From: L.A. Moran
To: All Msg #72, Jan-23-93 02:19PM
Subject: DEFINITION OF EVOLUTION
Organization: UTCS Public Access
From: lamoran@gpu.utcs.utoronto.ca (L.A. Moran)
Message-ID:
Newsgroups: talk.origins
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)
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From: L.A. Moran
To: All Msg #73, Jan-23-93 02:23PM
Subject: THE MODERN SYNTHESIS
Organization: UTCS Public Access
From: lamoran@gpu.utcs.utoronto.ca (L.A. Moran)
Message-ID:
Newsgroups: talk.origins
THE MODERN SYNTHESIS
A Theory of the Mechanism of Evolution
(version 1.5, January 22, 1993)
Many people do not understand current ideas about evolution. The following
is a brief summary of the modern consensus among evolutionary biologists.
The idea that life on Earth has evolved was widely discussed in Europe in
the late 1700's and the early part of the last century. In 1859 Charles
Darwin supplied a mechanism, namely natural selection, that could explain how
evolution occurs. Darwin's theory of natural selection helped to convince
most people that life has evolved and this point has not been seriously
challenged in the past one hundred and thirty years.
It is important to note that Darwin's book "The Origin of Species by Means
of Natural Selection" did two things. It summarized all of the evidence
in favor of the idea that all organisms have descended with modification
from a common ancestor, and thus built a strong case for evolution. In
addition Darwin advocated natural selection as a mechanism of evolution.
Biologists no longer question whether evolution has occurred or is occurring.
That part of Darwin's book is now considered to be so overwhelmingly
demonstrated that is is often referred to as the FACT of evolution. However,
the MECHANISM of evolution is still debated.
We have learned much since Darwin's time and it is no longer appropriate
to claim that evolutionary biologists beleive that Darwin's theory of Natural
Selection is the best theory of the mechanism of evolution. I can understand
why this point may not be appreciated by the average non-scientist because
natural selection is easy to understand at a superficial level. It has been
widely promoted in the popular press and the image of "survival of the
fittest" is too powerful and too convenient.
During the first part of this century the incorporation of genetics and
population biology into studies of evolution led to a Neo-Darwinian theory
of evolution that recognized the importance of mutation and variation within
a population. Natural selection then became a process that altered the
frequency of genes in a population and this defined evolution. This point of
view held sway for many decades but more recently the classic Neo-Darwinian
view has been replaced by a new concept which includes several other
mechanisms in addition to natural selection. Current ideas on evolution are
usually referred to as the Modern Synthesis which is described by Futuyma;
"The major tenets of the evolutionary synthesis, then, were
that populations contain genetic variation that arises by random
(ie. not adaptively directed) mutation and recombination; that
populations evolve by changes in gene frequency brought about
by random genetic drift, gene flow, and especially natural
selection; that most adaptive genetic variants have individually
slight phenotypic effects so that phenotypic changes are gradual
(although some alleles with discrete effects may be advantageous,
as in certain color polymorphisms); that diversification comes
about by speciation, which normally entails the gradual evolution
of reproductive isolation among populations; and that these
processes, continued for sufficiently long, give rise to changes
of such great magnitude as to warrent the designation of higher
taxonomic levels (genera, families, and so forth)."
Futuyma, D.J. in EVOLUTIONARY BIOLOGY
Sinauer Associates, 1986; p.12
This description would be incomprehensible to Darwin since he was unaware
of genes and genetic drift. The modern theory of the mechanism of evolution
differs from Darwinism in three important respects:
1) It recognizes several mechanisms of evolution in addition
to natural selection. One of these, random genetic drift,
may be as important as natural selection.
2) It recognizes that characteristics are inherited as discrete
entities called genes. Variation within a population is due
to the presence of multiple alleles of a gene.
3) It postulates that speciation is (usually) due to the gradual
accumulation of small genetic changes. This is equivalent to
saying that macroevolution is simply a lot of microevolution.
In other words, the Modern Synthesis is a theory about how evolution works
at the level of genes, phenotypes, and populations whereas Darwinism was
concerned mainly with organisms, speciation and individuals. This is a major
paradigm shift and those who fail to appreciate it find themselves out of
step with the thinking of evolutionary biologists. Many instances of such
confusion can be seen here in the newsgroups, in the popular press, and in
the writings of anti-evolutionists.
The major controversy among evolutionists today concerns the validity of
point #3 (above). The are many who believe that the fossil record at any one
site does not show gradual change but instead long periods of stasis followed
by rapid speciation. This model is referred to as Punctuated Equilibrium and
it is widely accepted as true, at least in some cases. The debate is over
the relative contributions of gradual versus punctuated change, the average
size of the punctuations, and the mechanism. To a large extent the debate is
over the use of terms and definitions, not over fundamentals. No new
mechanisms of evolution are needed to explain the model.
