BIOLOGICAL EVOLUTION

CHAPTER FIVE

CHAPTER SUMMARY

Larmarck, Darwin, Mendel, MacLeod & McCarthy

II. MECHANISM OF EVOLUTION

a. Mendel and the Green Pea experiment.

b. MacLeod & McCarthy [1944] experiment.

c. DNA, RNA and Protein synthesis

d. Mutations: chromozomal, genetic.

e. Mitosis, meiosis.

III. PROCESS OF EVOLUTION

a. Influence of the environment

b. Soviet-Michurin biology

c. Phylogenesis

d. Cladogenesis

e. Extinction

IV. MOLECULAR EVOLUTION

V. CONCEPT OF A SPECIES

a. Gamodeme

b. Biospecies and paleospecies

c. Morphospecies

d. Chronodeme and Holomorphospecies

Lamarck

Believed the environment altered the hereditary makeup of an organism e.g. Giraffes have long next because they had to reach leaves at the tops of high trees.

Darwin and Wallace

Established to process of evolution as survival of the fittest

Mendel G., was a Moravian monk interested in variation in plants.

Established the mechanism of evolution as controlled by genes

MacLeod and McCarty

Established DNA as the carrier of genetic information.

THE MECHANISM OF EVOLUTION

Mendel was interested in how individuals varied one from another and how such variations inherited. His method was breeding experiments using pure breeding strains. The classic mendalian experiment is the garden pea experiment using pure breeding strains of green and yellow peas. Pure breeding strains are types that always give descendants the same as the ancestors, no matter how many generations pass.

Upon interbreeding the F2 generation green peas [second generation offspring] Mendal always got only green peas and repeated generations derived from interbreeding the offspring continued to give only green peas. Thus the green type produced in F2 were pure-breeds as were the ancestral green peas.

Upon interbreeding the F2 generation yellow peas he did not get similar results but found that although some of the yellow peas proved to be pure-breeding the rest of the yellow peas produced mixtures of yellow and green. The nature of F2 yellow-peas could only be determined by breeding experiments.

These experiments of Mendel led to 4 conclusions:

(1) That the seed color is controlled by a pair of hereditary factors.

(2) That the factors of each pair in the descendant are derived from the parents: one member of the pair from each parent.

(3) Each reproductive cell bears only one factor, and on fertilization two factors are brought together.

(4) The factors for yellow seed and green seed are alternative forms of the factor for seed color.

Because when the yellow factor is present the offspring is always yellow and only in pure-stains does the offspring appear green, we call the yellow the dominant factor and the green the recessive factor. Such alternative forms of each factor are called alleles: thus the factor for yellow seeds and the factor for green seeds are alleles of each other.

The important experimental work done by MacLeod and McCarty (1944) was directed at the nature of the information contained in a cell. They determined that nucleic acid [DNA] held the genetic material. MacLeod and McCarty recognized that the basic material of cells were carbohydrates, lipids, proteins and nucleic acid. They studied a bacterium Diplococcus pneumoniae which had two strains. A smooth form which had a coating of carbohydrate (polysaccharide), and a rough form which lacked the polysaccharide coating.

Smooth forms of D. pneumoniae occasionally changed to produce rough forms (1 per 10⁶ or 10⁷). Rough forms almost never changed to smooth forms. MacLeod and McCarty disrupted "R" forms into their carbohydrate, lipid, protein and nucleic acid constituents, separated out these individual chemical components and injected them into "S" cells. When the nucleic acid (DNA) was injected into "R" cells those cells began to produce "S" cells. They concluded the nucleic acid held the genetic material. This type of experiment has since been done on a large number of organisms.

