Rates of Evolution: Punctuated Equilibrium & Molecular Clock Hypothesis

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  • 0:07 Evolution Takes Time
  • 1:11 Punctuated Equilibria
  • 3:10 Molecular Clocks
  • 4:47 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

In general, evolution is a very long process. But rates of evolution can be different for different organisms. In this video lesson, you will identify how scientists study rates of evolution and fill in some of the missing 'steps' in the fossil record.

Evolution Takes Time

Generally, evolution is not something that happens quickly. On average, it takes about 6.5 million years for a new species to develop. It takes such a long time for populations to evolve because the changes occur slowly over many generations. But that doesn't mean that all evolutionary changes happen at the same rate. In fact, relatively speaking, evolution can happen quickly (4,000 years), slowly (40 million years) and everything in between.

Scientists are able to study how quickly or slowly populations evolve through different means, but most of the information we have comes from the fossil record. The fossil record tells us a lot about the history of life on Earth, because as organisms die, they become part of the ground. They may leave behind bones, imprints or even footprint fossils that we find millions of years later. And since each layer of ground builds on top of the one below it, the fossils create a timeline through the earth, essentially telling us a story of how things changed over time.

Punctuated Equilibria

Often, new species come about slowly in the fossil record as they evolve over time. Take these butterflies for example. Two new species come about from a common ancestor and then slowly change over time. You can see the gradual change as each species continues to diverge from its original ancestor on the left.

Butterflies evolving over time
Evo Butterflies

Sometimes though, fossil species appear quite suddenly and then remain unchanged through many layers of the fossil record. These species may even disappear just as suddenly as they came about, leaving scientists with many questions.

These types of events in the fossil record are called punctuated equilibria. They describe long periods of equilibrium (little change) with abrupt episodes of speciation. Let's look at the butterflies again. This time, the common ancestor on the left suddenly gave rise to two new species that appear to have little change once they appear in the fossil record, reaching a sort of evolutionary 'equilibrium.'

Butterflies and Punctuated Equilibria
Punctuated Equilibrium

It may be tempting to make assumptions about the species that came about suddenly and changed very little, but we need to be careful. The new species may not have originated as suddenly as the fossil record suggests because shorter periods of time cannot be easily distinguished between the layers. For example, if the butterflies in the punctuated equilibria model had most of their changes occur within the first 50,000 years of existence (a relatively short time period in evolutionary standards), these would not show up well in the fossil record.

Additionally, if the change that led to the new species occurred in a remote location or from a small, isolated population, the chances of those fossils being discovered is quite low. So, while the fossil record does help us understand where some new species came from, it can be misleading because we do not always have all of the details about the changes that occurred.

Molecular Clocks

The fossil record is very useful for understanding rates of evolution, but we can also use molecular data to track evolutionary time. Some parts of an organism's genome, or all of its genetic information, accumulate changes at constant rates. This allows us to compare genomes of different organisms to determine evolutionary relationships.

For example, just by looking at them you might think that sharks and tuna are more closely related than bats and dolphins. Actually, the reverse is true! By comparing their DNA, we know that bats and dolphins are more closely related because their homologous genes, the ones that are similar in location, structure or function, are more alike than the homologous genes of sharks and tuna.

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