Is there evidence for evolution?
In the 150 years since Darwin proposed the theory of evolution by natural selection, a mountain of evidence has accumulated to support the theory. A greatly expanded fossil record since Darwin's time, the discovery of DNA and the process of genetic replication, an understanding of radioactive decay, observations of natural selection in the wild and in laboratories, and evidence in the genomes of many different organisms, including humans, have all bolstered the validity of the theory of evolution.
How can you know what happened millions of years ago if no one was there to see it?
Evidence and observation are the building blocks of all scientific inquiry; evolutionary science is no different. Evidence in the form of the fossil record, geological formations, and genetics attest to change having taken place and give clues to how evolution works. The theory of evolution puts these clues together into a cohesive explanation of the diversity of living things. Like all theories, the theory of evolution relies on tangible evidence as well as inference for those things that can't be observed directly. It is important to remember that Earth itself contains evidence of life in the past, and that this evidence provides critical support for the theory of evolution.
Does the fossil record tell us the whole story?
Opponents of evolution point to gaps in the fossil record as proof that the theory is invalid. They say the fossil record fails to show what are called "transitional forms," generally the in-between stages as one type of creature evolved into another. The fossil record certainly has gaps, mostly because the conditions required to create fossils have been rare ever since life began on Earth. A very small percentage of animals that have lived and died ever became fossils. Thus, many pieces of the puzzle are missing; some will never be found. Nonetheless, we have many, many fossils that illustrate evolutionary transitions between fish and amphibians, between reptiles and mammals, between dinosaurs and birds, and in many lineages such as whales and horses. And new fossils continue to reveal transitional forms that some said don't exist.
Can you observe evolution happening?
Because for many species, humans included, evolution happens over the course of many thousands of years, it is rare to observe the process in a human lifetime. Usually only laboratory scientists studying quickly reproducing life forms, like single-celled creatures and some invertebrates, have the opportunity to see evolutionary change happen before their eyes. All of us can and do experience the indirect effects of evolution nearly every day, however. One of the more important evolutionary concerns facing humans today is the emergence of antibiotic-resistant microbes. A battle against bacteria that we have been winning with medicine for the last 50 years or so is now an even race, according to some scientists -- because of the rapid rate of bacterial evolution. Similarly, the use of pesticides in agriculture has driven the evolution of resistant insects that require more or harsher chemicals to be killed. Scientists studying Galapagos finches have seen evolutionary changes in beak size and shape in just a few years. Major evolutionary transformations take much, much longer.