A year ago, I told Evil Rachel I’d write an explanation of polytopic origin of species.
Because it is pretty cool.
The way most speciation happens is that you have a single population (one species), then you separate it extrinsically, by a river, mountain range, several birds blown off-course, et cetera, so you have two populations (one species). Then natural selection, evolution, whoo-- but because the two populations are separate, different forces act on them. On one side of the river, squirrels become larger than on the other. The wet side of the mountain has more vegetation, so deer can breed earlier or later in the year than in the rain shadow, where there’s only a small window of opportunity. The birds that landed on the island sing a different song from those on the mainland.
This eventually becomes an intrinsic barrier to reproduction. The two populations, formerly one species, come into contact, but cannot reproduce together. The river changes course and squirrels from one side are too small to mate with squirrels from the other side. Deer from the rain shadow come to the verdant side of the mountain but aren’t fertile at the same time as their better-fed cousins. Mainland birds arrive on the island, but since they sing different songs, the two populations don’t recognize each other and never mate.
This is how one species becomes two most of the time. Extrinsic barrier to reproduction, evolution, intrinsic barrier to reproduction. If you repeat the experiment in its roughest form, you won’t get the same results. Speciation is a side effect of evolution.
There’s a reason I used animals for my examples, and it’s not just that they come to mind more easily (the examples are made up, but still). Polytopic speciation happens in plants. The usual methods apply too, of course.
But plants are weird.
A bit of plant biology: like closely-related animal species, closely-related plant species can cross-fertilize and produce hybrids. Because of chromosomal mismatches, these hybrids are often sterile. If the parent plants have 24 and 36 chromosomes each, they’ll contribute 12 and 18 chromosomes to the offspring, which will have 30, but the 30 chromosomes won’t match up properly to become egg and sperm for the next generation. Here’s where the spiffy chromosome diagrams go, but I am lazy. If you want them, comment and I’ll draw some.
Anyway, 30 chromosomes that don’t match up to make egg and sperm. How sad.
Except plants occasionally double their entire genetic complement. A great many very useful plants are polyploid, meaning that they make extra copies of all their chromosomes-- it’s basically cell division, but something goes wrong and the cell doesn’t divide in two. This means that the parent plants, 24 and 36, donate 12 and 18 chromosomes to their offspring, who has 30 mismatched, which then double to 60, each of which has a partner so gametes can form. Go plants. This part also has spiffy chromosome diagrams.
The hybrid is then fertile, but not with either parent. It cannot interbreed with the parent species. Like the squirrels, the deer, and the birds, there is now an intrinsic barrier to reproduction. The hybrid is a new species.
This is all very interesting, you say, but what is the point? So now we have one little plant that is its own species. This is not enough.
Well, with plants, it is. Plants generally have to work to keep from self-fertilizing-- the mechanisms evolved are really fascinating and elegant. One hybrid can become a lot of hybrids.
Or you can look at the word ‘polytopic’ and see that it means ‘in many places’. There’s no reason that multiple pollen grains don’t fall on multiple stigmas and produce multiple hybrids. Not all of them will double their chromosomes, but some will. Like spam, you don’t need a really high success rate to see a result.
Many plants have very broad ranges. Hybrids can arise huge distances from one another, double, and be the same species. Unlike typical speciation, polytopic speciation is replicable. Populations of a species can exist that have never mixed with other populations of the same species. That are a year or a century old. That go extinct for whatever reason, only to be followed by a new population of the same species.
And, because naturalists have been wandering around taking samples of plants continuously over time, we can see it happening.
The example we used in class was milkweed. Milkweed is weird for many reasons, but in this, it’s good. The wooded-area milkweed and the open-area milkweed interbred and produced a hybrid, which became its own species. Except the hybrid only shows up in recent years-- ‘recent’ in evolutionary terms, meaning that the first naturalists wandering west didn’t collect any, but current ones do. A couple centuries, tops. Suddenly, the hybrid species is popping up all over the place.
Why? The wooded-area species stays in the woods. The open-area species stays in the open. They only have the opportunity to hybridize when they’re close together-- disturbed ecosystems and the wooded-open interface.
If there is one thing humanity is good at, it is disturbing ecosystems. We have created way more interface between wooded and open areas than before. We mess with ecology. The milkweed species come into contact far more than they did before our interference, and there’s more opportunity for interbreeding.
So the hybrid species turns up all over the place.
I have to keep two channels of brain open for this sort of knowledge. On the one hand, really nifty mechanism of speciation, different from the typical one. On the other hand, because humanity moved in, new species have arisen within a few human generations. Because we chop down trees and put in roads, new species exist. And will keep existing even if we pull up every single member. The species could go extinct and pop up again, and probably has in the past.
Part of me says, “Well, yes,” and goes out to run errands. Part of me is speechless but for punctuation and boggles in wonder. Biology is a weird weird thing.
