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.
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.
EXAMPLES OF MICROEVOLUTION
from notes by Bonnie Dalzell
One example of "evolution as we watch" is the rapid adaptation of pest species to resist pesticides and herbicides. Another is the adaptation of weed grains that are harvested and replanted with desirable grains. Some wild grasses grow in fields of cultivated grains and the ripening times and seed weight have come to match the ripening time and seed weight of the desirable grains so that the weeds are cultivated along with the cultivated grains.
Bacteria evolve resistance to our antibiotics even more rapidly but they can assimilate genes from other bacteria by ingestion so their evolution takes a different course than the evolution of sexually reproducing, multicellular 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.
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 species through time.
What is the evidence for macroevolution? The evidence is abundant, and found in several fields:
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: