is in a sense a poor term to use. It gives the impression that it
is a distinctive type of evolution, or that there are different
levels of evolution, from that of macroevolution (which we will
discuss in the next chapter). In fact, there is only one
"type" of evolution, and evolution acts only at one
level that of the individual. Evolution acts on the individual
through a selection process, termed by Charles Darwin as
best definition for natural selection is found in Endler(1986)
Endler is saying that if you have an isolated population of
reproducing individuals and there is a shift in the environmental
pressures, then subsequent offspring will differ -the gene
frequency within the population will shift-in response to the
changing forces acting on the phenotype (physical characteristics)
of each individual. In a little simpler explanation, organisms
tend to increase in numbers, but resources such as food and space
are limited. This leads to competition amongst individuals within
the population --the so called "struggle for existence".
Organisms always vary -for most characteristics. Thus no two
individuals are alike, even though they belong to the same
some will be more successful than others in the "struggle for
existence", in the sense that they will leave more off
spring. This is natural selection. In many cases the
characteristics which make some individuals successful will be
heritable, so that they will also be expressed by their offspring.
Then the alteration in the
composition of the population caused by natural selection will be
permanent. This is evolution.
term "struggle for existence" should not be taken too
strictly. That is, it is a human metaphor for a natural
phenomenon. One gets the impression of conscience conflict between
members, which indeed does take place from time to time, but a
tree certainly does not consciously conflict with its neighbour
for sunlight. The "conflict" and "struggle"
for the most part is an elusive natural force, acting decisively
upon the individuals of the population.)
examples of selective pressures include rain, wind, soil type,
temperature, water, mate selection, predation, and food supply.
Essentially any and all encounters an individual of a population
has with the world around it will either select for, or against,
or have no effect, on an individual's phenotype. Selective
pressures are on individuals in a population, not on populations
collectively. (see figures 2 and 3 for the definition and an
example of a population.)
selection is not absolute. That is, fitness of a particular trait
does not guarantee reproductive success. Natural selection is
statistical. On a statistical basis a fit trait has a greater
chance of propagating through the gene pool than a lower fit
notation of "fitness" is greatly misunderstood by
creationists, and the rest of the public for that matter. The term
is thought to denote brute strength. But how a tree can have brute
strength is beyond me! Instead the term fitness is a genetic
expression of how well the genotype, expressed as the phenotype,
is suitable for the environment that the individual lives in (Dobzhansky
et al, 1977; Arnold, 1983;Keller, 1987). The different variations
within members of a population will mean that different responses
to the same selective pressures will occur.
fitness of an organism is in retrospect to the contribution a
mating pair leaves to subsequent generations. Thus you can only
determine if a mating pair were fit if their genes are still found
in some members of the population several generations later.
often. it is just one or two characteristics that will be left. Thus
one in mating pair would leave their genes for toe length, other
pairs for hair length, and so on. Each member who produces
offspring leaves different contributions to the overall fitness of
importance of fitness to the survival of the individual can be
seen in the reproductive rates and strategies of various species.
Organisms produce far more offspring than can possibly survive.
Take for example, a stable population --that is. the
frequency at which a population fluctuates is low. Add to that a
mating pair of birds producing 4 offspring per year for 10 years.
Since the population is stable, 38 of the 40 offspring
produced from that pair must perish without reproducing.
example of reproductive strategy is the shot-gun approach. A sea
urchin may produce millions of potential offspring during its
life. But again all but 2 must perish. (Realistically the rate of
survival for individual families will vary. These are average
survivals over the whole population.)
governs the elimination? Obviously selection pressures the
environment inflicts on the population.
take an unstable population where most of the producing members
die off due to an atypical stress, leaving behind very few
atypical members. More of their offspring will survive
perpetuating their atypical genes into a new population.
statistical basis of selection cannot he overstated. For example,
in a herd of red deer a few dominant males mate with all the
females. Rival younger males are driven off each year in combat
with the dominant male. How ever much they control the gene pool,
being exclusive male contributors, tie males' dominance is usually
short lived -a few seasons. There are instances where while the
dominant male is defending his properly against another male, a
third male can slip in unnoticed and fertilize a female. Who
is more fit? The dominant male with his physical strength,
or the smart third party who lakes advantage of the situation and
quickly mates unseen? Also the dominant male may succumb to an
accident and die prematurely. Thus natural selection is
statistical in nature.
charge that natural selection, which some admit does exist, can
only confine organisms to maintaining their "pureness".
and cannot produce new species (Hatlson. 1986). However true
this is in a stable environment, it most definitely is not true in
an environment that is changing (see figure 3 & 4 and page
of the best examples of natural selection changing a population
under a changed environment is the peppered moth Biston
betularia. Most of us know the story: Prior to industrialization
in England the moths which spent the day alighted on tree trunks
were the same colour as the lichen growing on the tree --gray.
