If evolution happens gradually as genetic changes are passed from one generation to the next, you’d expect the fossil record to show the transitional forms along the way. But it doesn’t always do that. 

In some cases, the fossil record clearly shows a pattern of gradual change from ancestor to descendent. But more often, it seems to show jumps from one species to another with no transitional forms in between. 

“It’s quite rare to actually have examples in the fossil record where something has changed gradually in one particular direction over a long period of time,“ says Melanie Hopkins, curator and chair of the Division of Paleontology at the American Museum of Natural History. 

How Long Does Evolution Take?

There are several ways to explain evolutionary changes and their patterns in the fossil record. One is called phyletic gradualism. According to this model, species gradually change over time, little by little. Small genetic variations in an individual cause it to be more or less fit for its environment. Those genes that help the individual survive are passed on. As these genetic variations accumulate over time, the species gradually changes and becomes a new species. This change is slow and steady.

The transitional forms, or steps along the way, don’t always show up in the fossil record. One reason is that conditions have to be just right for fossilization to occur; most living things come and go without leaving any record at all. This can create the illusion of sudden change, but in fact, the species gradually changed over many millions of years. The transitional forms were simply not preserved in the fossil record. 

However, sometimes species do change in what is, by geological standards, the blink of an eye. This can be explained by an idea called punctuated equilibrium, a hypothesis developed in the 1970s by paleontologists Stephen Jay Gould and Niles Eldredge. 

According to this idea, species go for many millions of years with very little change and then undergo considerable change in a short (at least in geological terms) period of time, say a few hundred thousand years. The reason we don’t see the transitional forms is that those forms didn’t last long enough to be preserved in the fossil record. 

Read More: What Is the Difference Between Convergent and Divergent Evolution?

The Rate of Evolution is not Always Gradual

Of course, evolution doesn’t always take millions of years. For changes in individuals to result in a change to the species, those genetic changes must be passed on to new generations. How long that takes depends on how fast the species reproduces. Bacteria, for example, reproduce very quickly (not sexually, but by dividing in two), so any changes — such as the ability to develop resistance to antibiotics — can spread with alarming speed and, as we’ve learned, to our peril.

Rapid environmental changes can also goose evolution. A 2022 study found that as environmental disruptions caused by climate change put pressure on wildlife to adapt, genetic changes occur much more quickly than previously thought, though it’s not yet clear if this will result in long-term evolutionary change.

The important point here, says Hopkins, is that there’s not only one driver of evolution. Species respond at different rates based on how much stress there is in the environment, as well as how much they’re capable of changing, either through mutations or variations within the species.

“It depends a lot on what kind of change is happening and how fast that change is happening,” she says. “And, of course, it also depends on what mutations might arise, as well as what we refer to as evolvability, for example, what aspects of variation already exist that might help those populations or species respond to the environmental change.”

So, is evolution fast or slow? The answer is both.

Read More: How Are Humans Still Evolving?

Article Sources

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Sara Novak is a science journalist based in South Carolina. In addition to writing for Discover, her work appears in Scientific American, Popular Science, New Scientist, Sierra Magazine, Astronomy Magazine, and many more. She graduated with a bachelor’s degree in Journalism from the Grady School of Journalism at the University of Georgia. She’s also a candidate for a master’s degree in science writing from Johns Hopkins University, (expected graduation 2023).