From: lamoran@gpu.utcs.utoronto.ca (L.A. Moran)
Subject: DEFINITION OF EVOLUTION
Organization: UTCS Public Access
Date: Sat, 23 Jan 1993 22:19:15 GMT


                            WHAT IS EVOLUTION?
                         version 2, January 22, 1993

Most non-scientists seem to be quite confused about precise definitions of
biological evolution. Such confusion is due in large part to the inability of 
scientists to communicate effectively to the general public and also to
confusion among scientists themselves about how to define such an 
important term. When discussing evolution it is important to distinguish 
between the existence of evolution and various theories about the mechanism
of evolution. And when referring to the existence of evolution it is important
to have a clear definition in mind. What, exactly, do biologist mean when they
say that they have observed evolution or that humans and chimps have evolved
from a common ancestor.

One of the most respected evolutionary biologists has defined biological
evolution as follows,

     "In the broadest sense, evolution is merely change, and so is 
      all-pervasive; galaxies, languages, and political systems all
      evolve. Biological evolution ... is change in the properties
      of populations of organisms that transend the lifetime of a 
      single individual. The ontogeny of an individual is not considered
      evolution; individual organisms do not evolve. The changes in 
      populations that are considered evolutionary are those that are
      inheritable via the genetic material from one generation to the 
      next. Biological evolution may be slight or substantial; it embraces
      everything from slight changes in the proportion of different
      alleles within a population (such as those determining blood 
      types) to the successive alterations that led from the earliest
      protoorganism to snails, bees, giraffes, and dandelions."

      Douglas J. Futuyma in Evolutionary Biology, Sinauer Associates 1986

It is important to note that biological evolution refers to populations and
not to individuals and that the changes must be passed on to the next 
generation. In practice this means that,

      Evolution is a process that results in heritable changes in a 
      population spread over many generations

This is a good working scientific definition of evolution; one that can be 
used to distinguish between evolution and similar changes that are not 
evolution. Another common short definition of evolution can be found in 
many textbooks,

     "In fact, evolution can be precisely defined as any change in the
      frequency of alleles within a gene pool from one generation to 
      the next."

      Helena Curtis and N. Sue Barnes BIOLOGY 5th ed. 1989
      Worth Publishers, p.974

One can quibble about the accuracy of such a definition (and we have often
quibbled on these newsgroups) but it also conveys the essence of what 
evolution really is. When biologists say that they have observed evolution 
they mean that they have detected a change in the frequency of genes in a 
population. (Often the genetic change is inferred from phenotypic changes that
are heritable.) When biologists say that humans and chimps have evolved from
a common ancestor they mean that there have been successive heritable changes
in the two separated populations since they became isolated.

Unfortunately the common definitions of evolution outside of the scientific
community are different. For example, in the Oxford Concise Science Dictionary 
we find the following definition;

     "evolution: The gradual process by which the present diversity
      of plant and animal life arose from the earliest and most primitive
      organisms, which is believed to have been continuing for the
      past 3000 million years."

This is inexcusable for a dictionary of science. Not only does this definition
exclude prokaryotes, protozoa, and fungi, but it specifically includes a term
"gradual process" which should not be part of the definition. More importantly
the definition seems to refer more to the history of evolution than to 
evolution itself. Using this definition it is possible to debate whether
evolution is still occurring but the definition provides no easy way of
distinguishing evolution from other processes. For example, is the increase
in height among Caucasians over the past several hundred years an example of
evolution? Are the color changes in the peppered moth population examples
of evolution? This is not a scientific definition.

Standard dictionaries are even worse. 

     "evolution: ...the doctrine according to which higher forms of
         life have gradually arisen out of lower.."   Chambers

     "evolution: ...the development of a species, organsim, or organ
         from its original or primitive state to its present or
         specialized state; phylogeny or ontogeny:  Webster's

These definitions are simply wrong. Unfortunately it is common for non-
scientists to enter into a discussion about evolution with such a definition
in mind. This often leads to fruitless debate since the experts are thinking
about evolution from a different perspective. When someone claims that they 
don't believe in evolution they cannot be referring to an acceptable 
scientific definition of evolution because that would be denying something 
which is easy to demonstrate. It would be like saying that they don't believe 
in gravity!

Recently I read a statement from a creationist who claimed that scientists
are being dishonest when they talk about evolution. This person believed that
evolution was being misrepresented to the public. The real problem is that
the public, and creationists, do not understand what evolution is all about.
This person's definition of evolution was very different from the common
scientific definition and as a consequence he was unable to understand
what evolutionary biology really meant. This is the same person who claimed
that one could not "believe" in evolution and still be religious! But once
we realize that evolution is simply "a process that results in heritable 
changes in a population spread over many generations" it seems a little silly 
to pretend that this excludes religion!

Scientists such as myself, must share the blame for the lack of public 
understanding of science. We need to work harder to convey the correct
information. Sometimes we don't succeed very well but that does not mean that
we are dishonest. On the other hand, the general public, and creationists
in particular, need to also work a little harder in order to understand
science. Reading a textbook would help.


Laurence A. Moran (Larry)

Newsgroups: talk.origins