March 17, 2023 | Jerry Bergman

How Could Sex Evolve?

Still baffling after all these years:
a new PNAS article reinforces the difficulty
of explaining the evolution of sex


by Jerry Bergman, PhD

Explaining the origin of sex is widely recognized as a major dilemma after 150 years of attempts to answer it by some of the world’s leading evolutionists. Since Darwin revolutionized the world with his theory, this “masterpiece of nature” is acknowledged as one of evolutionists’ most difficult evolutionary problems, second only to the origin-of-life problem.[1]

Sexual differences are widespread in animals, but no single rule explains them.

The dominant theory is that asexual reproduction somehow slowly evolved into sexual reproduction. However, the evidence is both overwhelming, and widely recognized even by evolutionists, that evolution by small steps cannot bridge the transition from asexual to sexual reproduction. Sexual reproduction cannot occur until both functional and compatible male and female reproductive systems exist. If any part of any component does not exist, reproduction will not occur. Nonetheless, evolutionists continue to look for ways to solve the problem of the origin of sex. One current example is a study by Yadav et al.[2] This study, rather than solve the problem, actually illustrates how difficult it is.

Evolutionists not only readily admit that “eukaryotic sexual reproduction is a mystery,” but also that the “ubiquity of eukaryotic sexual reproduction is a mystery.” In other words, the fact that eukaryotic sexual reproduction exists everywhere in life, from invertebrates to vertebrates, from plants to insects and animals must be explained. Furthermore, a variety of very different types of eukaryotic sexual reproduction systems are observed (see list below). For example, fungi “undergo alternative modes of sexual reproduction (unisexual, pseudosexual, and parasexual) in the laboratory and in nature that share features with alternative sexual processes observed in animals and plants (parthenogenesis, hybridogenesis, gynogenesis, and apomixis).”[3]

Most animals, including humans, after birth live out their entire lives and reproduce as either one sex or the other. With some animals, and many plants, a variety of sex types exist. These will now be briefly described to illustrate the problem this poses for evolution.

The Basic Kinds of Sexual Designs

Unisexual refers to an organism that can reproduce without requiring both male and female gametes. Unisexual plants’ flowers contain either stamens or carpels, but not both. Examples in the plant kingdom include papaya, cucumber, maize, tapioca, pumpkin, musk melon, castor bean, birch, pine (using cones), and watermelon.

Bisexual plant flowers contain both stamens and carpels and require both male and female gametes to reproduce. Common examples include rose, sunflower, hibiscus, lily, and mustard. Attempts to determine patterns related to why some plants can reproduce unisexually, while others require bisexual support, have failed.

Simultaneous hermaphroditism exists in a single organism which has both types of reproductive organs when mature. Consequently, they produce both male and female gametes. In simultaneous hermaphrodites, self-fertilization is possible in some species, but absent in others. Examples include vascular plants, worms, snails, slugs, barnacles, bryozoans (moss), and trematodes (flukes).

Sequential hermaphroditism produces eggs (female gametes) and sperm (male gametes) at different stages in their life. The change from one sex to another is a normal event as part of the organism’s reproductive cycle.  The change from male to female is called protandry or protandrous hermaphroditism, and from female to male is called protogyny or protogynous hermaphroditism. Sequential hermaphroditism is actually common in many fish, gastropods, and certain plants.

Bidirectional hermaphrodites possess the capacity for sex change in either direction, male to female and female to male, an alternation potentially repeated several times during the organism’s lifetime.

Pseudosexual includes animals that experience a tertiary physical attraction which mimics sexual attraction but no transfer of gametes occurs. The problem with this behavior is that it does not normally involve successful reproduction.[4]

Parasexual reproduction is a system that results in the recombination of genes from different individuals, but does not involve meiosis nor the formation of a zygote by fertilization as in sexual reproduction. The main examples include fungi and many unicellular organisms.[5]

Parthenogenesis, is a form of reproduction in which an egg develops into an embryo without being fertilized by sperm. It usually results in the development of a female; and very rarely males.  Rotifers, along with several insect species, including aphids, bees, wasps, and ants can reproduce by parthenogenesis.

Hybridogenesis, also called sexual parasitism, involves the selective transmission of one of the parental genomes, while the other genome is renewed by mating with the corresponding species. [6]

Gynogenesis is a system of asexual reproduction that requires the presence of sperm but not the contribution of its DNA. The paternal DNA dissolves, or is destroyed by another means, before it can fuse with an egg. The egg cell then is able to develop, unfertilized, into an adult using only maternal DNA. Most gynogenesic animals are fish or amphibians. Why this reproductive mode even exists, given that it combines the disadvantages of both asexual and sexual reproduction, remains another unsolved problem in evolutionary biology.[7]

Androgenesis is the male equivalent of gynogenesis, where the father is the sole contributor of DNA. Thus a zygote is produced with only the paternal nuclear genes.[8]

Apomixis is asexual reproduction in which seeds are produced from unfertilized ovules. Examples include the genera Crataegus (hawthorns), Amelanchier (shadbush), Sorbus (rowans and whitebeams), Rubus (brambles or blackberries), Poa (meadow grasses), Nardus stricta (doormatgrass), Hieracium (hawkweeds) and Taraxacum (dandelions).

