  We study a model of selection acting on a diploid population (one in which
each individual carries two copies of each gene) living in one spatial
dimension. We suppose a particular gene appears in two forms (alleles) $A$ and
$a$, and that individuals carrying $AA$ have a higher fitness than $aa$
individuals, while $Aa$ individuals have a lower fitness than both $AA$ and
$aa$ individuals. The proportion of advantageous $A$ alleles expands through
the population approximately according to a travelling wave. We prove that on a
suitable timescale, the genealogy of a sample of $A$ alleles taken from near
the wavefront converges to a Kingman coalescent as the population density goes
to infinity. This contrasts with the case of directional selection in which the
corresponding limit is thought to be the Bolthausen-Sznitman coalescent. The
proof uses 'tracer dynamics'.
