Ah spring! The season when a young man’s (and woman’s) fancy turns to love. What a noble sentiment. However in reality this is a season of intense competition and reproduction fervor. Wilson warblers sing from every willow thicket, red-winged blackbirds dive-bomb ravens, male mule deer are becoming horny (antlery?), newborn fawns frolic in the mustard, teenagers skip school and go skinny dipping, purple-spotted shore crabs burst with eggs, and wildflower pollen drifts in the wind. The entire world seems involved in duplicating itself.
An organism can accomplish this duplication by either reproducing asexually or sexually. Asexual reproduction is easy; it does not require a partner. An amoeba for example simply divides into two parts. Strawberries leapfrog into new territory with baby plants called runners. A female aphid eating a rose bush doesn’t need a male; she lays unfertilized eggs that hatch into other females. The resulting offspring of all these organisms are genetically identical to the parent and are called clones.
Aren’t you glad your kids can’t clone? Imagine if they went into their room and then split in two. “Dad! Set another plate for dinner.” But humans only propagate sexually.
The sexual method of reproduction isn’t new or confined to the “higher” forms of life. About 570 million years ago, primitive multicellular organisms evolved some specialized cells that rearranged the genetic material (chromosomes) of the organism. If these precursor cells were male, they then split into sperm; if female, then into eggs. The sperm and the eggs contain only one half of the original genetic material of the parent. When they come together during fertilization, the result is a full complement of chromosomes and a genetically unique new individual. Not a clone.
Therein lies the paradox of sex. If the biological bottom line is to pass on to the next generation as many genes as possible, then why does a strategy persist, that requires the loss of one half of the parent’s genetic material? Asexual reproduction is more efficient, less risky and quicker. It results in offspring 100 percent like the parent. Yet sexual reproduction is widespread and common; there must be some powerful advantage to it.
There are several theories to explain this dilemma. One is called the “best man” and was developed by George William in the early 1970s. For an organism that is thriving and well suited to its environment, asexual reproduction is the best choice. For example an aggregating anemone living on an intertidal rock just splits in two and forms large clonal groups. Why should it sexually reproduce and give up half of what makes it successful?
But what if the environment changes? Let’s assume the sea temperature increases. The clonal anemone group is doomed; there is no flexibility, no genetic diversity to cope with a changing world. However this particular species of anemone also reproduces sexually. By reshuffling its chromosome deck and spilling sperm and eggs into the ocean, the anemone produces offspring that are genetically variable. A few of these anemones, the “best men,” may have what it takes to survive in the higher sea temperatures. Sexual reproduction may enable this organism to flourish in a dynamic and changing environment.
Another theory proposes that disease pathogens are the motivating force behind the evolution of sex. In the primordial ooze some microorganisms became parasites of other microorganisms. The victims constantly check the “passports” of all cells and if they are invaders, destroy them. Occasionally a pathogen mimics the passport and succeeds in parasitizing the host. By sexual reproduction, the host can reshuffle the genetic deck and change the “passport.” The next generation of potential hosts is now protected from the pathogen. Sexual reproduction has again saved the day.
Confusing? You bet. We still don’t really understand why sex evolved or why it persists. But it’s definitely here to stay. Might as well enjoy the paradox..