Evolution of Sexes

PARTHENOGENESIS AND EVOLUTION OF SEX

An interesting unsolvable puzzle to the evolutionists to explain is the phenomena of parthenogenesis. The origin of the word are the Greek words "parthenos" (virgin) + "genesis" (generation) what all-together means reproduction of usually female gamete (egg) without polination or fertilization. Parthenogenesis mostly occurs in lower plants and according to statistics of all plants 80% display some form of asexual reproduction and approximately 50% are mainly or exclusively asexual. Amazingly the offspring is genetically or with other words in all inherited respects identical to the mother plant having e.g. egg with an unreduced chromosome number. Now, the puzzle in all this is that how did evolution continued at all from this kind of plants if these species cannot reshuffle genes to produce subspecies variations? With other words, how evolution developed without meiosis or the absence of cell division that results in two daughter cells each with half the chromosome number of the parent cell. If this could not somehow happen that means that practically there was no possibility of development of sexes.

Actually, the discussion about how the first fully functional female and the first fully functional male developed to begin the process of reproduction, is not the favorite topic of discussion in the circle of the evolutionists. Graham Bell in his book, The Masterpiece of Nature: The Evolution of Genetics and Sexuality, describes the problem in the following way:

”Sex is the queen of problems in evolutionary biology. Perhaps no other natural phenomenon has aroused so much interest; certainly none has sowed as much confusion. The insights of Darwin and Mendel, which have illuminated so many mysteries, have so far failed to shed more than a dim and wavering light on the central mystery of sexuality, emphasizing its obscurity by its very isolation”.

In this regard Dobzhansky and his colleagues made an interesting remark:

”With respect to the origin of sexual reproduction, two challenging questions present themselves. First, in what kinds of organisms did sex first arise? And second, what was the adaptive advantage that caused sexual reproduction to become predominant in higher organisms?”

Now in the following sections we would like to briefly describe few evolutionary theories, which try to solve the above-mentioned problems but of course ultimately without any success.

THE LOTTERY PRINCIPLE

Although this theory doesn’t give the explanation how asexual living entities developed into sexual ones, still we like to mention it because of its attempt to explain sexual reproduction as better then asexual. It was in 1975 that George C. Williams an emeritus professor of biology at the Princeton University of New Jersey for the first time suggested an idea that because sexual reproduction results in greater genetic variety it more successfully enables the species to survive in changing novel environments. To describe this idea he compared the asexual breeding to purchasing of many lottery tickets but all with the same numbers; and sexual reproduction to purchasing of a few lottery tickets with different numbers. The point of this analogy is that since asexual reproduction does not produce genetic variations but rather the offspring are the exact duplicates of their parents, there is a little chance for them to quickly adapt to the changing environmental circumstances and thus successfully survive. This idea was also described by Carl Zimmer in his book Parasite Rex:

"A line of clones might do well enough in a forest, but what if that forest changed over a few centuries to a prairie? Sex brought the variations that could allow organisms to survive change."

Looking more closely the species in the variable and non-variable surroundings, it is clearly observed that in static environmental conditions e.g. in the tropics sexual reproduction is the most common contrary to the unstable environment where asexual reproduction is prevailing. Conclusively, it turns out that genetic variation gained through sexual reproduction is not necessarily crucial for the survival of the living entity in changing environments. Thus obviously, the lottery theory fails as an explanation for the possibility of sexual reproduction being more beneficial then asexual.

THE TANGLED BANK HYPOTHESIS

Briefly this theory assumes that out of a great variety of offspring produced by the parent (animal or plant or bacteria) at least few will survive the hardships in their struggle for existence. About this, Darwin In his book “Origin of the Species” gives the following description:

"It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth, and to reflect that these elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us"

The expression "tangled bank" originated from this last paragraph of Darwin’s book. Inspired by this description of many creatures striving to gain sufficient light and food on "tangled bank", in 1982 Graham Bell made up his Tangled Bank theory in which he suggests that the reason sex developed was to more effectively prepare the offspring for the survival of the life’s hardships. In other words, sexual groups may be more capable to exploit more living places then the parthogenetic species. This circumstance was also nicely pictured by Carl Zimmer:

”In any environment a tidal flat, a forest canopy, a deep-sea hydrothermal vent the space is divided into different niches where different skills are needed for survival. A clone specialized for one niche can give birth only to offspring that can also handle the same niche. But sex shuffles the genetic deck and deals the offspring different hands. It’s basically spreading out progeny so that they’re using different resources”.

