A thought experiment

A thought experiment can help is to decide whether natural selection works on genes, or larger phenotypic units. Consider a phenotypic change such as a new hunting skill.

Can natural selection work on it if it is produced genetically? Consider a new mutation which improves the speed and therefore the hunting ability of a jaguar (pictured opposite). This genetic advantage will now be heritable and can replicate and change its frequency in the population. In other words, it is subject to selection.

Can natural selection work on it if it is produced non-genetically? Suppose that a jaguar's increased hunting ability was caused by a non-heritable phenotypic change instead, such as individual learning or some developmental accident in the lion's nervous system. The individual jaguar with improved hunting ability will survive and produce more offspring than an average jaguar; but the trait will not be passed on to the next generation. Natural selection cannot directly work on organisms.

The change in gene frequency over time, therefore, is not just a passive 'book-keeping' record of evolution. Genes are crucial if natural selection is to take place. The need for inheritance, and the fact that acquired characters are not inherited, gives the gene a priority over the organism as a unit of selection.