"Evolutionists have been very clear about this distinction of fact
and theory from the very beginning, if only because we have always
acknowledged how far we are from completely understanding the
mechanisms (theory) by which evolution (fact) occurred. Darwin
continually emphasized the difference between his two great and
separate accomplishments: establishing the fact of evolution, and
proposing a theory - natural selection - to explain the mechanism
Stephen J. Gould "Evolution as Fact and Theory"; Discover, May 1981
"Since Darwin's time, massive additional evidence has accumulated
supporting the fact of evolution - that all living organisms present
on earth today have arisen from earlier forms in the course of
earth's long history. Indeed, all of modern biology is an affirmation
of this relatedness of the many species of living things and of
their gradual divergence from one another over the course of time.
Since the publication of The Origin of Species, the important
question, scientifically speaking, about evollution has not been
whether it has taken place. That is no longer an issue among the
vast majority of modern biologists. Today, the central and still
fascinating questions for biologists concern the mechanisms by
which evolution occurs."
Helena Curtis and N. Sue Barnes, BIOLOGY 5th ed. 1989,
Worth Publishers, p.972
AN INTRODUCTION TO EVOLUTIONARY BIOLOGY -- BY CHRIS COLBY
Evolution is one of the most powerful theories science has ever known.
For a variety of reasons, however, it is also one of the most
misunderstood. One common misunderstanding is that the phrase "survival
of the fittest" summarizes evolutionary theory. In fact, it does not.
The phrase is both incomplete and misleading. Two other common
misinterpretations are that evolution is progress and organisms can be
arranged on an evolutionary ladder from bacteria to man.
This post is an outline of the basics of evolutionary biology. It is
intended to be a brief overview of the concepts and mechanisms of
evolution. Creationist arguments are not addressed directly here; nor
is a "laundry list" of reasons to believe in evolution provided. Many
interesting topics in evolutionary biology are not covered (symbiosis
and endosymbiosis, origins of life, evolution of sex, human evolution
and much more) because I can't include everything and keep this down
to a readable length.
WHAT IS EVOLUTION?
Evolution is a change in the gene pool of a population over time. The
gene pool is the set of all genes of a species or population. The
English moth, _Biston__betularia_, is a frequently cited example of
observed evolution. In this moth, rare black variants spread through
the population as a result of their habitat becoming darkened by soot
from factories. Birds could see the lighter colored moths more readily
and ate more of them. The moth population changed from mostly light
colored moths to mostly dark colored moths. Since their color was
determined by a single gene, the change in frequency of dark colored
moths represented a change in the gene pool. This change was, by
The kind of evolution documented above is called "microevolution".
Larger changes (taking more time) are termed "macroevolution". Some
biologists feel the mechanisms of macroevolution are different from
those of microevolutionary change. Others, including myself, feel the
distinction between the two is arbitrary. Macroevolution is cumulative
In any case, evolution is defined as a change in the gene pool. This
means that evolution is a population level phenomena. Only groups
of organisms evolve. An individual organism does not evolve, nor
do subunits of organisms evolve (with limited exceptions). So, when
thinking of evolution, is neccessary to view populations as a collection
I have defined evolution, here, as a process and that is how I will
use the term in this essay. Keep in mind, however, that in everyday use
evolution often refers to a variety of things. The fact that all
organisms are linked via descent to a common ancestor is often called
evolution. The theory that life arose solely via natural processes is
often called evolution (instead of abiogenesis). And frequently, people
use the word evolution when they really mean natural selection -- one
of the many mechanisms of evolution.
WHAT ISN'T EVOLUTION?
For many people evolution is equated with morphological change, i.e.
organisms changing shape or size over time. An example would be a
dinosaur species slowly turning into a bird species. It is important
to note that evolution is often accompanied by morphological change,
but this need not be the case. Evolution can occur without
morphological change; and morphological change can occur without
evolution. For instance, humans are larger now than in the past few
hundred years, but this is not an evolutionary change. Better diet and
medicine brought about this change, so it is not an example of
evolution. The gene pool did not change -- only its manifestation did.
An organism's phenotype is determined by both its genes and its
environment. Phenotype means the morphological, physiological, bio-
chemical, behavioral and other properties exhibited by a living
organism. Phenotypic changes induced solely by changes in
environment do not count as evolution because this change is not
heritable. In other words the change is not passed on to the organism's
offspring. Most changes due to environment are fairly subtle (e.g. size
differences). Large scale phenotypic changes (such as dinosaur to
bird) are obviously due to genetic changes, and therefore are
WHAT EVOLUTION ISN'T
Evolution is not progress. Organisms simply adapt to their current
surroundings and do not necessarily become "better" over time. A
trait or strategy that is successful for an organism at one time may be
deleterious at another. Studies in yeast have shown that "more evolved"
strains of yeast can sometimes be competitively inferior to "less
evolved" strains. An organism's success or failure depends to a great
deal on the behavior of its contemporaries; for most traits or
behaviors there is likely no optimal design or strategy, only
contingent designs or strategies.
