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THE EVOLUTIONARY UNIT IN LIVING SYSTEMS



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 reproduce indefinitely, and comply with other essential features of basic Darwinian theory. They contain all the information necessary for the maintenance of their reproductive cycle. The haploid reproductive cycle is subjected to selection at many points, but the selection is ultimately applied to the haploid reproducing cell. From this point of view I will very briefly examine a few aspects of living systems.


The value of sex and multi cell bodies



When we regard the haploid reproducing cells as the units of evolution, we can see that the variety created by sex is of immediate advantage to gametes in every generation, and there is no difficulty in seeing how species that evolved this mode of reproduction would have benefited.
Vast numbers of organisms nevertheless remain haploid and reproduce asexually (1,6). Regular sexual exchange may not be a great advantage to unicellular organisms. The haploid cell that has a regular cycle of sex, mitosis and meiosis may gain in variety, but its survival then depends on this more complex form of reproduction, particularly if the gametes no longer have the capacity to reproduce asexually.
The development of large complex organisms with multiple tissues and organs does not seem to be a common feature of haploid cells. The male bee, the drone, is an exception, and, although it is probably not a primary development, it shows that it is perfectly possible for a haploid cell to grow into a gamete producing body. There must be a reason for the rarity of this phenomenon and the only logical one is that greater adaptability is necessary for the formation of complex organisms. Cell differentiation involves an enormous number of intercellular relationships and intracellular processes that have to be switched on or off in harmony. Sexually reproducing gametes, because of the variety they possessed, were able to evolve the capacity to create bodies. This in turn supported their reproductive cycle, and proved advantageous.


Sexual selection



Gene selection theory has great difficulty in explaining how a gene ‘for’ a sexual feature survives in cases where it is clearly detrimental to the survival of the organism. Various ingenious attempts have been made to resolve this problem, but none has achieved universal acceptance (12).
If we regard gametes as units of evolution the problem does not exist. Sexually selected features are simply part of copulation, and they are selected because they promote the getting together of sexually compatible gametes. The external signals from one sex to the other are as much part of the physiology of the organism as are the internal changes. The justification for these characteristics is simply that they promote zygote formation, an essential step in the reproductive cycle. There is thus no natural limit to their development except that imposed by natural selection applied to the whole reproductive cycle and that imposed by the occurrence of the necessary mutations. It is like a physiological language between the sexes, and is as different a language in different species as is the language spoken by one race from that of another.
However extraordinary the language may be, and whether it is forceful, pugnacious, decorative, gymnastic, musical or in some other form, those gametes that make bodies that express it most successfully proliferate preferentially. And even in cases where this expression is sacrificial, or even fatal, provided it is advantageous to gametes it needs no further explanation. That is what bodies are for.
Sexual selection does not need to be tied to fitness to survive. Selection is not taking place only at the point of courting and copulation. Those that reach this point prove their fitness in other parts of the cycle. Efficient copulation is just one part of the fitness of the whole cycle. Sexually selected features may in addition have survival value, or may have a contrary effect.
This explanation does not preclude the possibility that there may in addition be differences in fitness that can be detected in courting behaviour and that can thus separate the very fittest males. Quite apart from any inherited difference in fitness there will inevitably be disease and debility which will affect courting behaviour. But it is not necessary to postulate this additional benefit in order to justify sexual selection.
Sexual selection is, of course, a form of natural selection. It is not essentially different from natural selection applied to other parts of the reproductive cycle. Successful copulation is vitally important to the cycle. But sexual selection is simply one step in the reproductive cycle, no more critical than many other parts of the cycle which may also result in failure to procreate.
An important distinguishing feature of sexual selection is its consistent and persistent nature. Most aspects of the environment are constantly changing (over millions of years) and the interaction with the reproductive cycle will select different things at different times. The distinguishing feature in the case of sexual selection is the constancy of the selector, the necessity to get gametes together.
Dimorphic gametes and different bodies to produce them can also be explained as developments which evolved because they aided the process of getting gametes together.
There are, of course, differences between the sexes that are not specifically sexually selected. Features related to parental care can be taken as an example, but there are many others.
When we choose a real unit of evolution, explanation becomes simple and clear. Because, as already mentioned, bodies are a good marker of gamete success, the view expressed here agrees almost entirely with that expressed by Charles Darwin (2) in The Descent of Man: ‘We are, however, here concerned only with sexual selection. This depends on the advantage which certain individuals have over others of the same sex and species solely in respect of reproduction.’
Elsewhere he also makes clear the distinction between natural selection and sexual selection; the fact that natural selection limits or dominates sexual selection, and may be concordant or discordant with it; and the fact that some features that are different in the two sexes are due to natural selection and not sexual selection.


Altruism



For a characteristic to become established and maintained it must bring benefit to the reproducing unit of evolution, and it must be an immediate direct benefit that increases the chance of successful reproduction. If a mutation arises in a sexually reproducing gamete that induces the body to which it contributes to behave altruistically towards others of the species who are in close proximity, this directly benefits the current reproductive cycle by aiding the progeny, and would therefore stand a good chance of becoming established. Once it had become established there would in addition be reciprocal benefit, immediate and direct, between different sexually created bodies of the species.
In order to understand how a characteristic that is detrimental to a body is beneficial to the reproductive cycle, it is helpful to remember that the whole somatic portion of the body has evolved because it promotes the essential process of gamete reproduction. The trillions of somatic cells have no long term future. They are all sacrificed, and are all ‘altruistic’. If the gametes that create a body program it to aim for self preservation, this ‘selfishness’ is often the best tactic in promoting the reproductive cycle. But this is not always so.
There are other situations in which altruism plays a part. For example, sterile worker bees cannot benefit reproductively from any of their work because they do not reproduce. But the reproductive cycle of the gametes benefits immediately and directly. In this respect sterile worker bees are comparable to the somatic cells of a body whose reason for existence is their value to the gametes in the reproductive cells. They have evolved because gametes that encode this capacity proliferate preferentially. Certain sexually selected characteristics can also be regarded as altruistic because they may impair survival fitness, but from the point of view of the gamete reproductive cycle this is outweighed by the benefit of more efficient copulation.