Predatory birds cannot see the moths and hence leave them to carry
on reproducing grey moths. However, a small mutation frequently
occurs where a black or darkened morph of the moth arises and
would have been quite obvious, and become a quick meal --obviously
selected against. Now, with pollution having killed off the lichen
and the trunks of the trees darkened, these dark morphs find
themselves at a selective advantage. The light ones, which were
the population's dominant colour, are quickly gobbled up by the
the change-over we would have seen a collapse in the population as
the light moths were consumed, the dark ones being the only
the population size of the birds must have increased with a short
interim of abundant food supply. Slowly over subsequent
generations, the black morphs increase the population of moths
back to its original equilibrium with its food supply. The
surrounding countryside, however, provided new gene influx of the
light morph (Bishop &Cook,1975).
of a population (black outline) of an organism (a small
rodent) isolated by various geographic conditions. This
organism must live in grassland and is not very mobile. To
the south is an ocean. On the north a high cliff. Westward
is dense forest and on the east a river separating the
population from available niches.
at location "C" will be the "type"
members under uniform selection pressures while those
individuals at the periphery will be influenced by different
selection pressures. Those at the sea margin (location B to
D) will be under different stress than those members at
location "A" at the waterfall. Gene flow from the
interior will keep the peripherals from diverging too much
from the central population.
for example, a small peripheral group invades the islands of
the delta (location B), or even gets to the other side of
the river, and becomes reproductively isolated from the rest
of |the population, then the differing selective pressures
would select for different traits producing a new species.
populations collectively have larger variability
for selection to act upon. (See figure 4) See also
Figure 3, next page, for a definition and dynamics of
All this seems
so logical that even creationists are forced to agree. But to them
it is not evolution. Creationist Gary Parker states:
the peppered moths do seem to provide strong evidence of natural
selection. But is that evidence of evolution? Notice I've
changed the question. That's a key point. First I asked if there
was any evidence that Darwin was correct about natural
selection. The answer quite simply is 'Yes, there is.' But now
I'm asking a radically different question, 'Is there any
evidence for evolution?' Many people say, 'Isn't that the same
question? Aren't natural selection and evolution the same
thing?' Answer: NO, absolutely not. ... The answer really
depends on what the person means by evolution. In one sense,
evolution means 'change'.... But change isn't the real question,
of course. Change is just as much a part of the creation model
as the evolution model. The question is, what kind of change do
we see: change only within type (creation) or change from one
type to others (evolution).... After 100 years of natural
selection, what did we end up with? Dark and light varieties of
the peppered moth, species Biston betularia. All that
changed was the percentage of moths in two categories -that is,
just variation within type. [Morris& Parker, 1982, p.48]"
accepts Darwinism! But is evolution natural selection, or natural
selection part of evolution?
Dynamics of a population of grass-grazing rodents
over time. Natural selection acts upon individuals in populations,
not at any other taxonomic level. Since populations are so
important in evolutionary theory, then it is important that a
definition for a population be explained in detail.
No two members of a population are identical, and
it is this difference amongst members of a population
-variability- that natural selection can act on to discriminate
for or against specific members of the population. Successive
generations of the members of a population are from a mixing
between a female's and male's genetic code. Thus, a population is
a group of freely interbreeding organisms. A species my have one
or more populations, both isolated and connected depending upon
geographical conditions. Populations are not static, and change in
size and morphology from generation to generation depending upon
changing environmental conditions.
What determines the maximum number of members in
a population is the Carrying Capacity. This is the total available
food energy al! the organisms can extract from their niche. The
population density -the number of members per area" and the
range of the population change over time (due to, for example,
predators) even if the Carrying Capacity does not. "A"
is a frame in such an environment (hatched area denotes the
population). The rodent's range is restricted to only where grass
is plentiful, in the creek valleys and along the lake shore.
Frame "B" is where there is a dramatic
drop in the Carrying Capacity (drought caused), and the subsequent
collapse in population density and range. Frame "C" is a
continuous improvement in conditions where sufficient rain
produces a bumper crop of grass far exceeding the original range,
and a subsequent increase in the population of rodents in their
range and density.
Thus one can see that populations are very
dynamic and always changing due to environmental changes. From
here it is not too difficult to conceive that a permanent split of
a population into two, and subsequent environmental changes in the
different areas, will produce speciation.
A theoretical, simplified, example of how a
change in the environment can select for a peripheral
subpopulation. In this example, fir colour is the varying
phenotypic trait (denoted by different patterns on the diagram).
The type population lives in the center grasslands and is green.
The peripheral members of the population, the subpopulations, are
under different selection pressures.
These pressures have an effect of selecting
for different traits, even though the dominant green trait filters
through. Individually these peripheral subpopulations will have a
narrower phenotypic variability than the type members, but
collectively all the peripheral subpopulations have a greater
variability than Hie type.
In the second figure, a selection pressure wave,
in the form of a change in the environment, moves from left to
right. This change in the environment selects for the pink trait
of the pink-green peripheral population such that only pink
survives. As the environmental change front moves over the
population, the members of the type, and the members of the other
peripheral populations, are entirely selected against because they
do not have the pink trait.
