How does evolution affect plants over time? Are there patterns in the way plants diversify? A paper last year by Ofir Katz examined the startling rise of angiosperms. They’ve exploded in diversity over a relatively short time. Today, they’re the most species-rich group of land plants.
One of the points Katz made was that biodiversity creates biodiversity. As life gets more complex, so it creates new niches for other life to inhabit. Effectively, evolution is operating within a positive feedback loop. Each iteration drives on change elsewhere in the natural world.
The result is, if you were to travel back in time, you would probably see that plant life is not only different, but also less complex. The farther back you go, the less diverse the forests would be. You’d see some species that are now extinct, but these new species would be fewer in number compared with the variety we have in the modern day.
So the evolution of life has been compared to an intricate machine. So what happens to that machine when you fix it with a (metaphorical) shovel?
Donald Levin has taken a look into the future, and he sees evolution taking a strikingly different turn. What could cause such an unprecedented change? Well, he gives that away in the title of his paper, “Plant Speciation in the Age of Climate Change“.
Carbon dioxide concentrations are changing faster now than at any known time in Earth’s past. Some people argue that carbon dioxide concentrations go up and down, and current concentrations are within norms. Likewise, the M1 motorway climbs hundreds of feet up and down as it runs from London to Leeds. So what possible harm could an unexpected 6-inch brick wall in the fast lane do? Rate of change matters and an unprecedented rate of change could have an unparalleled change on evolution.
Prof Levin has looked at data that says climate change on this scale could drive up to 33% of plant species to extinction. If we are living in a world where biology is becoming poorer and less diverse, what will drive evolution? If biodiversity creates biodiversity, is evolution over?
Looking at the evidence, Prof Levin has concluded that over the next 500 years, the most common way speciation will happen is through polyploidy.
Polyploidy is what occurs when plants get extra copies of genomes. Typically they are haploid, with single chromosomes, or diploid. Diploid organisms, like you, have pairs of chromosomes. Plants are often diploid in their flowering state, but they don’t have to be. They can be triploid, with three copies of their genome. Or Tetraploid, with four copies. Some strawberries are even octoploid, having eight copies of their genome.
Polyploids can tackle environments in a different way to their diploid ancestors. Piyal Karunarathne and colleagues examined how the grass Paspalum intermedium interacted when polyploids and diploid lived in the same range. They found the older, diploid, populations were adapted to a narrower set of habitats than the tetraploid grasses. So while the diploids ruled the core areas, the polyploids were able to expand into new habitats.
Prof Levin believes that, with habitats changing rapidly, ‘core areas’ could become marginal. This change means that polyploids could drive their diploid ancestors to extinction. If you were to ask Prof Levin if this were usual when polyploids form, you would get a firm, “No.”
However, the new polyploids will not have things all their own way and could lead to some conservation headaches of their own. Prof Levin explained: “Unlike the products of lineage splitting, young polyploids will have relatively few populations, and so they are/will be more prone to extinction than diploid species arising from lineage splitting.”
But what of the future? If Prof Levin were to borrow HG Wells’s time machine and pilot it 500 years into the future, what would he search for? For Prof Levin, the first question on his lips would not be of Morlocks or Eloi. Instead, it would be a question of academic standards. “Was my speculation correct, or should this paper have been rejected in 2019?”
Karunarathne, P., Schedler, M., Martínez, E. J., Honfi, A. I., Novichkova, A., & Hojsgaard, D. (2018). Intraspecific ecological niche divergence and reproductive shifts foster cytotype displacement and provide ecological opportunity to polyploids. Annals of Botany, 121(6), 1183–1196. https://doi.org/10.1093/aob/mcy004
Katz, O. (2017). Extending the scope of Darwin’s “abominable mystery”: integrative approaches to understanding angiosperm origins and species richness. Annals of Botany, 121(1), 1–8. https://doi.org/10.1093/aob/mcx109
Levin, D. A. (2019). Plant Speciation in the Age of Climate Change. Annals of Botany. https://doi.org/10.1093/aob/mcz108