Honors Project by Aaron Weinblatt Evolution of Sex

Why is 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 , 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 . In human beings, the male gametes are called sperm and the gametes are called ovum (eggs). Sexual reproduction occurs when a male 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.

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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/ ​

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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 . 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/ ​

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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 sex cells (egg and sperm) are the end result of meiosis; they combine to create new, genetically different offspring. Meiosis occurs in sexual reproduction where the division of a germ cell involving two fissions of the nucleus gives rise to four gametes/sex cells, each possessing half the number of chromosomes of the original cell (see diagram on page 2).18

Mitosis copies the genetic code of the nucleus during reproduction whether these chromosomes generates advantageous or disadvantageous traits. There is always the chance that these disadvantageous traits would not be passed down to the offspring but that chance is slim. Natural selection can further prevent these unfavorable traits from being passed to the next generation. As with the sexually reproducing individuals with the genetic ability to survive an outbreak of disease, the individuals without unfavorable trait(s) will reproduce, passing on yet

13 https://courses.lumenlearning.com/boundless-biology/chapter/reproduction-methods/ ​ 14 https://vittana.org/22-advantages-and-disadvantages-of-asexual-reproduction ​ 15 https://greengarageblog.org/20-big-advantages-and-disadvantages-of-asexual-reproduction ​ 16 https://futureofworking.com/12-advantages-and-disadvantages-of-asexual-reproduction/ ​ 17 https://futureofworking.com/7-advantages-and-disadvantages-of-sexual-reproduction/ ​ 18 https://www.diffen.com/difference/Meiosis_vs_Mitosis ​

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another genetic asset to their offspring, ensuring that the species will eventually be free of these unprofitable genes.19

Natural selection is the only evolutionary process resulting in conformity between a sexually reproducing organism and its environment. This allows for adaptations in species. If an organism produces offspring with varying genetics, the genetically superior offspring would dominate the gene pool and the inferior offspring will succumb to extinction.20 Natural selection operates only by the differential reproductive success which refers to the difference between individuals in a given generation and the amount of offspring they are able to leave. The fittest should leave more offspring and the inept leave fewer or none.21

As explained above, asexual organisms have genetically identical offspring and parents pass on the same types of chromosomes to their offspring, whether these chromosomes carry advantageous or disadvantageous traits. This means that if one individual develops a certain type of illness, the disease can easily spread to the rest of the population and affect them all in the same way. This could be the downfall of their species, rendering them extinct.

Although pleasure can be described as a benefit of sexual reproduction, it does not qualify as an advantage or explanation. This is because the first eukaryotes to engage in sex were single-celled protists22 that appeared over 1.3 billion years before the first animals with neurons capable of assessing pleasure appeared. These single-celled prokaryotes engaged in genetic exchange23 through processes such as conjugation,24 transformation,25 and transduction.26 These genetic exchange processes all fall under the umbrella of parasexuality.27 For approximately 700 ​

19 Ibid. 20 http://www.zo.utexas.edu/courses/thoc/Natural.Selection.html ​ 21 https://www.thoughtco.com/differential-reproductive-success-1224662 ​ 22 A protist is any eukaryotic organism that is not an animal plant or fungus. https://en.wikipedia.org/wiki/Protist 23 Genetic exchange is one mechanism by which new genotypes of species are formed. Genetic exchange results in an organism or cell with an altered genotype. https://www.atsu.edu/faculty/chamberlain/Website/Lects/Genexch.htm#intro 24 Conjugation is a type of genetic exchange mechanism where two bacteria lie close together and cytoplasmic forms between them. A plasmid or a part of the bacterial chromosome passes through this connection from one cell to the other. W. H. Freeman and Company. All rights reserved. Pierce, B. Genetics: A conceptual approach. 2nd Edition. ​ ​ 25 Transformation is another type of genetic exchange mechanism where DNA found in the medium is taken up by the cell. After transformation occurs, recombination may take place between the genes and the bacterial chromosome. 26 Transduction is another type of gene exchange that takes place when a virus carries genes from one bacterium to another. Once it is inside the cell, the newly introduced DNA may undergo recombination with the bacterial chromosome. Ibid. 27 Parasexuality is any reproductive process in which recombination of genes from genetically distinct cells or individuals occurs without meiosis or fertilization. https://www.collinsdictionary.com/us/dictionary/english/parasexual

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million years of sexual reproduction, pleasure was not in the realm of experience for these unicell protists.

