Natural Selection

Evolution is change over time. Specific to biology, it is the idea that living things from long ago changed over long periods of time into the organisms that are alive today. Natural Selection describes how evolution works. First described by Charles Darwin in the early 1800s, the basic idea is that organisms with good traits survive better and pass on their good traits to their offspring. Traditionally, it is taught as four or five individual steps or conditions:

There is variation in a population.
- Individuals have different traits cause by different genes/DNA
Life is dangerous; death is always lurking.
- Species tend to over-reproduce; they have more offspring than will survive to adulthood
Individuals with better traits will survive more often than those with less desirable traits.
- Fitness is how well an individual "fits" in its environment - how well it can survive.
The individuals that survive more often pass on their good traits.
- The genes that caused the good traits become more common in future generations
Over long periods of time, differences can add up to new traits and new species
- Different environments favor different traits, and environments also change over time.

Artificial Selection

The term "Natural Selection" means that nature chooses which traits are good (selected for) and which traits are bad (selected against). It is often helpful to first understand artificial selection - when humans make the choices.

Evolution of Corn

10,000 years ago, there was no such thing as corn
Humans in what is now central Mexico "discovered" a wild grass called teosinte that made sugary seeds in a single row on a stalk.
At some point, a mutation occurred in the plant and it made a few more seeds.
The "farmers" planted those seeds and started to only grow teosinte plants with the bigger stalks.
More mutations occurred over the years. If they were good, the farmers planted the seeds; if they were bad, the farmers killed the plants to make room for good plants.
Over thousands of years, farmers selected plants with more and more seeds and eventually grew maize, which has now become corn.

Natural Selection

In natural selection, there is no farmer choosing which seeds to plant. Those that are "most fit" can survive in nature and pass on their traits. One of the best examples of a species that has to deal with both of these demands of natural selection are the guppies on the island of Trinidad. Guppies are little fish that live in the pools of mountain streams. The pools are separated by swift currents and waterfalls making it nearly impossible for guppies to move upstream.

Lower Pools

The lowest pools of the streams tend to have more predatory fish that eat guppies.

Guppies are brown/drab color similar to the mud and plants at the bottom of the pool.
Camouflage helps them hide from predators and survive.
Those that survive to adulthood can mate and have brown/drab colored babies that are also likely to survive.
The predators are the main selection factor - if a guppy stands out, it will die.

Upper Pools

The pools higher up on the mountain tend to be smaller and don't have any predatory fish.

Guppies tend to have bright colors and patterns that DO NOT blend in with the mud at the bottom of the pool.
Female guppies find the bright colors more attractive.
Brightly colored males pass on their traits while brown/drab males do not.
Guppies of any color tend to survive equally, but only the brightest pass on their genes to the next generation.