DNA, RNA and Protein synthesis

DNA

The important characteristics of the DNA molecule were established by Watson and Crick and include:

a. it is a chain molecule

b. it is long and thin [width = 20^o A]

c. there are 4 links (nucleotides) 10 per turn and the molecule is dextrally spiralled.

i) adenylic acid [A] A <+> T (2 H bonds)

ii) guanylic acid [G] G <+> C (3 H bonds)

iii) cytidylic acid [C]

iv) thymidylic acid [T] (uracilic acid [U] in m RNA)

d. all 4 links are connected in the same way.

e. the exact order of the links is the code which resides in the DNA sequence. This is a code of 4 letters (compare Morse code with only 2).

f. a bacterium has about 2,000 genes each of about 1,000 letters (equivalent to two million letters); a human has about one million genes [equivalent to one billion letters]

PROTEINS

The important characteristics of protein are:

a. chain molecules

b. long but not as long as DNA.

c. 20 links (amino acids)

d. all 20 links connected in same way.

e. exact order of links = the specific kind of protein translation. DNA provides the information to make protein by simply translating a 4 word code (compare Morse code into English). The translating machines are the Ribosomes (rRNA). The process works as follows.

1. gene is copied by an enzyme (called mRNA = short chain molecules similar to DNA and representing only 1 gene).

2. the mRNA passes into the cytoplasm and connects with a ribosome. The ribosome "reads" the sequence of nucleotides in the mRNA and as it "reads" the series of nucleotides it begins to create a protein molecule.

Remember proteins are made from amino-acids. Within the cytoplasm are numerous scattered small RNA molecules called tRNA molecules, the purpose of which is to form an amino acid.

i.e. a specific amino acid attaches to a specific tRNA. The tRNA then is attracted to the mRNA or the ribosome and attaches to it. However, the mRNA has a specific sequence of nucleotides and as tRNA gradually fit into place so specific amino acids are brought to lie next to one another. When the mRNA chain is complete the amino-acid sequence is released (i.e. a protein has now been formed). Each gene can produce a specific kind of protein. A human has some 200,000 genes and thus some 200,000 proteins.

The amino-acids in the protein each have their own chemical characteristics. The protein molecule is not simply a straight chain but is folded. The exact way in which the chain folds is determined by the way in which the amino-acids attract one another as the protein molecule forms. This is entirely a result of the sequence of amino-acids. For example, muscle protein has the amino-acids folded in such a way that long-flat molecules form. Hemoglobin protein has a unique shape that is capable of holding and the releasing oxygen. Providing no change takes place in the DNA molecule or the RNA molecules from one generation to another then the offspring should have the same kinds of genes and features as the parent.

MUTATIONS

Changes in the nucleic acids that occur when one cell gives rise to another are referred to as mutations. It would be surprising if the genetic material was passed on perfectly forever and we know that a mutation takes place randomly about 1:0.5 million cases of the production of new cells. Also, we know that mutations take place by physical alteration of the nucleic acid chains i.e. X-ray bombardment; and by chemical alteration of the chains. By this we imply that although random mutations take place; some chemical and physical conditions in the environment can cause mutations e.g. cancer. For example, Muller won a Nobel prize for his studies on mutations. He put hundreds of fruit flies in capsules and bombarded them with X-rays. The irradiated flies were then bred with untreated ones and thousands of mutants were generated; bulging eyes, flat eyes, different colored eyes, no eyes, etc. He had disrupted the molecular structure by high energy bombardment.

MITOSIS

When cells reproduce asexually there is a splitting of chromosomal material so that chromosomes replicate themselves, i.e. the DNA molecules replicate themselves. This is a particular time when the DNA can mutate i.e. when DNA is forming during replication.

Both genetic mutation (physical and chemically introduced changes) and chromosomal mutation (one offspring getting too much the other too little DNA) can occur.

MEIOSIS

When a new animal is produced sexually, a male cell (O gamete) and a female cell (O gamete) unite to form a fertilized egg (zygote). Theoretically, we would have double the number of chromosomes (e.g. man 96). Fortunately, this is not so because the reproduction cells of the body undergo a process of reduction of chromosome number during their formation. The chromosomes are reduced by half and hence the offspring (zygote) has the correct number e.g. man has 48: half are of maternal origin and half of paternal origin. The process of reduction in number is termed meiosis.

