What Is Evolution? How It Works — Mechanisms & Evidence

What is evolution?

In every field of scientific endeavor there sometimes comes a quantum shift in knowledge, a grand denouement that changes how we see the world. In physics we have Newton’s laws, Einstein’s Relativity and Quantum Mechanics; in biology we have the discovery of DNA and Darwinian evolution.

Why evolution matters

It’s hard to overstate the importance of the theory of evolution. It did not produce the immediate technological effects that relativity produced (for example, nuclear power) or the medical applications that genetics now provides (gene therapy, GM crops), but as a piece of fundamental science it ranks with the great conceptual advances.

Evolution gives us keys to the past and present of life, and it frames our understanding of where life may go. Questions that have long occupied philosophers and religions—why we are here, how we arose, what we might become—find a scientific answer in evolution. The patterns we observe in nature provide testable explanations rather than myth or guesswork.

Evolution also drives modern biology. The theory stimulated progress across microbiology, immunology, and genetics by removing mystical explanations and replacing them with mechanisms that can be tested and applied. Many advances in medicine and public health ultimately depend on an evolutionary understanding of organisms, pathogens and genomes. In short, evolution has both deep explanatory power and practical consequences.

How does evolution happen?

At its most basic, evolution requires three components: random variation, genetic inheritance, and natural selection. These components interact over generations to change the frequency of traits in populations and, over long periods, produce new species.

1. Random variation

Individuals within any species show variation. Some differences arise from environment (for example, illnesses or nutrition), but others are genetic. Two main genetic sources of variation are the reshuffling of existing genes during sexual reproduction and random mutations.

During sexual reproduction alleles (different versions of the same gene) are mixed: an offspring receives one allele from each parent, producing new combinations of traits. This reshuffling alone creates genetic diversity in a population.

Mutations are random changes in DNA that occur during cell division or because of chemical interactions or radiation. Most mutations are neutral or harmful and are repaired or eliminated; some cause disease (for example, certain cancers), and a few are beneficial. When mutations affect gametes (sperm or eggs), they can be passed to the next generation and contribute new genetic variants to the gene pool.

2. Genetic inheritance

Inheritance is the mechanism that passes genetic variation from parents to offspring. Genes come in pairs (one copy from each parent). Alleles can be dominant or recessive. A dominant allele is expressed if at least one copy is present; a recessive allele is expressed only when both copies are recessive.

Example: suppose allele A is dominant and allele b is recessive. Then:

• AA → trait determined by A
• bb → trait determined by b
• Ab → trait determined by A

Sexual reproduction combines half the alleles from each parent, so offspring inherit a unique mix. Recessive alleles can “hide” in carriers and reappear later, which affects how traits spread through populations.

3. Natural selection

Natural selection is the non-random process that changes trait frequencies based on differential survival and reproduction. Organisms compete for limited resources—territory, food, mates—and those with traits that confer an advantage are more likely to survive and reproduce.

Environmental changes (weather, new diseases, predators, human-made pressures such as antibiotics) alter which traits are advantageous. For example, the widespread use of antibiotics has selected for resistant strains of bacteria. Over generations, successful traits become more common, neutral traits stay roughly the same, and disadvantageous traits decline.

Natural selection also disfavors wasteful traits that reduce fitness. Crucially, selection has no final goal: it favors incremental advantages (or neutrality) in given environments, not perfection.

From variation to new species

Evolution is the cumulative effect of random mutations and recombination passed down by inheritance, filtered by natural selection and other processes (genetic drift, gene flow, sexual selection). Over long timescales, these processes can cause populations to diverge and form new species, or lead to extinction when lineages fail to adapt.

Although the underlying processes include chance, the patterns they produce—fossil sequences, shared anatomy, genetic similarities—provide strong evidence that evolution is the historical process that produced Earth’s biodiversity. For further reading on evidence for evolution, see five lines of evidence.