A Biosphere of One

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Even a seed can be an ark

Biodiversity is a hard concept to pin down. The word evokes a rich ecosystem brimming with plant and animal species. But is it just the number of species that matters, or their dissimilarity? Picture a pond with twenty kinds of frog and no other vertebrates. Surely you’d call it less diverse than a pond with twenty total species of frogs, snakes, turtles, muskrats, carp, ducks, and newts. And what if one species has a lot of genetic variation? Is the kaleidoscopic western ground snake, with myriad shades and stripe patterns, weighted the same as a population of genetically identical salamander clones? What about individuals? Can one organism be more biodiverse, by itself, than another? I once had a t-shirt that proclaimed “BIODIVERSITY” with a picture of a lone, multicolored frog. It implied that a single animal can be biodiverse if it only has enough distinct pigments. Nonsense? Or is there something to this idea?

A genome contains, among other things, a set of genes that each encode a different molecular product. Not every genome has the same number of genes. A bacterium might have a few thousand, while a mammal has tens of thousands. But all living lineages, simple or complex, have spent the exact same amount of time evolving to fit Earth’s conditions. A germ is as adapted as a geranium. Like the fox of fables who knows many things while the hedgehog knows one important thing, the genome of a mudpuppy “knows” 10 million times more information than that of a cold virus. Yet both are equally successful. If all you care about is endurance over the eons, elaborate genomes aren’t necessarily better than streamlined ones.

But sometimes we care about more than that. On the rapidly changing planet of the Anthropocene, the first priority might be the ability to acclimatize over the short term. To have the right gene in your toolkit for whatever the future throws at you. And we humans should also value biological information per se if want to be responsible technocratic stewards of the Earth. A species with more DNA has more biotechnological potential: more chances that it secretes the precursor of a new drug or nanomachine. Consider the antimicrobial peptides. Many species produce a cocktail of these bacteria-killing chemicals, but the exact number varies. Imagine two species of, say, dragonflies. One produces five different antimicrobial peptides, and one produces fifty. All else being equal, I’d prefer to conserve the peptide-rich insect, since it represents forty-five more chances to wipe out the next superbug. Of course, not all is ever equal, and we probably wouldn’t be able to tell even if it were. But in theory, at least, some genomes are more equal than others.

Where do all these genes come from? Gene families can spread across the genome via a copy-and-paste mechanism, with each new copy developing a unique sequence. Entire genomes can even duplicate. Our own human genome has doubled itself at least twice, deep in our evolutionary past. We don’t have eight copies of every gene, though, because extra copies have been discarded if they didn’t prove beneficial. But some species, like the cultivated strawberry, have recently replicated genomes. They still have eight nearly-complete copies of every chromosome. This isn’t mere redundancy. These copies originated from at least two different species which hybridized. (Perhaps confusingly, the resultant hybrid then split into several wild species, two of which which again hybridized to form the strawberry we eat). You could say that your shortcake topping represents twice the biodiversity of one of the original wild ancestors. But it’s actually a bit more than that, because even copies with the same background have begun to diverge from each other. Does this molecular bonanza have practical consequences? It’s hard to say for sure. Species with duplicated genomes have certainly flourished. Intuitively, you might expect these species to be more flexible. If part of your genome is itself a complete genome that originally adapted to a cold climate, and another part is an entire additional heat-adapted genome, maybe you can just turn the appropriate genes on or off depending on the weather. On the other hand, plenty of genetic paupers do just fine. The carnivorous corkscrew plants don’t suffer despite having a tenth of the DNA of a strawberry. It’s an open question. We’re looking into it.

Genes within gene families. Genomes within genomes. Even our own mitochondria, the calorie furnaces within our cells, have their own genomes and independent origin. Every animal is already an ecosystem of a sort, a corporation with more than one shareholder. The botanical realm is even more bureaucratic. Not only do plants have mitochondria too, but their light-harvesting chloroplasts are another genetically distinct entity. Hybrid ancestries like the strawberry’s are also common. Where will all this lead? Could every genome in the world be combined into a supergenome, a blueprint for a zygote that could choose to grow into a petunia or a sperm whale depending on the conditions? No, that’s not where evolution is going, and such a task would elude even the more skillful genetic engineer. But it’s a nice thought experiment. A planet inhabited by only that monstrosity would still be very biodiverse.

In the end, biodiversity is just a convenient rule of thumb, a rough guideline for setting conservation priorities. We can’t predict the future, but we can be pretty sure of this: the more of evolution’s accomplishments we can keep around, the better. The fine points of how we measure it aren’t so important. What is important is to remember that so many living things are cornucopias of evolutionary innovation. If we eliminate even one obscure wildflower, we don’t just lose one basic set of genes. We lose a full suite of genomes with unique proliferations of gene families. We lose a whole world.

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