During meiosis, the chromosomes come out of the nucleus, associate in twisted pairs and unite at certain points. Parts of the chromosomes may be exchanged so giving new combinations in terms of the chromatids. The nuclear membrane breaks down, the chromosomes arrange themselves in the center and then split. Two new reproductive cells are now formed, each with half the correct number of chromosomes.

Other breeding experiments along the lines of Mendel showed that certain characters are coupled in that they appear to be controlled by the same part of the chromosome. An example is the classical work by Morgan of Columbia University (in 1910) on the fruit fly Drosophila melanogaster. Morgan in his experiments produced a white-eyed male Drosophila [they normally have red eyes]. He then bred the white eyed male to a red-eyed female and produced many red-eyed offspring. Thus suggesting red-eye coloration was dominant. To bring out the underlying heredity, Morgan then cross-bred the red-eyed hybrids. The mating produced 50% red-eyed females, 25% red-eyed males, and 25% white-eyed males. The conclusion was that the white eye coloration is linked with the sex factor.

If chromosomes are really the controller of heredity all the factors carried by a single chromosome must remain together during mitosis and meiosis. If this is true it is possible to make a chromosome map. Different points on the chromosome control different characteristics. These locations are the genes. In many cases many genes are active in producing a particular character. However, inheritance is definitely particulate in that it operates through the transmission of definite bits of self-producing matter.

These relatively simple ideas of the way in which the chemistry of DNA and RNA works are responsible for our understanding of the evolutionary mechanism. They help to explain the following:

a. How offspring come to resemble parents (heredity)

b. How new varieties arise

PROCESS OF BIOLOGICAL EVOLUTION

In looking at the processes of evolution we must explain:

a. How do the new varieties continue and eventually become a characteristic of the population.

b. How has life become so diverse.

c. Why have successful evolutionary lines suddenly become extinct.

d. Why are similar evolutionary tendencies repeated at widely different periods in earth's history.

Influence of the environment.

Gene mutation provides the means of getting the great variation necessary to explain evolution. However, the development of particular characteristics of the parent are not handed down to the offspring but merely the potential to develop the parents' characteristics.

It is the environment, in its widest sense, that controls what characteristics will develop in the offspring. The environment draws-out those characteristics most needed by the organism in its struggle for survival within the particular environment. Such characteristics are drawn from the total potential variation possessed by the organism. Essentially the individual fertilized egg has a wide range of potential variability. It could survive and grow in a large number of possible environments. These are however, initial possibilities only. Once the organism begins to grow it is affected by only one specific environment (or a sequence of environments) and its potentialities for developing specific characteristics are narrowed down as it matures. Thus its resultant growth comes to be directed along a definite channel.

So far we have been dealing with individuals. These are, of course, the ultimate units in the evolutionary process. However, neontologists normally regard the basic unit not the individual but the total inter-breeding population or gamodeme. Such an assemblage of individuals is a genetic group and continuos gene exchange within the total genetic pool has tended to level out any irregularities in characteristics. Thus the tendency within a gamodeme is for a general unison of characters because a certain gene-combination is dominant: that which is most suited to the environment.

The variation displayed by a gamodeme is due to exactly the same cause as that which influences the development of the individual. There is of course a more extended variability in the gamodeme than in a single individual, because the individuals in a gamodeme do not inhabit the same specific environment. The precise environment is never the same but is changed by such factors as food supply, crowding and temperature. However, the majority of the adults are very similar, because they have adapted themselves to the same general conditions; only a small number show extreme variability.

If one takes a gamodeme and analyses any individual characteristics and then draws a graph plotting the value of that characteristics (size, shape, etc.) against the number of individuals in the gamodeme showing the characteristic the curve is normally a monomodal curve results.

