Similarity among relatives may be either environmental or genetic.

The two effects may have different relative importance in different kinds of species.

• In a species such as chimpanzees, in which parents and offspring live together in social groups (shown opposite), much of the similarity will be due to the family’s common environment (i.e., there is gene-environment correlation).

• At the other extreme, in a species like a bivalve mollusc in which eggs are released into the sea at an early stage, relatives will not necessarily grow up in similar environments, and shared environments will not be such a strong influence on similarity among relatives.

Darwin's finches are probably somewhere between these two extremes. It is easy to understand the similarity among relatives that is caused by similar environments: in so far as relatives grow up in correlated environments, and there is environmental variation in a character, relatives will be more similar than non-relatives.