Why can a population evolve but an individual can not?

Answer 1

Evolution and natural selection is a long process covering many hundreds, thousands, even millions of generations. It is based upon random mutations that occur in the offspring of organisms. In other words, almost by definition, the evolutionary process cannot take place in an individual insofar as the individual represents only one generation. If the individual has experienced some sort of random mutation that affords it some sort of advantage over its un-mutated peers in its environment, then perhaps it will be more likely to procreate and pass on its genes to its offspring who, in turn, will benefit from the advantages the mutation has created and procreate more successfully. This process can lead to an overall improvement of the species or an entirely new species altogether.

Answer 2

Evolution is a phenomenon best expressed in terms of either changes between generations or changes in allele frequencies from one generation to the next. It is individual variants that are subject to natural selection, but evolution is most easily measured in terms of allele frequencies in the population gene pool. My answer would therefore be: Both.

Although genetic information DOES change through the course of an individual's life (which is why even the DNA of identical twins grows less and less alike as they age), in general one's genotype is considered predominantly fixed at conception. In that respect, individuals do not evolve, but populations do. While individuals are subject to natural selection, they are not subject to inheritable germ line mutations. That would only happen to their offspring.