Finally, we are left with a small population of
all pink traits. Since the niche has been left vacant from the
extinction of the rest of the population, the pink species will
enlarge in numbers, and it too will eventually become the type
population which will have many peripheral subpopulations slightly
The rate at which environmental change occurs,
such as this one. will be different in speed and intensity,
depending upon the change. It can take a few generations to occur,
or much longer. Some environmental changes can totally eliminate a
species, such that not even a peripheral subpopulation will
survive. Thus extinction
is accepted definition of evolution.
Organic evolution is a series of partial or
complete and irreversible transformations of the genetic
composition of populations, based principally upon altered
interactions with their environment. It consists chiefly of
adaptive radiations into new environments, adjustments to
environmental changes that take place in a particular habitat,
origin of new ways for exploiting existing habitats. These
adaptive changes occasionally give rise to greater complexity of
developmental pattern, of physiological reactions, and of
interactions between populations and their environment"
Dohzanxky et al. 1977. p. 8
change at all in a population as a result of the influences of
environmental pressures, is evolution -period.
selection is one of three components of the mechanism by which
evolution occurs. The other two are micromutation and
reproduction. Let's look at these others in some detail.
This is mutation in the DNA during cell division --called
micromutation. It is important for evolution during sex cell
division (meiosis). Contrary to what creationists will tell you,
micromutations are mostly neutral; they have little net effect on
the individual's ability to produce offspring, or have little net
effect on the population. What mutations do provide is variation
amongst individuals in a population.
mutations may have a beneficial effect on the individual. For
example, prior to human habitation, a population of mosquitoes
would have produced individuals resistant to DDT, but back then
the mutation producing this trait would have been neutral. Now
when DDT is used, only those in the population who have the trait
live to produce offspring. Thus that mutation did have a positive
effect (for the mosquito, not for us!) when the environment
changed due to the introduction of DDT. Did the mosquito know that
DDT would have been used against it and unconsciously mutate to
resist? Obviously not. Are populations of mosquitoes intrinsically
resistant to any and all pesticides? Again, obviously not. But
insect population sizes coupled with micromutation occurring in
high and fast offspring production will obviously be a strategy
for insects to cope with changes in their environment.
flow quickly over subsequent generations. Mayr (1963) states
"...genes accumulate in a population, independently of. each
other, in accordance with the contribution they make to fitness, [p.216]"
positive contribution of mutations reproducible in the lab? Yes,
it has been. For example, two isolated populations of fruit flies
--one subjected to low dose radiation, the other kept normally--
were subjected to the same reproductive stress forces --artificial
selection. After successive generations which do you suppose
produced the better surviving population? The irradiated
The introduction of radiation produced more micromutations which
provided more variability for selection to choose from. The
reference is Dobzansky et al(1977), see also figure 5.
alter only the chemical sequence of the proteins' amino acids, but
rarely alter the protein's function. A protein is a folded chain
of amino acids coded from the DNA. It is the sequence, NOT the
chemical make up, of a protein that determines the function a
protein will perform. Only part of the outer structure performs
the function. Thus a small mutation in the coding for that protein
will only slightly, if at all, change the outer structure, which
in most cases would not alter the normal function. It could,
however, give the protein a new function in addition to the old
Reproduction: The second component of evolution is sexual
reproduction. Sexual reproduction allows these variable traits in
individuals to be combined with traits from others in the
population. Also much alteration of the genetic code takes place.
You are not the average of the traits of your parents, nor are you
the sum of the traits. You have some traits from each of your
parents, while your siblings may have different traits from your
parents. Such is the case with my own children. But on the other
hand, you certainly have seen cases where all the children in the
family look like only one of the parents @dominant traits.
Selection: Natural selection can alter a population in
response to directional pressure, or maintain a population in a
static environment Creationists charge that evolution is a random,
chance series of events. They will charge that the animal world
shows purposeful design, as if the "designer", which is
God, knew what He was doing. Make them be specific about this.
Have them commit themselves into saying that animals cannot evolve
by chance, and evolution is a chance event. Make sure you confine
the argument to the evolution of organisms, and leave the origin
of life as a separate matter.
mutation and sexual reproduction are chance events --in most
cases. For example, where mutations will occur before and during
sex cell division appears to be random.
Who one mates
with is random (except in the case of sexual selection in some
species). Which sperm fertilizes which egg is also random. Had
your parents decided not to have sex the day you were conceived,
you would not be here. On the other side, because they missed
contraception that day you are here! There definitely is chance in
these two components of evolution. Creationists cannot deny this.
Graph comparison of populations of fruit flies to
see the effects of increased mutation, from radioactivity,
on a population's ability to survive environmental stresses.