Attempts to Explain the Variety of Reproductive Methods Fail

In their PNAS paper (ref. 2), the authors attempt to theorize how and why organisms could have evolved so many different systems for mating-type determination. This, they claim, could advance the understanding of the evolution-of-sex problem itself. Actually, their attempt creates additional major difficulties for understanding the evolution of sex. For example, they write:

the systems by which sex is defined are highly diverse and can even differ between evolutionarily closely related species. While the most commonly known form of sex determination involves males and females in animals, eukaryotic microbes can have as many as thousands of different mating types for the same species. Furthermore,… several examples are also present among vertebrates suggesting that alternative modes of sexual reproduction evolved multiple times throughout evolution.[9]

It is widely recognized that the evolution of sex is an enormous problem: “no other problem has sowed as much confusion” as have attempts to explain the origin of sexual reproduction.[10]  As Richard Dawkins asked, “why did sex, that bizarre prevision of straightforward replication, ever arise in the first place? … This is an extremely difficult question for evolutionists to answer” which he admitted he was “going to evade” due to “the difficulty which theorists have with explaining the evolution of sex.”[11] The late Lynn Margulis added in the introduction of her book on sex that the subject was so difficult that “becoming sexual [beings] is one [topic] which we will try to steer well clear of throughout this book.”[12]

How Yaiv et al., in their PNAS Article Deal with the Origin of Sex Problem

Yaiv et al. proposed that the variety of sex behaviors they documented did not evolve from some hypothetical original sexual reproduction system, but rather evolved multiple times. They openly stated that “sexual reproduction evolved multiple times throughout evolution. The problem is, if sex is unlikely to have evolved once, it is far more unlikely to have evolved as many as 12 different times to explain the different sexual systems listed by Yaiv and noted above.

The authors’ phraseology implies that animals can choose their method of reproduction, as if it were a conscious choice made by the organism. They write,

some species have found alternatives to sexual reproduction, and prefer to grow clonally and yet undergo infrequent facultative sexual reproduction. These organisms are mainly invertebrates and microbes.[13]


Most evolutionists believe that evolution explains the origin of all types of sexual reproduction but struggle to determine when, how, and why sex evolved. The PNAS paper reviewed here is no exception. All past attempts fail, and the paper reviewed here, published in a leading American science journal, is another example of the norm. Now evolutionists have to explain the evolution of over a dozen types of sexual reproduction. But they must admit that sexual reproduction is evolutionarily conserved, meaning that, when examined historically, it has been shown to have not changed.[14] In other words, no evidence exists that any of the sexual systems the authors discussed have evolved. All evolutionists can do is attempt to speculate how one sex system could have evolved into another reproductive method.


[1] Trivers, Robert. The evolution of sex: A review of the masterpiece of Nature: The evolution and genetics of sexuality. The Quarterly Review of Biology 58(1):62-67, March 1983.

[2] Yaiv, Vikas, et al. On the evolution of variation in sexual reproduction through the prism of eukaryote microbes. Proceedings of the National Academy of Sciences 120(10). 3 March 2023;

[3] Yaiv et al., 2023; ref. 2.

[4] Dias, Brian, and David Crews. Regulation of pseudosexual behavior in the parthenogenetic whiptail lizard, Cnemidophorus uniparens. Endocrinology 149(9):4622–4631, September 2008.

[5] Mishra, Abhishek, et al. Parasexuality of Candida species. Frontiers in Cell Infection Microbiology 11:796929; doi: 10.3389/fcimb.2021.796929, 2021.

[6]  Lavanchy, Guillaume, and Tanja Schwander.

Hybridogenesis. Current Biology 29(3, 4):539, February 2019.

[7] Schlupp, Ingo. The evolutionary ecology of gynogenesis. Annual Review of Ecology, Evolution, and Systematics 36:399–417, 2005.

[8] Komma, Donald, and Sharyna Endow. Aploidy and androgenesis in Drosophila. PNAS-Genetics. 92:11884-11888; December 1995.

[9] Yaiv, et al., 2023; ref. 2, italics added.

[10] Bell, Graham. The Masterpiece of Nature: The Evolution and Genetics of Sexuality. University of California Press, Berkeley, CA, p. 19, 1982.

[11] Dawkins, Richard. The Selfish Gene. Oxford University Press, New York, NY, p. 46, 1976.

[12]Margulis, Lynn, and Dorion Sagan. Origins of Sex. Yale University Press, New Haven, CT, p. 3, 1986.

[13] Yaiv, et al., 2023; ref. 2. Emphasis added.

[14] Yaiv, et al., 2023, p. 1; ref. 2.

Dr. Jerry Bergman has taught biology, genetics, chemistry, biochemistry, anthropology, geology, and microbiology for over 40 years at several colleges and universities including Bowling Green State University, Medical College of Ohio where he was a research associate in experimental pathology, and The University of Toledo. He is a graduate of the Medical College of Ohio, Wayne State University in Detroit, the University of Toledo, and Bowling Green State University. He has over 1,300 publications in 12 languages and 40 books and monographs. His books and textbooks that include chapters that he authored are in over 1,800 college libraries in 27 countries. So far over 80,000 copies of the 60 books and monographs that he has authored or co-authored are in print. For more articles by Dr Bergman, see his Author Profile.

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