One of the most illuminating examples of the evolutionary mechanism in action is that of the British Pepper Moth. Before the British Industrial Revolution trees covered with lichen occurred over most of the British Isles. Such trees continued to occur only in the unpolluted areas such as Western England and Highland Scotland. Pepper moths shows two varieties, one white in color and the other black (the melanic form). The white variety when resting on a trunk covered in lichen is almost invisible to the moths chief enemy (birds) whereas the melanic variety is extremely conspicuous and rapidly eliminated. Before the Industrial Revolution the light colored variety was abundant throughout the country. With the on-coming of the Industrial Revolution Britain became progressively polluted with smoke - around industrial areas it was actually measured in tons per square mile per month. As a consequence the lichens died and the trees become blackened with soot - the situation became such that the light colored variety was conspicuous and the dark camouflaged. The melanic form began to dominate. A small number of melanic varieties remained in the restricted white areas and the white varieties remained in the melanic areas, but these normally die before maturity. With the new smoke abatement laws, enacted starting in the 1950's, the situation is once more reversing as lichen begins to survive on the trees again.

The British Pepper Moth example explains quite a lot of the bizarre forms met with in the diversity of life - particularly why such wonderfully camouflaged forms of insects are found. In the case of the Peppered Moth only two alleles were being dealt with i.e. the moth is either white or melanic. However, when many characteristics are plotted on a graph or analyzed statistically, significant groups of variations can be recognized.

Soviet-Michurin Biology

Of some interest to our discussion on the effect of genetics on evolution are the ideas held by some Soviet scientists under the general heading of Michurin Biology.

Michurin biology believed that the formation of the sex cells and their heredity factors depends on the metabolic conditions under which the sex cells build themselves up. If organisms develop under external conditions usual for their type, their sex cells form in accordance with their inherent heredity factors. The organism assimilates from the general environment the same components as their progenitors did and it is this feature that ensures the continuity of the parent characteristics in the off-spring. When the environmental conditions are severe enough to deprive the sex-forming cells of their power to assimilate the nutrients that are usual to them these cells are compelled either to stop developing or to undergo change. If they undergo a change they acquire different heredity characteristics. Michurin biology held that a change in heredity always and necessarily follows only from those environmental changes which deeply affect the metabolic processes in the reproductive cells.

The difference between the Michurin and traditional Western view was simply that the Soviets believe that if an organism is subjected to a severe change its sex cells can be altered to accommodate the offspring to the new environment. The Western view believed that changes in the sex cells come about by random mutations and are then selected by the environment.

Phylogenesis

This is the mere process of descent with or without modification. Any continuous history of ancestors and descendants is technically termed a phylogeny. If a number of phylogenies are examined it is observed that modification when it occurs - occurs at very diverse rates. There seem to be four recognizable types of phylogenetic lineage:

a. stasigenesis

This is the type of evolution which shows little or no modification with descent. e.g. the phylogeny of Lingula.

b. orthogenesis

This is the moderate to rapid type of evolution that most organisms seem to have followed. e.g. the phylogeny of Titanotheres.

c. anagenesis

This is a type of phylogeny which superficially appears to indicate that a new type of species suddenly arises with intermediate types. It is similar to orthogenesis but the process is much quicker so that intermediate stages are lost in the imperfection of the paleontological record. e.g. the phylogeny of Homo.

d. typogenesis

This is a real jump in phylogenetic lineage - a new form being introduces between one generation and another. e.g. the Marsh fritillary butterfly.

The type of phylogenesis that will take place is determined by the selection pressure - which is simply the leniency of the total environment to the development and growth of the individuals within the species.

Paleontology shows that each time-plane is characterized by a variety of morphologic types in each species population; and that succeeding time-planes are marked by the appearance of new morphologic types. However, there is always overlap with the ancestral population - that is some of the descendants are always morphologically similar to their immediate ancestors.

Orthogenesis - the typically steady natural selection - is often termed directional selection because such a phylogeny when looked at as a whole shows a strong constant trend from beginning to end.

