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Sex evolved as a strategy against cancer?


sperm fertilizing egg//© nobeastsofierce, Adobe Stock

From ScienceDaily:

One of the greatest enigmas of evolutionary biology is that while sex is the dominant mode of reproduction among multicellular organisms, asexual reproduction appears much more efficient and less costly. However, in a study publishing on June 6 in the open-access journal PLOS Biology, researchers suggest that sexual reproduction is favored by selection because, unlike asexual reproduction, it not only provides important evolutionary advantages in constantly changing environments, but also prevents the invasion of transmissible cancer, or “cheater” cells. …

Not only did first multicellular organisms have to deal with their own cheater cells, they also had to evolve adaptations to prevent them being colonized by foreign malignant cells (i.e. infectious ones). Because asexual reproduction leads to identical (“clonal”) organisms, this mode of reproduction is risky due to the possibility of being invaded by clonal infectious cell lineages (i.e. transmissible cancers). Conversely, sexual reproduction decreases the compatibility of contagious cancer cells with their hosts, limiting individual infection risk, as well as the risks of transmission between parent and offspring. Sexual reproduction also generates genetic variation that facilitates the detection of foreign cells, the first and critical step of immune protection.

Although relatively rare, transmissible cancers do exist (e.g. Tasmanian devils, dogs, bivalves), and increasing evidence suggests that most, if not all, malignant cells are potentially transmissible provided a suitable transmission route is offered. Given the ubiquity of cancer in multicellular organisms, in combination with the plethora of potential transmission routes, sexual reproduction may have been favoured as a less risky, more profitable option to produce viable offspring despite its associated costs. Paper. (open access) – Frédéric Thomas, Thomas Madsen, Mathieu Giraudeau, Dorothée Misse, Rodrigo Hamede, Orsolya Vincze, François Renaud, Benjamin Roche, Beata Ujvari. Transmissible cancer and the evolution of sex. PLOS Biology, 2019; 17 (6): e3000275 DOI: 10.1371/journal.pbio.3000275 More.

It’s an interesting theory but the obvious problem is that transmissible cancers are, as the authors admit, rare. They may always have been rare, relative, say, to predation or extinction—whether sex was part of a life form’s organization or not. And sex is a big development, with many consequences, including a heightened risk of extinction.

Cancer cells, on this view, are seen as a form of parasite:

… As early cells banded together to form single, eukaryotic organisms, these organisms would have needed to guard against member cells that refused to subordinate themselves to the whole—“internal cheater cells,” or cancer cells. Early multicellular organisms would also have needed to develop defenses against invading malignant cells from other organisms, or transmissible cancers…

Such early immune systems would have had an easier time differentiating between healthy cells and malignancies, the study argues, if sexual reproduction created offspring that were genetically distinct from surrounding organisms. Targeting cancerous cells could have created an evolutionary pressure to embrace sex, similar to the pressure from parasites and other pathogens.

Jon Kelvey, “The Evolution of Sex Could Have Provided a Defense Against Cancer Cells” at Smithsonian Magazine

Ah yes, that “evolutionary pressure” again. It says nothing about how exactly the machinery develops. The question isn’t what would benefit an organism but for what paths did resources actually exist?

When we get down to that, we are talking about science. The rest is all speculation.

See also: Can sex explain evolution?

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Does there exist a reasonable, step-by-step Darwinian process for going from asexual reproduction to the sexual type? That would seem to require at least one major saltation that Darwin so disliked. Fasteddious
Geneticist Giuseppe Sermonti wrote:
Sexuality has brought joy to the world, to the world of the wild beasts, and to the world of flowers, but it has brought an end to evolution. In the lineages of living beings, whenever absent-minded Venus has taken the upper hand, forms have forgotten to make progress. It is only the husbandman that has improved strains, and he has done so by bullying, enslaving, and segregating. All these methods, of course, have made for sad, alienated animals, but they have not resulted in new species. Left to themselves, domesticated breeds would either die out or revert to the wild state—scarcely a commendable model for nature’s progress.
(snip a few paragraphs on peppered moths)
Natural Selection, which indeed occurs in nature (as Bishop Wilberforce, too, was perfectly aware), mainly has the effect of maintaining equilibrium and stability. It eliminates all those that dare depart from the type—the eccentrics and the adventurers and the marginal sort. It is ever adjusting populations, but it does so in each case by bringing them back to the norm. We read in the textbooks that, when environmental conditions change, the selection process may produce a shift in a population’s mean values, by a process known as adaptation. If the climate turns very cold, the cold-adapted beings are favored relative to others.; if it becomes windy, the wind blows away those that are most exposed; if an illness breaks out, those in questionable health will be lost. But all these artful guiles serve their purpose only until the clouds blow away. The species, in fact, is an organic entity, a typical form, which may deviate only to return to the furrow of its destiny; it may wander from the band only to find its proper place by returning to the gang. Everything that disassembles, upsets proportions or becomes distorted in any way is sooner or later brought back to the type. There has been a tendency to confuse fleeting adjustments with grand destinies, minor shrewdness with signs of the times. It is true that species may lose something on the way—the mole its eyes, say, and the succulent plant its leaves, never to recover them again. But here we are dealing with unhappy, mutilated species, at the margins of their area of distribution—the extreme and the specialized. These are species with no future; they are not pioneers, but prisoners in nature’s penitentiary.
Cures cancer AND prevents universal common descent... ET
There is a a function behind "sex" that is not clearly understood. Amongst almost all eukaryotes, cells can reproduce a finite number of times before the "clone" senesces and dies. This is well known by scientists who maintain cultures of protists such as Paramecium. The cells can reproduce by binary fission for numerous generations but they eventually die off. However, if two cells get together and undergo a meiotic process, their progeny are rejuvenated and can continue to reproduce asexually for numerous additional generations. But it becomes even more strange. For some protists, a mate is not required for this rejuvenation process. The nucleus divides by meiosis and then the two "gametes" fuse, restarting the clock. Brother Brian
I thinks it’s better explained as a horizontal transfer of DNA AaronS1978

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