HOW DOES EVOLUTION WORK?
If evolution is a change in the gene pool; what causes the gene pool to
change? Several mechanisms can bring about a change in the gene pool,
among them: natural selection, genetic drift, gene flow, mutation and
recombination. I will discuss these in more detail later. It is
important to understand the difference between evolution (change in the
gene pool) and the mechanisms that bring about this change.
Bringing about a change in the gene pool assumes that there is genetic
variation in the population to begin with, or a way to generate it.
Genetic variation is "grist for the evolutionary mill". For example, if
there were no dark moths, the population could not have evolved from
mostly light to mostly dark. In order for continuing evolution there
must be mechanisms to both increase genetic variation, or create it,
(e.g. mutation) and decrease variation (e.g. natural selection and
HOW IS GENETIC VARIATION DESCRIBED?
Genetic variation has two components: allelic diversity and non-random
associations of alleles. Alleles are different versions of the same
gene at a given locus. For example, at the blood group locus (locus means
location) humans can have an A, B or O allele. There are subtypes of
these alleles as well. Most animals, including humans, are
diploid. This means they contain two alleles for every gene at every
locus. If the two alleles are the same type (for instance two A
alleles) the individual would be termed "homozygous" for that locus. An
individual with two different alleles at a locus is called
Allelic diversity is simply the number of alleles at each locus scaled
by their frequency in the gene pool. At any given locus there can be
many different alleles in the gene pool. It is important to realize
that there can be more alleles in the gene pool at any locus than
any single organism can possess.
Linkage disequilibrium is a measure of association of alleles at
different loci. If each gene assorted entirely independently, the gene
pool would be at linkage equilibrium. However, if some alleles were often
found together in organisms (ie. did not assort randomly) these alleles
would be in linkage disequilibrium. Linkage disequilibrium can be the
result of physical proximity of the genes or maintained by natural
selection if some combinations of alleles work better as a team.
HOW MUCH GENETIC VARIATION IS THERE?
Considerable variation has been detected in natural populations.
At 45 percent of loci in plants there is more than one allele
in the gene pool. Any given plant is likely to be heterozygous at
about 15 percent of it's loci. Levels of genetic variation in animals
range from roughly 15% of loci having more than one allele (polymorphic)
in birds, to over 50% of loci being polymorphic in insects. Mammals
and reptiles are polymorphic at about 20% of their loci -- amphibians
and fish are polymorphic at around 30% of their loci.
Most loci assort independently (i.e. they are at linkage equilibrium).
In most populations, there are enough loci and enough different alleles
that every individual (barring monozygotic (identical) twins) has a
unique combination of alleles.
EVOLUTION WITHIN A LINEAGE (ANAGENESIS)
The following sections deal with evolution within a population or
lineage -- this is called anagenesis. Several mechanisms can bring
about anagenetic change. I have grouped them into two classes --
those that decrease genetic variation and those that increase it.
MECHANISMS THAT DECREASE GENETIC VARIATION
MECHANISMS OF EVOLUTION: NATURAL SELECTION
Natural selection is the only mechanism of adaptive evolution; it is
defined as differential reproductive success of pre-existing classes
of genetic variants in the gene pool. In other words some genotypes
are (on average) better at contributing their alleles to the subsequent
generations gene pool.
Selection is not a force in the sense that gravity or magnetism is.
However, biologists often, for the sake of brevity, refer to it that
way. Selection is not a guided or cognizant entity; it is simply an
Natural selection allows organisms to adapt to their current environment
only; it does not have any foresight. Structures or behaviors do not
evolve for future utility. The organism must be, to some degree, adapted
to its environment at each stage of its evolution. As the environment
changes, new traits (new combinations of alleles) may be selected for.
Of course, this raises the question; how do complex traits evolve? If
half a wing is no good for flying, how did wings evolve? Half a wing
may be no good for flying, but it may be useful in other ways. Feathers
are thought to have evolved as insulation (ever worn a down jacket?)
and/or as a way to trap insects. Later, proto-birds may have learned to
glide when leaping from tree to tree. Eventually, the feathers that
originally served as insulation now became co-opted for use in flight.
This illustrates the point that a trait's current utility is not always
indicative of its past utility. It can evolve for one purpose, and be
used later for another. A trait evolved for its current utility is
called an adaptation; one that evolved for another utility is an
exaptation. An example of an exaptation is a penguin's wing. Penguins
evolved from flying ancestors. Now they are flightless and use their
wings for swimming.
Natural selection works at the level of the individual. The example I
gave earlier was an example of evolution via natural selection. Dark
colored moths had higher reproductive success because light colored
moths suffered a higher predation rate. The decline of light colored
moths was caused by light colored individuals being removed from the
gene pool (selected against). It is the individual organism that
either reproduces or fails to reproduce. Genes are not the unit
of selection (because their success depends on the organism's
other genes as well); neither are groups of organisms a unit of
selection. There are some exceptions to this "rule".
