Life evolves!

Learning Objectives

  1. Students will list the conditions that cause populations of living organisms to evolve.
  2. Students will distinguish biological evolution of populations from changes to individual organisms over a lifetime.
  3. Students will be able to name the 3 domains of life.
  4. Students shall be able to cite evidence that all life on earth have a common origin.

Evolution as an emergent property of life

A key part of any definition of life is that living organisms reproduce. Let’s now add a couple of observations:

  • The process of reproduction, while mostly accurate, is imperfect. When cells divide, they have to replicate their DNA. Although DNA replication is highly accurate, it still makes about 1 mistake in 10 million nucleotides. Over generations, the population will contain lots of heritable variation.
  • The population of a given type of organism will tend to grow exponentially, but will reach a limit, where the individuals have to compete with each other for the limiting resource (food, space, mates, sunlight, etc.)

Suppose some heritable variations (speed, strength, sharper claws, bigger teeth) make some individuals more competitive for the limiting resource – what will happen?
The individuals with superior variants will acquire more resources, and have more progeny. If the superior variants are heritable, then their progeny will have the same superior variants. Over generations, then, a larger and larger proportion of the population will consist of individuals with the superior heritable variants. This is biological evolution.
Definition: Biological evolution is change in the heritable characteristics of a population. In more technical terms, evolution is defined as change in the gene pool of a population, measurable as changes in allele frequencies in a population.
So for evolution to occur, all that is needed is heritable variation in a population, and conditions where the heritable variation makes a difference in the survival and reproduction of organisms. If these conditions exist, and they do for all natural populations of living organisms, evolution must occur. Life evolves!
Charles Darwin called this process natural selection. He and Alfred Wallace were the first to propose that evolution by natural selection could explain the origin of all the multitudes of species on Earth and how they appear so well-adapted in form and function to their particular environments. Moreover, Darwin proposed that all of life on Earth descended from a common ancestor, via slow, incremental accumulation of heritable (genetic) changes.

Evolution is a theory, not just a hypothesis

Darwin published his theory of evolution in the Origin of Species (1859), with carefully reasoned evidence to support this theory that all life on earth evolved from a common ancestor, by natural selection. This theory has been tested in numerous ways by the work of many thousands of scientists. Every test has produced results that confirm or elaborate or refine the theory. Evolutionary theory now forms a framework for biological thinking, so that one famous evolutionary biologist wrote that “Nothing in Biology Makes Sense Except in the Light of Evolution” (Dobzhansky, 1973).

Title page of Darwin’s The Origin of Species, 1859 from Wikipedia

A few key lines of supporting evidence:

  • geological and fossil record, showing that the Earth is about 4.5 billion years old, and sequential changes in the kinds and forms of living organisms over geological time scales
  • a common biochemistry for all life on Earth – the same amino acids, the same biological building blocks, the same genetic code
  • inference of evolutionary relationships from gene sequence comparisons largely agree with the fossil record, and are consistent with a common origin for all extant life on Earth.

Three domains of life

DNA sequence comparisons and structural and biochemical comparisons consistently categorize all living organisms into 3 primary domains: Bacteria, Archaea, and Eukaryota. Both Bacteria and Archaea are prokaryotes, single-celled microorganisms with no nuclei, and Eukaryota includes us and all other animals, plants, fungi, and single-celled protists – all organisms whose cells have nuclei to enclose their DNA apart from the rest of the cell. The fossil record indicates that the first living organisms were prokaryotes (Bacteria and Archaea), and eukaryotes arose a billion years later.

Tree of life showing the three domains of life. Image credit: UC Museum of Paleontology (

For thought and discussion:

Think of some ways that evolution can be or has been tested. What testable predictions arise from evolutionary theory?
How does the work of many geologists or some physicists test evolutionary theory?
What are some common misconceptions about evolution?

Evolution Resources:

Evolution 101 University of California Berkeley evolution site, a complete resource for learning and teaching about evolution. Engaging, well-illustrated, accurate.
How did feathers evolve? Carl Zimmer’s TED-Ed video, 3 1/2 minutes.
Evolution animation by Tyler Rhodes, produced from drawings made by children copying a drawing of a salamander-like animal with successive generations of variation, mass extinction and selection. The process is described in this Scientific American blog post and Tyler Rhodes blog describes both the drawing “game” and his animation process. His “wheel of life” is an amazing phylogenetic tree of the drawings.
Newly found: the world’s oldest fossils A post in the Why Evolution Is True blog by Jerry Coyne, explaining the paper by Wacey, D.,M. R. Kilburn, M. Saudners, J. Cliff, and M. D. Brasier.  2011.  Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia.  Nature Geoscience online: doi:10.1038/ngeo1238
Darryl Cunningham Investigates: Evolution A lucid, inviting comic-strip presentation of basic evolutionary theory and evidence. Aimed at beginning learners.
Evolution Made Simple BBC Bang illustrates evolution beginning with a simple straight line, and replication with errors leads rapidly to diversification.
Nothing in Biology Makes Sense Except in the Light of Evolution Dobzhansky’s 1973 essay in The American Biology Teacher 35:125-129, just as relevant today as then, and I have yet to read a better explanation.

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