[Preview] Why There Are ‘Males’ & ‘Females’

Just why are there ‘males’ and ‘females’?

Why split up a species into distinct mating types—or sexes—such that they must expend time, effort and various resources searching for mates? Why split up a species such that both sexes are required to produce children, but only one sex can actually “give birth”?

At first glance, the sexual divide does not seem feasible as a means to efficiently maintain a population. And yet, this curious division is ubiquitous in the natural world, and commonly featured in most complex species.

It follows that there must be some profound advantage to such configuration.

* * *

On the Role of Reproduction in Biology

All things in nature are subject to physical laws. The living thing—the organism—is no exception. The organism will wear, tear and physically deteriorate until it can no longer sustain its own life. And when the organism inevitably dies, the “blueprint” for its design is lost with it, unless the organism could somehow preserve its “blueprint” beyond its individual lifespan.

That mechanism is reproduction. Reproduction is the process by which the “blueprint” of an organism—encoded in genetic material—is transferred from one generation to the next. It is a fundamental feature of life, as old as the earliest base of life. So it is said that every organism—no matter how complex it appears to be—is first and foremost designed as a machine to reproduce; the survival of the organism itself is secondary, because it only needs to live long enough to pass on its genetic material.[1]

The role of reproduction in nature, because of an obvious consequence of such process, extends much further. As genetic material is passed down, it creates lineage: successive lines of organisms, each one inheriting genetic material from its predecessor, and all of them drawn from the same genetic legacy as the original ancestor. It is this descent of organisms that gives way to the evolution of life.[2] The evolutionary history of life can therefore be thought of as subsequent forms of life, where the higher species are those more adept at carrying out their biological imperative to reproduce.

Reproduction is thus central to biology, not only as a means for life to persist, but also as the key to what brings about all higher levels of biology.


Origin of the Sexes


For most of evolutionary history, reproduction was an asexual affair. There weren’t any males, females, or sexual mechanisms of any kind. Early species reproduced by replication: they split themselves into duplicate copies to create offspring. These species were genetically simple, so this was the ideal way to reproduce: cheap, easy and very little scope for replication errors. Consequently, their populations were large in numbers. Even if errors in replication did occur, the affected individuals could be left to their doom with negligible impact to the reproductive efficiency of the population.

However, the physical environments of early Earth were frequently marked by drastic changes resulting from volcanic activity, continental drift, glaciation, impact events, and changes to atmospheric and marine chemistry. Some of these factors caused extinction-level events that wiped out huge numbers of species in the blink of an eye. To survive such treacherous and unpredictable surroundings, it is not enough just to maintain large populations; rather, populations must maximize reproductive efficiency such that nearly all individuals are healthy and fully functional. In this way, at least some of them may evolve to become more robust—or especially well-adapted—to survive against extinction.

With the arrival of more genetically complex species, reproduction by simple replication was becoming an increasingly expensive, difficult and error-prone process. The more complex an organism is genetically, the more expensive it is to reproduce and greater the chances of replication errors. Consequently, there were fewer of them in a population. This meant that random changes to genetic code—or mutations—caused by replication errors were potentially disastrous because the resulting malfunctioning individuals could negatively impact the reproductive efficiency of the population.

A genetically complex organism will have thousands of genes that are replicated a huge number of times during its development, and then during the course of its life. Given the odds, random replication errors are inevitable, occur frequently and rapidly accumulate. Some genetic damage can be repaired, but there are limits to the type and extent of damage that can be repaired correctly: an incomplete repair becomes a mutation. Environmental factors such as radiation also cause mutations. Left unchecked, deleterious mutations will result in large numbers of malfunctioning individuals in a population.

Dealing with the accumulation of deleterious mutations thus becomes the principal problem for a population.

The Evolution of Anisogamy

The need for novel mechanism to deal with deleterious mutations prompted the evolution of sexual reproduction. Sexual reproduction involves the fusion of sex cells—called gametes—from two parents to produce offspring. This process of recombination separates and then shuffles together genetic material from two parents such that clusters of deleterious mutations are dissolved, reducing their potential for damage and increasing the possibility for genetic repair. Recombination also creates genetic variation, increasing the potential for enhancing mutations as new combinations of genes are discovered.

