– PATTERN & PROCESS
Out near the branch tips of an evolutionary tree, we see a more or
less continuous range of variation. One genotype is relatively similar to others in the same
population, populations are a bit more divergent, species are similar enough to be groupable into
genera, and genera are similar enough to be groupable into families, and so on.
– The gaps get larger as we move inward toward the roots of the tree, back in
time. The gaps get big enough that they require some systematic explanation. No one has ever
seen an intermediate form between a bat and a whale; I can’t even imagine what one would look
like. How can we say they are connected? We could imagine several different answers:
– Distinct groups of organisms are
from the same ancestors. Life
may have arisen more than once. This one is discounted, these days. Everything we know
about life, from the universality of “the code” to ontogenetic and developmental patterns,
suggests that all life comes from previous life. Life arose once, for practical purposes.
– The gaps exist because there never were any intermediates. What we are looking
at is a set of rare mutations that caused massive phenotypic changes. We are talking about
, Goldschmidt’s “hopeful monsters”. These things are rare, but even rare
events count, over evolutionary time. It only has to happen once, every now and again.
– The gaps represent extinction of the ancestral forms. If we had a
complete fossil record, all the missing links would be there. The paleontological record is so
sparse that it is more gap than record, and most of what was there is not seen in the record.
We see bits and pieces, and we fill in the gaps for ourselves. We’ll never see them all.
– Most evolutionary biologists subscribe to the gradualist school established by
Darwin’s writing. Change is thought to be gradual at any given point in time, small – relative
to the scale of the paleontological or taxonomic gaps we see. We have lots of empirical
evidence that minor variation is present in populations, and we can select on almost anything.
– I should probably say just a
bit more about this business of intermediate
forms. I mentioned the bats and whales, and
there have probably never been intermediate
forms, so what are we talking about? Look at
the Figure on the right to get the picture.
We have an ancestral species that diverges
along two different lineages. Once we get to
the ‘now plane’, at the top, the two forms are
not even similar. The ancestor of bats and
whales was probably a ground pounder; the
continuity is around the (now missing) loop.