Download Macroevolution 11/18/04 Macroevolution

Macroevolution 11/18/04
Macroevolution – large scale patterns in evolution
Adaptive radiation
Convergent evolution
Evolutionary trends and patterns
Cope’s Rule of size increase
Escalation
Macroevolution
Large-scale patterns in evolution; sometimes said to be “evolution above the species level”.
Basically referring to patterns and trends in the evolutionary record.
1. Bursts of diversification. To measure origination rates, paleontologists calculate the
number of first appearances of a taxon (species, genus, family, etc.) within a time
interval. Or the number of first appearances divided by the total number present in order
to correct for differences in overall diversity.
As shown in the figure above graph of the rate first appearances of marine genera through the
Phanerozoic, diversification rates are not constant. Some periods of time are characterized by
higher rates than others.
Note, for starters, the rapid rates of origination in the Cambrian and Ordovician and the rapid
rates in the Triassic. In the case of the Cambrian and Ordovician, origination seems to have been
a rapid infilling of available ways of life (the “Cambrian Explosion” of “Biological Big Bang”) a phenomenon known as adaptive radiation.
In the case of the Triassic, a rapid exploitation of biological opportunities after the major mass
extinction of the end-Permian. This was also an adaptive radiation, but also a replacement of the
pre-existing biotas.
Similarly, note below the rapid diversification of mammals into many different habitats and
adaptive types following the end-Cretaceous extinction of the dinosaurs
2. Convergent evolution is a macroevolutionary pattern in which unrelated groups are
morphologically similar to each other, that is, they seem to “converge” on a similar
morphology, probably because that morphology is particularly well-suited for that way of
life.
For example, sharks, ichthyosaurs swimming birds and dolphins all have similar, streamlined
body shapes for swimming, but they are unrelated to each other.
3. Evolutionary trends and patterns
a. Cope’s “Rule” of evolutionary size increase. Ancestors tend to be small and their
descendants can be either small or larger. As a result, thru time, the average size
of a species in the clade may increase (even though small species are still present.
Notice in the figure above that small horses persist this illustrates that horse evolution is not a
case of gradual, directional evolution from small to large horses because small forms also persist.
Rather, evolution proceeds by branching (cladogenesis) that produces large as well as small
forms.
Although it’s easy to think of the advantages of large size, there are clearly also advantages to
small size.
So, is bigger better? Or is this trend toward larger size simply the artifact of evolution from a
small ancestor? See the graph below of maximum size through time: If bigger is better, why are
there still small species? Maybe the question should be: Why do ancestors tend to be small?
b. Escalation, or an evolutionary ”arms race”. Predators and prey are sometimes thought to be
in an arms race, where the improvements in offense are countered by improvements in defense,
and so on through time. Is there evidence for this in the fossil record? Possibly.
Gary Vermeij, a paleontologist at UC Davis has observed that there has been an increase through
time in the proportion of gastropod families that have thickened or narrowed apertures.
Vermeij argues that this trend is the result of continuing adaptation against predators. Fish and
crabs tend to peel away at the aperture of gastropods in order to get at the tasty bits inside, so a
strengthened and narrower aperture is a defense against predation that started, he says in the
early Mesozoic.