Ecosystems and the cycle of haploid cell and gamete reproduction



An ecosystem is a mutually beneficial relationship between two or more biological components. An ecosystem possesses function which is not achievable by the members acting alone. The primary function is benefit to the members but there may be additional function which allows it to become part of a larger ecosystem. Each component of an ecosystem has a function that is at least important, and often essential, to survival of the ecosystem. A purely parasitic relationship is possible but relatively rare (1,6,8,10,11).
Different ecosystems, because they possess particular functions, are able to take part in higher level ecological relationships, to which the same principles apply. Ecosystems obviously may vary from relatively simple to vastly complex. An entity might possess several different functions and thus might be part of many different ecosystems. Even if it only has one function it might still take part in different combinations.
As many people have pointed out in recent years, almost all living organisms can only survive for any significant length of time within ecosystems. There are relatively few exceptions, all single cell asexual organisms. (1,6,8,10,11)
Because ecosystems possess variable function they are subject to selection. When the environment changes some will survive and others will not. However, it is difficult to determine how much selection occurs at each level because ecosystems are so complex. We can, nevertheless, make some deductions. The gametes and other haploid cells are selected on the basis of their capacity to survive and reproduce in their immediate environment. At the same time their survival may be affected because an ecosystem of which they are a part is successful or unsuccessful and is possibly lost altogether. The same argument applies at every level. There is selection at that level and the effects of selection at higher levels.
There is nothing new in this brief description of ecosystems. In The Origin of Species Charles Darwin (2) discusses complex biological interrelationships on a number of occasions, and the effects of damage to them. Nor is there anything new in the fact that most forms of life are dependant upon ecosystems involving other living organisms and that their relationships are the result of selection and adaptation.
What I want to emphasize here is that, at whatever immediate level selection is taking place, it finally operates on the haploid reproducing cells, including gametes, because they are the only living information bearing units that reproduce indefinitely and therefore they must contain and transmit all the information necessary for participation in ecosystems.


The body as an ecosystem



We can consider sex as creating an ecosystem. Gametes of sexually reproducing species are slightly different and they unite and do something together that neither could do alone and which benefits both. What they primarily do is create a degree of variety in their offspring that neither could achieve alone. They also create bodies which consist of complex branching ecosystems which, like leaves on a deciduous tree, have no long term future but perform a temporary function and are then discarded. Nevertheless, these bodies are high level ecosystems and feed back ripples of influence upon lower levels and thus influence the selection and evolution of gametes. And they take part in higher level ecosystems.
If we view sexual reproduction in this way as an ecosystem formed between gametes, it is very distinctive. The components actually combine into a single cell. One could regard it as a form of symbiogenesis, except that this term is usually reserved for the combination of different species (11). The way in which bodies are then created is also distinctive. The diploid cells have the ability to differentiate and to create multiple complex ecosystems and thus create tissues which cooperate to form organs and a body. All the information required for differentiation and formation of the body is contained in the gametes that form it.


Secluded and nebulous ecosystems



The ecosystem within a body is very similar to ecosystems in general. We could perhaps call the body a secluded ecosystem and those outside nebulous ecosystems. The secluded ecosystem is tightly packaged in a skin within which the internal environment is controlled. It is discrete and self contained. It is a more concrete entity. Although it contains many different interdependent ecosystems bound together, it interrelates with the environment only as a distinct, separate, secluded, coherent whole. It is dependant upon all its component parts; each one has a limited life span; each moves and lives independently. It is mortal and it lives or dies as a whole.
When we think about the nebulous ecosystems, we can imagine that many may be intermittently present. They can dissipate and reform if a part is temporarily absent. They may vary in size and geographical spread. Their component parts may be shared with other ecosystems. Although potentially immortal, many may be short lived.


Reproduction and evolution of haploid cells including gametes



A haploid reproducing cell is also a secluded ecosystem that can be described in very much the same way as that of a body. The essential difference between haploid cells and all other ecosystems is that they can reproduce themselves. This is the crucial element that allows them to evolve. Once achieved, order can be perpetuated. And each time that mutation produces better haploid cells the improvement is again maintained, as if by a ratchet. Thus the evolution of haploid cells produced a degree of order that was consistent; that was persistent through each life span; and that was maintained by reproduction.
On the other hand, sexually created bodies do not reproduce; they are not even recreated. Each one is a new creation. It is the gametes which make up that body that will reproduce if a body is successful and those gametes that create the best bodies are the ones that will proliferate.
Nebulous ecosystems are subject to selection and one might even refer to them as units of selection; their selection undoubtedly influences evolution; they undoubtedly influence the survival of haploid cells, gametes and bodies. But they do not reproduce; they are constantly recreated by the supply of new units as the old ones die. Even though they can be selected and they can be recreated, they can, like bodies, only exert an influence on evolution through the haploid cell.
Selection can occur at many levels but it is ultimately expressed in selection of haploid reproducing cells and the information they contain. Whether selection operates directly on the unit of evolution or indirectly through other levels of organization, it leads to the preferential proliferation of fitter units.


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.


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