2011-05_Patt - SMOUSE SPRING 2011 EVOLUTION (11:704-486)...

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– PATTERN & PROCESS SMOUSE SPRING 2011 1 P ATTERN AND P ROCESS Gradualism and Saltation: 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. Gap Hypotheses – 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: Independent Origins – Distinct groups of organisms are not 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. Saltation – 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 macromutations , 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. Paleontological Gaps – 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. Gradualism – 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. Intermediates – 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. ancestor
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2011-05_Patt - SMOUSE SPRING 2011 EVOLUTION (11:704-486)...

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