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The Basic Theory
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THE EVOLUTIONARY UNIT



The basic theory



Darwin described how organisms, which vary to some extent and which reproduce themselves, are subjected in the natural state to an adverse environment, which includes physical factors as well as competition and aggression from their own and other species. The result is that the organisms which are best able to cope (the fittest) survive to reproduce more frequently. Because new organisms inherit the features of survivors the organisms evolve to become better adapted to their environment, or they become extinct. He supported this theory with factual information derived both from controlled breeding, that is, artificial selection; and from nature, which he referred to as natural selection (2).

For Darwin’s theory of evolution to be true certain conditions must exist.
  • aa   There are a number of entities which have a finite length of life.
  • aa   Each is able to reproduce itself.
  • aa   Reproduction is accurate enough to maintain a stable form.
  • aa   Reproduction is subject to some inaccuracy so that variation occurs.
  • aa   There are adverse environmental factors which ensure that the entities that are best suited to that environment survive to reproduce more frequently than those less suited.
  • aa   The reproduction rate, the mortality rate, the length of life of the entities, the accuracy of reproduction, the frequency of advantageous variations, the severity and nature of adverse environmental effects and the rate of change in the environment may all vary but must be within certain limits if extinction is to be avoided.

If these conditions are met it is inevitable that the entities will evolve to become progressively better adapted to the environment, and this will occur naturally without any external control. But if the criteria are not met the theory is not applicable.

Darwin did not need any facts to support the basic theory. It is logically true whatever the entities may be. The reason he required all the factual support was to show that organisms in the natural world did indeed possess these essential features. As an extension of his theory he argued that new species could be explained by the process of evolution over very long periods of time (2), and evidence was required to show that this had in fact occurred.

The word evolution is often used simply to mean change, or to mean progressive change, and so I must make it clear that in this essay I am using the word evolution to mean change that is occurring according to Darwin’s theory. The entities that evolve, the evolvers, are also limited to those that fulfil these criteria.

A persisting difficulty with Darwin’s theory of evolution is in deciding which parts of life possess the essential features outlined above. We cannot point to an entity, or a class of entities, and agree that it is the unit of evolution. Some say it is the gene, and others variously say it is the genome, the organism, the group, the population, the species, the clade, James Lovelock’s Gaia, or a hierarchy including all of them (7).

The problem with all these suggestions is that they do not comply with the criteria necessary for the application of Darwin’s theory. For example, none of them truly reproduce themselves. This results in confusion and paradox.