Those populations that were irradiated did better at
surviving laboratory induced selection pressures than the
control populations. Thus, mutations do provide populations
with increased variability for natural selection to act
upon. From Dobzhansky et al, 1977, p. 67.
selection is quite different. It is directional and anti-chance.
Certain physical characteristics are forced to be propagated
throughout generations due to the discriminating function of
natural selection. However, there still are chance events which
interfere with natural selection, such as a mass extinction caused
by a meteorite's impact. Thus, even though natural selection is
directional, it is blind to the future (Dawkins, 1987),which is
how diversity evolves. Thus if there is purposeful design at work
in nature, one must explain why the "designer" used, and
still uses, random chance in the creation.
are two possible results of the effects of natural selection on a
population --extinction or speciation. Extinction is the most
radical and final environmental influence on a population.
Speciation is the more interesting for evolution.
when a single population is split into two or more reproductively
isolated populations. Let us now look at the evidence for
SYSTEMATICS-A THEORY OF LINEAGES
In 1966, Willi
Hennig published a book on a new method of classification of
organisms entitled Phylogenetic Systematics. It latter became a
small revolution in our understanding of evolutionary
relationships of organisms. It has now become so important that we
simply must cover it before we go too far into this monograph.
systematics is not a new theory of evolution, but a mechanism by
which we can derive evolutionary relationships between organisms
--extant and extinct. The basic premise of this classification
system is that it is based upon a series of closely related
species having a mosaic of ancestral and derived characteristics.
From this mosaic of characters with closely related organisms, one
can derive the evolutionary progression from one form to another.
Thus, this classification system is one by whiche volution itself
tells us who is related to whom.
McLennan's (1991) book Phytogeny, Ecology, and Behavior
is a detailed text of the methods and mechanisms of how to do this
analysis. Simply, you gather a group of taxa (species, genera,
family or class) and compare who has what character traits. Each
taxa either has a trait, or it does not. Traits can be as simple
as a long or short wing, the number of hairs on an insect's legs,
or the colour of a structure. There is some new terminology that
must be learned to understand what is going on.
If a group of
organisms has a single ancestral species, then they are called a
monophyletic group and is considered natural. Unfortunately, as
you will see later on, the current system of classification lumps
some organisms together to produce non-natural groups.
birds, reptile sand dinosaurs are a natural group based on
phylogenetic analysis, but the current classification system
places birds as a Class instead of a group within the dinosaurs, a
discussion we will get to later. Such non-natural groupings are
slowly being changed.
themselves have terminology depending upon whether they are old,
shared or new. If a particular trait is a very old one (for
example, the vertebral column in
is it called a plesiomorphic trait. If a traitis shared between a
monophyletic group, then it is called a synapomorphy. For example,
the existence of mammary glands is a synapomorphy and defines the
monophyletic group of mammals.
If only one
species in a monophyletic group has a trait, then it is called an
apomorphy. For example, spots on the leopard is an apomorphy
because it is a characteristic trait of only leopards.
of a new trait is directly from a speciation event. Because
phylogenetic systematics is based on a has-or-has-not situation,
numerical coding can be used to model the relatedness of the
traits. This, then, is placed into a table consisting of the taxa
with their traits, and from the logic of the coding a phylogenetic
tree is derived (see figure 6).
Basic idea of how cladistics and a cladogram
works. In this hypothetical case, six taxa (can be species
to Class) plus an unrelated taxa called the "Out
Group" (OG) are correlated with 10 character traits
observed in the field (or lab). Each trait is designated as
either old (because it exists in the Out Group) and thus is
designated as 0 or is a newly evolved change in the trait
Working out the cladogram is a simple matter of
establishing who has what, and is therefore related, to
produce the phylogenetic tree. The tree represents where
once lived ancestral species evolved the change in the
trait. The length of the Sine between taxa and to taxa can
represent the length of time from the last speciation from
the ancestor. The junction between the lines of two taxa
represents where a speciation event took place. The
placement of taxa into the tree is dependent upon who has
what character and the assumption that the change in the
character propagated through the other taxa.
For example, between the Out Group and taxa
"A" is a change in character 8 from rearing of
young being performed only by females, to provisioning
performed by both parents. Thus only the Out Group does not
have both parents involved. Next, taxa "A" is
separated by the others in the group because it retains
building nests in trees (trait 7) and still nests in early
June (9). All the others have the opposite trait. Only Taxa
"C" has tail plumage, none of the others have it.
And so on.
This example does show two possible problems that
emerge and make the picture somewhat more complex. You wilt
note that trait 10 (Breeding years) is 4 years for A and 4
years for B,E. Thus we assume that this is a trait that
shows parallel evolution. That is, they independently
evolved and are not from a common ancestor, You will also
note -9 trait for taxa "D". This means that all
the others evolved to breed in Mid June except for A, but
that taxa D reverted back to the ancestral condition of
breeding in Early June.
|FIGURE 7: Classification can produce a family that
is either natural, or non-natural. Cladistics attempts to achieve
the most natural family tree. It does this by comparing derived
and ancestral characteristics to produce the family tree that
produces monophyletic groups or clades.
