Yesterday I was thinking about sex.
Not like that. My intention is to keep this blog PG-13. I meant sexual reproduction in general, and the topic comes up because I just finished reading Riley Black's lovely new book When the Earth Was Green: Plants, Animals, and Evolution's Greatest Romance, which looks at paleontology through the lens of botany. It's a brilliant read, the writing is evocative and often lyrical, and it needs to be added to your TBR list if you've even the slightest bit more than a passing interest in the past.
One of the topics she looks at in some detail is how sexual reproduction in plants -- better known as pollination -- led to an inseparable relationship between flowering plants and their pollinators. A famous example is Darwin's orchid (Angraecum sesquipedale), a Madagascar species with night-scented white flowers whose nectaries are at the base of an impossibly long tube:
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Its discovery prompted Charles Darwin to predict that there must be a moth on the island whose mouthparts fit the flower, and which was responsible for pollinating it. Sure enough, in a few years, biologists discovered the Madagascar hawk moth (Xanthopan morganii):
The problem is, such dramatic specialization is risky. If something happens to either member of the partnership, the other is out of luck. In fact, sexual reproduction in general is a gamble, but its advantages outweigh the risk, and I'm not just talking about the fact that it's kind of fun.
Asexually-reproducing organisms, like many bacteria and protists, some plants and fungi, and a handful of animals, have the advantages that it's fast, and only requires one parent. There's a major downside, however; a phenomenon called Muller's ratchet. Muller's ratchet has to do with the fact that the copying of DNA, and the passing of those copies on to offspring, is not mistake-proof. Errors -- called mutations -- do happen. Fortunately, they're infrequent, and we even have enzymatic systems that do what amounts to proofreading and error-correction to take care of most of them. A (very) few mutations actually lead to a code that works better than the original did, but the majority of the ones that slip by the safeguards cause the genetic message to malfunction.
It's called a "ratchet" because, like the handy tool, it only turns one way -- in this case, from order to chaos. Consider a sentence in English -- space and punctuation removed:
TOBEORNOTTOBETHATISTHEQUESTION
Now, let's say there's a random mutation on the letter in the fourth position, which converts it to:
TOBGORNOTTOBETHATISTHEQUESTION
The message is still pretty much readable, although the second word is now spelled wrong. But most of us would have been able to figure out what it was supposed to say.
Now, suppose a second mutation strikes. There is a chance that it would affect the fourth position again, and purely by accident convert the erroneous g back to an e, but that likelihood is vanishingly small. This is called a back mutation, and is more likely in DNA -- which, of course, is what this is an analogy to -- because there are only four letters (A, T, C, and G) in DNA's "alphabet," as compared to the 26 English letters. But it's still unlikely, even so. You can see that at each "generation," the mutations build up, every new one further corrupting the message, until you end up with a string of garbled letters from which not even a cryptographer could puzzle out what the original sentence had been.
Sexual reproduction is a step toward remedying Muller's ratchet. Having two copies of each gene (a condition known as diploidy) makes it more likely that at least one of them still works. Many genetic diseases -- especially the ones inherited as recessives -- are losses of function, where copying errors have caused that stretch of the DNA to malfunction. But if you inherited a good copy from your other parent, then lucky you, you're healthy (although you can still pass your "hidden" faulty copy on to your children).
This, incidentally, is why inbreeding -- both parents coming from the same genetic stock -- is a bad idea. It doesn't (in humans) cause problems in brain development, which a lot of people used to think. But what it does mean is that if both parents have a recent common ancestor, the faulty genes one of them carries are very likely the same ones the other does, and the offspring has a higher chance of inheriting both damaged copies and thus showing the effects of the loss of function. It's this mechanism that explains why a lot of human recessive genetic disorders are characteristic of particular ethnic groups, such as cystic fibrosis in northern Europeans, Tay-Sachs disease in Ashkenazic Jews, and malignant hyperthermia in French Canadians. It only happens when both parents are from the same heritage -- which is why "miscegenation laws," preventing intermarriage between people of different races or ethnic backgrounds, are exactly backwards. Mixed-race children are actually less likely to suffer from recessive genetic disorders -- the mom and dad each had their own "genetic load" of faulty genes, but there was no overlap between the two sets of errors. Result: healthy kid.
The difficulty, of course, is that despite its genetic advantages, sexual reproduction requires a genetic contribution from two parents. This is tough enough with mobile species, but with organisms that are stuck in place -- like plants -- it's a real problem. Thus the hijacking of animals as carriers for pollen, and the evolution of a host of mechanisms for preventing self-pollination (which cancels out the advantage of higher variation, given that once again, both sets of genes come from the same parent).
What's most curious about sexual reproduction is that we don't know how it started. Even some very simple organisms have genetic exchange mechanisms, such as conjugation in bacteria, which help them not to get clobbered by Muller's ratchet, and something like that is probably how it got going in the first place. We know sexual reproduction is evolutionarily very old, given that it's shared by the majority of life on Earth, but how the process of splitting up and recombining genetic material every generation first started is still a mystery.
Anyhow, that's our consideration of birds, bees, and others for the day. I'll end by saying again that you should buy Riley Black's book, because it's awesome, and gives you a vivid picture of life at various times on Earth, not from the usual Charismatic Megafauna viewpoint, but from the perspective of our green friends and neighbors. It's refreshing to consider how life is experienced from an entirely different angle every once in a while.
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