Although this theory looks quite logical, one of it’s deficiency is that it still does not explain any steps how asexual species developed sexes. Beyond this there are many other problems this theory cannot solve and answer. In his book, Evolution and Human Behavior, John Cartwright comments:

”Although once popular, the tangled bank hypothesis now seems to face many problems, and former adherents are falling away. The theory would predict a greater interest in sex among animals that produce lots of small offspring that compete with each other. In fact, sex is invariably associated with organisms that produce a few large offspring, whereas organisms producing small offspring frequently engage in parthenogenesis [asexual reproduction]. In addition, the evidence from fossils suggests that species go for vast periods of [geologic] time without changing much”.  [emp. added].

This statement fully matches the concept of stasis, which was explained before to be non-changing phenomena of the species for very long time period. There are quite some examples for this and we will mention them in other place but still lets see two interesting ones. Recently an enlightening discovery was published in the jurnal Nature by Teresa Pawlowska and John Taylor, the biologists of the University of California at Berkeley, about asexual reproduction of Arbuscular mychorrhizal (AM) fungi. They found an evidence that the nuclei in AM fungi are identical - what is typical for asexual reproduction. Further, although the present scientific opinion holds that regarding the capability to obtain nutrients and to tolerate diseases, sexual reproduction is more beneficial for long-term survival of species still, this seems not to be the case with (AM) fungi on which no any sex organs were found by any scientists. According to the fossil records dating back to the Ordovician period (460 million year before) they look just like the modern species what means they could successfully survive.

Another examples are the bacteria that underwent almost no change. Margulis and Sagan remarked that bacteria supposed to be the origins of sexual production but still remained unchanged for millions of years. Here we can note that asexual reproduction is in many ways more advantageous then sexual e.g. it allows beneficial combinations of characteristics to continue unchanged and eliminates the often-vulnerable stages of early embryonic growth. It is found in most plants, bacteria, protists (free-living or colonial organisms) and the lower invertebrates.

But to continue with bacteria, there are rocks containing bacteria that have been dated at 3.5 billion years old and they really defeat the tangled bank theory because e.g. the archaebacteria can survive the very harsh environments such as salt lakes, hot springs, and hydrothermal vents at the bottom of the ocean. Beyond this, there are many other organisms with asexual reproduction system and their existence naturally raise a question, why they did not change to sexual type of reproduction. Especially, don’t they need to prepare their offspring for the life’s hardships in this world -  how it is explained in the Tangled Bank Hypothesis?

THE RED QUEEN HYPOTHESIS

The title of this hypothesis was invented by van Valen in 1973 as an analogy to Lewis Carroll's popular children's novel, Through the Looking Glass (1872). What he meant is that Just as the Red Queen told to Alice in Through The Looking Glass "It takes all the running you can do, to keep in the same place", so, the Red Queen Hypothesis similarly has the idea, that sex evolved and became dominant so that life forms could shuffle their genes and thus become a moving target for diseases and parasites. The phrase `Red Queen' signifies a metaphor for necessary evolutionary change of the species for the purpose of successful survival in the struggle for existence, while competing with other species. Interestingly, according to this theory an organism can never become perfectly developed because of the constant changes and moreover, cessation of changes may result in extinction.

  Now, regarding the evolution of the sexes the Red Queen theory suggests that in ever transforming environments the adaptive value of sex is to create genetic variation so that large organisms can out-evolve their parasites and thus maintain their species. In other words, they have to fight against disease causing pathogens which break into the cells either to eat them, as fungi and bacteria do, or like in the case of viruses, which ruin their genetic structure in order to produce new viruses. To grow, all these alien pathogens within the attacked cell, use protein molecules that bind to other molecules on cell surfaces. For their nourishment both the parasites and the animals or plants need and fight for these proteins.

  There is an interesting example of freshwater snail and a tiny parasite from the glacial lakes of New Zealand's South Island that illustrates how sexual reproduction more successfully continues to exist in changing environmental circumstances but not in steady one. A professor of biology at IUB (Indiana University Bloomington) Norman R. Pace and Gill Fellow Curt Lively found that in the lakes where there are no many parasites, asexual (or clonal) populations of snails are more common. They also discovered that when the asexual snails increase their population at that time attacked by parasites their number significantly becomes decreased. However, without the presence of many parasites, the clonal snails are able not only to survive, but also to replace the sexual individuals in their lake. Conversely, in the lakes with many parasites, sexual populations of snails are more common because they can more easily survive the attacks of parasites. From this observation they inferred `that because the genetic makeup of the sexual population is constantly changing, the parasite must also evolve to attack successfully, but it always remains one step behind the snail. Clonal snails, which by definition always retain the same genetic makeup, cannot adequately fight off the large numbers of rapidly changing parasites.

  This example supports the general consideration of the Red Queen hypothesis that sexual reproduction continues to exist because it enables both in plants and animals the generation of new genetic formulas for the sake of defense against the parasites. Parasites find new keys to brake into the cells, but hosts change the locks. They all, "run to try to improve". And so, this supposed to be the reason why sexes developed long time ago in the history of evolution.