However, in the long term, sexual reproduction exacerbated the very problem it was supposed to solve. Diluted deleterious mutations are distributed among the population over time, which results in progressive degradation and malfunction, preventing defective individuals from being efficiently purged by natural selection.

Compounding this problem was the cost of requiring gametes from two parents to produce one offspring. The first sexually-reproducing species were isogamous: their gametes were similar in size, so there were no sexes. As they became more genetically complex, larger gametes were needed to provision the development of offspring; but large gametes can only be produced in fewer numbers—given the limited resources for producing gametes—and are also weighed down from the provisions they carry. It is therefore not feasible for both parents to produce large gametes. On the other hand, smaller gametes can be produced in much larger numbers and they can be highly motile; but if both parents produced small gametes, provisioning offspring becomes a problem.

With the distinct advantages conferred by small and large gametes, the larger gametes got even larger, the smaller gametes got even smaller and sexually-reproducing species became anisogamous. The gametes then polarized into two types in an equilibrium where one gamete type is small, highly numerous and motile, to complement the other large, rare and sedentary gamete type.[3][4][5][6] The large gametes became specialized to store the tissue and resources required for the development of offspring and, in order to keep mutations away from the large gametes, the process of dealing with mutations was confined onto the small gametes. By separating the process of reproduction from the process of dealing with mutations, anisogamy solved both the problems that sexual reproduction had failed to solve.

As the gamete types polarized, so too did the adults they produced.

Sexual Dichotomy of Function

The large gametes are the egg cells and the adults that produce them are the females; the small gametes are the sperm cells and the adults that produce them are the males. Adult male and female organisms serve as elaborate extensions of their gametes, because, unlike their gametes, they are greatly exposed to the wider environment and for much longer.

Females allocate their efforts into producing and conceiving their expensive eggs, and then nurturing the developing offspring to adulthood. Females thus function as “genetic vessels” specialized to carefully carry genetic material from one generation to the next.

Males are specialized to function as “genetic filters.” Genetic material in males is radically exposed to natural selection, rendering them the prime target for mutations and thereby confining mutations away from loading on to the females. This exaggeration of mutations in males serves to skew their reproductive success according to their individual genetic quality: males loaded with deleterious mutations will increasingly fail to reproduce—taking their mutations with them out of the gene pool—while those males with enhancing mutations will have increasingly greater reproductive output. The male lineage thus becomes a genetic ‘sieve’ in which enhancing mutations are caught while deleterious mutations fall through until they end up in malfunctioning / dead males.[7][8]

Males and females, with their symbiotic specializations, finally solve the problem of curating genetic material in biological systems: genetic material filtered through males are carried from one generation to the next by females, and so on, perpetually cleansing the gene pool.[9][10]

[See phenomenal works by evolutionary psychologist Steve Moxon on the origin of the sexes: www.stevemoxon.co.uk.]


Design of Males & Females


The dichotomy of function with males as “genetic filters” and females as “genetic vessels” forms the basis for the physiology and behaviour of the sexes.

At every level of biology, there exist various mechanisms for sexual differentiation that have been finely tuned into the rigid division now seen in nature. Greater selection on genetic material in males than on females is the key to how sexual specializations are maintained.[11][12][13][14][15][16][17] The most obvious example is the elaborate process by which the sperm must fertilize the egg. Genetic material in the sperm is subject to selection on a massive scale as hundreds of millions of sperm from the male race through various obstacles to fuse with the one egg of the female; only the most robust sperm finally fuses with the egg.

