What is evolution and why do biologists think it’s important?

Learning Objectives

  1. List the conditions that cause populations of living organisms to evolve
  2. Distinguish biological evolution of populations from changes to individual organisms over a lifetime.
  3. Know that two mechanisms of evolution are mutation and natural selection, and recognize examples of each.
  4. Cite evidence that all life on earth has a common origin
  5. Identify common misconceptions about evolution

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 over succeeding generations. 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.

Suppose there is heritable variation in a population, and the heritable variation makes a difference in the survival and reproduction of individual 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 evolution by natural selection. In On the Origin of Species by Natural Selection (1859), Charles Darwin described four requirements for evolution by natural selection:

  1. the trait under selection must be variable in the population, so that the encoding gene has more than one variant, or allele.
  2. the trait under selection must be heritable, encoded by a gene or genes
  3. the struggle of existence, that many more offspring are born than can survive in the environment.
  4. individuals with different alleles have differential survival and reproduction that is governed by the fit of the organism to its environment

Darwin 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.

Because the definition of evolution is change in the heritable characteristics of a population over generations, evolution can occur by means other than natural selection. Evolution can also occur via random processes, especially in small populations, where the frequency of some heritable traits may rise or fall just by chance.

One of these other mechanisms of evolution is called mutation

Mutation generates variation

Evolution by mutation occurs when the heritable cells of organisms make a mistake when they replicate their DNA. In single-celled asexual organisms, such as bacteria and archaea, the whole cell and its DNA is passed on to the next generation because these organisms reproduce via binary fission. For sexual organisms, mutations are passed to the next generation if they occur in the egg or sperm cells used to create offspring. Mutations occur at random in the genome, but mutations of large effect are often so bad for the organism that the organism dies as it develops, so mutations of smaller effect or even neutral mutations are theoretically more common in a population.

The random process of mutation generates standing genetic variation in a population, and this variation must be present for evolution to occur. Mutation is the raw stuff of evolution because it creates new heritable phenotypes without regard for how good those phenotypes are for the survival and reproduction of the organism. How frequent are mutations? Mutation rates are actually pretty low for most genes, ranging from 1 in a million for the average human gene to less than 1 in a billion for the average bacterial gene (from http://bionumbers.hms.harvard.edu/). Because mutation rates are low relative to population growth in most species, mutation alone doesn’t have much of an effect on evolution. 

The video below defines and gives examples of biological evolution, and ends with a teaser about the role of natural selection in biological evolution.

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. This theory has been tested in numerous ways by the work of many thousands of scientists. Every test has produced results that are consistent with the theory. Evolutionary biologists conduct research to elaborate or refine the theory and understand the mechanisms at work in specific populations. 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).

The scientific use of the word theory is very different from the casual, every-day use.  A scientific theory is an overarching, unifying explanation of phenomena that is well supported by multiple, independent lines of evidence – i.e., composed of hundreds or thousands of independent, well-supported hypotheses.  For example, germ theory is the theory that explains how microorganisms cause disease, and cell theory explains how cells function as the basic unit of life.

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
  • homologies in body plans, structures, and DNA sequences indicative of common ancestry
  • 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.

The video below highlights some of this key supporting evidence in the context of the evolution of whales:

Common misconceptions about evolution

There are many common misconceptions about evolution in general, and evolution by natural selection:

Here are corrections to some common misconceptions about evolution by natural selection:

  1. The individual undergoing natural selection does not evolve–it just lives or dies! Instead, the population of organisms is evolving. Recall that evolution is the change in allele frequencies, and only populations have allele frequencies. Individuals just have alleles.
  2. Evolution is not a directed process with a fixed end point, or a best phenotype. Rather, the environment serves as a selective agent. No amount of planning on the part of the organism can predict whether an organism will be a good fit for the environment it finds itself in. An individual cannot “try” to evolve or “anticipate” the types of mutations it should have for future environmental change.
  3. Organisms, and the genes they contain, do not behave for the ‘good of the species.’ Rather, each individual lives and reproduces, which increases its representation in the gene pool, or it dies or fails to reproduce and is not represented in the gene pool. Those most represented after encountering a selective agent are considered the “most fit” for that environment, in that time and place.
  4. Selection doesn’t always result in the best possible fit of an organism to its environment because of constraints and trade-offs. Sometimes the same genes that code for a trait also cause a second, suboptimal trait to occur.
  5. Mutations are not caused or induced as a result of environmental change. Variation is already present in the population. When the environment changes, those individuals that already have some beneficial variation (mutations) that is well suited to the new environment are more likely to survive and reproduce; organisms do not develop new mutations in response to the environmental change.  (And if there is no variation present in the population such that some individuals survive and reproduce, then the population is likely to go extinct).

At its simplest, evolution distills down to the idea that as long as there is variation in a population, as long as that variation is heritable, and as long as there is differential reproductive success (not everyone reproduces equally), then the next generation will be genetically different from the previous generation. We will explore the mechanisms that contribute to evolution over the next class sessions.


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 http://blogs.scientificamerican.com/psi-vid/2012/02/29/an-evolution-animation-unlike-any-youve-seen-before/ and Tyler Rhodes blog http://evolutionanimation.wordpress.com/ 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.