  So, is the Red Queen hypothesis the ultimate explanation of how sexes developed from asexual forms? Certainly not. Let's examine some of its deficiencies.

1) In this theory there is again no answer how sexual reproduction arise just the assumption why it appeared. With other words there is a luck of elaborate description of all the steps of how asexual species evolved into sexual species. This is what Dobzhansky found to be an imperfection of this theory saing, `there is no answer to the question in what kinds of organisms did sex first arise.'

2) Simply because the genes of the offspring are 50 - 50% from both of the parents and that means there is fifty percent of chance of losing a beneficial mutation. In contrary, in the steady asexual population there is in average one surviving offspring per parent, so asexual reproduction is twice as efficient at passing on genes to the next generation.

3) Leigh Van Valen  the author of the Red Queen Hypothesis proposed that the probability of organisms becoming extinct bears no relationship to how long they already may have survived. Similarly, John Cartwright also remarked: "It is a sobering thought that the struggle for existence never gets any easier; however well adapted an animal may become, it still has the same chance of extinction as a newly formed species." With other words although it is favorable for maintenance of the species that the organisms create genetic variations through sex to more easily out-evolve their parasites still, this doesn't grants automatic guarantee of winning the struggle known as "survival of the fittest. For example in the process of cell division in the sexual organisms there is always a possibility that some "unfit" combinations of genes will be generated thus disabling the organism to be resistant to the attacking parasites. And another surprising thing is that instead of increasing their resistance to diseases for better survival on this planet, according to statistics many species in the wild - in the forests, oceans and rivers –are becoming victims to new diseases. Surprisingly, humans are going through similar problems. The every year increasing number of medicines in the pharmacies is a strong proof of people's weak health and their not very strong resistance to diseases. All together, it becomes very questionable whether the Red Queen hypothesis is correct.

4) Dobzhansky wrote: `With respect to the origin of sexual reproduction, two challenging questions present themselves. First, in what kinds of organisms did sex first arise? And second, what was the adaptive advantage that caused sexual reproduction to become predominant in higher organisms?' It would appear that in some circumstances, there is no advantage and even some disadvantage. Although according to evidential proves species with sexual reproduction have greater resistance to parasites, as the Red Queen hypothesis propose, still it has also disadvantages. In normal circumstances when the parasites are not very active, asexual forms have all facilities for survival and are more able to maintain their species then sexual ones, just as it was described in the above example of clonal snails, which were able not only to survive, but also to replace the sexual individuals in their lake. 

Let’s summarize the whole theory briefly.   

1)       Sex evolved and became dominant so that life forms could shuffle their genes and thus become a moving target for diseases and parasites.

This explanation doesn’t give the detailed description how sexes evolved and in which life form did they evolve first.

2)       Sexual reproduction continues to exist because it enables both in plants and animals the generation of new genetic formulas for the sake of defense against the parasites.

Here we have to point out that the offspring that receives 50-50% of genes from both parents have a great chance to lose beneficial mutations. Therefore it is wrong to say that all the new offspring will always have better resistance to diseases caused by parasites. One obvious proof is that e.g. millions of people are born with some disease although their parents were healthy. Moreover, there is even a statistic of increasing number of diseases. 

 3)       In other words, sexual species are more successfully surviving compared to asexual species.

Well, as above mentioned and as seen in reality even the well adopted sexual species become extinct. In the example of snails, we mentioned before, in normal circumstances without the presence of many parasites, the clonal snails are able not only to survive, but also to replace the sexual individuals in their lake. It’s interesting that the study of the environmental changes doesn’t always result in improving the positive health conditions of man and animals (sexual ones) but rather is the cause of diseases like temperature-related illness and even death. To elaborate on this last point we would have to write an other essay.

 4)       Sexual reproduction more successfully continues to exist in changing environmental circumstances.

It’s interesting that the study of the environmental changes doesn’t always result in improving the positive health conditions of man and animals (sexual ones) but rather is the cause of diseases like temperature-related illness and even death.

5)       Clonal snails, which always retain the same genetic makeup, cannot adequately fight off the large numbers of rapidly changing parasites.

This is true. Still, this doesn’t explain the cause of sexual species but rather it is describing only the effect. 

   The greatest blunder of all is in the calculations of the modern scientists that the rate of speciation in asexual beings took almost four billion years, about eighty five percent of the earth's existence so far, to evolve up to the complexity of protozoans, an early group of, initially, asexual, single-celled organisms. 

 Considering this slow development it turns out that not even till nowadays there would be any sexual species. Moreover, the calculation of four billion years development is completely contradictory to many archeological discoveries. Some of them we will mention in one other essay. But in general balance of things, it appears that we still do not have an adequate evolutionary explanation for sexual reproduction.