The individual organism is what reproduces or fails to reproduce. It
competes primarily with others of it own species for its reproductive
success. For this reason organisms do not perform any behaviors that
are for the good of their species. Natural selection favors selfish
behavior because any truly altruistic act increases the recipient's
reproductive success while lowering the donors. Altruists would quickly
disappear from a population as the non-altruists would reap the
benefits, but not pay the cost, of any altruistic act.
Of course, many observable behaviors appear, at first glance, to be
altruistic in nature. Biologists, however, can demonstrate (in the
cases they have studied) that these behaviors are only apparently
altruistic. Cooperating with or helping other organisms is often the
most selfish strategy for an animal. Often this is called "reciprocal
altruism", an oxymoron of there ever was one. A good example of this is
blood sharing in vampire bats. In these bats, those lucky enough to
find a meal will often share part of it with an unsuccessful bat by
regurgitating some blood into the others mouth. Biologists have found
that these bats form bonds with other bats and help each other out when
the other is needy. If a bat is found to be a "cheater", (ie. he
accepts blood when starving, but does not donate when his partner is)
the partner will abandon the cheater.
Helping closely related organisms can appear altruistic; but this is
also a selfish behavior. An organisms reproductive success (or fitness)
has two components; direct fitness and indirect fitness. An organisms
direct fitness is a measure of how many alleles it contributes to the
subsequent generation's gene pool by reproducing. An organisms indirect
fitness is a measure of how many alleles that are identical it's own
it helps enter the gene pool. An organism's direct fitness plus its
indirect fitness is called its inclusive fitness. Natural selection
favors behaviors increase an organism's inclusive fitness.
Closely related organisms share many of the same alleles.
For example, in a diploid species, siblings share at least 50% of
their alleles -- the percent is higher if the parents are related.
So, helping close relatives to reproduce gets an organisms
own alleles better represented in the gene pool. The benefit of
helping relatives increases dramatically in highly inbred species.
In some cases, organisms will completely forgo reproducing and
only help their relatives reproduce. Ants, for example, have sterile
castes that only serve the queen and allow her to reproduce. The
sterile workers are reproducing by proxy.
Keep in mind that the words "selfish" and "altruistic" have
connotations in everyday use that biologists do not intend. "Selfish"
simply means behaving in an attempt to maximize ones' own inclusive
fitness; "altruistic" means behaving in an attempt to increase
anothers fitness without regard to ones' own. This is not meant to
imply that organisms consciously understand their motives.
The opportunity for natural selection to operate does not induce genetic
variation to occur, selection only distinguishes between existing variants.
Variation is not possible along every imaginable axis, so all possible
adaptive solutions are not open to populations. For example, a steel
shelled turtle would probably be an improvement. Turtles are killed
quite a bit by cars these days because when confronted with danger, they
retreat into their shells -- this is not a great strategy against a two
ton automobile. However, there is no variation in metal content of
shells, so it would not be possible to select for a steel shelled turtle.
Natural selection does not necessarily produce individually optimal
structures or behaviors. Selection targets the organism as a whole,
not individual traits. So, specific traits are not optimized, but
rather combinations of traits. In addition, natural selection may not
necessarily even select for the the most optimal set of traits. In any
population, there would be a certain combination of possible alleles
that would produce the most optimal set of traits (the global optima);
but are probably several other sets of alleles that would yield a
population almost as adapted (local optima). Transition from a local
optima to the global optima may be hindered or forbidden because the
population would have to pass through less adaptive states to make the
transition. So, natural selection only works to bring populations to
the nearest optimal point.
SEXUAL SELECTION -- A SUBSET OF NATURAL SELECTION
Darwin, and others, noticed that in many species males developed
prominent secondary sexual charactoristics. A few oft cited ex-
amples are the peacock's tail, coloring and patterns in male birds in
general, voice calls in frogs and flashes in fireflies. Many/most of
these traits are a liability from the standpoint of survival, mainly
because any ostentatious trait or noisy, attention-getting behavior
will alert predators as well as potential mates. How then could
natural selection favor these traits?
Natural selection can be broken down into many components, of which
survival is only one. Sexual attractiveness is a very important
component of selection, so much so that biologists use the term
sexual selection when they talk about this subset of natural selection.
Sexual selection occurs when the sexual attractiveness of a trait
outweighs the liability incurred for survival. A male who lives a short
time, but produces many offspring is much more successful than a long
lived one that produces few. His genes will eventually dominate the
gene pool of his species. In many species, especially polygynous
species where only a few males monopolize all the females, sexual
selection has caused pronounced sexual dimorphism. In these species
males compete against other males for mates. The competition can be
either direct (i.e. the largest males guarding their harems and
fending off other males physically) or mediated by female choice.
In species where females chose, males compete by displaying striking
phenotypic charactoristics and/or performing elaborate courtship
behaviors or both. The females then mate with the males that
most interest them, usually the ones with the most outlandish
There are many competing theories as to why females are attracted
to these displays. One model, the "good genes" model, states that the
display indicates some component of male fitness. A "good genes" advo-
cate would say that bright coloring in male birds indicates a lack of
parasites. The females are cueing on some signal that is correlated
with some other component of fitness.
