is introduced, the resistant bacteria are more likely to survive and reproduce, passing on their resistance gene to their offspring. Over time, the frequency of the antibiotic resistance gene will increase in the population, demonstrating evolution.