Selection of the evolvers may occur indirectly. Selection applied to an entity, for example the body of an organism, can, logically, affect selection and survival of anything that is a part of that entity. Likewise if a part of an entity is altered it may affect the selection of the whole entity (4). For example, a cell which is part of a body may be well fitted to its local environment, but if the body of which it is a part is destroyed by the wider environment, the cell will die. On the other hand if a cell develops a heritable feature that adversely affects the contribution of that cell and its progeny to the whole body, it influences the selection of the body. Through a ‘Russian doll’ of entities a change that affects selection at any level may ripple up and down the hierarchy of ‘dolls’ to affect many other levels (4).
Selection at a higher level in the hierarchy dominates selection at a lower level. However well a cell and its progeny function in the local environment, they will die if the body dies.
Selection at any level can only affect evolution inasmuch as it affects the entity that evolves. This is true whether it is a part of the evolver which is primarily selected or whether selection operates primarily on some entity of which the evolver is a part.
The final common path of all relevant selection is those entities which evolve. It is these entities that we seek to define. They are the units of evolution.
When we apply the theory to the evolution of life on earth, it is obvious that the evolvers must possess certain other characteristics. Individual units may be different and there may be groups of units that have definable differences, for example the units of different species, but we are seeking to define all that fulfil the essential characteristics that Darwin’s theory defines.
A unit is the smallest part that is not a fraction. If an entity can separate into two or more reproducing parts which are essentially different from itself, then that entity is not a unit, the parts are potential units.
The unit must be an entity functioning and reproducing as a whole; if divided, the parts are, by definition, fractions, which do not fulfil the criteria required of units. They cannot function and reproduce independently; otherwise they would be units.
The units must be living because we are discussing units of life. We are concerned here with evolution of life over billions of years and in units that help us understand this. Whatever units we choose must be potentially capable of reproduction over this period of time and for as far into the future as we expect life to continue. There are various circumstances in which evolution may occur for a limited period of time, and we need to exclude these. For example, a somatic cell in a body can reproduce itself, and because it can only undergo mitosis and not meiosis it cannot divide into parts of itself, even though it is diploid. But the reproduction of somatic cells by mitosis is limited in duration. Somatic cells can evolve but only for an insignificant period of time.
If copies of one entity are created by another entity, then the controlling entity possesses the information necessary for the replication. What is replicated may change, but it cannot evolve in a Darwinian sense. Only the entity which possesses and reproduces the information can evolve.
An evolutionary unit is the only link that exists between one generation and the next. It is an information bearing unit, and it must possess and transmit all the essential information. If it does not reproduce its whole self then the information must lie in the portion that is reproduced, which we could then consider as a possible unit of evolution. Information in the portion that is not reproduced cannot evolve.
It is possible for new entities to be formed which might be considered as units. Organisms may live in symbiosis and reproduce together. They remain separate reproducing entities, and possibly units of evolution, unless and until they are only able to reproduce as a unit. This is something additional to the process of evolution as described by Darwin. Change is occurring as a result of combination, not as a result of reproduction with variation and natural selection. New units are created, a process that has been termed symbiogenesis (10,11). Symbiogenesis is clearly important to evolution but I will not discuss it further because in this essay I am concerned with defining those parts of life that we can consider to be units of Darwinian evolution.


Sexual reproduction

In order to understand which entities truly evolve we need to establish that in sexually reproducing species it is gametes, and not bodies, that reproduce. We usually regard individual plants and animals as the entities that reproduce sexually (1,3,6,9,12), but it is more logical, and it more accurately reflects reality, to regard the gametes as the reproducing entities. Haploid asexually reproducing organisms were present on earth for many millions of years before regular and consistent sexual reproduction evolved. It has been firmly established that many, if not all, of these organisms occasionally share their content in various ways and have done so in the past. This might be regarded as a form of sex, but the consequences are variable and inconsistent. Gametes are haploid organisms that, over millions of years and through many stages (10,11), evolved the ability to reproduce sexually in a regular and consistent manner. A few haploid organisms, for example, some fungi, have a regular sexual cycle and still retain the ability to reproduce asexually, but most organisms that have a regular sexual cycle have lost this ability.
Similarities between the cells of all living organisms in structure, biochemistry and genes preclude any other explanation. They indicate a common ancestral origin (5), and it is inconceivable that diploid organisms developed first and later evolved the ability to create all the haploid organisms. Sexual reproduction is indeed a cycle, but the haploid cells came first and haploid cells are reproduced. The gametes are the smallest entities in the cycle and they must therefore possess all the information necessary to produce the whole cycle. They fulfil the requirements of a unit of evolution. The diploid cell from which gametes are created cannot be a unit of evolution because it divides into gametes which are essentially different; they only contain half of its genetic information. Diploid cells truly reproduce themselves when they undergo mitosis.
Gametes reproduce, bodies do not. Just as Ptolemaic confusion cleared when it was appreciated that the earth and the other planets move around the sun, and that the earth is not the centre of the universe, so, when we take this small theoretical step, evolution becomes clear and simple to understand, although we must again adapt to a more peripheral role.
We are so used to accepting that organisms reproduce sexually that one’s immediate reaction is to resist the conclusion that it is gametes that reproduce; and to try to explain how organisms in sexually reproducing species do reproduce themselves. Such explanations are always defeated by these considerations:
  • aa   The haploid gametes are the smallest living organisms in the regular sexual reproductive cycle and possess the capacity to recreate the whole cycle.
  • aa   The haploid cell is the origin of the diploid cell.
  • aa   The diploid cell is not a unit. It is a particular combination of gametes, which are whole indivisible living entities.
  • aa   If a gamete that possesses a certain structure is reproduced, the whole entity is reproduced. A diploid cell that undergoes mitosis also reproduces its whole self, but it does not do so if it undergoes meiosis. The somatic portion of a body obviously does not reproduce itself.
  • aa   Almost never will more than one part of a particular diploid cell form a zygote. The diploid cell can therefore reproduce only a half of itself, and only the part that is reproduced, in essence a gamete, can evolve. ‘Almost never’ in this context means that the chance is infinitesimal, excluding laboratory manipulation. In humans, for example, in a female body each germ cell undergoing meiosis only produces one ovum; the other products of division become polar bodies. On copulation the chance of more than one sperm forming a zygote is very small; when this occurs, the chance that the multiple successful sperm come from the same diploid cell is remote.