Another model, proposed by Fisher, is called the "runaway sexual
selection" model. In his model he proposes that females develop a
preference for some male trait (without regards to fitness) and then
mate with these males. The offspring of these matings will therefore
have the genes for both the trait _and_ the preference for the trait.
Note, these genes would be expressed in the males and females respect-
ively. As a result, the process snowballs out of control until natural
selection brings it into check. Here is an example to clarify.
Suppose that, due to some quirk of brain chemistry, female birds
of one species prefer males with longer than average tail feathers.
Males with longer than average feather will therefore produce more
offspring than the short feathered males. In the next generation,
the average tail feather length will increase.
As the generations progress, feather length will increase because
females do not prefer a specific length tail, but a longer than
average tail. Eventually tail feather length will increase to the
point were the liability to survival is matched by the sexaul attract-
iveness of the trait and an equilibrium will be established. Note
that in many exotic birds male plumage is often very showy and many
species do in fact have males with greatly elongated feathers. In
some cases these feathers are shed after the breeding season.
A third model, called "the handicap hypothesis" states that males
with the most costly displays (in terms of detriment to survival)
are advertising the fact that, despite their "handicap", they still
had what it took to survive.
None of the above models are mutually exclusive. There are millions
of sexually dimorphic species on this planet and the forms of sexual
selection probably varies amongst them.
Natural selection is the only non-random mechanism of evolution. It
is the only mechanism that causes adaptive evolution. The phrase
"survival of the fittest" is often used synonomously with natural
selection. IMHO, the phrase is both incomplete and misleading. For
one thing, survival is only one component of selection -- and perhaps
one of the less important ones in many populations. For example, in
polygynous species, a number of males survive to reproductive age,
but only a few ever mate. Males may differ little in their ability
to survive, but greatly in their ability to attract mates -- the
difference in reproductive success stems mainly from the latter
consideration. Also, the word "fit" is often confused with physically
fit. Fitness, in an evolutionary sense, is the average reproductive out-
put of a class of genetic variants in a gene pool. Fit does not mean
biggest, fastest or strongest -- sexiest might be closer to the truth
in most animal species.
Of all the mechanisms of evolution, natural selection has the potential
to change gene frequencies the fastest. It usually acts to keep gene
frequncies constant, however. This led a famous evolutionist, George
Williams, to say "Evolution proceeds in spite of natural selection".
MECHANISMS OF EVOLUTION: GENETIC DRIFT
Another important mechanism of evolution is genetic drift. Drift is
a binomial sampling error of the gene pool. What this means is, the
alleles that form the next generation are a sample of the alleles in
the current generation.
Drift is a rather abstract concept to some; I will try to explain it
via an analogy.
Imagine you had a swimming pool full of one million marbles (this will
represent the parental gene pool), half are red and half are blue. If
you repeatedly picked ten marbles out, do you think you would get five
red and five blue every time (assume you replaced your sample to the
pool each time)? If you picked one hundred marbles out, do you think
you would get fifty red and fifty blue out every time? In both cases
the answer is no, some times the frequency of red marbles in the sample
would deviate from 0.50. In the case of the 100 marble sample, the
frequency of red marbles would deviate much less, however.
If, after picking out ten or one hundred marbles, you refilled the pool
with marbles at the frequency of that sample and repeated the process
over and over; what do you think would happen? What would happen is
that the frequency of red to blue would fluctuate over time.
Eventually, there would be only one color marble left in the pool. This
is roughly analogous to how genetic drift works.
In small populations, the rate of change in the frequency of alleles is
greater than in large populations. However, the overall rate of genetic
drift is independent of population size. If the mutation rate is
constant, large and small populations lose alleles to drift at the
same rate. This is because large populations will have more alleles
in the gene pool, but they will lose them more slowly. Smaller
populations will have fewer alleles, but these will quickly cycle
through. This assumes that selection is not operating on any of these
Sharp drops in population size can greatly affect the gene pool.
When a population crashes, the alleles in the surviving sample
may not be representative of the pre-crash gene pool. This change
in the gene pool is called the founder effect, because small
populations of organisms that invade a new territory (founders)
are subject to this. Many biologist feel the genetic changes
brought about by founder effects may contribute to isolated
populations developing reproductive isolation from their parent
Both natural selection and genetic drift decrease genetic variation. If
they were the only mechanisms of evolution, populations would
eventually become genetically homogeneous and further evolution would
be impossible. There are, however, mechanisms that replace variation
depleted by selection and drift. These are discussed below.
MECHANISMS THAT INCREASE GENETIC VARIATION
MECHANISMS OF EVOLUTION: MUTATION
A mutation is a change in a gene. There are many kinds of mutations. A
point mutation is a mutation in which one "letter" of the genetic code
is changed to another. Lengths of DNA can also be deleted or inserted
in a gene; these are also mutations. Finally, genes or parts of genes
can become inverted or duplicated.
