Some thoughts about plant evolution


Well, I’ve been brewing a post about ISL ethics for a few days now, but besides my chronic procrastination due to school and work, I had another reason to postpone this post, which is that just today I got the first paper from Cocoon about ISL ethics and I think this one really calls for some homework!

Anyhow, as a brief hors d’oeuvre, I would like write a brief post on some nagging thoughts and doubts I had about plant evolution due to this week’s biology class.

First, some background: this semester is “botany-semester”, meaning that all labs and all classes that are specifically about biology (and not, say, math 101, physics 101 etc.) are mainly focused on plants, algae, etc.
Second, before I write a about the nitty-gritty of my argument, let me just say that every single one of my professors, lab chiefs and even the guy who instructs our lab team have either your garden variety yarmulke, or in the case of the PhD student who instructs our lab team, a yarmulke and those curly braids that the hassidic Jews have. I’m really not too privy to the that whole “Hassidic spectrum”, but since he doesn’t wear those penguin suits the Jerusalem Hassidim wear, I can’t say he’s as fundamentalist as those kooks in Jerusalem are.

But anyway,

What I wanted to say is that every single one of my professors, lab chiefs and lab instructors is, well, REALLY JEWISH, really religious and god-fearing, and completely, unabashedly, evolutionist. These guys speak of evolution as if the fact that it’s true is so ho-hum that it doesn’t even worth a second thought. Shiesh. It’s only the major kooks in Israel who have any qualms with it, I guess.

And now, to the batmobile!

The theory of plant evolution goes roughly like this: a great number of yonks ago, prokaryotes endosymbiontly evolved into eukaryotes (something I find totally reasonable), and the variety of prokaryotes that evolved photosynthesis (namely, Cyanobacteria or Cyanophytae, or blue algae), coupled with endosymbiosis, turned into the first eukaryotic algae. So far so good, but the problems I have with plant evolution start here.

A good analysis of algae evolution can be done by looking at the various evolutionary pathways observed in various algae phyla. It’s probably no coincidence that all green algae and all plants have the same preservative polysaccharide (namely starch, unlike our glycogen), all have the same (and rather unsually so for the 7 or so algae phyla) characteristic photopigment (chlorophyll a), all have similar sexual reproduction and all are surrounded by cell walls composing of cellulose (also an “anomaly” among algae phyla)

This is a good and credible explanation for the origin of land plants (plantae or metaphyta). However, things start to get really shaky when you look at the other algae phyla, who have indiscrepant levels of development, which make it rather futile to try to pinpoint who evolved when. For example, the multicellular alga “Chara” has a superficial “stalk” and a complex sexual reproduction system, but it does, however, use isogamy as a means for zygogenesis (the production of zygotes from gametes). Isogamy is rightly considered to be archaic, as it is less efficient, less specialized and is more characteristic of primitive organisms than of evolved ones.

So how come Chara has an unevolved sexual reproduction while Volvox, which is a microscopic colonial alga that has no sexual organs, uses oogeny for zygogenesis, which is strikingly reminiscent of human zygogenesis (the male gamete is small and motile, the female gamete is large and static). The professor merely said that certain things evolve at different paces, and this is a good explanation and a very reasonable one, but I find it hard to accept it while at the same time claiming that this or that phylum evolved before or after based on comparative anatomy.

In cases like this, I prefer to say “I don’t know, but…” rather than to firmly put my finger on a phylogeny (which I can comfortably do regarding green algae and plantae).

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