Trends in Evolution
- What is a trend?
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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