In Stasigenesis the natural selection process differs only in degree form that operative in orthogenesis. Because it is highly restrictive on diversification of morphology it is called stabilizing selection.

The accompanying diagram showed the varying effect of selection pressure. One of the more important aspects is that the positive action of selective pressure is always towards a reduction in variation; the more intense the selection pressure, the more uniform the adult population becomes. This is simply because increased selection pressure causes the early death of many of the young individuals that differ from the environmentally controlled norm. Conversely the lowering of selection pressure results in the survival to maturity of a greater number of the variable young. This process was actually observed by the famous geneticist E. B. Ford and his father during their studies of an isolated butterfly colony. Observations on this colony were conducted for 50 years (1881-1935) and during this period of observation the numbers in the colony fluctuated between extremes. The results of the Fords' observations are tabulated in the accompanying diagram.

The fluctuations in numbers were accompanied by a marked effect on the amount of morphologic variations in the population. While the population was numerically stable (moderate selection pressure) and during its period of decline and rarity (increasingly high selection pressure) the amount of variation decreased to a minimum. During the period of decrease morphologic variation ran rife, even deformed young that were hardly able to fly reached maturity (lowering of selection pressure). As soon as the population built its numbers up to a maximum size it's morphologic variation became fairly constant (moderate selection pressure once more).

Apart from the importance of selection pressure that these observations show there is another significant feature. In the initial population of 1881 a particular morphologic form (type A) was the normal type. but after the period of high selection pressure a new type came on the horizon and gradually increased in importance until in the 1935 population it was normal morphologic type (type B). The form B was not at all like the type A and here we have a case of typogenesis.

Cladogenesis

This is the process of branching of the phylogenetic lineage to give diversity to life. The process of Cladogenesis is the means whereby ancestral populations give rise to a number of descendant groups, each of which remains discrete from each other throughout their subsequent history: it is the process by which new species and higher Taxa arise. The process of Cladogenesis has been investigated to a large extent by neontologists who have determined that one of the most important factors involved in Cladogenesis is the geography of the area in which the process is taking place. Such studies by the neontologists, as well as by the paleontologists, have shown that there are four recognizable stages in Cladogenesis:

1. Phase of stabilization.

2. Phase of eruption.

3. Phase of disruption.

4. Phase of divergence.

PHASE OF STABILIZATION:

During this phase selection pressure is moderate and the ancestral species is confined to a constricted habitat, with a closely controlled population size.

PHASE OF ERUPTION:

During this phase the selection pressure is decreasing and the species increases its numbers and inhabits a wider geographic area. This results in a wider range of morphologic types, living in a wider range of environmental conditions.

PHASE OF DISRUPTION:

During this phase selection pressure is increasing and the species undergoes a drastic drop in numbers. The individuals living in the less favorable parts of the environmental range are wiped out. If cladogenesis is to take place, two or more groups must survive in slightly contrasting environments that remain isolated from one another.

PHASE OF DIVERGENCE:

During this phase the selection pressure is moderate once more. The surviving groups start to diverge from each other. At first the differences are only slight but they continue to become more and more pronounced until they reach specific, generic or even familial distinction.

The process of Cladogenesis is universal and has been in operation since diversification began. It does not always proceed in the same way but differences involving the number of descendant branches and the actual morphologic differences between the initial branch populations give slightly different results. Thus we can say that the degree of Cladogenesis achieved is influenced by:

1. genetic make-up of the organism;

2. environmental conditions in the new environments.

