A diagram of the tree of life, showing the evolutionary relationships among major groups of organisms

The Tree of Life: All living things are related by common ancestry. This diagram shows the evolutionary relationships among major groups of organisms.
Credit: NASA Astrobiology / Wikimedia Commons

Scientific Definitions

Nature
"Evolution is a process that results in changes in the genetic material of a population over time."
Nature Scitable
University of California Museum of Paleontology
"Biological evolution, simply put, is descent with inherited modification."
Understanding Evolution (UC Berkeley)
National Human Genome Research Institute
"Evolution, as related to genomics, refers to the process by which living organisms change over time through changes in the genome. Such evolutionary changes result from mutations that produce genomic variation, giving rise to individuals whose biological functions or physical traits are altered. Those individuals who are best at adapting to their surroundings leave behind more offspring than less well-adapted individuals. Thus, over successive generations (in some cases spanning millions of years), one species may evolve to take on divergent functions or physical characteristics or may even evolve into a different species."
NHGRI Genetics Glossary
Encyclopedia Britannica
"Evolution [is a] theory in biology postulating that the various types of plants, animals, and other living things on Earth have their origin in other preexisting types and that the distinguishable differences are due to modifications in successive generations."
Encyclopedia Britannica
BioLogos
"The theory of evolution states that all the lifeforms on earth share a common ancestor as a result of variation and selection over a very long time (currently thought to be around 4 billion years)."
BioLogos
National Academies
"Evolution consists of changes in the heritable traits of a population of organisms as successive generations replace one another. It is populations of organisms that evolve, not individual organisms."
National Academies

Observable Facts Supporting Evolution

  1. Life forms show variation within populations due to genetic differences.
  2. These variations are heritable, passed from parents to offspring via DNA.
  3. The fossil record reveals changes in life forms over geological time, including extinct species and transitional forms.
  4. The geographic distribution of species reflects their history, with related species often found nearby.
  5. Organisms share fundamental genetic, molecular, anatomical, and behavioral similarities.

The theory of evolution scientifically explains these interconnected observations about life's diversity and history.

Conclusion

As these definitions make clear, evolution is a unitary process operating at multiple scales. The scientific community is unanimous in recognizing that the mechanisms that drive small-scale changes (microevolution) are the same mechanisms that, over longer time periods, produce large-scale changes (macroevolution) including the origin of new species, genera, and higher taxonomic groups.

The distinction between microevolution and macroevolution is simply one of time scale and degree of change, not a difference in mechanism. Attempts to accept microevolution while rejecting macroevolution are inconsistent with the scientific understanding of evolutionary processes.