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| Sphagnum Moss, Volo Bog, Lake Co, IL 7/16/2003 |
This is Sphagnum Moss from Volo Bog.
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| Liverworts, Devil's Lake State Park, Sauk Co, WI 8/9/2013 |
These little plants are liverworts, from Devil's Lake State Park, Sauk Co, WI.
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| Ground-Pine (Lycopodium clavatum), Ridges Sanctuary, Door Co, WI 5/29/2012 |
This one is a Ground-Pine (Lycopodium clavatum) from The Ridges Sanctuary, Door County, WI.
All of these share an interesting trait: They don't produce seeds! Instead, they reproduce by spores, which are smaller, much more numerous, and carry almost no start-up capital with them. They share another interesting trait as well, but this one will take a bit of explanation.
You, presumably, are a diploid organism. That means you carry two copies of each chromosome, and therefore of each gene. (That would be one copy from Dad and one from Mom, of course.) In humans, aside from the sex chromosomes, having too many or two few chromosomes causes serious problems. (Mostly fatal ones, in fact.) We (like most animals) only have a very short haploid phase, in our case as sperm and eggs.
Plants are different -- not only can they tolerate extra chromosomes much better than we can, they actually develop as both haploid and diploid organisms, one after the other. In fact, all of the plants in those photos were haploid! (The brown part of the Ground Pine is diploid.) In these plants, the haploid phase is dominant, while the diploid phase is typically small and completely dependent upon the haploid phase. By contrast, in flowering plants and pines, the dominant phase is diploid, while the haploid phase does develop into a multicellular structure it does so inside the cones or flowers.
The dominance of diploid phases in "advanced" plants and animals makes it easy to assume that there is some advantage to being diploid (1) , but is that really true? Anderson et al. found that haploid populations of yeast evolved resistance to anti-fungal agents considerably faster than diploid populations, presumably because recessive mutations in diploid populations were masked and therefore invisible to selective pressure. (2) On the other hand, being diploid means that a) heterozygote advantage is possible, and b) any deleterious recessive mutations are tolerable. The flip side of that, of course, is that those same mutations can be inherited, whereas in a haploid population, any deleterious mutation is immediately selected out of the population. This sounds rather callous, but it's the way that wild populations actually work - if your infant mortality rate is going to be high, you can afford a very high level of deleterious mutations, since those individuals were probably going to die anyways. So it turns out that in a given generation, haploid organisms often show higher fitness within a population than diploid ones.
While animals are mostly diploid, we find every variation in this life cycle in protists, so it seems quite clear that there are advantages to both conditions. And although the "advanced" plants do make up the majority of today's plant species (~250,000), there are 9,000 species of liverworts and 12,000 species of mosses, so they aren't exactly on the short road to extinction. It is true that they are all small, but that's explainable by the fact that most of them lack any sort of vascular tissue. So it's not even the case that these plants, by their stature and evolutionary success, are an illustration for the advantages of diploidy.
This is an active area of theoretical and experimental research, which means that we don't actually have any solid answers. Which makes the whole question so much more fun!
(1) Mable, B. K., & Otto, S. P. (1998). The evolution of life cycles with haploid and diploid phases. BioEssays, 20(6), 453-462.
(2) Anderson, J. B., Sirjusingh, C., & Ricker, N. (2004). Haploidy, diploidy and evolution of antifungal drug resistance in Saccharomyces cerevisiae. Genetics, 168(4), 1915-1923.
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