Honors Project by Aaron Weinblatt Evolution of Sex Why is Sexual Reproduction so Commonplace? Introduction The topic of reproduction has been one of my interests for a while. Evolution, in general, has always fascinated me and I pursued more knowledge of the topic with extracurricular reading and research. I was inspired to write this honors project after reading The Red Queen: Sex and ​ the Evolution of Human Nature by Matt Ridley during the summer before sophomore year. ​ Ridley pointed out that initially, all life forms reproduced asexually. As lifeforms evolved, so did reproduction and eventually, sexual reproduction was practiced. Humans have since evolved their human nature, however, the two forms of reproduction have stayed the same. This begs the question of why life forms have two separate forms of reproduction and why we humans, the most recent species, reproduce sexually. Life has been around for approximately 4.250 billion years and it wasn’t until about 1.250 billion years ago that life began reproducing sexually. Before then, life forms on earth reproduced asexually. Around nine hundred million years after the first sexually reproducing organisms evolved, land life emerged. An estimated 99.99% of all eukaryotes1 can (and often do) reproduce sexually including us humans. So how and why did life begin reproducing sexually after three billion years and why has it prevailed? The explanation for the predominance of sexual reproduction is inconsistent and includes many theories, however, by studying and comparing the facts, a conclusion can materialize. How Sexual Reproduction Works Unlike asexual reproduction, sexual reproduction produces offspring that resemble their ​ ​ parents but are not identical. Asexual offspring are genetic clones (identical) to their parents. All organisms have sex cells called gametes. In human beings, the male gametes are called sperm and the female gametes are called ovum (eggs). Sexual reproduction occurs when a male gamete and a female gamete connect. This connection is known as fertilization. Human gametes contain 23 chromosomes each, half the number of chromosomes found in the other cells in the body. When male and female gamete join, they become a zygote (see diagram right), which ultimately becomes an embryo containing the combined 46 chromosomes. In this way, sexual 1 A eukaryote is an organism whose cells have a cell nucleus and other organelles enclosed within membranes. 2 reproduction allows some of the genetic information from each parent to mix, producing offspring that resemble both parents but are not identical to either of them. How Asexual Reproduction Works As explained above, sexual reproduction requires two parents. Contrary to sexual reproduction, asexual reproduction needs only one parent meaning that asexual organisms do not require a mate to reproduce.2 Therefore, the fusion of the gametes and mixing of genetic information as described above do not take place during reproduction. Asexual offspring are genetically identical to the parent and to each other and are often described as clones.3 The Ability to Reproduce both Sexually and Asexually There are many organisms that have the ability to reproduce sexually and asexually. The ​ ​ ability to do this is called heterogamy.4 Some of these species include aphids, slime molds, sea anemones, and some species of sea stars and fish. Most commonly, asexual reproduction is employed to exploit suitable conditions for survival when environmental factors are favorable. These conditions include ample food supply, suitable shelter, and climate, disease, optimum pH,5 and/or a fitting mix of other lifestyle necessities. In order to take full advantage of affluent resources populations of these organisms increase exponentially via asexual reproduction. These organisms will switch to sexual reproduction when food sources have depleted, the climate becomes inhospitable, or individual survival is threatened by some other unfavorable change in living conditions.6 Comparing How Asexual and Sexual Reproduction Work The main difference between asexual reproduction and sexual reproduction breaks down to the number of parents. Sexual reproduction is the combination of genetic material from two ​ parents whereas asexual reproduction produces offspring that are genetically identical to their one parent.7 ​ 2 http://www.biology-online.org/dictionary/Asexual_reproduction ​ 3 http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel_pre_2011/genes/reproductionandcloningrev2.shtml ​ 4 http://www.biology-online.org/dictionary/Asexual_reproduction ​ 5 In chemistry, pH is a logarithmic scale used to specify the acidity. Extremely high or low pH values generally ​ result in complete loss of activity for most enzymes. pH is also a factor in the stability of enzymes. As with activity, for each enzyme, there is also a region of pH optimal stability. http://www.worthington-biochem.com/introbiochem/effectsph.html 6 https://courses.lumenlearning.com/boundless-biology/chapter/reproduction-methods/ ​ 7 https://www.ck12.org/biology/reproduction/lesson/Asexual-vs.