If the process of Cladogenesis takes place with only one species origination at a time, it is called simple dichotomy. If many new species originate at or about, the same time it is called explosive evolution. The differences between these two types are illustrated in the accompanying diagram. The best examples of explosive evolution occur when whole new environments suddenly become available - such as the sudden appearance of volcanic islands, (e.g. Hawaiian Archipelago); or of new lakes (e.g. Lake Baikal and the Rift Valley Lakes); or with the extinction of a former dominant group of organisms (e.g. the extinction of the major reptile groups was followed by the explosive evolution of the placental mammals). The phylogeny of the Ammonids (Phylum Mollusca) is one of the most illustrative of both explosive evolution and simple dichotomy. The Ammonoids were derived from another group of Molluscans (the Nautiloids) in the Middle Devonian Period and there was an immediate explosive burst but, thereafter, until the close of the Permian Period the Ammonoids underwent simple dichotomy. The late Permian Period was a very critical stage in the evolution of the Ammonoids and only three families survived into the Triassic Period. Of these three survivors, two continued for a little while without diversification and then became extinct but the third underwent what is perhaps the most astonishing burst of explosive evolution known to the fossil record of the invertebrates. Forty-five families arose before the end of the Triassic from the single original family. Many of these branches failed to survive but their places were immediately filled by new bursts of evolutionary activity. At the end of the Triassic Period the Ammonoids underwent a second crisis -this time all the families but one dying out. This remaining family duplicated the patterns of the Triassic with another explosive burst. Repeated branches re-populated the seas wherever extinction occurred in some group or other and it was not until the upper part of the Cretaceous Period that living Ammonoids disappeared from earth in the third crisis during the group's history.

The cause of Extinction

Although the stream of life has been continuous since it originated some 3 million years ago, the fossil record of the past does not indicate uniform evolution. One of the most interesting phenomena is that of extinction. We have seen that the environment is the determining factor in selection pressure. Environment disrupts the biological equilibrium on both a local and world-wide scale. If the new conditions put high selection pressure on a given organism, it will be considerably reduced in number and morphologic diversity. If the selection pressure is sufficiently high the population will become extinct. The fossil record indicates that eventual extinction comes to all organisms. Roughly 2,500 families of animals with an average longevity of somewhat less than 75 million years have left a fossil record. Of these about a third are extant [still living], although a few families became extinct by evolving into other families, a majority dropped out of sight without descendants.

Whereas the odd extinction of a single species or even family is easy to explain, it is difficult to account for the recordings of mass extinction known from the geological record. In fact, many of these episodes of mass extinction - particularly among the marine invertebrates - provide world-wide stratigraphic reference dates which have been adapted as boundaries of the main divisions of geologic time. An excellent example of mass extinction is given by the abrupt extinction of nearly two thirds of the families of Trilobitamorphae at the close of the Cambrian Period.

Why should the whole group become extinct more or less abruptly? Why indeed do numerous groups of organisms become extinct at or about the same time, as they did at the end of the Permian Period? There have been many speculations as to the causes of mass extinction-- these range from world-wide cataclysms to bursts of high-energy radiation from a nearby super nova. One of the most plausible of all explanations of mass extinction rests on fluctuations in sea level. There is much evidence to show the very close relationship between fossil-zones and major advances and retreats of the seas across the continents. Newell (1962) has pointed out: During much of Palaeozoic and Mesozoic time, spanning some 540 million years the land surfaces were much lower than they are today. An appreciable rise in sea-level was sufficient to flood large areas; a drop of a few feet caused equally large areas to emerge producing major environmental changes. At least 30 major and a hundred minor oscillations of sea-level have occurred in the past 600 million years. It is obvious that repeated expansion and contraction of the sea has a profound effect on the various environments. Selection pressure would be low during the periods of flooding and high during periods of retreat by the sea (for shallow water marine animals). Furthermore, the simple withdrawal of the environment was not the only factor involved. We know that to survive, an organism must be adapted to its environment but it also must be adapted to live with the other organisms in the environment. The balance of nature is very complicated and many species owe their existence in a particular environmental niche to the fact that some other organism on which it feeds, also lives in this environment. The extinction of the food supply could cause the extinction of the predator organisms. Any changes involving even a single greater or lesser degree. Because of this interdependence of organisms, local extinction in an area of high selection pressure might cause a wave of extinction's passing through to many distant habitats.

