The first individual of the new species would be very unlikely to find a
mate. Hybrids are infertile, so a newly evolved individual would not be
able to breed successfully with the original species. The mutation that
caused that individual to be a new species would also have to occur in an
individual of the opposite sex.
This objection falsely assumes that speciation must happen suddenly when
one individual gives rise to an individual of another species. In
fact, populations, not individuals, evolve, and most speciation occurs
gradually. In one common mode of speciation ("allopatric" speciation),
two populations of the same species are split apart geographically.
Small changes accumulate in both populations, causing them to be more
and more different from each other. Eventually, the differences are
great enough that the two populations cannot interbreed when they do get
together (Otte and Endler 1989).
It is also possible for speciation to occur without the geographical
separation (sympatric speciation; see Diekmann and Doebeli 1999;
Kondrashov and Kondrashov 1999; Otte and Endler 1989), but the process
is still gradual.
Sometimes new species can form suddenly, but this occurs with species
that are asexual or hermaphroditic and do not need to find mates.
References:
Dieckmann, Ulf and Michael Doebeli. 1999. On the origin of species by
sympatric speciation. Nature 400: 354-357.
Kondrashov, Alexey S. and Fyodor A. Kondrashov. 1999. Interactions
among quantitative traits in the course of sympatric speciation.
Nature 400: 351-354.
Otte, D. and J. A. Endler, eds. 1989. Speciation and Its
Consequences. Sunderland, MA: Sinauer Assoc.
Further Reading:
Schilthuizen, Menno. 2001. Frogs, Flies, and Dandelions: the making of
species, Oxford Univ. Press.
