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Trends in evolution An evolutionary trend can
be either directional change within a single lineage or parallel
change across lineages, in other words, several lineages undergoing
the same sort of change. However, not just any change counts as a
trend. After all, if the weather gets warmer one day, you wouldn't
call it a warming trend; warming would have to go on for some length
of time before you'd call it a trend. Biologists think about
evolutionary trends in the same way — there has to be something
about the change that suggests that it's not just a random
fluctuation before it counts as a "trend."
For example, titanotheres (a cool, extinct clade related to
modern horses and rhinos) exhibit an evolutionary trend.
Titanotheres had bony protuberances extending from their noses. The
sequence of fossil skulls from these animals shows that evolutionary
changes in the size of these "horns" were not random; instead,
changes were biased in the direction of increasing horn size. And in
fact, several different titanothere lineages experienced the same
sort of change in horn size.
The titanothere reconstructions shown here range from about 55
mya (A) to 35 mya (D).The cause of this trend is not obvious. It may
be a by-product of selection for increasing body size, and/or it may
be a result of selection on horn size directly: big-horned
individuals may have had an advantage in "butting" contests for
females, as in sheep and goats.
Other evolutionary trends are not so consistent across lineages.
For example, many different animal lineages have undergone
cephalization, basically "the evolution of a head." Cephalization
involves concentrating neurons into a brain at one end of the animal
and evolving sensory organs at that same end. Arthropods
(crustaceans, insects, and family), annelids (segmented worms), and
chordates have all undergone increasing cephalization. However, many
animal lineages have not undergone much cephalization (where's the
head on a starfish?), and other lineages, such as many internal
parasites, have gone in the reverse direction, losing the "heads"
they started out with.
Is evolution progressive?
This is
not an easy question to answer. From a plant's perspective, the best
measure of progress might be photosynthetic ability; from a spider's
it might be the efficiency of a venom delivery system.
The
problem is that we humans are hung up on ourselves. We often define
progress in a way that hinges on our view of ourselves, a way that
relies on intellect, culture, or emotion. But that definition is anthropocentric.
It is tempting to see evolution as a grand progressive ladder
with Homo sapiens emerging at the top. But evolution produces
a tree, not a ladder — and we are just one of many leaves on the
tree.
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