The gamete reproductive cycle

The basic sexual reproductive cycle consists of gametes, zygote formation to create a diploid cell, mitosis to produce increased numbers of diploid cells, and finally meiosis to produce numerous gametes. The reason that this is a successful evolutionary development is that it produces very large numbers of unique gametes, that is, enormous variety within a stable range; a range which is defined by the necessity to remain sexually compatible with other gametes of the species.
Gametes whose mutations enabled them in addition to evolve to form multi celled bodies gained many further advantages, for example, production of gametes over a prolonged period of time, nutritional support, defense, mobility, a means of delivery to suitable gamete partners, and so on. There cannot be any evolutionary advantage to the body in these developments because all the somatic portion of the body dies. The only entities that gain advantage are the gametes; and the ability to produce useful, slave bodies must be contained in the gametes which evolve as they acquire mutations and their life cycle is subjected to selection. The capacity to produce diploid cells and bodies must reside in the gametes because they are the smallest link in the reproductive cycle. There is no other way for this information to be transmitted.
All parts of the sexual reproductive cycle are subjected to selection, but ultimately this selection is applied to the gametes because they are the essential information bearing units that reproduce themselves and pass the information down the generations. Of those that succeed in forming a zygote, the ones bearing the most effective information proliferate preferentially.
Gametes reproduce themselves with only slight variation which in most cases appears to be almost entirely restricted to the allelic portion of their DNA. The degree of variation depends upon the number of allelic genes and upon the degree of mixing that occurs in meiosis, but the principle is clear: gametes truly reproduce themselves, with their whole structure and biochemistry, and with only slight unique alteration in their genes. If 10% of genes have alleles that are different in the two parent gametes, and if mixing in meiosis is random, the genes of the baby gametes will be 5 - 10% different from each parent. Units of evolution must vary, and if they are to be unique this variation must be more than trivial. More extensive variation is one of the advantages of sexual reproduction over asexual reproduction.
Variation is indeed important, but so is stability. The gametes of a sexually reproducing species are all very similar; they are indeed comparable to a clone of asexual organisms except that they possess a small variable portion, which is essential to gain the value of sex.
One might object that one cannot follow a gamete as an unchanging identity in the way that one can follow an allele through many generations and in many copies until it is discarded or is fixed as the only form of that gene (3). But it is an illogical expectation that the units from generation to generation should usually be identical. For Darwinian evolution units should be similar but they must vary. They also need not be identifiable. What is required is a basic form with limited variation, and selection decides the degree of variation that is most beneficial. Shuffling of the genetic material which is identical in both gametes, and the sharing of other aspects of the cell, make it impossible to identify each gamete all the time. But this is, after all, the essence of sex. The outcome is that gametes are reproduced.
One of the reasons for rejecting the bodies of sexually reproducing species as units of evolution is the mixing of genes in each generation, confusing us with regard to the inheritance of beneficial features (3). One’s first inclination is to think that gametes suffer the same problem. But there is a real difference: if a body is regarded as reproducing, a part of it divides and then one half unites with a half from another body; in contrast, whole gametes unite; shuffle and share a small portion of one of their components; and then divide to produce whole gametes.
It does not matter to the logic of the argument what precisely varies from generation to generation. The basic form must be reproduced with some variation. In many species it may only be the allelic portion of the genome that varies, and this may be due to exchange, as in meiosis, or due to mutation; in others there may, or may not, be some variation in cell structure, organelles and biochemistry; or there may have been in the past. It is only important that there must be sufficient stability to prevent error catastrophe but sufficient variation for the required degree of adaptation.
At first the fact that the sperm and ova of a species appear so different makes it difficult to see them both as manifestations of one entity. However, their essential structure and biochemistry is the same and their differences are relatively minor; this must be so for zygote formation and development to succeed. Darwin’s theory would suggest that dimorphic gametes evolved because the relevant mutations proved advantageous to gamete sexual reproduction. Selection decides the most beneficial course. Provided that gametes remain similar enough to form a common zygote, differences that aid conjugation are beneficial; and are likely to be selected if they are of immediate benefit to the reproductive cycle in which they arise. The essential unity of ovum and sperm is illustrated by the drone whose haploid cells differentiate from an unfertilized ovum to create a body which has reproductive cells that produce sperm: the ovum produces sperm.


