Introduction Chapter 1
Microevolutionary Mechanisms
Chapter 2
Macroevolutionary Mechanisms
Chapter 3
Filling Those Gaps -- Fossil Transitions
Chapter 4
Other Lines of Evidence
Chapter 5
Taxonomy and the Origin of Higher Taxa
Chapter 6
Objections and Answers
Conclusion Refernces

 

CHAPTER 1

Microevolutionary Mechanisms

 

Microevolution 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 "Natural Selection".

NATURAL SELECTION

The best definition for natural selection is found in Endler(1986)

Essentially 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 species. 

Therefore, 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. 

(The 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.) 

Some 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.)

Natural 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 trait. 

The 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. 

The 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.


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Most 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 the population. 

The 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. 

Another 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.) 

What governs the elimination? Obviously selection pressures the environment inflicts on the population. 

Now 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. 

The 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. 

Creationists 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 40).

One 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 birds. 

During the change-over we would have seen a collapse in the population as the light moths were consumed, the dark ones being the only survivors. Theoretically 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).

FIGURE 2:

Example 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.

Members 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. 

If. 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.   

Small 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 a population.

 

All this seems so logical that even creationists are forced to agree. But to them it is not evolution. Creationist Gary Parker states: 

"Well, 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]"

Clearly Parker accepts Darwinism! But is evolution natural selection, or natural selection part of evolution?


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FIGURE 3: 

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.


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FIGURE 4: 

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 different. 

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

Here 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, and the 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, any change at all in a population as a result of the influences of environmental pressures, is evolution -period. 

Natural 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. 

Mutation: 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. 

Occasionally 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.

Positive genes 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]" 

Is this 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 one!  Why? 


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Simple. 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. 

Micromutations 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 one. 

Sexual 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.

Natural 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. 

Indeed 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.

FIGURE 5

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.

 

Natural 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. 


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There 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. 

Speciation is when a single population is split into two or more reproductively isolated populations. Let us now look at the evidence for speciation.

 

PHYLOGENETIC 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.

Phylogenetic 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. 

Brooks and 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. 

For example, 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. 

Traits 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 

mammals) then 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. 

The emergence 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).

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 (=1).

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.


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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 public? 

Example A from Brooks and McLennan (1991), B from Brooks pers. comm(1992) and C from Eldredge and Stanley (1985).

Today 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 species together.

This arises 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 condition.

In science, 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 evolutionary steps.

There are 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. 

Relationships 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?

 One can 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.

 


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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).

Creationists 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. 

Specific 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 

"Here 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)."

Three phenomena give evidence for geographic speciation. They are:

1) Levels of speciation: That is a range of degrees of isolation of various populations.

 

2) Geographic variation of species characters dependent upon differing habitats the population occupies.

3) Borderline cases and distribution patterns. That is, isolated sub-populations that show some characteristics of species, but retains old characteristics. Isolating mechanisms are not completely operative. 

Another 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. 

Eventually, 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. indifferens races.

The cherries 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 European cherry 

 

 

FIGURE 8:  

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.


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selected 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. 

They both 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,

 

but 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 important. 

FIGURE 9

Example of 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.

 

 

Mayr (1976) discusses at length each of the required modes of isolation for a new species to be distinct from the ancestral.

1. PREDATING ISOLATION: Mechanisms that prevent interspecific cross breeding 

  • a) 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.
  • b) 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.
  • c) 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.
2. POSTDATING ISOLATION: Mechanisms that reduce success of crossbreeding.
  • 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 is infertile.
  • 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. 

One example 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 to hybridize.

 


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FIGURE 10: 

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 variation.

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.

Creationists 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 possible.

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". 

Clines, as 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 impossible.

FIGURE 11: 

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 environment.

 

FIGURE 12: 

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.


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RATES OF EVOLUTIONARY CHANGES

Classical 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 against us. 

They claim 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 they represent?

Fossilization 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 habitats.

Niles Eldredge (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 trilobites. 

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".

Creationists 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.)

Gould, obviously outraged by the blatant misrepresentation of punctuated equilibrium by creationists, said this

"Since 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....

Continuing 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]."

Punctuated 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 occur. 

Small 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 rates. 

This change 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 this? 

Eldredge may 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.

What he 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? 

A 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.

 


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What 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. 

It appears 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.

Creationists 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: 

The 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].

In 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 range [Gould,1984,p.24].

When 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.

Creationists 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:

What 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].

Thus "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 Eldredge's trilobites.


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