Trends in evolutionAn 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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Titanothere illustration from Osborn, H.F. 1929 The titanotheres of ancient Wyoming, Dakota and Nebraska. U.S. Geol. Surv. Monogr. 55
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