In short: the theory of evolution says that living organisms change over generations because heritable genetic variation is filtered by processes such as natural selection, genetic drift, mutation, and gene flow, so populations become better adapted (or simply different) to their environments and can diverge into new species. All living things share common ancestry—the diversity of life today arose by descent with modification from earlier life forms.

Key points
- Variation exists among individuals in a population, and some of that variation is heritable (passed to offspring).
- Mutations and genetic recombination create new genetic variation.
- Natural selection is the non-random process by which individuals with traits that increase survival and/or reproduction tend to leave more offspring, so those traits become more common.
- Other mechanisms (genetic drift, gene flow, sexual selection) also change allele frequencies, especially in small populations.
- Over many generations these changes can accumulate, producing adaptations and, eventually, new species (speciation).
- The result is a branching “tree of life” showing how species are related by common descent.

Evidence supporting the theory
- Fossil record: shows sequences of forms over time and transitional fossils.
- Comparative anatomy and embryology: shared structures and developmental patterns indicate common ancestry.
- Biogeography: geographic distributions of species match evolutionary histories (e.g., island endemics).
- Genetics and molecular biology: DNA sequences reveal patterns of relatedness and mechanisms for inheritance and change.
- Direct observation: documented cases such as bacterial antibiotic resistance, pesticide resistance in insects, and rapid changes in some wild populations.

What “theory” means here
- In science, a theory is a well-supported, coherent explanation for a wide range of observations and data. The theory of evolution is supported by multiple, independent lines of evidence and is a central unifying principle of biology.

Common misconceptions
- Evolution is not a claim that life arose by chance alone or that it has a goal; it’s a natural process producing change over time.
- Individuals do not evolve—populations do, across generations.
- Humans did not evolve “from” modern apes; humans and other apes share common ancestors.

Everyday example
- When bacteria evolve resistance to an antibiotic, resistant variants survive treatment and reproduce, so the population becomes more resistant—a direct, observable instance of evolution by natural selection.

If you want, I can explain any part in more detail (e.g., how natural selection works mathematically, examples of speciation, or the genetic mechanisms that produce variation).