Monophyletic clades, such as above for the North
American fresh water fish genus Nortropis, are where all the
members of the group can trace their origin to a single species
that lived in the past.
The old Linnaean classification is very non-natural.
Two possibilities arise shown here with specific examples. The
bottom right is polyphyletic for the horseshoe crab Limulus. Four
descended species are designated two different genera names within
Limulus. This cannot be true. Either the other genera are changed
to Limulus, or, as has been suggested, that name not be used so
far back to designate fossils.
The top right is an example of paraphyletic. In this
case the cladogram compares traits between humans and the two
species of chimps. The current system of classification has the
two chimps in the genus Pan, and humans as Homo. However,
since they are actually a monophyletic clade, the usage of the
term "Homo" is incorrect (So is Australopithicus).
Strictly speaking, we should be Pan sapiens
-the smart chimp. Would this be accepted by the general
Example A from Brooks and McLennan (1991), B from
Brooks pers. comm(1992) and C from Eldredge and Stanley
it is not the subjectiveness of physical traits that are used, but
gene sequences. Differences in gene sequences between the
different taxa are compared to get phylogenetic trees.
Because those sequences are so large and complex, computers must
be used to derive the tree pattern. This cladogram is a
representation of how the traits show an evolutionary lineage, and
relate the taxa together. However much this system looks great,
complications can fog the issue. There is the possibility that the
methods hows one or more possible trees of the phylogeny relating
when certain traits evolve independently (called homoplasy) and
are not derived from a common ancestor. Complications can also
arise when a trait in one species reverts back to the ancestral
there is an axiom that the theory with the most amount of data and
the least number of assumptions is the theory one uses. The term
for this is parsimony. The cladogram chosen amongst many possible
cladograms from the derived data must be the most parsimonious.
That is, the tree that has the fewest homoplasies and fewest
reversals is the one used. This will give us the closest theory of
the relationships between organisms with the fewest number of
mathematical calculations that can be performed on the collected
data that will highlight that tree with the fewest possible
evolutionary steps. We go with that tree as a representation of
the phylogeny until more traits show otherwise.
not just between related organisms, but the interacting
relationships of a community of unrelated organisms can also be
done. This has become very important in our understanding of why
organisms have the characteristics they have. Did the trait of one
organism evolve at the current habitat, or did the trait evolve
elsewhere and migration had taken place?
derive some answers from phylogenetica nalysis using more than one
organism in a community. The complexity of this part of the
subject is far more than we can get into here, but it is important
when one wants to known why organisms are the way they are.
This has also
had profound effects on ourunderstanding of the problem of
bio-diversity loss due to environmental degradation (Brooks &
McLennan, 1991; Novacek & Wheeler, 1992).
The advent of
the cladistic method has certainly put a wrench into the Linnaean
system currently used. It gives us a very complex evolution of
related taxa, and gives us far more levels of hierarchy than the
current system supports. It has also, as you will soon see, placed
some organisms where the current system says otherwise. What the
final outcome of all of this on taxonomy will surely unfold as the
years role on and cladistics gets more and more used.
SPECIATION:MECHANISMS AND EVIDENCE
There are two
ways speciation can occur. The first is geographic splitting of a
population into two and is called allopatric speciation. This is
when a portion of a population is physically cutoff from the rest
of the population, or the rest of the population becomes extinct.
This is the way most speciation takes place (Mayr, 1976;Endler,
1977; Eldredge, 1989; Brooks & McLennan, 1991).
That is, most
speciation takes place when a population is physically split into
two and is thus referred to as vicariance. In fact, measurements
have shown that some80 of all speciation is by this mechanism
(Brooks &McLennan, 1991).
who argue that speciation does not take place will have trouble
with you after this. Ask them what they would demand as undeniable
and acceptable evidence for one species to become two. After they
give you what would satisfy them, then give them these next
examples of allopatric speciation can be found in (Mayr, 1963;
Endler, 1977; Brooks & McLennan, 1991). Here is one such case
: A Central American fish, Xiphoporus maculatus, that lives
in rivers up the east coast exibits various stages of speciation,
from simple diversity of a single population, to subspecies, to
full isolated species. Mayr (1963, p.281) points out
then we have a series of related, allopatric populations showing
every stage from the local genetic race, to the ordinary
subspecies, to the almost specifically distinct subspecies (X.xiphidium),
to the full species (couchianus)."
phenomena give evidence for geographic speciation. They are:
1) Levels of
speciation: That is a range of degrees of isolation of various
Geographic variation of species characters dependent upon
differing habitats the population occupies.
cases and distribution patterns. That is, isolated sub-populations
that show some characteristics of species, but retains old
characteristics. Isolating mechanisms are not completely
striking example of speciation occurred in the Australian mallee
thickhead Pachycephala. In the first stage a wide ranging
population became split into two because of changes in the
vegetation of southern Australia.
the two populations were allowed to come into contact, but were
reproductively isolated from each other --two new species (Keast,
1961; see figure 8).