SUMMARY

In summary it may be said that Phylogenesis takes place by the changing environment gradually drawing out different characteristics from the gene-pool. Additional variation is given by gene mutation. It should be remembered that in a large proportion of organisms the reproductive rate is enormous. For example, an oyster may lay 6 - 9 million eggs at a single spawning. Many of the earlier deaths in such a mass may be random and accidental but the vast majority are because of an inability to adapt to the environment. Those forms that can adapt to the environment will survive. The amount of variation allowed from the ideal type will depend on the selection pressure operating in the particular environment. In the next generation, assuming a similar or an increased selection pressure, the characteristics which were selected by the environment in the ancestral population will assume a greater prominence. With succeeding generations there is a gradual shift in the morphology of the population in the direction of better adaptation to the environment. When selection pressure decreases a greater variation can be displayed and different environments can be inhabited. Isolation of the variant in a new environment away from the main genetic pool can cause, in time, the new population to differ radically from the ancestral population and the eventual creation of new species, genera or higher taxa.

MOLECULAR EVOLUTION

Large gaps remain in our knowledge of phylogenetic lines - any phylogenetic lines. The evidence for phylogeny and cladagenesis rests on the confirmation of comparative anatomy and the fossil record. One of the Darwinian concepts is that evolution is forever divergent i.e. when species split off from a common ancestor they grow more different with time. This concept should apply to the molecular make-up of the organism i.e. the DNA, RNA and Protein. This led to the idea that protein structure offers a useful tool for comparing phylogenetic relationship. One can get proteins which have the same function in different organisms but which are chemically different is their amino-acids e.g. Cytochrome C (a basic respiratory cellular protein) is the same in humans as in chimpanzees but differs by 44 amino-acids out of 104 from the fungus Neurospora.

In a real sense an organism can be quantitatively defined as the sum of the proteins it manufactures. The basic idea behind molecular evolution was put forth by Nuttoll in 1902 report in the British Medical Journal but was not confined until 1962 when Morris Goodman of Wayne State University (Detroit) showed the close link between Homo Gorilla and Chimpanzees. Fitch and Margoliash demonstrated how phylogenetic trees can be developed in this way using mathematical rigor. Linus Pauling and Emile Zuckenkandl (1962) suggested proteins evolved at a steady rate and we can develope a probabilistic molecular clock on the basis of taking similar proteins comparing amino-acids differences and extrapolating back to the time at which the divergence took place. The method is similar to the radioactive method in that mutations take place at a statistically regular rate but we cannot predict when one will take place in nature.

Wilson and Sarich of Berkeley showed that the data definitely documents the existence of the clock. Different proteins tick at different rates e.g. Cytochrome C has a 1% change in 6 million years. Hemoglobin has a 1% change every 20 million years (very large molecules of more than 500 amino-acids). Such measurements put the hominid/hominoid divergence at about 1 million older than then the oldest known hominid Australopithecus afarensis. Between 4-8 million years the record is non-existent. Dryopithecus which some regard as the ancestor of the Hominids last occurred 8 million years ago. Molecular data suggests that divergence was from the Dryopithecines. Others (Goodman for example) have suggested that whereas the molecular evolution shows the amount of differences and similarities the mutation is not regular but is irregular and thus cannot be used as a clock. Molecular evolutionists offer two modes of development at the molecular level. Selectionists use the principle of natural selection to maintain that fixed change in protein amino-acids occur because of selective advantage. Neutralists maintain that most substitutions are neutral from the viewpoint of evolutionary advantage. With this random drift of chance mutations environmental changes still act on the organism and can lead to extinction. The neutralist approach requires steady molecular evolutionary change.

More recently Finch and Langely (1975) used an improved test to answer the question of degree of protein change with time. They concluded that many changes were natural but some were not and the molecular clock was not as precise as the radiogenic mineral clock (twice the variance) nevertheless a molecular clock does exist.

Current work is interested in getting DNA sequence measurements -involving the millions of neucleiotides that can be used for sequencing.

