A Valentine’s Day guide to reproduction, sexual and otherwise
Science by Gregory Beatty
While this is a Valentine’s Day feature, it’s not your usual hearts and flowers ode to love and romance. Oh, there’s sex — don’t get me wrong. But the actual topic is the evolutionary history of reproduction.
And in that age-old drama, sex actually plays a small role, says University of Saskatchewan biologist Timothy Sharbel.
“If you look at life on Earth, 99 per cent of reproduction is asexual,” says Sharbel. “That includes things like bacteria and viruses. But if you look at mammals, 99 per cent of reproduction is sexual.” [see sidebar]
Fossil records show that life began to emerge within a few hundred million years of Earth forming 4.3 billion years ago. The organisms were extremely primitive, granted. But they could function, change and reproduce, so they met the test to be considered life.
From those humble beginnings, nature proceeded over the next three billion-plus years to create an amazing diversity of life. Not all experiments were successful — it’s estimated that 99 per cent of all species that ever existed on Earth are extinct.
Still, it’s a wonderful story and reproduction is a key part.
Asexual & Sexual
At its core, says Sharbel, reproduction is a transfer of information. “That information, as we know it, is DNA,” says Sharbel “But when you go back billions of years, the hypothesis is that the first lifeforms were RNA-based.
“They were prokaryotes [lacking internal organelles], so they were cells with no nucleus,” he says. “Some were chemotrophic, so they derived energy from chemistry in their environment. Others learned to harvest energy from sunlight. Then there was an event where one prokaryote cell engulfed another and developed a partnership with it, and out of that arose eukaryote cells which had a nucleus, and that led to all the complex life we see today.”
To reproduce, cells would do things like bud or split in two to replicate their genome and produce new cells.
Compared to the intricacies of sexual reproduction, asexual reproduction has obvious advantages. First, you don’t have to devote precious time and energy to finding, wooing and copulating with a partner. Instead, it’s a DIY job.
That makes asexual lifeforms adept at exploiting new environments. When glaciers retreat, for instance, they can colonize the area far quicker than their sexual brethren.
That’s not the only advantage, says Sharbel.
“Imagine you’re a plant or animal, and you’re perfectly adapted to your environment. If you reproduce sexually, all your offspring are going to be different. The probability is they will be a little less adaptive than you. But if you reproduce asexually, they would be genetic clones. So, like you, they will be perfectly adapted to the environment.”
That’s fine when the environment is stable. But Earth, as we well know, is incredibly dynamic. Change, on various scales over various time periods, is constant.
It’s not that asexual life can’t adapt. Just look at the virus SARS-CoV-2, which causes COVID-19 and has evolved several new strains since the pandemic started in late 2019. There are even advanced species such as strawberries, tulips, fungi, amphibians and more that reproduce asexually. But there are limitations.
The chief advantage of sexual reproduction, says Sharbel, is that it pairs genetic material from two individuals. “If the environment changes every year, sometimes you might be adapted, and other times not. That makes sexual reproduction advantageous because you would create variability in your offspring, and the probability is some would be better adapted than you.”
Another advantage of combining genomes, says Sharbel, is that it can purge mutations.
“When mutations show up that are super bad, the embryo might abort after the first week of conception” he says. “No one notices it, but the mutations don’t get passed down.”
What happens after that depends on natural selection.
“It can work very slowly, or very quickly,” says Sharbel. “But it’s an extremely complex process which involves linkage or conflict between genes in the genome.
“Some genes may want to do one thing, while others want to do something else,” he says. “All this variation is thrown out there. On certain occasions, it can be adaptive and positive. But 99 per cent of the time the changes are negative, so they tend to be lost.”
Egg & Sperm
Evidence of sexual reproduction goes back a long way, says Sharbel. “It’s well-documented in brown algae, which are quite primitive. Some species made gametes, or what we would call egg and sperm.”
Gametes contain DNA. In some species they are indistinguishable, says Sharbel.
“You can’t tell the difference between male and female. All you can do is label them plus or minus. Nonetheless, plus can only mix with minus and vice versa. So even though the gametes look alike, there’s a genetic mechanism underlying how they combine.”
Other species evolved a gendered approach to sexual reproduction, with recognizably “male” and “female” gametes. And without getting bogged down in gender stereotypes, it’s had a profound evolutionary effect, says Sharbel.
“The female makes this enormous egg, and makes very few of them, whereas the male makes billions of sperm,” he says. “It all comes down to energy. Females put a ton of energy into one gamete, whereas males produce lots of gametes.”