Mutation is a mechanism of evolution because it changes allele
frequencies very slightly. If an allele "A" mutates to another allele
"a", the frequency of "a" has increased from zero to some small number
(1/2N in a diploid population where N is the effective population
size). The allele "A" will also decrease slightly in frequency.
Evolution via mutation alone is very slow; for the most part, mutation
just supplies the raw material for evolution -- genetic variation.
Most, but not all, mutations are slightly deleterious or neutral. The
genome of most organisms (certainly all eukaryotes) contains enormous
amounts of junk sequences. In addition, even in coding regions, many
sites can undergo mutation and still maintain its original meaning. In
other words, the genetic code is redundant. So, most mutations are
neutral or nearly so; but, the overwhelming majority of mutations that
produce any detectable phenotypic effect are deleterious. "Good"
mutations, however, do occur.
One example of a beneficial mutation comes from the mosquito _Culex_
_pipiens_. In this organism, a gene that was involved with breaking
down organophosphates - common insecticide ingredients - became
duplicated. Progeny of the organism with this mutation quickly swept
across the worldwide mosquito population. There are numerous examples
of insects developing resistance to chemicals, especially DDT - which
was once heavily used in this country. And, most importantly, even
though "good" mutations happen much less frequently than "bad" ones,
organisms with "good" mutations thrive while organisms with "bad"
ones die out.
Mutations occur at random with respect to their adaptive significance.
Organisms cannot "decide" that they need a mutation and have it occur.
The frequency of a mutation occurring is independent of the potential
effect it would have, with one exception.
A new class of mutation has recently been documented in bacteria
and yeast. It appears that unicellular organisms can undergo directed
mutagenesis to repair "broken genes". The reversion mutation
that restores a gene to normal functioning occurs several orders of
magnitude more frequently when the gene is needed than when it isn't.
The mechanism of directed mutagenesis is unknown at this time, but
it has been shown that it is under genetic control -- i.e. directed
mutations are not errors like normal mutations are, the are actively
created (or selectively retained) by the organism in response to
The importance of directed mutagenesis is not yet known. Biologists
have not yet studied if directed mutations can produce novel
solutions to environmental challenges. It is also unknown if it
can occur in multi-cellular organisms with separate germ
and somatic cell lines. In any case it appears that in at least
a few instances, the potential for selection to operate induces
adaptive genetic variation to occur.
MECHANISMS OF EVOLUTION: RECOMBINATION
Recombination can be loosely thought of as gene shuffling. Most
organisms have linear chromosomes and their genes lie at specific
locations (loci) along them (bacteria have circular chromosomes). In
most sexually reproducing organisms, there are two of each chromosome
type in every cell. For instance in humans, there are two chromosomes
number one (through 22 and two sex chromosomes), one inherited from the
mother, the other inherited from the father. When an organism produces
gametes, the gametes end up with only of each chromosome per cell.
Haploid gametes are produced from diploid cells by a process called
In meiosis, homologous chromosomes line up. The DNA of the chromosome
is broken on both chromosomes in several places and rejoined with the
other strand. Later in meiosis, the two homologous chromosomes are
split in to two separate cells that divide and become gametes. But,
because of recombination, both of the chromosomes are a mix of alleles
from the mother and father.
For example, lets say an organism has a chromosome with three genes,
(A,B and C -- in that order). Assume that at each of these three loci
there are at least two alleles. From the father, the organism inherited
a chromosome with the alleles A1, B1 and C1. From the mother the
organism inherited A2,B2 and C2 alleles. In meiosis the two chromosomes
would line up and the two A alleles would line up, as would the B and C
alleles. If recombination occurred between locus A and locus B, the
resulting chromosomes in the two gametes would be; one chromosome
carrying A1, B2 and C2 alleles and one chromosome carrying A2, B1 and
Real chromosomes carry many more than three genes and recombination
occurs at many locations along the chromosome. The end result is that
the two homologous chromosomes have "shuffled" alleles.
Recombination can occur not only between genes, but within genes as
well. Recombination within a gene can form a new allele. Recombination
is a mechanism of evolution because it adds new alleles and
combinations of alleles to the gene pool.
A beneficial aspect of recombination is that beneficial new alleles can
be brought together onto the same chromosome, even if the mutations
originally occurred in separate organisms.
MECHANISMS OF EVOLUTION: GENE FLOW
Gene flow simply means new genes added to a population by migration
from another population. In some closely related species, fertile
hybrids can result from interspecific matings. These hybrids can vector
genes from species to species.
Gene flow between more distantly related species occurs infrequently.
One interesting case of this involves genetic elements called P
elements. In the genus _Drosophila_, P elements were transfered
from some species in the _willistoni_ group, to _D. melanogaster_.
These two species of fruit flies are distantly related and hybrids
do not form. Their ranges do, however, overlap. The P elements
were vectored into _D. melanogaster_ via a parasitic mite that
targets both these species. This mite punctures the exoskeleton
of the flies and feeds on the "juices". Material, including DNA,
from one fly can be transfered to another when the mite feeds.