Some scientists continue to refer to modern thought in evolution as
Neo-Darwinian. In some cases these scientists do not understand that the
field has changed but in other cases they are referring to what I have
called the Modern Synthesis, only they have retained the old name.
Laurence A. Moran (Larry)
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From: L.A. Moran
To: All Msg #74, Jan-23-93 02:25PM
Subject: GENETIC DRIFT
Organization: UTCS Public Access
From: lamoran@gpu.utcs.utoronto.ca (L.A. Moran)
Message-ID:
Newsgroups: talk.origins
RANDOM GENETIC DRIFT
version 1, January 22, 1993
The two most important mechanisms of evolution are natural selection and
genetic drift. Most people have a reasonable understanding of natural
selection but they don't realize that drift is also important. The anti-
evolutionists, in particular, concentrate their attack on natural selection
not realizing that there is much more to evolution. Darwin didn't know about
genetic drift, this is one of the reasons why modern evolutionary biologists
are no longer "Darwinists". (When anti-evolutionists equate evolution with
Darwinism you know that they have not done their homework!)
Random genetic drift is a stochastic process (by definition). One aspect of
genetic drift is the random nature of transmitting alleles from one generation
to the next given that only a fraction of all possible zygotes become mature
adults. The easiest case to visualize is the one which involves binomial
sampling error. If a pair of diploid sexually reproducing parents (such as
humans) have only a small number of offspring then not all of the parent's
alleles will be passed on to their progeny due to chance assortment of
chromosomes at meiosis. In a large population this will not have much effect
in each generation because the random nature of the process will tend to
average out. But in a small population the effect could be rapid and
significant.
Suzuki et al. explain it as well as anyone I've seen;
"If a population is finite in size (as all populations are) and if
a given pair of parents have only a small number of offspring,
then even in the absence of all selective forces, the frequency
of a gene will not be exactly reproduced in the next generation
because of sampling error. If in a population of 1000 individuals
the frequency of "a" is 0.5 in one generation, then it may by chance
be 0.493 or 0.0505 in the next generation because of the chance
production of a few more or less progeny of each genotype. In the
second generation, there is another sampling error based on the new
gene frequency, so the frequency of "a" may go from 0.0505 to 0.501
or back to 0.498. This process of random fluctuation continues
generation after generation, with no force pushing the frequency
back to its initial state because the population has no "genetic
memory" of its state many generations ago. Each generation is an
independent event. The final result of this random change in allele
frequency is that the population eventually drifts to p=1 or p=0.
After this point, no further change is possible; the population has
become homozygous. A different population, isolated from the first,
also undergoes this random genetic drift, but it may become homozygous
for allele "A", whereas the first population has become homozygous for
allele "a". As time goes on, isolated populations diverge from each
other, each losing heterozygosity. The variation originally present
within populations now appears as variation between populations."
Suzuki, D.T., Griffiths, A.J.F., Miller, J.H. and Lewontin, R.C. in
An Introduction to Genetic Analysis 4th ed. W.H. Freeman 1989 p.704
Of course random genetic drift is not limited to species that have few
offspring, such as humans. In the case of flowering plants, for example,
the stochastic element is the probabilty of a given seed falling on fertile
ground while in the case of some fish and frogs it is the result of chance
events which determine whether a newly hatched individual will survive.
Drift is also not confined to diploid genetics; it can explain why we all
have mitochondria that are descended from those of a single women who lived
hundreds of thousands of years ago.
"This does not mean that there was a single female from whom we
are all descended, but rather that out of a population numbering
perhaps several thousand, by chance, only one set of mitochondrial
genes was passed on. (This finding, perhaps the most surprising
to us, is the least disputed by population geneticists and others
familiar with genetic drift and other manifestations of the laws
of probability.)"
Curtis, H. and Barnes, N.S. in Biology 5th ed. Worth Publishers 1989
p. 1050.
But random genetic drift is even more that this. It also refers to accidental
random events that influence allele frequency. For example,
"Chance events can cause the frequencies of alleles in a small
population to drift randomly from generation to generation. For
example, consider what would happen if [a]... wildflower population
... consisted of only 25 plants. Assume that 16 of the plants have
the genotype AA for flower color, 8 are Aa, and only 1 is aa. Now
imagine that three of the plants are accidently destroyed by a rock
slide before they have a chance to reproduce. By chance, all three
plants lost from the population could be AA individuals. The event
would alter the relative frequency of the two alleles for flower
color in subsequent generations. This is a case of microevolution
caused by genetic drift...
Disasters such as earthquakes, floods, or fires may reduce the
size of a population drastically, killing victims unselectively.
The result is that the small surviving population is unlikely
to be representative of the original population in its genetic
makeup - a situation known as the bottleneck effect.... Genetic
drift caused by bottlenecking may have been important in the
early evolution of human populations when calamities decimated
tribes. The gene pool of each surviving population may have been,
just by chance, quite different from that of the larger population
that predated the catastrophe."