-Sexual-Reproduction-BIO/ ​ 3 Sex is the solution to the conflict between the cytoplasmic8 genes of the two parents: All the cytoplasmic genes would come from the mother and none from the father. “Gender is a bureaucratic solution to an antisocial habit.”9 This explains why there are two separate sexes, one with small gametes, the other with large ones, however, those aren’t the only distinctions between males and females. Since the fathers’ gametes were made smaller by evolution, they could specialize in being more numerous and mobile, the better to find eggs.10 The differences between asexual and sexual organisms extend beyond the method and practicality of creating offspring––both means of reproducing warrant different conditions for survival among the relative species. The consequences of sexual reproduction must excel the consequences of asexual reproduction since, generally, only species which are rarely threatened by disease (such as microscopic creatures and arctic and high-altitude plants and insects)11 reproduce asexually. Advantages of Asexual Reproduction It is reasonable to assume that creating offspring that are genetic clones of the parent is a better system than sexual reproduction. If a parent organism is successfully residing in a habitat, offspring with the same traits will, too, be successful in the habitat. Species that reproduce sexually must maintain males and females, two different types of individuals. This can limit the ability to settle into new habitats because both sexes would need to be present. In asexual populations, every individual is capable of producing offspring. Asexual reproduction does not require another organism of the opposite sex to reproduce which allows asexual organisms to produce offspring whenever conditions are favorable. In contrast, in sexually reproducing populations, both sexes are necessary to create new offspring. The offspring of asexual organisms are automatically equipped to survive in their habitat because they are a genetic clone of their parent. On the other hand, even if the offspring of two sexually reproducing parents are suited to survive in an environment, there is no guarantee that their offspring will be similarly capable since they are a combination of two parent genetics. In theory, an asexual population could grow about twice as fast as a sexual population.12 Disadvantages of Asexual Reproduction 8 The cell substance between the cell membrane and the nucleus,containing the cytosol, organelles, cytoskeleton, and various particles. 9 Matt Ridley, The Red Queen: Sex and the evolution of human nature (New York: HarperCollins, 2003). p 104 ​ ​ 10 Ibid. 11 https://www.pbs.org/wgbh/evolution/sex/advantage/page06.html ​ 12 https://courses.lumenlearning.com/boundless-biology/chapter/sexual-reproduction/ ​ 4 Since, often, asexual reproduction is used to take advantage of favorable environmental factors, once an asexual organism has settled in a suitable territory, it won’t leave it.13 Therefore, if the conditions of the environment within the territory change, the entire species of asexual organisms could be eliminated.14 Similarly, parasites and predators that have evolved to kill as few as one of these asexual organisms can kill all of them since they all possess the same traits, and thus, the same weaknesses.15 There are limited movement capabilities within most asexual species and as a result, asexual organisms are not fully in control of their survival, often leading to a difficult struggle for existence.16 Advantages of Sexual Reproduction As explained above, asexual organisms have offspring genetically identical to them. This is not the case with organisms that reproduce sexually. Each individual in a sexually reproducing species is genetically different and, therefore, some members would not be affected by diseases that are capable of killing others. The individuals that have the genetic ability to survive will reproduce, passing on that genetic asset to their offspring, guaranteeing that their species will not go extinct from said disease.17 The reproductive procedure which achieves this is known as Meiosis. The procedural equivalent to this operation practiced by asexual organisms is known as Mitosis. Mitosis is a process of cell division that occurs in asexual reproduction of organisms. It results in two genetically identical daughter cells developing from a single parent cell (see diagram on page 3). The male and female