User:Silence/Introduction to evolution

Evolution is the change in groups of living things over time. Living things (organisms) have children (offspring) which differ from their parents in minor random ways. Many of these differences, called traits, can be passed down to future generations of offspring during reproduction. Evolution is the process of these inheritable differences becoming more common or rare within large groups (populations) of organisms.

Evolution occurs in two different ways. The first way is random &mdash; when a population's traits change by chance. The second way is called selection. Selection happens when a trait helps an organism to have more offspring, such as by keeping the organism from dying early. This helpful trait will tend to become more common in the population, because organisms with the trait produce more offspring &mdash; who may inherit the same trait.

Selection and random change can cause more and more differences to accumulate in a population, eventually resulting in new species. Every living thing is distantly related &mdash; every organism is part of an enormous family tree. This means that all differences between species have arisen through this gradual process of change, as different populations have evolved in different ways.

What evolution is not
In clearly defining evolution, it is helpful to also clarify what evolution is not.

Evolution is not a theory, as defined in biology. Evolution is an observed process and natural phenomenon in the world, akin to gravity or aging. As such, it is a scientific fact. The world evolution is also sometimes used as shorthand for "theory of evolution", a well-supported scientific theory which explains how the observed process of evolution occurs. To avoid ambiguity, the term evolution will here signify the observed fact, and not the broader theory.

Evolution is not progress. Evolution is simply change, and change can be good, bad, or neutral. Evolution may seem progressive at times, because beneficial traits tend to out-compete less helpful traits under selection. However, evolution does not aspire toward any goal; there is no such thing as 'backward evolution' or 'de-evolution' because there is also no 'forward evolution' &mdash; evolution does not move in any particular direction. Even natural selection is not progress, since a trait that is helpful in one environment may be harmful after the environment changes. And lastly...

Evolution is not sudden. It is an extremely gradual, incremental process, usually requiring millions of years to effect obvious or dramatic change. Any major evolutionary change requires thousands of intermediary transitional forms. However, an organism can only be called "transitional" in retrospect, looking back over countless millennia; evolution does not make long-term plans, and is only guided by short-term, opportunistic selection on an individual level.

Evolution, then, is a directly observed occurrence. Although selection can result in adaptations which help organisms flourish in their environment, most evolution is not particularly helpful. And although evolution can eventually result in dramatic changes, in the short term it is a meandering process of minor modifications.

How does evolution work?
Evolution is the process of organisms' inherited traits changing from generation to generation. As such, evolution follows from two simple facts:


 * 1) Variation: Organisms in a population are different from each other.
 * 2) Heredity: Some of these differences are passed on to the next generation.

This means that populations can change &mdash; and that they do change, whenever a different trait increases or decreases in commonness. This change is evolution. Traits can even vanish entirely, and new traits can appear.

Variation
Evolution can only occur because offspring are not perfect clones of their parents &mdash; parents and children are different.

There are two ways that organisms can be different from each other: genetically and environmentally. These are equivalent to "nature" and "nurture", respectively. If an organism's environment changes, it will develop differently. However, this change on its own is not evolution: Dogs will grow bigger if they are fed more, but this size increase will not be passed on to offspring. The "nature" part of organisms, the characteristics they are born with, are what can evolve.

This type of variation, genetic diversity, is directly passed down to offspring during reproduction. Although this diversity is encoded in a tiny molecule called DNA stored in each cell of an organism, it has a major impact on the visible traits of an organism.