Campbell, N.A. in Biology 2nd ed. Benjamin/Cummings 1990 p.443
Several examples of bottlenecks have been inferred from genetic data. For
example, there is very little genetic variation in the cheetah population.
This is consistant with a reduction in the size of the population to only
a few individuals - an event that probably occurred several thousand years
ago. An observed example is the northern elephant seal which was hunted almost
to extinction. By 1890 there were fewer than 20 animals but the population
now numbers more than 30,000. As predicted there is very little genetic
variation in the elephant seal population and it is likely that the twenty
animals that survived the slaughter were more "lucky" than "fit".
Another example of genetic drift is known as the founder effect. In this case
a small group breaks off from a larger population and forms a new population.
This effect is well known in human populations;
"The founder effect is probably responsible for the virtually
complete lact of blood group B in American Indians, whose
ancestors arrived in very small numbers across the Bering Strait
during the end of the last Ice Age, about 10,000 years ago. More
recent examples are seen in religious isolates like the Dunkers
and Old Order Amish of North America. These sects were founded
by small numbers of migrants from their much larger congregations
in central Europe. They have since remained nearly completely
closed to immigration from the surrounding American population.
As a result, their blood group gene frequencies are quite different
from those in the surrounding populations, both in Europe and
in North America.
The process of genetic drift should sound familiar. It is, in
fact, another way of looking at the inbreeding effect in small
populations ... Whether regarded as inbreeding or as random
sampling of genes, the effect is the same. Populations do not
exactly reproduce their genetic constitutions; there is a random
component of gene-frequency change."
Suzuki et al. op. cit.
There are many well studied examples of the founder effect. All of the cattle
on iceland, for example, are descended from a small group that were brought to
the island more than one thousand years ago. The genetic make-up of the
icelandic cattle is now different from that of their cousins in Norway but the
differences agree well with those predicted by genetic drift. Similarly,
there are many pacific islands that have been colonized by small numbers
of fruit flies (perhaps one female) and the genetics of these populations
is consistant with drift models.
Thus, it is wrong to consider natural selection as the ONLY mechanism of
evolution and it is also wrong to claim that natural selection is the
predominant mechanism. This point is made in many genetics and evolution
textbooks, for example;
"In any population, some proportion of loci are fixed at a
selectively unfavorable allele because the intensity of
selection is insufficient to overcome the random drift to
fixation. Very great skepticism should be maintained toward
naive theories about evolution that assume that populations
always or nearly always reach an optimal constitution under
selection. The existence of multiple adaptive peaks and the
random fixation of less fit alleles are integral features
of the evolutionary process. Natural selection cannot be
relied on to produce the best of all possible worlds."
Suzuki, D.T., Griffiths, A.J.F., Miller, J.H. and Lewontin, R.C. in
An Introduction to Genetic Analysis 4th ed., W.H. Freeman, New York 1989
"One of the most important and controversial issues in population
genetics is concerned with the relative importance of genetic drift
and natural selection in determining evolutionary change. The key
question at stake is whether the immense genetic variety which is
observable in populations of all species is inconsequential to survival
and reproduction (ie. is neutral), in which case drift will be the
main determinant, or whether most gene substitutions do affect
fitness, in which case natural selection is the main driving force.
The arguments over this issue have been intense during the past half-
century and are little nearer resolution though some would say that
the drift case has become progressively stronger. Drift by its very
nature cannot be positively demonstrated. To do this it would be
necessary to show that selection has definitely NOT operated, which
is impossible. Much indirect evidence has been obtained, however,
which purports to favour the drift position. Firstly, and in many
ways most persuasively is the molecular and biochemical evidence..."
Harrison, G.A., Tanner, J.M., Pilbeam, D.R. and Baker, P.T. in
Human Biology 3rd ed. Oxford University Press 1988 pp 214-215
The book by Harrison et al. is quite interesting because it goes on for
several pages discussing the controversy. The authors point out that it is
very difficult to find clear evidence of selection in humans (the sickle
cell allele is a notable exception). In fact, it is difficult to find good
evidence for selection in most organisms - most of the arguments are after
the fact (but probably correct)!
The relative importance of drift and selection depends, in part, on estimated
population sizes. Drift is much more important in small populations. It is
important to remember that most species consist of numerous smaller inbreeding
populations called "demes". It is these demes that evolve.
Studies of evolution at the molecular level have provided strong support for
drift as a major mechanism of evolution. Observed mutations at the level of
gene are mostly neutral and not subject to selection. One of the major
controversies in evolutionary biology is the neutralist-selectionist debate
over the importance of neutral mutations. Since the only way for neutral
mutations to become fixed in a population is through genetic drift this
controversy is actually over the relative importance of drift and natural
selection.
Laurence A. Moran (Larry)