The parts of life that qualify as units of evolution

Sexually reproducing gametes originally evolved from asexually reproducing haploid cells; and they are conceptually equivalent as units of evolution. The whole cycle of haploid cell reproduction interacts with the environment and is subjected to selection. The haploid cells encoding the fitter cycles proliferate preferentially to create evolution. This provides a unit that is simple and logical.
Haploid reproducing cells exist throughout the living world in all species. Even before they evolved RNA and DNA, primitive cells could still be regarded as equivalent units with a different form of variation but, of course, cannot accurately be described as haploid because they had no DNA.
Most haploid organisms are potentially capable of indefinite reproduction. An exception is the somatic portion of the drone, which consists of haploid cells, but has no long term reproductive future. For accuracy we therefore need to refer to the unit of evolution as the single haploid reproducing (or reproductive) cell. Single haploid reproducing cells, including gametes, all fulfil the criteria necessary for them to be units of evolution.
Diploid or polyploid cells generally do not qualify as units of long term evolution, because they do not, or cannot, reproduce indefinitely. Cells that are only reproduced by parthenogenesis do fulfil the criteria, and might be regarded as exceptions, but cells so created might also be regarded as ultimately becoming haploid. If a parthenogenetic species survives for a very long time, mutation will ensure that the two sets of chromosomes, which no longer mix, do not remain homologous (5). The same argument applies to any diploid cell that reproduces indefinitely without meiosis. In those cases where meiosis occurs occasionally, the gametes must be the unit that evolves. Symbiogenesis is similar. When cells produced by symbiogenesis are only able to reproduce as a unit, they, or their gametes if they reproduce sexually, are essentially haploid.
Although there are many different reproductive cycles, sex is essentially an exchange of content between cells and this must of necessity involve some form of combined state, but it is always haploid cells that have the ability to reproduce indefinitely and to evolve.
‘Single haploid reproducing cell’ is not an entirely satisfactory term, but I believe I have made clear the sense in which it is being used.