The concept of a species

The ability to sub-divide life into taxonomic categories depends greatly on evolution and ultimately on the genetic make-up of an organism. Natural taxonomic units e.g. species, cannot truly be grouped in as rigid a way as many scientists would wish. The neontologist (botanist, zoologist) uses the interbreeding factor as the basis for defining a species and we get the definition of the biospecies as:

A group of Gamodemes, potentially capable of interbreeding one group with another, and reproductively isolated from other such groups. This definition of a species has a sound genetic basis. The diagram below illustrates this concept of a species.

GAMODEME 1 GAMODEME 2 GAMODEME 3 GAMODEME 4

REPRODUCTIVE ISOLATION

CAPABLE OF INTERBREEDING

Here four gamodeme (1, 2, 3, 4) are present. 3 and 4 are adjacent and capable of interbreeding one with the other: they show morphologic overlap. 1 is capable of breeding and giving fertile offspring with either 3 or 4 but is geographically isolated from these two gamodeme. 2 is incapable of interbreeding with either 1, 3 or 4. Thus we have two biospecies present: the first is composed of gamodeme 1, 3, and 4: and the second of gamodeme 2.

Because of the very nature of evolution it is not surprising that slight anomalies occur in this single-dimension concept of a species. Thus, cases of partial genetic compatibility are known. For example, the European crested newts and Marble newts of central and western France. In this case F1 hybrids composed of sterile males and partially fertile females occur: the latter allowing a certain amount of gene exchange. Thus the neontological concept of a species is not always as easy to apply as one would like it to be, but the irregularities that are noted indicate that biospecies are all part of the evolutionary pattern.

On the other hand the paleontologist defines his species entirely on morphological factors: but morphology is usually directly related to breeding factors. The characteristics chosen by a paleontologist to define a species could be a few of many. However, normally those characteristics which have proved themselves diagnostic in living organisms are used: particularly features which had some functional use. Moreover, the factor of spatial isolation is usually coupled with morphologic divergence in the definition of a paleospecies. A useful definition of a paleontological species is that given by Mayr (1942) defining a morphospecies thus: A group of individuals with similar or the same morphological characters, the limits of variation allowed in such a species being arbitrarily defined by a competent worker. There is more than one type of paleontologic species: if only a few specimens are available the term morphospecies is used. However, one can envisage and actually find, cases of more widespread and better preservation, such that virtually the entire fossil population is found - this is very near to a gamodeme and is obviously a better group to give the name species to :it is termed a chronodeme (Peter. C. Sylvester-Bradley 1951). This is not equivalent to the entire biospecies but such an equivalence is possible: all fossil gamodemes of one stratum that can be grouped around a single morphological type are called a holomorphospecies.

Because evolution is a cumulative process one species will grade into another unless abrupt extinction takes place. We can thus actually define a species that extends into time: this we call a chronospecies and it is a far better concept of a species than any other, including a biospecies.

T 5 ----------------------e f g h i j k l m n o ------------Chronodeme

T4 --------------------d e f g h i j k l m n ---------------Chronodeme

T3 ------------------c d e f g h i j k l m -----------------Chronodeme

T2 ----------------b c d e f g h i j k l --------------------Chronodeme

T1 --------------a b c d e f g h i j k---------------------Chronodeme

TIME --------<-----VARIATION----->

In the course of time offspring come to differ from their original ancestors to such a degree that they cannot be classed in the same species. However, this process is only possible when individual variants arise in the gamodeme which are novel, and moreover, only if these novelties can be passed on to succeeding generations. As far as actual inability to interbreed is concerned, i.e. genetic separation, it seems to arise by the fact that the internal chemical environment is too different for fertilization to take place...the meeting of the male gametophyte and the female gametophyte to form the zygote is, even within a good species, a tricky thing. It seems that divergence to form new species, which although it is expressed in morphology is basically a chemical effect, reaches a stage where certain reproductive cells are unacceptable to the receiving organisms.