The second way
speciation can take place is called synipatric speciation. This is
when speciation --reproductive isolation-- takes place within a
large population. There was some difference of opinion amongst
scientists if this could actually occur. Until, that is, specific
examples were observed. However, it is still pretty controversial.
A case well
documented by Bush (1975) is the fruit fly Rhagoletis
infestation on cherry trees (See figure 9). The western cherry
fruit fly R. indifferens normally infests the Califomian
native cherry Prunus enarginata in August. The plant grows
at altitudes over 400 meters and fruits from August to October.
But when a domesticated orchard of P.avium or P. cerasus
(introduced from Europe) are grown within the upper boundary of
the wild cherry there is occasional out-break of infestation by R.
are never infested at the lower altitudes. On Mt. Shasta there is
an interogression zone of wild and domesticated cherries at
altitudes between 1050 and 1500 meters. During the last two weeks
in July R. indifferens can shift from the native Prunus
to the introduced species. The different growth period of the
Example of vicariance speciation in the bird Pachycephala.
The original population (1) became split due to arid conditions
isolating the vegetation and the bird population into two groups
(2).Different environmental conditions selected the two groups in
different directions (3 & 4). Subsequent invasion of the
western population into the eastern's range (5) showed
reproductive isolation had occurred (6). From Keast, 1961.
for a small number of early rising flies in a race of Rhasoleti.
If not for the European cherries, these early rising flies would
have been selected against due to the absence of cherries. Now
this race has infested the domesticated cherries, and because of
the offset fruiting, we have a new species of fly.
occupy the same area, but infect cherries of differing fruiting
times --late June for the domesticated cherry, August for the wild
cherry. Speciation at the same locality,
isolated by a shift in breeding and egg laying by an atypical
group of the fly population.
These are a
clear examples of speciation, and there is no way creationists can
argue that they are the same species. They do indeed look the
same, but their life-cycles are now so divergent that there is no
way of testing for inbreeding -they are truly isolated and by
definition separate species. It is reproductive isolation that is
sympatric speciation in the wild cherry fruit fly Rhagoletis
indifferens. Infestation in the wild cherry fruit occurs
during August when the flies lay their eggs in the fruit.
Introduction of a domesticated cherry with a June fruiting time
allowed a small number of early egg laying flies to be selected
for and propagate into a new species of flies.
(1976) discusses at length each of the required modes of isolation
for a new species to be distinct from the ancestral.
PREDATING ISOLATION: Mechanisms that prevent interspecific
Potential mates do not come into contact either due to
seasonally or geographically different habitats. For example.
Lake Victoria in Africa 3,500 years ago was much higher than
now. A reduction in the level of the lake isolated smaller
lakes, like Lk. Nabugabo, in which the cichlid fish became
isolated and diverged independently from the ancestral
population. Five new species evolved (Mayr, 1963). The case of
the cherry tree fly is an example of seasonal isolation.
Potential mates occupy the same geographic areas, but do not
recognize each other as mating partners. This is the case with
song birds and many insects. The song birds occupy the same
geographic area but it is the difference in their songs which
attracts the correct mate. Frogs are the same. In the case of
insects, it is the chemical equivalent of the song, their pheromones,
which attracts the correct mate.
Attempted mating by two individuals fails due to differing
mechanical parts preventing sperm transfer. In the case of
insects, the mechanical parts are very complex and specific to
each species. For example there is a mimic firefly, Photuria.
where the female is capable of flashing the code of another
species of firefly, Photinus. The male of the other
species comes to what he expects is the flashing code of a
female of his own species, but instead when he attempts to
mate becomes a meal. While consuming one male if another male Photinu.'i
arrives and attempts to mate with the female mimic then
differing mechanical parts prevents sperm transfer between the
two different species. What is also interesting is that
premating isolation has been achieved experimentally through
selection for habitat choice in the lab for the fly Drosphila.
POSTDATING ISOLATION: Mechanisms that reduce success of
- a) Sperm
cannot fertilize egg. Either the sperm cannot enter the egg,
or the genetic makeup is different. This would be true when
the number of chromosomes are different, for example, so even
if mating does occur then there will be no offspring.
- b) Zygote
dies after fertilization.
- c) Hybrid
in viability and death after birth. The best case here is the
mule being a crossbreed between a horse and a donkey. The mule
- d) Hybrid
lives but is partly or fully infertile, or produces an in
viable second hybrid. Creationists will charge that if an
artificial mating of two species shows fertility, then really
they are not separate species, and that they could have
diverged after the ark landed.
creationist lan Taylor (1984) gives is the horse and zebra. I
doubt that Taylor would agree that they are actually the same
species. It is not just the criterion of hybrid ability of two
species, but the other isolating mechanisms too.
