The theory of evolution, primarily articulated by Charles Darwin in the 19th century, posits that species of organisms change over time through a process known as natural selection. Key concepts of this theory include:

1. **Variation**: Within any given population, individuals exhibit variation in their traits (e.g., size, color, shape, behavior). This variation can be genetic and is often influenced by mutations.

2. **Inheritance**: Traits that are beneficial for survival and reproduction can be inherited from one generation to the next. Offspring tend to resemble their parents, but genetic variation can introduce new traits.

3. **Natural Selection**: Organisms with traits that give them a reproductive advantage in their environment are more likely to survive and reproduce. Over time, these advantageous traits become more common in the population. This is often summarized as "survival of the fittest," where "fittest" refers to those best suited to their environment, not necessarily the strongest.

4. **Speciation**: Over long periods, the accumulation of changes can lead to the emergence of new species. When populations become isolated or face different environmental pressures, they can diverge and evolve independently.

5. **Common Descent**: All life on Earth is thought to share a common ancestor. Through the process of evolution, diverse forms of life have arisen from this common lineage, which is supported by evidence from various fields such as genetics, paleontology, and comparative anatomy.

The theory of evolution is a fundamental concept in biology and provides a framework for understanding the complexity of life on Earth. It is supported by a vast body of scientific evidence and continues to be refined and expanded through ongoing research.