Genetic material is arranged, interpreted, copied and regulated in a very sex-specific manner.[18] The genomes in sexual species are organized such that an overall greater proportion of genes are involved in male function, and those genes are located in favourable positions. This process has been dubbed the “masculinization of the genome.”[19][20][21][22][23] In order to curate genes, they doesn’t necessarily have to be placed in the male more than in the female. The vast majority of genetic material is shared between the sexes, however, the same genetic material is expressed differently in males and females.[24][25][26][27] Genetic material can also be tagged such that it is expressed differently according to whether it is inherited from the male parent, or the female parent.[28]

X–Y sex chromosome pairing reveals a similar purpose. Females possess two X chromosomes, but males have one X chromosome and a Y chromosome. The Y chromosome, which is clonally passed down the male lineage, is itself a blatant example of how genetic material is confined to males. The XY configuration necessarily subjects genes on male chromosomes to greater selection than genes on female chromosomes. Furthermore, XY and XX cells are intrinsically different, which means they exert direct sex-specific effects on development, independent of prenatal hormonal action: male XY cells will express genes on the Y-chromosome that are not present in female XX cells and XX cells may receive a higher dose of X-chromosome genes that escape inactivation.[29][30][31][32][33][34][35][36][37]

The distinct genetic “blueprints” of males and females code for their distinct physiology. And this is largely implemented through hormones. Sex-determining genes on the sex chromosomes initiate the differentiation of male and female sex organs, which then secrete sex-specific hormones, generating sex differences in external genitalia, the brain, the central nervous system, the immune system, the cardiovascular system and the skeletal system. Hormones will first exert organizational effects on the developing fetus to create permanent “wiring” in preparation for structural changes as per the pre-existing genetic “blueprint.” Then, during key periods in the offspring’s development into adulthood, hormones exert activational effects upon the pre-rendered “wiring” to complete the process of sexual differentiation.

The distinct physiology of males and females drives the most important aspect of sexual differentiation: behaviour.

Mechanisms of Sexual Selection

Each sex is driven to behave differently from the other by their distinct physiology, continuing in the vein of their sexual specializations.

Males may actively seek out environments that put their physiological systems to the ultimate test, in order to expose their individual genetic quality. They may compete fiercely with other males in order to gain access to females, or exhibit elaborate ornamentation and mating displays in order to attract females. Males may choose females with higher reproductive capacity as reliable vessels for their genetic material. Females, on the other hand, select males with higher genetic quality whose genetic material they will carry to the next generation. In this way, sexual selection—where the preferences of each sex selects for the other—compounds natural selection in the curation of genetic material.[38]

The differences between males and females, driven by sexual selection mechanisms, generates the incredible diversity of the animal kingdom.

………………..

[End of Preview]

The full article will be posted sometime in the future.


References

  1. Dawkins R. (1976) The Selfish Gene. Oxford: Oxford University Press. 

  2. Darwin, C. (1859) On the origin of species by means of natural selection. Murray, London. 

  3. Roughgarden J & Iyer P. (2011) Contact, not conflict, causes the evolution of anisogamy (pp. 96–110). In Togashi T & Cox PA (ed), The Evolution of Anisogamy: A Fundamental Phenomenon Underlying Sexual Selection. Cambridge: Cambridge University Press. 

  4. Iyer PL. (2009) Evolution of sexual dimorphism from gametes to ornaments. Dissertation. Department of Biological Sciences, University of Stanford. MI Number: 32941. 

  5. Kodric-Brown A & Brown JH. (1987) Anisogamy, sexual selection, and the evolution and maintenance of sex. Evolutionary Ecology, 1(2):95–105. 

  6. Parker GA et al. (1972) The origin and evolution of gamete dimorphism and the male-female phenomenon. Journal of Theoretical Biology, 36. 

  7. West-Eberhard MJ. (2005) The maintenance of sex as a developmental trap due to sexual selection. Quarterly Review of Biology, 80(1):47–53. 

  8. Atmar W. (1991) On the role of males. Animal Behaviour, 41(2):195–205. 

  9. Moxon SP. (2016) Sex difference explained from DNA to society: Purging gene copy errors. New Male Studies, 5. 

  10. Moxon SP. (2012) The origin of the sexual divide in the ‘genetic filter’ function: Male disadvantage and why it is not perceived. New Male Studies, 1(3):96–124. 

