 We may be observing the first steps of parapatric speciation in the grass species Anthoxanthum odoratum (at right).
Some of these plants live near mines where the soil has become contaminated with heavy metals. The plants around
the mines have experienced natural selection for genotypes that are tolerant of heavy metals. Meanwhile, neighboring
plants that don't live in polluted soil have not undergone selection for this trait. The two types of plants are
close enough that tolerant and non-tolerant individuals could potentially fertilize each other — so they seem to
meet the first requirement of parapatric speciation, that of a continuous population. However, the two types of
plants have evolved different flowering times. This change could be the first step in cutting off gene flow
entirely between the two groups. |
In parapatric speciation there is no specific extrinsic barrier to 