Since P elements actively move in the genome (they are themselves
parasites of DNA), one incorporated itself into the genome of a
_melanogaster_ fly and subsequently spread through the species.
Laboratory stocks of _melanogaster_ caught prior to the 1940's
are devoid of P elements. All natural populations today harbor
OVERVIEW OF EVOLUTION WITHIN A LINEAGE (ANAGENESIS)
Evolution is a change in the gene pool of a population over time.
It can occur due to several factors. Three mechanisms add new
alleles to the gene pool: mutation, recombination and gene flow.
Two mechanisms remove alleles, genetic drift and natural selection.
Drift removes alleles randomly from the gene pool. Selection removes
deleterious alleles from the gene pool. Natural selection can also
increase the frequency of an allele (or combination of alleles)
in the gene pool. Selection that weeds out harmful alleles is called
negative selection. Selection that increases the frequency of
helpful alleles is called positive, or sometimes positive Darwinian,
A new allele can also drift to high frequency. But, since the change
in frequency of an allele each generation is random, nobody speaks
of positive or negative drift.
Except in rare cases of high gene flow, all new alleles enter the gene
pool as a single copy. Most new alleles added to the gene pool are lost
almost immediately due to drift or selection; only a small percent ever
reach a high frequency in the population. Even moderately beneficial
alleles can be lost due to drift when they appear.
The fate of any given new allele depends a great deal on the organism
it first appears in. This allele will be linked to the other alleles
near it for many generations. A mutant allele can increase in frequency
in the gene pool simply because it is linked to a beneficial allele at a
nearby locus. This can occur even if the mutant allele is deleterious,
although it must not be so deleterious as to offset the benefit of the
other allele. Likewise a potentially beneficial new allele can be
eliminated from the gene pool because it was linked to deleterious
alleles when it first arose.
An allele "riding on the coat tails" of a beneficial allele is called
a hitchhiker. Eventually, recombination will bring the two loci to
linkage equilibrium. But, the more closely linked two alleles are,
the longer the hitchhiking will last.
The effects of selection and drift are coupled. Drift is intensified
as selection pressures increase. This is because increased selection
(i.e. a greater difference in reproductive success among organisms in
a population) reduces the effective population size, the number of
individuals contributing to the next generation's gene pool.
AN INTRODUCTION TO EVOLUTIONARY BIOLOGY -- BY CHRIS COLBY
EVOLUTION AMONG LINEAGES (CLADOGENESIS)
The following sections deal with how single populations ramify to
become multiple populations and eventually separate species - this is
called cladogenesis. The overall pattern of macroevolution is also
SPECIATION -- INCREASING BIOLOGICAL DIVERSITY
Speciation is the process of a single species becoming two or more
distinct species. Many biologists feel speciation is key to
understanding evolution. These biologists believe certain evolutionary
phenomena apply only at speciation and macroevolutionary change cannot
occur without speciation.
Other biologists think major evolutionary change can occur without
speciation. Changes between lineages are only an extension of the
changes within each lineage. In general, paleontologists fall into the
former category and geneticists in the latter.
MODES OF SPECIATION
Biologists recognize two types of speciation: allopatric and sympatric
speciation. The two differ in geographical distribution of the
populations in question.
Allopatric speciation is thought to be the most common form of
speciation. It occurs when a population is split into two (or more)
subdivisions that organisms cannot bridge. The two populations are
geographically isolated; organisms from subdivision A can only breed
with organisms from subdivision A and B organisms can only breed with B
organisms. Eventually, the two populations' gene pools change
independently until they could not interbreed even if they were
brought back together. In other words, they have speciated.
Sympatric speciation occurs when two subpopulations become
reproductively isolated without first becoming geographically
isolated. Monophytophagous insects (insects that live on a single host
plant) provide a model for sympatric speciation. If a group of insects
switched host plants they would not breed with other members of their
species still living on their former host plant. The two subpopulations
could diverge and speciate. Some biologists call sympatric speciation
microallopatric speciation to emphasize that the subpopulations are
still physically separate not at a geographic level, but on an
Biologists know little about the genetic mechanisms of speciation. Some
think series of small changes in each subdivision gradually lead to
speciation; others think there may be a few key genes that could change
and confer reproductive isolation. One famous biologist thinks most
speciation events are caused by changes in internal symbionts. Most
doubt this, however.
Populations of organisms are very complicated. It is likely that there
are many ways speciation can occur. Thus, all of the above ideas may be
correct, each in different circumstances.
It comes as a surprise to some to hear that speciation has been
observed. In the genus _Tragopogon_ (a plant genus consisting mostly
of diploids), two new species (_T._ _mirus_ and _T._ _miscellus_)
have evolved. This occured within the past 50-60 years. The new
species are allopolyploid descendants of two separate diploid
Here is how this speciation occured. The new species were formed
when one diploid species fertilized a different diploid species and
produced a tetraploid offspring. This tetraploid offspring could
not fertilize or be fertilized by either of it's two parent
species types. It is reproductively isolated, the definition of
Two other plant species have also arizen within the past 110
years in this manner, _Senecio_ _cambrensis_ and _Spartina_
EXTINCTION -- DECREASING BIOLOGICAL DIVERSITY
Extinction is the ultimate fate of all species. The reasons for
extinctions are numerous. A species can be outcompeted by a
closely related species. The habitat a species lives in can
dissappear and/or the organisms that the species eat could come
up with an unbeatable defense.