  11. Campos JL, et al. (2012) Molecular evolution in nonrecombining regions of the Drosophila melanogaster genome. Genome Biology & Evolution, 4(3):278–88. 

  12. Roze D & Otto SP. (2012) Differential selection between the sexes and selection for sex. Evolution, 66(2):558–574. 

  13. Mallet MA, et al. (2011) Experimental mutation-accumulation on the X chromosome of Drosophila melanogaster reveals stronger selection on males than females. BMC Evolutionary Biology, 11(1):156. 

  14. McGuigan K, et al. (2011) Reducing mutation load through sexual selection on males. Evolution, 65(10):2816–2829. 

  15. Whitlock MC & Agrawal AF. (2009) Purging the genome with sexual selection: reducing mutation load through selection on males. Evolution, 63(3):569–582. 

  16. West-Eberhard MJ. (2005) The maintenance of sex as a developmental trap due to sexual selection. Quarterly Review of Biology, 80(1):47–53. 

  17. Atmar W. (1991) On the role of males. Animal Behaviour, 41(2):195–205. 

  18. Wright AE & Mank JE. (2013) The scope and strength of sex-specific selection in genome evolution. Journal of Evolutionary Biology, 26(9):1841–53. 

  19. Singh RS & Artieri CG. (2010) Male sex drive and the maintenance of sex: Evidence from Drosophila. Journal of Heredity, 101:100–106. 

  20. Singh RS & Kulathinal RJ. (2005) Male sex drive and the masculinization of the genome. Bioessays, 27:518–525. 

  21. Arnqvist G & Rowe L. (2005) Sexual Conflict. Princeton: Princeton University Press. 

  22. Tomkins JL, et al. (2004) Genic capture and resolving the lek paradox. Trends in Ecology & Evolution, 19:323–328. 

  23. Rowe L & Houle D. (1996) The lek paradox and the capture of genetic variance by condition dependent traits. Proceedings of the Royal Society of London: Biological Sciences, 263:1415–1421. 

  24. Ellegren H & Parsch J. (2007) The evolution of sex-biased genes and sex-biased gene expression. Nature Reviews Genetics, 8:689–698. 

  25. Voolstra C, et al. (2007) Contrasting evolution of expression differences in the testis between species and subspecies of the house mouse. Genome Research, 17:42–49. 

  26. Zhang Y, et al. (2007) Constraint and turnover in sex-biased gene expression in the genus Drosophila. Nature, 450:233–237. 

  27. Khaitovich P, et al. (2005) Parallel patterns of evolution in the genomes and transcriptomes of humans and chimpanzees. Science, 309:1850–1854. 

  28. Crowley JJ, et al. (2015) Analyses of allele-specific gene expression in highly divergent mouse crosses identifies pervasive allelic imbalance. Nature Genetics, 47(4):353–60. 

  29. Chen X, et al. (2013) X and Y Chromosome Complement Influence Adiposity and Metabolism in Mice. Endocrinology, 154(3): 1092–1104. 

  30. Lentini E, et al. (2013) Sex Differences in the Human Brain and the Impact of Sex Chromosomes and Sex Hormones. Cerebral Cortex, 23(10):2322–36. 

  31. Berletch JB, et al. (2011) Genes That Escape from X Inactivation. Human Genetics, 130(2): 237–45. 

  32. Ngun TC, et al. (2011) The Genetics of Sex Differences in Brain and Behavior. Frontiers in Neuroendocrinology, 32(2):227–46. 

  33. Kopsida E, et al. (2009) The Role of the Y Chromosome in Brain Function. Open Neuroendocrinology Journal, 2(2009):20–30. 

  34. Xu J & Disteche CM. (2006) Sex Differences in Brain Expression of X- and Y-Linked Genes. Brain Research, 1126(1):50–55. 

  35. Arnold AP & Burgoyne PS. (2004) Are XX and XY Brain Cells Intrinsically Different? Trends in Endocrinology and Metabolism, 15(1):6–11. 

  36. Carruth LL, et al. (2002) Sex Chromosome Genes Directly Affect Brain Sexual Differentiation. Nature Neuroscience, 5(10):933–34. 

