In positive frequency-dependent selection the fitness of a phenotype increases as it becomes more common.

Ignoring frequency-dependent selection, then genetic load () may be calculated as:

In negative (or purifying) frequency-dependent selection, the fitness of a phenotype increases as it becomes rarer.

In behavioral ecology, negative frequency-dependent selection is often responsible for maintaining multiple behavioral strategies within a species.

The relative abundance of the two morphs in populations is regulated by frequency-dependent selection.

This causes frequency-dependent selection in which forms with rarer colours and patterns are less likely to be killed.

The two major and most studied are heterozygote advantage and frequency-dependent selection.

The balance between these three morphs is maintained by frequency-dependent selection.

The book also pioneered the concept of frequency-dependent selection and introduced the term "aposematism".

And he is recognised as the first scientist to identify frequency-dependent selection, as described in this book.