Doubling up DNA
You’ve got two sets of DNA: one from your mother, one from your father. All animals, and most plants (except mosses) have half their DNA from each parent. But sometimes, accidents in reproduction can mean that living things end up with more sets of DNA, most commonly four (double the usual two). Most examples are plants—wheat, cabbages, strawberries, and plenty more—but it can happen to animals, such as goldfish. The technical term for this is ‘Polyploidy‘.
We’ve known for some time that polyploidy can form new species. There’s a couple of ways it can happen, but the simplest is that it lets two different species occasionally breed together. For plants, breeding involves sending out pollen to the flowers of another plant, so it’s not unusual for a hybrid to grow, but it will usually be sterile. To make the sex cells (pollen and eggs, for plants), the DNA needs to be divided up, so that each sex cell only has one of the normal two sets. Dividing it up depends on having two similar sets of DNA; if the DNA from Mum is too different from the DNA from Dad, it gets in a mess.
An occasional lucky hybrid can get round this by having an extra copy of all its DNA. If it came from an egg with DNA set A, and pollen with set B, a normal hybrid would have AB DNA, but a polyploid will have AABB. Each of its sex cells then has AB: all the DNA from both parents. It can breed with itself, or others like it, but not with either of the parent species—in one step, it’s formed a new species. The classic example in biology lectures is common cordgrass, a hybrid between a European cordgrass species and an American one.
We know of many polyploid plant species, and it’s generally thought to be important in plant evolution, but an key part of science is checking what’s “generally thought”, and putting numbers to “important”.
A group of scientists used a database of how many chromosomes (bits of DNA) different plant species had to estimate which ones were polyploid. By linking this with the plant evolutionary tree, they estimated how often groups had separated with one of them becoming polyploid: 15% of the separations in flowering plants, and 31% in ferns. That suggests it’s not the one major cause for plant species to separate, but it’s certainly a pretty important one.
It’s also been suggested that species that are already polyploid might separate into new species more often than those that aren’t. This study didn’t find any evidence that that was true: it looked like polyploid lines had similar numbers of species to related non-polyploids.
This study on the EEB and Flow blog (where I read it)
Original paper (in PNAS)