If any of
these mechanisms above prevent hybridization, then we have
separate species-period. As for hybridization of two separate
species under artificial conditions only shows that the two
species have yet to become phenotypically divergent enough for the
second set of mechanisms to kick in, in addition to the first
set.It is the time separation and the effects of directional
selection which would determine when the species would be unable
How speciation looks in the three dimensions of
space, time, and phenotypic variation. X is phenotypic variation
of all characteristics of all members of the population. Y is time
or number of generations. Z is geographic location or selection
Populations "defined by the X and Z
axis" are dynamic objects, constantly fluctuating during
successive generations. Budding off of peripheral members of the
population can result in extinction for that sub-population, or a
new species depending upon environmental pressures.
Three important factors exist for recognizing
geographic speciation: 1) Different degrees of speciation
occurring, 2)geographic variation of species characters, and 3)
borderline cases and distribution patterns.
will argue that what to them is a separate "kind" is
whether or not cross-breeding can take place. Ask them, then, if
they would think it possible for a series of populations which, at
their contact with other populations of the same species,
interbreed. But at the two extreme ends there can be no inbreeding
For example, a
series of populations: A, B, C, D, E, F, G. A breeds with B, B
with C and so on. But A cannot breed with G. Ask them if that
would fit their "model".
they are called, do indeed occur (See Figure12). For example,
Endler (1977) gives many examples where the gradual change in
environmental conditions over a distance, for example latitude or
up the side of a mountain, will select for different
characteristics within the same species. Each successive
population over the gradient has slightly different averages for
the population's phenotype, until the morphology is so great that
interbreeding between the extreme distant populations is
Two examples of how speciation works. A static
environment produces a static population where atypical members
(the stippled sides of the bell graph) are selected against each
generation, to be replaced with new atypical members through gene
mutations. "Scenario A" has the parent population
completely dying off due to a selection barrier, like DDT
introduction, except for a small peripheral atypical group. That
new group later becomes the dominant type. "Scenario B"
is when a small atypical peripheral group becomes geographically
isolated from the parent group due to some change in the
Clines occur when a series of subspecies are
linked together and inbreed across the boundary. The extreme
range, however, cannot interbreed. This occurs in many specie; of
organisms including insects and birds. For example, the above
illustration is of the various species of sea gulls around the
north pole. Species A, B, C can inbreed at their boundaries with
each other. However, the invasion of A2 into Europe shows that
they cannot inbreed with B3 and B4. D1, D2, D3 (Greenland) cannot
inbreed with A2. From
Mayr, 1969, p.292.
RATES OF EVOLUTIONARY CHANGES
Darwinism is portrayed as saying that species change gradually
over time. The fossil record does not entirely support that. There
are gaps in the record, and creationists do their best to use that
that these gaps are proof of the "abrupt appearance" of
organisms. This is nothing more than their supernatural creation.
But still the creationists have a fallacy here. They are claiming
that a lack of evidence is evidence itself. False. A lack of
evidence says nothing at all. But the gaps are real, so what do
is a very rare event in many environments, such as grasslands and
forests. But the low probability of fossilization coupled with the
large numbers of fossils found attests to one thing --large
population sizes. Large populations are stable, and resist changes
from selection pressures due to the large numbers of genes flowing
through the population.
On the other
hand in small isolated peripheral populations, where the real part
of evolution --speciation-- is taking place, one would expect not
to find any fossils. Unless, that is, even the peripheral
population is large enough, or is living in ideal fossilization
(1986) found the test case with his work on the trilobite Phacops
rana. He was trying to find a continuous gradual evolutionary
pattern in this trilobite found in the Middle Devonian seas, 380
million years ago. From several locations around Ohio,
Southwestern Ontario and Michigan, Eldredge sampled several
successive layers of the shales and limestones for these
In looking at
specimens from different horizons he could see no difference in
the morphology across a geological boundary that should have
indicated a gradual evolutionary change. He eventually discovered
something quite remarkable in the trilobite eyes that would set
the stage for his theory of "Punctuated Equilibrium".
have greatly distorted punctuated equilibrium. They wiil tell you
that it is the same as what the evolutionist Richard Goldschmidt
postulated --that a reptile laid an egg and a bird flew out!
(Creationists have also misrepresented, or not understood,
Goldschmidt's "hopeful monster" theory. Since his
saltationist view of new species is rejected and has no real
evidence we can dispense with it here.)
obviously outraged by the blatant misrepresentation of punctuated
equilibrium by creationists, said this
we proposed punctuated equilibria to explain trends, it is
infuriating to be quoted again and again by creationists
-whether by design or stupidity, I do not know- as admitting the
fossil record includes no transitional forms. The punctuations
occur at the level of species; directional trends (on the
staircase model) are rife at the higher level of transitions
within major groups....
the distortion, several creationists have equated the theory of
punctuated equilibrium with a caricature of Goidschmidt's belief
that major transitions are also accomplished suddenly by means
of 'hopeful monsters. (I am attracted to some aspects of then
on-caricatured version, but Goldschmidt's theory has nothing to
do with punctuated equilibrium) [Gould,1984a]."