Some species enjoy a long tenure on the planet while others are
short-lived. Some biologists believe species are "programmed" to
go extinct in a manner analogous to organisms being destined to die.
Most, however, disagree with this. The majority believe that if the
environment stays fairly constant, a well adapted species could
continue to survive indefinately.
Mass extinctions shape the overall pattern of macroevolution. If
you view evolution as a branching tree, it's best to picture it as one
that has been severely pruned a few times in its life. The history of
life on this earth includes many episodes of mass extinction in which
many taxa (groups of organisms) were wiped off the face of the planet.
Mass extinctions are followed by periods of radiation where new species
evolve to fill the empty niches left behind. It is probable that
surviving a mass extinction is largely a function of luck. Thus
contingency plays a large role in patterns of macroevolution.
The most famous extinction occurred at the boundary between the
Cretaceous and Tertiary Periods (the K/T Boundary- 65MYA). This
extinction eradicated the dinosaurs. Following this extinction the
mammalian radiation occurred. Some hypothesize that the K/T event
was caused by environmental disruption brought on by a large impact
on earth. Several lines of evidence point to a large collision
at the time of the extinction, but attempts to link the two have
not been convincing to some biologists.
The largest mass extinction came at the end of the Permian(250MYA);
it is estimated that 96 percent of all species died out at this time.
This event coincides with the formation of Pangea II, when all
the worlds continents were brought together by plate tectonics.
A worldwide drop in sea level also occured at this time. Currently,
human alteration of the ecosphere is causing a global mass extinction.
THE PATTERN OF MACROEVOLUTION
Evolution is not linear progress. The popular notion that evolution can
be represented as progress from simple cells through complex life forms
to humans (the pinnacle of evolution), can be traced to the concept of
the scale of nature. This view is incorrect.
Evolution is better viewed as a branching tree or bush, with the tips
of each branch representing currently living species. Populations or
species of organisms split and become two or more species as time goes
on. No living organisms today are our ancestors. Every species we see
today is as fully modern as we are; each has its own unique
evolutionary history. No extant species are "lower life forms",
atavistic stepping stones paving the road to humanity.
A related fallacy about evolution is that humans evolved from apes.
This is not the case -- humans and apes share a common ancestor.
Both humans and apes are fully modern species; the ancestor we
evolved from is now extinct and was not the same as present day
apes (or humans for that matter). Our (human's) closest relatives are
the chimpanzee and the pygmy chimp. Our next nearest relative is
Whereas microevolution can be studied directly, macroevolution is
studied by examining patterns in biological populations and clades
(groups of organisms) and inferring process from pattern. Given
the observation of microevolution and the knowlege that the earth
is billions of years old -- macroevolution could be postulated.
However, this extrapolation in and of itself does not really
provide a compelling explanation of the patterns of biological
diversity we see today. Evidence for macroevolution, or common
ancestry and modification with descent, comes from several other
fields of study. These include: comparative biochemical and
genetic studies, comparative developmental biology, patterns of
biogeography, comparative morphology and anatomy and the fossil
record. These are not discussed here, see the references for
Further strong evidence for macroevolution comes from the fact that
suites of traits in biological entities fall into a heirarchical pattern.
For example, plants can be divided into two broad catagories,
non-vascular (mosses) and vascular. Vascular plants can be divided
into seedless (ferns) and seeded. Vascular seeded plants can be
divided into gymnosperms (pines being one example) and flowering
plants or angiosperms. And angiosperms can be divided into monocots
and dicots. Each of these types of plants have several characters
that distinguish them from other plants -- traits are not "mixed
and matched" in groups of organisms. For example, flowers are only
seen in plants that carry several other characters that distinquish
them as angiosperms. This pattern arises due to lineages splitting
(speciation) and retaining ancestral traits and deriving new traits.
Derived traits only appear in lineages descended from the population
that first displayed the trait.
This heirarchical pattern of diversity is what one expects to see
if species branch into new species and are modified with descent.
Here is a diagram to illustrate. Capital letters designate traits --
these are shown both where they arise and how they are distributed
A A A A A A A A |traits
B B B B C C C C |carried
D D E E F F G G |by
H I J K L M N O |species
| | | | | | | |
\ / \ / \ / \ / present
H I J K L M N O ^
\ / \ / \ / \ / |
| | | | |
D E F G |
\ / \ / |
\ / \ / |
| | |
B C |
\ / |
\ / |
\ / |
A heirarchical pattern can be produced even if the process
responsible is not heirarchical. For example, microevolution
leads to heirarchical patterns of genetic diversity even though
it works at a single level. The question of heirarchical processes
in evolution is still being debated.