In biology, there’s a famous graph drawn in 1948 by Angus Bateman. “On one axis, you plot reproductive success, on the other mating success,” says Sharbel. “Mating success is your ability to find a partner with whom you’d like to reproduce, while reproductive success is your ability to produce offspring with another individual.
“With females, it doesn’t matter how many partners they have, because they only have one or two eggs, so their reproductive success doesn’t go up,” Sharbel says. “But males have billions of sperm, so the more mating success they have, the more reproductive success they’re likely to have. So the lines on this graph are very different for males and females.”
Because females put all their “eggs” in one reproductive basket, so to speak, their investment in mating is higher than males. To maximize that investment, females have evolved all sorts of “preferences” to assess the genetic fitness of potential mates.
With birds, plumage and singing/dancing ability are highly prized. For other species, characteristics such as antlers, tusks, manes and dominance over rival males are relied on.
Some characteristics, such as sharp horns or tusks to ward off predators, do confer a survival advantage. But others seem to work at cross purposes with survival. Think of a male peacock having to drag around a cumbersome tail, or a male moose trying to evade predators in a forest while burdened with heavy antlers.
Females, though, read the characteristics as signs of prosperity and cunning, which they value to improve the odds of their offspring being fit and hardy.
Still, sexual reproduction isn’t without pitfalls.
One is what biologists call runaway selection. “The Irish elk, which is now extinct, is an example,” says Sharbel. “Why did they go extinct? It was males having such enormous antlers that they could no longer hold their heads up. Why did they have those antlers? Because there was female choice for large antlers, which were used as a measure of the male.”
Speaking of measuring males, there’s a fish species where runaway selection went in the opposite direction. “The female is relatively normal, but the males have degenerated to a point where they can’t swim on their own,” says Sharbel. “All they basically are is a heart and testicle, and they attach to the female and derive nutrients to live that way.”
Nature Gets Kinky
Some species can switch between sexual and asexual reproduction. Generally, the switch is triggered by environmental conditions which favour one form over the other. Species known to do this include aphids, cape bees, hammerhead sharks, Komodo dragons and boa constrictors. Only females can reproduce asexually, not males.
In the sexual realm, some species are hermaphrodites. “A good example is oysters,” says Sharbel. “They are serially hermaphroditic, so the same individual sitting in the ocean could be male in the spring and female in the fall.”
Other species are simultaneously hermaphroditic, so they have both male and female characteristics. Self-fertilization defeats the purpose of sex, so most need partners. And sometimes it leads to interesting results.
How interesting? Well, Google “penis fencing” and see.
“When flatworms mate they stab each other with syringe-like penises, trying to deposit as much sperm as possible,” says Sharbel. “They will tear each other apart, but portions of the animal will survive. The sperm, no matter where it’s injected, will burn its way to the egg cells. If the worm doesn’t get killed by the mating process or the sperm burning through its body, they reproduce.”
Really, the range of strategies species have evolved to reproduce is awe-inspiring. And Sharbel enjoys sharing that awe with his students.
“There’s this huge Gaea-like phenomenon where everything is communicating with everything else. And we’re losing it. If they start seeing the wow and wonder, they’ll be more inclined to study it and protect species so those valuable partnerships and interactions aren’t lost.”
March Of The Placentals
Truly advanced life began to emerge with the Cambrian explosion 540 million years ago. As sexual reproduction evolved, female animals began producing eggs.
With some aquatic species such as fish and cephalopods, females lay eggs in a clutch, and males fertilize them. In other species, fertilization occurs internally, and females either lay eggs in a nest (as turtles and crocodiles do) or carry them internally until they hatch (such as sharks and frogs).
Mammals, which date back 300 million years, started out as egg-layers too. A few are still around, in fact. Typically, they caught some evolutionary break (like Australia being a remote island for the duck-billed platypus and echidna) that shielded them from selection pressure when placental mammals evolved.
Marsupials emerged 120 million years ago. Instead of birthing their young in nests, where eggs and offspring were highly vulnerable to predation, bad weather and other catastrophes, marsupials carry the fetus internally for a time before it’s “born” and crawls instinctively into a pouch in the female’s abdomen to grow to maturity.
Still, like egg-laying mammals, marsupials are largely confined to remote locations (most notably Australia) which buffered them from the dreaded placentals that arose around 70 million years ago.
Placentals took the marsupial breakthrough to its logical conclusion, with the fetus gestating internally until it was reasonably mature and was born. Parenting is still required, of course. But that’s a good thing, especially if the male helps out, which happens in some species, but not all.
Early placental mammals were rodent-like, but they survived the dinosaur extinction, and from there placentals began a steady rise on the evolutionary ladder, leading to us today.
So Happy Valentine’s Day everyone!