Relationship to other theories of evolution

This concept provides an understanding of the contribution made by different theories of evolution. The bodies created by the gametes of sexually reproducing species are an easily visible manifestation of a major portion of every successful reproductive cycle. Therefore, even though they do not represent individual gametes directly, they act as surrogates and appear to comply with the requirements of the basic theory of evolution. Variety in the gametes will often be reflected by variety in bodies. Selection does indeed operate directly upon the body phase of the gamete reproductive cycle, but only affects evolution through the indirect effect on gamete reproduction.
Both extant organisms and the fossil record tell us a great deal about how successful various species of gametes have been in different circumstances. Species are also good markers of a much wider range of evolutionary developments, and of longer lasting trends.
Genes, of course, provide the most detailed record of previous selection. As they are progressively deciphered we learn more about the history of evolution and the relationships between different species, because genetic similarities reflect the common ancestral origins of gametes and other haploid cells. Genes are copied by the cell; they do not reproduce themselves.
The genome is an important part of the haploid cell that is reproduced in each generation and it is the major, perhaps only, part that is altered in the sexual exchange in modern cells. The genome regulates virtually every part of the cell and so the overall function of the cell is vitally dependent on it. It will almost always be true that changes in gametes and the bodies they create, or in asexually reproducing haploid cells, can be correlated with changes in the genes. But the genome is copied by the whole integrated cell; it does not reproduce itself.

The value of sex and multi cell bodies, sexual dimorphism, altruism, the many different and complex sexual cycles, and ecosystems can be more easily explained without paradox when one understands that the evolving units are the haploid reproducing cells, including gametes. These aspects are dealt with more fully elsewhere (13).


Conclusion

It is a certain fact that sexual reproduction evolved from asexual haploid organisms. It follows inevitably from this that it is gametes that reproduce themselves in sexual reproduction, and that they are conceptually equivalent to asexually reproducing haploid organisms. Haploid reproducing cells are the only entities that truly possess the potential to reproduce indefinitely, and they comply with other essential features of basic Darwinian theory. They contain all the information necessary for the maintenance of their reproductive cycle, including that which enables them to take part in ecosystems. The haploid reproductive cycle is subjected to selection at many points, but the selection is ultimately applied to the haploid reproducing cell.

The essential features of Charles Darwin’s original conclusions are supported by this concept because bodies are a reflection of gametes; and we can create a logical synthesis between Darwin’s theory of evolution and our knowledge of genetics.
If this view of the mechanism of evolution is correct, then one can understand how evolution appears to be occurring in all the macroscopic life that we can see around us; but that it is in fact occurring at the microscopic level amongst the myriads of haploid reproducing cells. They form the river of life flowing through time; and all the macroscopic life, including humankind, is merely the means whereby some of them reproduce themselves. We are not the centre of the universe; not the centre of the solar system; and not the central focus of life.


References:
  • 1. Ayala, Francisco J, 2007. Evolution in Encyclopædia Britannica 2007 Deluxe Edition. Chicago: Encyclopædia Britannica.
  • 2. Darwin, Charles in Great Books of the Western World Number 49, Darwin, 1986. Chicago: Encyclopaedia Britannica.
  • 3. Dawkins, Richard, 2006. The Selfish Gene. New York: Oxford University Press.
  • 4. Dawkins, Richard, 1990. The Extended Phenotype. Oxford: Oxford University Press.
  • 5. Dawkins, Richard, 2004. The Ancestor’s Tale. London: Orion.
  • 6. Fortey, Richard, 2008. Life. London: The Folio Society.
  • 7. Gould, Stephen, 2002. The Structure of Evolutionary Theory. Cambridge, Massachusetts: Harvard University Press.
  • 8. Lovelock, James, 2007. The Revenge of Gaia. London: Penguin.
  • 9. Maddox, John, 1998. What Remains to be Discovered. London: Macmillan.
  • 10. Margulis, Lynn and Dolan, Michael, 2002. Early Life. Canada: Jones and Bartlett.
  • 11. Margulis, Lyn and Sagan, Dorion, 2003. Acquiring Genomes. New York: Basic Books.
  • 12. Ridley, Matt, 2003. The Red Queen. New York: Harper.
  • 13. Taylor, David, 2009, Escape of the Ufsapog. Portland (Victoria, Australia): Davis Printers.


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