  37. Burgoyne PS, et al. (1995) The Genetic Basis of XX-XY Differences Present before Gonadal Sex Differentiation in the Mouse. Philosophical Transactions of the Royal Society B, 350(1333):253–260. 

  38. Darwin, C. (1871) The descent of man, and selection in relation to sex. Murray, London. 


7 thoughts on “[Preview] Why There Are ‘Males’ & ‘Females’

  1. ”They may compete fiercely with other males in order to gain access to females, or exhibit elaborate ornamentation and mating displays in order to attract females. Males may choose females with higher reproductive capacity as reliable vessels for their genetic material.”

    What’s interesting is that in the humans females also have ornamentation in the form of feminine beauty. Whilst male ornamentation is in the form of high-status.

    This is the result of mild polygyny and high levels of monogamy.

    1. This is not the full article, hence “preview.” I’ve been running into some technical issues, that is why I’ve had to post a preview first. The full article goes into greater detail about sexual selection mechanisms and its quirks.

  2. What do you think of Ants and the role of sex in it? A single mother with multiple drones offering their sperm. Whilst sterile females do everything else.

    They seem to be very stable phenotypically over 10’s of millions of years.

    1. The Haplodiploidy sexual system in Hymenoptera is particularly interesting. Because the males are haploid, any recessive deleterious genetic material will automatically be expressed—exposing them to natural selection—and is rapidly purged from the gene pool. The underlying ‘design’ of males as “genetic filters” and females as “genetic vessels” holds. 🙂

      “Stable” is exactly right. It is a finely tuned, near-perfect system that helps some species propagate in specific environments, but it is not viable for most highly complex species.

  3. Hello, I am one passionate reader of this website and I find it interesting because it deals with important topics and reliable because it is compiled with a fact-based rigorous method.
    I now express one of my deepest concerns about the damages feminism is having on society at large, in particular of course against men. I’m talking about the concentrated efforts (that, read in their entirety, clearly show a well thought plan) to not only promote the idea of inferior men by artificially depicting them as dumb and bad in all the media, but also by grounding this inferiority in biology. So we all know that feminism, as one of the predominant ideas of today’s western regime, has polluted everything from tv to school and workplace, and that is on a social-cultural level, that you can ignore or preferably warn everyone you know against its vile lies. But the destruction of all male values as well as men themselves is also carried out by spreading ideas such as (this is in no way a complete list): the demise of the Y chromosome, men’s shorter lifespan solely due to biological factors, men’s weaker immune system, merciless attacks on testosterone, men’s uselessness due to technological advances (of any kind, e.g. robots that will pave roads instead of humans as well as artificial sperm). This is often accompanied by strong efforts to feminize men, both culturally and biologically, for example saying that we are all female in the womb and that men have nipples to also lactate.
    It took me a long while to recover from the pain of these inhuman offences, but then I tried to check if these claims were indeed as true as they were presented. From what I could see after months (if not years) of gathering info, none of them holds and they are all false. Nothing but lies. Most of the times with no evidence whatsoever, other times maybe a minuscule grain of truth but then completely blown out of proportion. My personal opinion now, after diving in this monstruosities, is that males (humans as well as most species’ males) are in no way biologically inferior to females.
    Now, why am I writing this? Since I don’t have a strong science background (although, as a lawyer, I have a clear sight and can spot solid evidence), don’t have a way to reach many people at once (like, say, with a website) and have no time to arrange a complete rebuttal of the aforementioned claims, gathering all the necessary sources and proofs, I am here to ask the creator of Science vs Feminism to endeavour in debunking these and other supposedly biologically grounded claims of men’s inferiority. These ideas are causing immense harm to real boys and men all over the world, brainwashing them and making them feel bad for no reason. Author of this page as well as some intelligent commenters that I have seen here, please take the burden to study these and other claims and debunk them officially, in the most solid way possible and them give them the greatest publicity.
    I hope my request of help won’t be unheard and I long for reading a great article here and elsewhere about this crucial topic.
    Keep fighting feminism

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