25 Paelozoology Will Clark

Paelozoology

Will Clark Upper Sixth

Charles Darwin

Throughout the evolutionary history of animals, there have been many integral driving forces that have combined to push animals to the evolutionary point they are at today, with millions of complex organisms deriving from once singlecelled organisms inhabiting the Earth. Each of these driving forces has certainly been integral and essential in its own right, making it difficult to pinpoint a “most influential” factor. It is unlikely that any of these developments could have happened without the others happening. However it is fair to say that some factors, such as mitochondria being absorbed into eukaryotic cells, has a distinctly more noticeable physical advantage than a factor such as the predator-prey relationship, as this is more of a behavioural evolutionary trait, compared to the drastic physical changes caused by the engulfment of mitochondria by eukaryotes. The symbiotic relationship formed as this event happened helped shape the modern cells as we study them today. Therefore it is a factor that cannot be overlooked and is clearly monumentally important in the development of animals. However, another factor that is perhaps often overlooked is the ability to reproduce sexually, and the numerous advantages this trait brings with it. Sexual reproduction is one of the most complex and bizarre scientific processes we can study, and the creation of a new lifeform that is genetically unique from its adapt to their environments, as natural predecessor brings some serious advantages selection does. However, as I will discuss in the evolutionary game, helping modern later, producing more variable offspring animals to push through the evolutionary is not always favourable, and, even more boundaries that other lifeforms have faced. surprisingly, sexual reproduction does not The origin of sexual reproduction is always increase the variability among the widely disputed, with the first species to offspring. utilise this evolutionary tool being highly uncertain. Despite 99.99% of eukaryotes Although the dawn of sexual reproduction being known to reproduce sexually as has always been a puzzle for scientists, it opposed to asexual reproduction, we still seemed to puzzle no one more than Darwin do not know exactly how and why this himself. Although making breakthroughs process first came about. One argument in the areas of natural selection and for the origin of sexual reproduction evolutionary studies, he wrote in 1862: states that organisms simply engage in “We do not even in the least know the sexual reproduction as final cause of opposed to asexual as it is sexuality; why pleasurable. However, on new beings the large-scale evolutionary should be picture, this theory seems produced by rather out of place and the union of far-fetched. Considering the two sexual that the first eukaryotic elements. The organisms to engage whole subject is in sexual reproduction as yet hidden in were single-celled protists that appeared darkness.” He could not explain the reasons over two billion years ago, this argument why many organisms and animals will go starts to seem rather unlikely. This is to great and troublesome lengths to attract further confirmed when considering the a mate and be completely preoccupied fact that organisms capable of producing by sex, even if it is time consuming and pleasure-inducing neurons only evolved troublesome for the creature. Why would around 1.3 billion years ago, casting animals make the transition from asexual doubt on the pleasurable aspect of sexual reproduction, a form that is known to be reproduction for this 700-million-year far more efficient and less troublesome, period. The eukaryotic cells that engaged to sexual reproduction, a time consuming in sexual reproduction in this time tended and often unpredictable lottery of creating to transfer genetic material via processes a new generation? This can partly be such as conjugation, transformation and explained again by August Weismann’s transduction, all of which come under the theory of “individual differences” upon umbrella of parasexuality. which natural selection acts. Reproducing sexually causes the offspring to carry a

Sexual reproduction is one of the most complex and bizarre scientific processes.

Another theory as to the original reason that sexual reproduction was brought into the behaviour of modern animals states that it is because of its ability to produce genetically variable offspring, allowing natural selection to take its course. This is a far more reasonable theory, and dates back to the work of German biologist August Weismann in the late 1800s. It is natural that organisms would reproduce in a way that best lets them evolve and

beneficial mix of good genes from both of the parents, causing them to respond better to the environmental stresses caused by a changing or harsh environment. This contrasts to the carbon copies of an organism produced by asexual reproduction, whereby this cloning process could leave the offspring in grave danger. This leads on to the fact that sexual reproduction is thought to speed

evolution up - which can be seen as an extreme advantage in a changing climate or environment. Whereas an asexual reproducer may be left behind and unable to cope, the natural variation caused by sexual reproduction allows the organism to respond to these changes and increase the frequency of this favourable phenotype in the gene pool, furthering its’ species survival.

Many organisms are able to reproduce both sexually and asexually, including aphids, slime moulds, sea anemones, and many more creatures which can do this. When environmental conditions are favourable, these organisms reproduce on a mass scale using asexual reproduction, in order to take advantage of this increase in survivability. This could be caused by a new and abundant food supply, adequate shelter, optimum pH or other climatic factors. However, it is only when the conditions become unfavourable that sexual reproduction is preferred, as it enables a mixing of the gene pool and allows variation. This variation in the

offspring naturally allows some to be better suited to this new, hostile environment and provides a mechanism for selective adaptation to occur. If organisms were not presented with this choice millions of years ago, then this would make it impossible for many of the modern animals we know today to exist at all. This is because it is extremely likely that one singular change in the environment, ranging from a dip in the food supply to a monumental event such as an asteroid hitting the Earth, could cause the entire species to die out, without any variation to utilise. Leading on from this point, is the idea of disease resistance within a population. An asexually reproducing population, although it may produce large quantities of offspring within a short time, would be far more susceptible to a new disease that there is not an immunological defence against yet. Species that reproduce asexually, even when faced with a deadly disease or infection, would have a far higher likelihood of some members having natural immunity due to random variation, and this would allow the population to survive. The members of the species that are less vulnerable to the disease would be able to survive and reproduce, therefore passing on these disease-resistant genes to their offspring and increasing the allele frequency of this characteristic in the gene pool. As more susceptible members die out, there would be a shift in the population towards organisms with the immunological response [S]exual against the disease, and the reproduction is thought population members would be able to survive and grow with this new gene embedded to speed evolution up. within their DNA. Asexually reproducing populations, however, would likely be wiped out due to the lack of variation, meaning that if one member is susceptible to the condition then there is certainly a high chance that the rest of the population is. However, as mentioned earlier, sex does not actually generate more variable offspring, contrary to popular belief. Although this may often be the case, there are examples where asexual reproduction is actually more favourable in producing variable offspring after selection has taken place. For example, with a gene that contributes to height in a diploid organism, here, individuals with the genotype aa are the shortest and those with Aa are of intermediate height, with AA being the tallest. If the shortest organisms are able to hide safely and the tallest are too large to be eaten by predators, then this implies that the Aa (intermediate height) organisms are going to be heavily consumed and preyed upon. Amongst the few extremely tall or small organisms that are able to survive, there will be a huge variation between heights, and this brings in the question of how sexual reproduction would actually be an advantage at this point. It is logical to use asexual reproduction in this particular circumstance in order to keep the extreme heights that prevent the species from being preyed upon rather than to use the Hardy-Weinberg principle of sexual reproduction to stabilise the height distribution, thus allowing more of the species to be preyed upon. This would result in variation actually being reduced by sexual reproduction and would certainly be unfavourable for the species involved. However, this argument is overly simplified and, although it illustrates a general point, these circumstances are rare and cannot account for all examples of sexual reproduction.

Having taken all of these points into account, it is clear that sexual reproduction is certainly one of the driving forces leading to the evolution of modern animals, and that it can be argued to be the singular most important reason that modern animals were able to evolve. The variation caused (most of the time) by sexual reproduction is extremely important and without it a huge number of organisms would not have been able to survive changes in their environments causing them to become hostile and dangerous.