Equilibrium basically states that if the environment does not
change over long periods of time then the animal forms that are ui
equilibrium with that environment will not be changed either.
However, if the environment starts to change, the population can
absorb a bit of that change, but eventually the stress becomes too
large and then radical and rapid evolutionary changes can
peripheral populations selecting for few atypical members are the
only ones capable of coping --the rest dying off. The time
in which this "rapid" change can occur is dependent on
several factors such as population size and reproductive
can take place in only a few hundred of thousands of years --a
mere instant in geological time. The small population and short
time interval for the event, makes the possibility of
fossilization very remote. But is there any evidence to support
have found the test case for this in the evolution of trilobite
eyes. Trilobites had eyes similar to insects, with some major
differences. The particular trilobites he was studying had eyes
arranged in columns of lenses. A typical eye would have 18 columns
with having from 1 to 7 or more lenses in each column.
discovered in the Mid-west was that below a disconformity the trilobites
had 18 rows. Directly above that rock unit, above a disconformity,
the trilobites had 17rows. What happened?
disconformity is a distinct break between two horizontally
successive sedimentary rock units. It is an erosional surface. The
shallow sea that was covering the continent in that area dried up
for an unknown period oftime. Later, the sea reinvaded the dry
land and the lush marine animals returned, except these trilobites
had one less column of lenses in their eyes!
In a remote
small quarry in New York Eldredge found a sequence of rocks that
also contained his trilobites. This unit is parallel in time to
the lower unit which had the 18 columns. What he found were a very
few trilobites, that had a variation of columns of lenses from 18
to 17. That is, the first column had different number of lenses in
individuals in the population. Small peripheral populations
collectively have higher variability.
he was apparently seeing was two populations of trilobites. One,
with 18 columns of lenses, living in the large shallow continental
ocean. The other, a much smaller peripheral population with a
variation in columns from 18 to 17 (that is containing lenses from
one to several), living in a near shore environment at the base of
the growing Appalachian Mountains to the east.
that when the continental sea dried up a small pocket of water,
with the smaller population, was left and they continued to
flourish. When the sea reinvaded the land the trilobites with the
17 columns found themselves a nice uninhabited environment to live
in. Eldredge was able to determine that the interlude lasted about
8 million years.
Further up the
rock column in the Mid-west Eldredge noticed a second
"event" where the 17 columns were overlain by a rock
unit that had trilobites with only 15columns. Looking back in New
York he found in another small pocket of rocks a population of trilobites
with variation in columns from 17 down to 15, that is having
151/2, 16, 16 1/2 etc. columns of eyes. A repeat of the previous
event. These transitions are true transitions. Exactly what the
creationist hoped would never be found.
will charge that this is a very small change and can easily be
incorporated into just variation within "created kind."
However, they have missed the boat on this. These are found in
what the creationists would claim are rocks from Noah's Flood. Ask
them how such a cataclismic and violent an event, as the Flood is
supposed to have been, could have neatly deposited the trilobites
so that only those with 18 columns lie together, then a break and
all the 17 columned together, and another break and finally the 15
columned trilobites together.
Ask them how a
small pocket of transitions could have come to lie together during
the Flood, in what looks like a perfect evolutionary sequence.
What this really shows us is that evolution, when it occurs, takes
place in unusual places, during unusual circumstances.
This has been
Eldredge and Gould's position all along:
essential idea here is that new species -new reproductive
communities- tend to bud off in some isolated region from a more
widely spread ancestral species [Eldredge, 1986, p. 189].
summary, most evolutionary change is concentrated in events of
speciation; speciation tends to occur rapidly in very small
subpopulations isolated at the periphery of their ancestor's
one finds a gap in the rock record, the transitions should be
found in another location -where evolution took place. Add to that
periodic mass extinctions, and one can see why explosive radiation
of organisms can, and did, occur.
have been away off base in their accusation that Gould and company
have completely destroyed Darwinism. Punctuated equilibrum is only
at the level of species; "Punctuated equilibrium is a model
for the level of speciation alone [Could, 1964, p.24]" and is
in perfect harmony with Darwinism:
needs to be said now, loud and clear, is the truth: that the
theory of punctuated equilibrium lies firmly within the
neo-Darwinian synthesis. It always did. It will take time to
undo the damage wrought by the overblown rhetoric, but it will
be undone [Dawkins,1987, p. 251].
"Punk Ek", as it is abbreviated, is nothing more than an
expression of the rate at which speciation and overall evolution
takes place. Once evolved, species tend to remain unchanged --stasis--
until there is a change in the environment, then only a small
peripheral sub-population survives. This new population is small
in size, but rapid in filling vacant niches. The transitions occur
during this rapid phase of evolution, on the order of hundreds of
thousands of generations. This is far too short for fossilization
to record, unless the organism lives in ideal conditions, such as