Why do trees live so much longer than, say, animals? Why do they live so long in general, sometimes thousands of years?
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This difference can be addressed through what’s known as the Evolutionary Theory of Aging.
Most animals are strongly subject to a quirk of population genetics where young individuals contribute more to the gene pool of the next generation than old individuals do. It’s a slightly subtle bit of math, but it might help to imagine two genetic variants:
**Variant A** – capable of living forever without aging, and has a few offspring each year
**Variant B** – generates more offspring each year than A, but ages and dies after a few years of life
Variant B will typically win out through sheer numbers, as the addition of those few extra B individuals each year quickly swamp out the slow trickle of the immortal A individuals. And there are lots of cases where this dichotomy arises — e.g. because individuals whose genes tell them to mature early and have lots of kids put stress on their bodies, accelerating their decline.
Trees have a couple of traits that lessen the benefit of the “live fast, die young” lifestyle that so easily becomes the most viable strategy in animals. For one thing, trees tend to get bigger (and can put out more offspring) the longer they live, i.e. they keep growing long after sexual maturity, which gives old individuals more of an edge over the competetion, and boost the spread of genetic variants that sacrifice high fecundity early in life for longevity. (This hypothesis is also supported by the fact that some of the longest-lived animals are ones that show the same pattern of growth after sexual maturation.)
For another thing, animals have this thing called germline/soma separation, where the cells that are to become sperm and eggs “branch off” from the rest of the body early in fetal development. A consequence of this is that mutations that make the body age but don’t affect the sperm/eggs can become quite common. In most plants, this separation doesn’t exist; flowers (which contain the sperm and eggs) are dispersed all over the body, and are produced by the same cell lineages that produce bark and leaves and so on. So there isn’t, to the same extent, such a thing as a mutation that affects the body but not the germline in a plant. This is believed to make plants less liable to evolve aging.
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