WHAT IS EVOLUTION?

Basically, biological evolution refers to genetic change in a population of organisms.

Evolving populations usually change from simpler states to more complex ones. Thus, the first living things were one-celled or even simpler, but today the most complex organisms are composed of many millions of cells. The first living things did little more than reproduce, but today living things can think and create. On Earth, surely the most "highly evolved" organism is the human, because of the human brain, but other plants and animals also have amazing adaptations and capabilities.

It is useful to recognize two basic forms of biological evolution: Microevolution and macroevolutuion.

MICROEVOLUTION

Let's say we start out with a population of reddish dragonflies such as the one at the right. Let's also say that the climate is slowly warming up, year after year. It happens that Bergmann's Rule (see our Ecological Rules page) asserts that geographic races of a species possessing smaller body size are found in the warmer parts of the range, and races of larger body size in cooler parts. Reflecting this dynamic, over many generations the average size of individuals of the reddish dragonfly species shown at the right become smaller, as they adapt to the growing heat. This is an example of microevolution.

Microevolution refers to adaptive change within a species.

The mechanism enabling microevolution is natural selection.  Here's how a particular instance of natural selection worked:

In Europe there is a speckled moth called the Peppered Moth, Biston betularia. Among its members are very light-colored ones, and very dark-colored ones. Before the mid 1800s, the pale-colored ones were by far the most common kind encountered. Since they were less visible when resting on pale tree trunks, birds had a harder time seeing them, and eating them than they did the dark ones. However, then came the Industrial Revolution with its pollution, which covered tree trunks with soot making them darker. Now the dark-colored moths  became the most common form because now they blended in the best with the dark, soot-covered tree trunks. In other words, with regard to the entire species, the Peppered Moth gradually evolved toward the darker form, because the darker form was better adapted for a landscape darkened with soot. This is an example of how selection can alter the relative proportion of an existing trait in a population. In other words, it demonstrates a mechanism for how microevolution works.

MACROEVOLUTION

You know that at one time in geological history dinosaurs existed (Daniel's hand is over a dinosaur footprint in Utah at the left) but now they don't. You also know that dinosaurs existed long before humans appeared on Earth. These facts relate to the macroevolution of species on Earth.

Macroevolution refers to the appearance and disappearance (extinction) of many species through time.

What is the evidence for macroevolution? The evidence is abundant, and found in several fields:

• The fossil record: As our Fossil Page explains, usually fossils are deposited with the oldest ones at the lower levels of geological strata. As you dig deeper and deeper, you can watch species come and go through geological history, with the older species becoming simpler and simpler in structure. You might enjoy referring to our Geological Time Scale where you can see how different kinds of organisms arose and went extinct throughout Earth's evolutionary history.

• The molecular record: When you study the fossil record and figure out when new species arose from their ancestors, you see that the longer they have been separated in geological time, the more differences there are in their DNA (genetic material) and protein.

• Homology: This interesting word refers to the fact that related species share similar patterns of bodily structure. For instance, all animals with backbones share the same pattern of bones, muscles, nerves, blood circulation and organs. Moreover, the arrangement of these things becomes more and more complex as one moves from the fishes to amphibians to reptiles and mammals. This suggests that over time progressive change has taken place.

• Development: Developing embryos pass through stages exhibiting traits of the species from which they arose. For example, human embryos have gill slits like fish and later  possess a tail. Human fetuses even possess a fine fur (the lanugo) during the fifth month of development.

• Vestigial structures: Many species retain structures that were important to the species from which they arose, but which no longer are useful. For example, the human appendix is what remains of the time when our ancestors needed a pouch off the large intestine for bulky food to be stored, perhaps where bacteria could break it down more before being sent on through the intestine. Think in terms of our primate ancestors eating roots, tough fruits and such. Take a look at Live Science's Top Ten Useless Limbs (and Other Vestigial Organs).

• Convergent Evolution: Since a specific form or manner of being may be the best for exploiting a certain ecological niche, unrelated organisms in different parts of the world often evolve to look like one another. For example, in Australia many marsupial mammals have evolved to look strikingly like very unrelated placental mammals in other parts of the world. Australia's marsupial Tasmanian Wolf looks much like North America's placental Wolf. Australia's Flying Phalanger looks just like North America's Flying Squirrel. On and on it goes. You may enjoy reading about a spectacular case of convergent evolution between two species of poison frogs, one in South America and another in Madagascar.

• Patterns of distribution: Charles Darwin pointed out that organisms living on oceanic islands resemble most closely the forms living on the nearest continent. That wouldn't make sense if all organisms were specially made, instead of having evolved from common ancestors.

An interesting point about macroevolution is that its rate does not appear to have been taken place at a constant rate throughout evolutionary history. Macroevolution has proceeded in jerks -- there were moments in time when it sped up, others when it slowed down. So far no general explanation for why this might be so has been accepted by all scientists. More on the pace of evolution here.

In 2007, a study appearing in PLoS Biology showed that though sexual reproduction has not occurred in a certain group of bdelloid rotifers for 40,000,000 years (Females lay eggs that are genetic clones of their mothers; the species has no males)  the group has evolved into different species. Therefore, sexual reproduction is not needed for evolution to take place and new species to arise. Read more about rotifers, including the bdelloids, here.

Well, there's still a lot not understood about evolution. About the only fact accepted by all serious, unbiased researchers is this: