  The problem of intermediates in the fossil record has been frequently
discussed ever since Darwin. The extent of `gaps' (missing transitional stages)
has been used to argue against gradual evolution from a common ancestor.
Traditionally, gaps have often been explained by the improbability of
fossilization and the discontinuous selection of found fossils. Here we take an
analytical approach and demonstrate why, under certain sampling conditions, we
may not expect intermediates to be found. Using a simple null model, we show
mathematically that the question of whether a taxon sampled from some time in
the past is likely to be morphologically intermediate to other samples (dated
earlier and later) depends on the shape and dimensions of the underlying
phylogenetic tree that connects the taxa, and the times from which the fossils
are sampled.
