Article: 16616 of talk.origins
From: firstname.lastname@example.org (Chris Colby)
Subject: What is evolution?
Summary: evolutionary biology in a nutshell
Date: 3 May 92 01:49:04 GMT
References: <email@example.com> <1992May2.142019@IASTATE.EDU> <firstname.lastname@example.org>
Organization: animal -- coelomate -- deuterostome
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. The notions that;
evolution represents progress and, that organisms can be arranged on an
evolutionary ladder from bacteria to man, are two other common
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, punctuated equilibrium, 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__Bistularia_, 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 moth color represented a
change in the gene pool. This change, by definition, was evolution.
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. Later
in this post I will discuss macroevolution as well as microevolution.
For the sake of brevity I will use the terms as if it is useful to draw
a distinction between them.
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 morphology is determined by both its genes and its
environment. Morphological 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 morphological 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 organisms 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
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 one eye color locus (locus means
location) humans can have the blue allele or the brown allele (there
are other alleles also). Most organisms, 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 blue eye
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 a given locus than
any single organism can possess.
Linkage disequilibrium is a measure of association of alleles in the
gene pool. 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
about 70% of gene loci, there is more than one allele present in the
gene pool. Any given individual is likely to be heterozygous at 30% of
its loci. Most loci have been found to be assorting independently (i.e.
they are at linkage equilibrium). In most populations, there are enough
loci and enough different alleles that every individual (barring
monozygotic 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.
MECHANISMS THAT DECREASE GENETIC VARIATION
MECHANISMS OF EVOLUTION: NATURAL SELECTION
Natural selection is held to be the only mechanism as far as _adaptive_
evolution is concerned; it is defined as differential reproductive
success. 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 effect. Some organisms have alleles that enable them to
reproduce more efficiently than others of their species. Organisms with
these alleles, therefore eventually replace the others of their species
without these alleles.
If environmental conditions change, new traits (new combinations of
alleles) will be selected for. Natural selection is a mechanism that
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.
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; a trait evolved for another utility than its
current use is termed an exaptation. An example of an exaptation would
be a penguins wing. Penguins evolved from flying ancestors. Now,
however, 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. Individual 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 by the
oxymoronic name "reciprocal altruism". 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.
Keep in mind that the words "selfish" and "altruistic" have
connotations in everyday use that biologists do not intend. "Selfish"
simply means behaving so that one's own best interest comes first;
"altruistic" means behaving so that anothers best interest comes
Natural selection does not induce genetic variation to occur, it only
distinguishes between existing variants. Variation along all possible
axes is not possible, so not every possible adaptive solution is open
to an organism. 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 --
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 works on 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 peacocks 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 catch the eyes/ears/nose/whatever of predators. How then could
natural selection favor these traits?
Natural selection can be broken down into many components, of which
survivability 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 or undergoing elaborate courtship
behaviors or both. The females then mate with the males that
most interest them, often the ones with the most outlandish
phenotypes or behaviors.
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, in this example color,
that is correlated with some other important trait (ex. parasite load).
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 in the population with longer than average feather will there-
fore produce more offspring than the short feathered males. So in
the next generation, the average tail feather length will increase.
As the generations progress, tail feather length will increase becuase
females prefer not a specific length tail, but tails a little longer
than average. 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 form of sexual
selection probably varies amongst them.
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.
Organisms produce more gametes than are needed. Females produce many
more eggs than are ever fertilized and males produce billions of sperm
that never fertilize an egg. The alleles in this sample of gametes are
likely to be slightly different than the alleles in the parental gene
pool due solely to chance. Drift is a rather abstract concept to some;
I will try to explain it via a somewhat simple 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.
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.
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.
Recently, certain exceptions have been found to the above "rule" in
some bacteria (E. Coli). It appears that these organisms can undergo
directed mutagenesis to repair "broken genes". The reversion mutation
that restores the gene to normal functioning occurs several orders of
magnitude more frequently when the gene is needed than when it isn't.
It is unlikely, however, that this could occur in multi-cellular
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 (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.
EVOLUTION AMONG LINEAGES (CLADOGENESIS)
The following sections deal with how single populations ramify to
become several 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 geological 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 (both
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 it 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
MACROEVOLUTION VS. MICROEVOLUTION
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 Linneaus' 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.
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
disappear 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 animals 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.
The most famous extinction occurred at the boundary between the
Cretaceous and Tertiary Periods (the K/T Boundary). This extinction
eradicated the dinosaurs. Following this extinction the mammalian
radiation occurred. Currently, human alteration of the ecosphere
is causing a global mass extinction. In terms of rate, this is the
largest mass extinction too date.
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? These are two
questions that biologists ask.
Evolution is the unifying theory of biology. The functions of
biological entities at all levels (populations, organisms, genes) are
a product of deterministic factors (such as natural selection) and
non-deterministic factors (such as mutation and mass extinction)
acting within a framework of historical constraint. For centuries
humans have asked, "Why are we here?". A question such as that 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, Mass
The text assumes some previous knowledge of biology, but reviews most
critical background material. It contains numerous references to the
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, Sunderland,
None of the math is very daunting (it's just an intro text after all)
but it's really critical (IMHO) to really understanding what evolution
is all about. And again, lots of refs.
A text that deals with the interface of molecular biology and evolution
is: Fundamentals of Molecular Evolution, Li and Graur, 1991, Sinauer,
A very concise introduction to this field.