Disadvantages of Sexual Reproduction A concern of the advantage of sexual reproduction is that sex does not always increase variation. One of the oldest explanations for sexual reproduction traces back to the work of German Biologist August Freidrich Leopold Weismann in 1896. In his book, Über ​ Germinal-Selection: eine Quelle bestimmt gerichteter Variation (On Germinal Selection as a ​ ​ Source of Definite Variation), Weismann proposed the theory of germinal selection as one ​ explanation for the theory of evolution by natural selection. He figures that variations resulted from different combinations of determinants28 in germ cells.29 The stronger determinants overrode the weaker determinants and thus germinal selection kept the stronger determinants in the germ cells and created adaptive variations. Weismann acknowledged that external factors, such as environment, play a role in the development of organisms. He concluded that sex is so commonplace because it generates variable offspring upon which natural selection can act. This explanation is far too subtle to explain the mediocrity of sexual reproduction without additional information since sex does not always increase the variability among offspring and, even so, producing more variable offspring is not necessarily favorable.

Take a single gene that contributes to the height of a diploid30 organism, for example, and say organisms with genotype aa are shortest, genotype ​ ​ AA are tallest and genotype Aa are mid-height. In ​ ​ ​ the case of this argument, the tallest party are too big to be eaten by predators and the smallest party are able to hide safely. This leaves the average height individuals to be densely preyed upon. Mating would produce fewer offspring at the extremes of height and more offspring in the

28 Gene https://www.merriam-webster.com/dictionary/determinant 29 Germ Cell: A germ cell is any biological cell that gives rise to the gametes of an organism that reproduces ​ ​ ​ ​ ​ sexually. ​ https://en.wikipedia.org/wiki/Germ_cell 30 A cell or an organism consisting of two sets of chromosomes: usually, one set from the mother and another set ​ ​ ​ ​ ​ ​ from the father. http://www.biology-online.org/dictionary/Diploid

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middle. Sex would reduce variation in height, as opposed to a population that reproduces asexually.31

Part of the Red Queen Hypothesis suggests that all species co-evolve with other organisms. This means that predators evolve with their prey and even parasites evolve with their hosts.32 Each and every advantage gained by favorable variation gives one species an edge over their close competitors (i.e. predators, parasites, and prey). Co-evolving species can only maintain its own share of resources is to continually improve its fitness as well. As one species gains an advantage, the other species must also develop an advantage or they will be out-competed. One species increases selection on the other species. “No single species progresses too far ahead of other species because genetic variation among the [offspring] of ​ ​ sexual reproduction provides all species with a mechanism to improve rapidly. Species that ​ ​ cannot keep up become extinct.”33

Evolution does not cause mass extinction because all species have the capability of reproducing and changing. Co-evolution is a system of checks-and-balances. One cannot rush ahead and stay ahead. If any can’t respond to changes, then they become extinct (individually).

Comparing the Disadvantages of Asexual Reproduction to the Advantages of Sexual Reproduction

The primary difference between asexual organisms and sexual organisms is that asexual reproduction generates offspring that are genetically identical to the single parent whereas sexual reproduction generates unique offspring with genetic information from two parents.34 Therefore, asexual organisms have incredibly minor variability when it comes to weaknesses; sexual organisms are genetically different and thus do not all share the same weaknesses. This means that sexual organisms have considerably more movement capabilities than asexual organisms. If the conditions in an inhabited environment would change or predators and diseases have evolved to kill a single organism, they could kill all asexual organisms and only the few sexual organisms that possess the weakness that this change or disease exploits would die. Whereas a whole asexual species is subject to extinction, sexual organisms with the genetic ability to survive will reproduce and immunize the species from that change or disease.

While it has been established that sexual organisms differ vastly from asexual organisms, sexual organisms differ among themselves as well. Sexual organisms can be simply categorized

31 © 2008 Nature Education ​ 32 A living animal or plant from which a parasite obtains nutrition. http://www.dictionary.com/browse/host?s=t 33 https://courses.lumenlearning.com/boundless-biology/chapter/sexual-reproduction/ ​ 34 http://learn.genetics.utah.edu/content/basics/reproduction/ ​

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as Male and Female in terms of sex. Among other physical features, males and females share the same method of reproduction, however, males and females have many physical differences as well. The term for these differences is “sexual dimorphism” and it doesn’t just apply to humans––peacocks (below35 are one of the most distinguishable vertebrate36 in terms of sex.

