Land Plant Evolution – Another Example of Mental Gymnastics

Spirogrya are part of the Zygnematales, which are now thought to be the closest living relatives of land plants. Click for credit
It is difficult to be an evolutionist these days. Back when morphology (the study of the form and structure of organisms) was the only way we could compare organisms to one another, evolutionists could spin tales of evolution unchecked. However, as science and technology improved, we became able to test those evolutionary tales against genetic data, and the results were devastating. Evolutionary relationships inferred from morphology are often quite different from those inferred from genetics.1


To make matters worse, evolutionary relationships based on genetics are confusing as well. For example, while most of an organism’s DNA is in its cells’ “control center” (the nucleus), some of an organism’s DNA resides in its cells’ powerhouses (the mitochondria). In most muticellular organisms, this DNA (not surprisingly called “mitochondrial DNA”) is thought to be inherited only from the mother. Well, when evolutionary relationships are inferred from mitochondrial DNA, they disagree with evolutionary relationships inferred from nuclear DNA.2 Even when you concentrate on nuclear DNA, the evolutionary relationships inferred from one set of genes disagree with those inferred from a different set of genes.3

These problems came to mind recently as I read a study on the supposed evolution of land plants. Evolutionists have long thought that the the Charales, a biological order of pondweeds, are the closest living relatives to the ancestors of land plants. This is because the Charales are algae, but they are thought to be the most complex form of algae. In addition, they sexually reproduce by making freely-moving sperm cells and much larger, stationary egg cells. This is also the way modern land plants reproduce.

It only makes sense, then, that the most complex algae with the same basic reproductive mode as land plants must be closely related to land plants, right? Wrong…at least according to the genetic data.

Sabina Wodniok and colleagues compared 129 genes among 40 different land plants and algae. They say that their selections included a wide range of land plant diversity, all major lineages of the Charales, and other green algae as well. They found that based on the 129 genes they studied, the Charales were not the closest living relatives to the ancestors of land plants. Instead, the Zygnematales probably are. The say that their results are consistent with a few other studies that looked at both genes and RNA.4

Now by itself, this might not sound like a big deal. After all, science is a self-correcting endeavor. New data regularly correct errant assumptions. Thus, why should it matter that genetic data are overturning a generally-accepted view in the supposed evolution of land plants? It matters because the Zygnematales are less complex than the Charales, but more importantly, the reproductive mechanism of the Zygnematales is fundamentally different from that of the land plants.

The Zygnematales do not produce freely-moving sperm cells and larger, stationary egg cells. Instead, they make cells that are roughly the same size and crawl like amoebae. When they reproduce, a filament from one individual lines up with a filament of another individual. A tube is formed between the two, and either one cell crawls through it to fuse with the other cell, or they both crawl into the tube together and fuse there.

Clearly, then, this reproductive method is rather different from the reproductive method of land plants, which just happens to resemble the reproductive method of the Charales. So how is it that two closely-related groups (land plants and Zygnematales) have significantly different reproductive mechanisms and different levels of complexity but two more distantly-related groups (land plants and Charales) have more similar reproductive mechanisms and more similar levels of complexity? Here is what the authors say:

It seems plausible that the simpler morphology of extant Zygnematales represents a secondary simplification, similar to the loss of flagellate cells in this group, which may actually represent an adaptation to ensure sexual reproduction in the absence of free water. Alternatively, the morphological complexity of the Charales and Coleochaetales might have evolved independently after the three evolutionary lines (Coleochaetales, Charales, and Zygnematales) diverged.

In other words, either the Zygnematales “devolved” after the land plants started to evolve, or this is all just another wild coincidence.

These are the lengths to which one must go in order to be an evolutionist in light of the most current data. I am glad I don’t have to do such mental gymnastics to be a creationist!

REFERENCES

1. Trisha Gura, “Bones, Molecules or Both?,” Nature 406:230-233, 2000.
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2. John J, Weins, Caitlin A. Kuczynski, and Patrick R. Stephens, “Discordant mitochondrial and nuclear gene phylogenies in emydid turtles: implications for speciation and conservation,” Biological Journal of the Linnean Society 99:445–461, 2010.
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3. Graham Lawton, “Why Darwin was wrong about the tree of life,” New Scientist January 21, 2009.
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4. Sabina Wodniok, et al., Origin of land plants: Do conjugating green algae hold the key?,” BMC Evolutionary Biology 11:104, 2011.
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4 thoughts on “Land Plant Evolution – Another Example of Mental Gymnastics”

  1. I wonder if the discipline of biology will undergo a major paradigm change – our current method of classifying species based on external characteristics is having a hard time bringing genetic evidence into the fold. Will the problem be solved by developing a whole new classification system, or creating a secondary system that takes genetic similarities into account, or will they just ignore the problem as long as possible?

    1. Shevrae, that discussion is already taking place in biology. There are those who want to expand the kingdoms to make a broader classification system in the hopes that it could take genetic similarities into account. They are called the “splitters,” because they want to split the classification system up. Others want the classification system to be less about details and more about broad, general things. That would make the genetic data less important. Those folks are called “lumpers,” because they want to lump the groups together.

      I personally think that if biologists want a classification system that reflects reality, they should follow what the young-earth creationists are doing. They are trying to classify things base on archetypes – the originally created organisms. The young-earth idea is that God created kinds of organism, and those organisms then varied and adapted as they were created to do, producing all that we see today. For example, it seems that God created a cat kind, and that Noah probably took two cats on the ark. Those cats held all the genetic information necessary to make a wide variety of specific cats. As they moved out and adapted to fit their specific surroundings, they became lions, tigers, panthers, etc., and even domestic cats. A good overview of this kind of classification can be found here.

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