Treebeard's Stumper Answer
The Birds and the Bees
To paraphrase Cole Porter, "Birds do it, bees do it, even mushrooms and trees do it..." Of course what they all do is exchange genetic material to produce unique offspring that are not identical with either parent. They're doing it right now, and I have some hay fever from all the pollen in this spring air. There are several alternatives like one dividing into two or cloning. That seems much easier than two cooperating to produce one! Why is sexual reproduction so universal, even among flowers, protists, fungi, and bacteria? What are the exceptions?
Here's a photo gallery of sexy flowers "doing it" or at least advertising that they are ready to exchange genetic material. From left to right, top to bottom, they are: Swallowtail butterfly on Spotted Knapweed (Centaurea maculosa); Pickleweed (Salicornia virginica); Western Sycamore (Platanus racemosa); Humboldt Lily (Lilium humboldtii); Fuschia-flowered Gooseberry (Ribes speciosum); Chaparral Pea (Pickeringia montana); Beavertail Cactus (Opuntia basilaris); Mariposa Lily (Calochortus venustus); and Horsetail (Equisetum laevigatum?). The first and last photos are from the North Umpqua River in Oregon, the cactus is from Owens Valley, California, and the rest are near home in Santa Barbara, California. I have more sexy flower photos at my Blowing in the Wind (6 April 2001) stumper about wind-pollinated plants.
"The birds and the bees" is the traditional euphemism for sexual reproduction, but it's a puzzling phrase. For one thing, the worker bees we always see are not fertile. Bart Simpson raises the question: "What a day, eh Milhouse? The sun is out, birds are singing, bees are trying to have sex with them - as is my understanding...." Why birds and bees??
Most animals, plants, and even fungi produce offspring that are not identical with either parent. Cloning or dividing into identical offspring seems easier. But in our world where climate and life itself are always changing, only creatures that can adapt will survive. Sexual reproduction is good for individual differences, and individual differences are good for survival. Maybe "love makes the world go 'round," but there is a growing controversy among evolutionary biologists that this is not the whole story. Keep reading for the details and some exceptions.
I have my DMS Science Fair project to work on this weekend, so I only have time for a short answer to a deep stumper!
I thought this would be easy to answer with the traditional wisdom that "Sexual reproduction is good for individual differences, and individual differences are good for survival." I borrowed the wording from Larry Gonick's fine Cartoon History of the Universe (vol.1). The author goes on to say "Therefore, sexual beings survived, and the ones who did best were the ones who liked sex the most - which is why sex felt good then, feels good now, and can only feel better tomorrow!!!"
I was surprised to find that there is much controversy about this answer. As I understand it, the stumper is that genetic diversity may be good for populations, but natural selection must happen through individual advantages. There are costs to being sexual that go beyond the price of dinner and a movie:
- The Cost of Meiosis: If one individual developes a favorable mutation, it might be lost when combining genes with a partner; whereas cloning or dividing or budding exact same offspring will always pass it on. Non-sexual beings pass on 100% of their genetic material, but sexual offspring only get 50% from each parent.
- The Cost of Males: If half of a population can't reproduce, it will take longer for a favorable adaptation to spread, and the environment must still support them all at the expense of more females.
In a non-sexual population, every individual can reproduce and pass on their genes to identical offspring. So the question remains, what is the individual advantage of making reproduction more difficult?
There are interesting exceptions which call the rule into question. Local whiptail lizards are all female. They reproduce witout males by parthenogenesis, a Greek word meaning "virgin birth." There are entire populations of these lizards without dads that are genetically identical except for very rare mutations. Aphids are born pregnant in the spring. They reproduce asexually all summer until they mate and lay eggs in fall for the next year. The beautiful Chaparral Pea (Pickeringia montana) in the middle right photo above has bisexual flowers, but it rarely (never?) produces seed, so (almost) all plants are clones. If it works for lizards and aphids and shrubs, then why not us? Why isn't this strategy more common?
Why "the birds and the bees"? It's an odd euphemism since birds don't have external genitalia and the worker bees we see (other than the queen) are all infertile females. Maybe it's from Samuel Taylor Coleridge's poem Work without Hope:ALL Nature seems at work. Slugs leave their lair -
The bees are stirring - birds are on the wing -
And Winter, slumbering in the open air,
Wears on his smiling face a dream of Spring!
And I, the while, the sole unbusy thing,
Nor honey make, nor pair, nor build, nor sing.
We know what Coleridge means. If you get away from your computer long enough on a beautiful spring day like today, you will see that the birds are busy reproducing themselves, and the bees really do work the flowers. Watching the hawks mate in the air is thrilling!
I'm happy to raise this stumper without answering it. Here are a few starting web links for your own research:
- I'm writing this on April 26, 2003. It was 50 years ago almost to the day that Crick and Watson figured out the double helix structure of DNA, and it was just announced that the human genome mapping project is "finished". Good timing, but stumpers abound!
- There's a discussion of "the birds and the bees" at the Straight Dope. My favorite is the memorable discussion of catkins in chapter 3 of D.H. Lawrence's Women in Love, and the Ken Russell movie.
- I learned a lot from R.D. Howard's Biol 597 lecture notes. There's more info at Brig Klyce's Why Sexual Reproduction?, M.T. Dorak's Evolution of Sexual Reproduction, Rosemary J. Redfield's Evolution: A Molecular Point of View, The Meaning of Life's Big Instruction Book, and this interesting Christian discussion that raises the right questions.
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Copyright © 2003 by Marc Kummel / firstname.lastname@example.org