Some paleontologists believe evolution is a heirarchical process.
The theory of punctuated equilibria attempts to infer the process
of macroevolution from the pattern of species documented in the
In the fossil record, transition from one species to another is
usually abrupt in most geographic locales -- no transitional forms
are found. In short, it appears that species remain unchanged for
long streches of time and then are quickly replaced by new
species. However, if wide ranges are searched, transitional forms
that bridge the gap between the two species are sometime found in
small, localized areas.
For example, in Jurassic brachiopods of the genus _Kutchithyris_,
_K. acutiplicata_ appears below another species, _K. euryptycha_.
Both species were common and covered a wide geographical area. They
differ enough that some have argued they should be in a different
genera. In just one small locality an approximately 1.25m sedimentary
layer with these fossils is found. In the narrow (10 cm) layer that
separates the two species, both species are found along with
transitional forms. In other localities there is a sharp transition.
Gould and Eldredge, the authors of punctuated equilibria, interpret
this in light of theories of allopatric speciation. They concluded
that isolated populations of organisms will often speciate and then
invade the range of their ancestral species. Thus at most locations
that fossils are found, transition from one species to another will
be abrupt. This abrupt change will reflect replacement by migration
however, not evolution. In order to find the transitional fossils,
the area of speciation must be found.
They also argue that evolution can proceed quickly in small populations
so that the tempo of evolution is not continuous. This has lead to some
confusion about the theory. Some popular accounts give the impression
that abrubt changes in the fossil record are due to blindingly fast
evolution; this is not what the theory of punctuated equilibria says.
Gould and Eldredge envision their theory as a heirarchical theory
of evolution because they see speciation as analogous to mutation
and the replacement of one species by another (which they call
species selection) as analogous to natural selection. Speciation
adds new species to the species pool just as mutation adds new
alleles to the gene pool. Likewise, they argue that species
selection favors one species over another just as natural selection
can favor one allele over another. This is the most controversial
part of the theory. Many biologists agree with the pattern of
macroevolution these paleontologists posit, but many disagree with
the mechanism -- species selection. Critics would argue that species
selection is not analogous to natural selection and therefore
evolution is not heirarchical.
The theory of punctuated equilibrium was designed to replace the theory
of phyletic gradualism. Phyletic gradualists held that a species would
slowly transform into another species over it's entire range. Phyletic
gradualism is often associated with the assumption of a uniform rate
of evolution, but this need not be the case.
ARE WE STILL EVOLVING?
Yes, evolution is still occurring; all organisms continue to adapt to
their surroundings and "invent" new ways of better competing with
members of their own species. In addition, allele frequencies are being
changed by drift, mutation and gene flow constantly. Studying the
process of evolution as it continues to occur is a major field of
biology today. Although evolution has been observed and all the
mechanisms have been shown to work, there is still no consensus on
the relative contribution of each of the mechanisms to the overall
pattern of evolution within a lineage. Likewise, although new species
have been seen to arise; biologists have many questions about what
influences the pattern of macroevolution. Are some groups "good" at
speciating? Who survives mass extinctions and why?
Evolution is the unifying theory of biology. The functions of
biological entities at all levels (populations, organisms, genes) are
the product of a non-random factor (e.g. natural selection) operating
in conjunction with random factors (such as mutation and mass extinction)
within a framework of historical constraint. For centuries humans have
asked, "Why are we here?". A question such as that probably lies
outside the realm of science. However, biologists can now provide
an elegant answer to the question, "How did we get here?"
SOME GOOD EVOLUTION TEXTS (IMHO)
A good introductory text in evolutionary biology is:
Evolutionary Biology, by Douglas Futuyma, 1986, Sinauer, Sunderland,
The text assumes some previous knowledge of biology, but reviews most
critical background material. It contains numerous references to the
primary literature. Most of the information in this file can be found
(along with the references to the primary literature) in this text.
A good introductory text into population genetics, the field that
mathematically describes changes in the gene pool is:
Principles of Population Genetics, by Hartl and Clark , 1989, Sinauer,
None of the math is very daunting (it's just an intro text after all)
but it's really critical (IMHO) to understanding what evolution is all
about. And again, lots of refs.
A text that deals with the interface of molecular biology and evolution
Fundamentals of Molecular Evolution, by Li and Graur, 1991, Sinauer,
A very concise introduction to this field.
A text that deals with theories of macroevolution is:
Macroevolutionary Dynamics, by Niles Eldrege, 1989, McGraw-Hill,
A text that documents the history of life on earth is:
History of Life, by Richard Cowen, 1990, Blackwell Scientific, Boston
A readable introduction to the history of our planet and
especially the changes that have occurred in the biota.
A popular introduction to the field that also debunks the most
common creationist arguments is:
The Blind Watchmaker, by Richard Dawkins, 1987, Norton, New York
Dawkins is (IMHO) a very engaging writer.
A close look at the creation/evolution debate can be found in:
Abusing Science, by Philip Kitcher, 1982, MIT, Cambridge, Mass
A meticulous critique of creationism.