Sexual dimorphism often insights competitive, often violent, practices between sexually reproducing species. These violent sub-practices include sexual cannibalism, where (in most cases) a female arachnid or insect cannibalizes her mate around the time of copulation. This can be credited to a sexual conflict which includes cannibalism as a result of an unwillingness to mate or objection to the capability of the male made to mate with another female. Additionally, males commonly compete amongst each other for a female mate and occasionally these competitions are violent. As can be seen in many sexual conflict/antagonism practices, organisms may die before reproducing or exhausting their reproductive abilities, thus limiting population growth among some sexually reproducing species.

Comparing the Advantages of Sexual Reproduction and Asexual Reproduction

The disadvantages of sexual and asexual reproduction largely relate to the struggle for existence. Sexually reproducing species must compete for a mate––which can be life-threatening intrinsically––and cannot breed for life, whereas asexual species have an overtly easier process of reproducing. However, this advantage does not override the disadvantage of lack of variability. Although the variation among sexually reproducing species is not entirely reliable or consistently advantageous, invariability is significantly more dangerous to the survival of a species as the plurality of the species cannot evolve defenses against threats.

The consequence of occasional advantageous variability among sexually reproducing organisms warrants higher chances of survival than very limited variability, a major consequence of asexual reproduction.

35 https://en.wikipedia.org/wiki/Sexual_dimorphism ​ 36 An animal of a large group distinguished by the possession of a backbone or spinal column, including mammals, ​ birds, reptiles, amphibians, and fishes. https://www.google.com/search?q=vertebrate&spell=1&sa=X&ved=0ahUKEwjw8vqRpcXZAhWMylkKHQLKCZ gQBQgjKAA&biw=912&bih=676

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Comparing the Disadvantages of Sexual Reproduction and Asexual Reproduction

The most crucial difference between asexual and sexual reproduction is the principle of variation: asexual reproduction produces offspring which are genetically identical to (clones of) their parent; sexual reproduction produces genetic mixtures of two halves of the genetics of two parents, allowing for much more variation. This genetic variability, which leads to evolution, is crucial for the survival of sexually reproducing organisms.

Diseases function by breaking into cells and simply eat them or overthrow genetic systems to make new viruses. This process requires bacterial protein molecules to bind to other molecules on a cell’s surface (right37) and what ensues is a competition to out-evolve the other group of molecules. Matt Ridley, author of The Red Queen: ​ Sex and the Evolution of Human Nature uses the ​ following simile: “parasites invent new keys; hosts change the locks.”38 Variation guarantees that if one “lock” is common in one generation, these hosts will die out and, thus, so will the species of the parasite with the “key” needed to extinguish them. However, since sexually reproducing organisms vary (and, therefore, evolve) the variation of the species with a different “lock” will prosper. This process will continue to occur as parasites evolve simultaneously, however, if the host organism was asexual, this situation would result in extinction.

Since asexual organisms cannot evolve as quickly as sexual organisms, as soon as one asexual organism is exposed to harmful bacteria, the entire population becomes vulnerable and will have a high chance of being driven to extinction.

Conclusion Without sexual reproduction, there is no little to no variation among species. Without variation among species, hosts cannot evolve

37 http://news.mit.edu/2015/using-entire-immune-system-halts-tumor-growth-0414 ​ 38 https://www.pbs.org/wgbh/evolution/sex/advantage/page03.html ​

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genetic defenses or advantageous physical characteristics (right39). Without this evolution, many species would be abated by natural change in habitat, more productive predators, bacteria/disease, etc. Asexually reproducing organisms have limited defenses regarding harmful conditions and their strength of reproductive productivity does not outweigh this weakness.

This explanation is justified best when studying a species that can reproduce both sexually and asexually, such as the Mexican topminnow. Topminnows’ biggest threat to survival is a parasitic worm which causes “black-spot disease” (right). Topminnows occasionally crossbreed with another similar species of fish, producing an asexual amalgam. In a study done by biologists Robert Vrijenhoek and Curtis Lively, it was observed after many generations that the asexually reproducing topminnows were much more susceptible to the parasitic worm than the sexually reproducing worms. This is because sexually reproducing topminnows rearranged their genetic material during gamete formation by joining segments of DNA from another organism, thus evolving a resistant gene pool. The asexually reproducing topminnows cannot evolve such a defense as quickly and thus are far more vulnerable to the “black-spot disease.”40

Sexual reproduction persists because superior survival capabilities are more important to the long-term success of a species than productive and continuous reproduction without the ability to adapt.

39 Grant, Peter R., and B. Rosemary. Grant. 40 Years of Evolution: Darwin's Finches on Daphne ​ ​ Major Island. Princeton University Press, 2014. ​ 40 https://www.pbs.org/wgbh/evolution/sex/advantage/page06.html ​

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