Antibiotic resistance in bacteria is a clear example of how natural selection works. It shows how populations of organisms can change over time in response to environmental pressures, like the presence of antibiotics.

Evolution and Progress

The statement, "Evolution by natural selection is progressive, it makes individuals 'better'" is not considered true in modern evolutionary biology. Here's why:

• Evolution is about adaptation to the current environment. What is 'better' depends entirely on the specific conditions.
• In the presence of antibiotics, being resistant is 'better' for survival and reproduction. But if antibiotics are removed, the metabolic cost of resistance might make resistant bacteria less competitive than non-resistant ones in that new environment.
• Evolution does not follow a set path towards increasing complexity or improvement in a universal sense. It's a branching process of diversification and adaptation to local conditions. Therefore, saying it's always 'progressive' or makes individuals inherently 'better' is misleading.

Natural Selection Mechanisms

Let's look at why the other statements accurately describe natural selection:

• Natural selection acts on individuals but it is populations that change with time: This is a fundamental concept. Individual bacteria either survive the antibiotic exposure or die based on their traits (like resistance). However, the frequency of resistant individuals increases in the population over generations, leading to a population that is, as a whole, more resistant than the original one.
• Natural selection does not cause genetic changes in individuals: Natural selection acts on the variation that already exists within a population (due to random mutations or gene transfer). The antibiotic doesn't cause a bacterium to become resistant; it simply selects for the individuals that already have resistance genes or advantageous mutations. Genetic changes (mutations) occur randomly, independent of selection pressure.
• Natural selection acts on phenotype: Phenotype is the observable characteristic of an organism, resulting from its genotype and environment. Antibiotic resistance is a phenotype. The antibiotic acts on this trait – it kills bacteria with a susceptible phenotype and allows bacteria with a resistant phenotype to survive and reproduce.

Considering these points, the statement that evolution is always progressive and makes individuals 'better' is not true in the context of natural selection.