DNA content and false notions of evolution

[L.A. Moran]

Cliff (cliff at WATSON.IBM.COM) asks,

    "Is it safe to say that the amount of DNA per cell appears to be in 
     proportion to the complexity of the organism?  Higher plant cells,
     contain about 2.5 pg per cell, compared with 6 pg for mammals, and 0.01
     pg for bacteria. The figures for plants and mammals are for somatic
     (diploid) cells."

No, it is not safe to say that the amount of DNA increases with complexity.
There are many complicated reasons why such a statement is wrong. One of
them is that "complexity" is not a very scientific term. For example, is
the fruit fly (Drosophila melanogaster) more complex than a seed fern? Is
a pine tree more complex than a frog? What about yeast and amoeba?

Unless we can arrive at an acceptable definition of complexity it will not
be possible to draw a conclusion about DNA content and complexity. However,
it appears to be true that eubacteria and archaebacterai have smaller
genomes than most eukaryotes. It is also safe to say that vascular plants
have larger genomes (on average) than any other organisms. Vertebrates
usually have larger genomes than arthropods. Molluscs, I believe, have
genomes that are about the same size as that of the average mammal. 

The genomes of bacteria range from about 800Kb to over 50,000Kb worth of
DNA and it is not obvious which bacteria are more complex. The range of DNA
content in algae varies over four orders of magnitude and some species of 
algae have as much DNA as any living organism! Within the vertebrates the DNA
content ranges from about one billion Kb to over twenty billion Kb with humans
at the low end of this range (three billion Kb). The record holders among the 
vertebrates are clearly the salamanders. Almost every species of salamander
has more DNA than any frog or other vertebrate. 

Not much is known about the DNA content of thousands of species of small 
eukaryotes, especially unicellular eukaryotes.

I'm worried that Chris equates complexity with "advanced". Many people 
believe that evolution can be represented as a ladder with humans at the 
top. They use terms such as "higher" organisms and "lower" organisms because 
their view of evolution is false. Often the average non-biologist believes 
that humans are the most complex and most advanced life form and they see a 
progression from living bacteria to Homo sapiens. (This is the same fallacy
that produces statements to the effect that man has evolved from monkeys
instead of saying that man and modern monkeys share a common ancestor.)
Many textbooks used to contain a graph of DNA content vs. "complexity"
with the curve rising steeply when we get to mammals. The implication was
that the more "advanced" organisms had more DNA. Such graphs have been
removed from more recent editions because they are wrong.

There is an interesting transition stage in Lewin's GENES III (1983). On
page 368 he says,

     "Figure 17.1 summarizes the range of C values found in different
      evolutionary phyla. There is an increase in the minimum genome
      size found in each phylum as the complexity increases.

      ...A further twofold increase in genome size is adaquate to support
      the slime mold D. discoidium, able to live in either unicellular
      or multicelllular modes. Another increase in complexity is necessary
      to produce the first fully multicellular organisms....

      Climbing further along the evolutionary tree (sic), the relationship
      between complexity of the organism and content of DNA becomes 
      obscure although it is necessary to have a genome of >10^8 bp to
      make an insect, >4 x 10^8 bp to assemble an echinoderm, >8 x 10^8
      bp to produce a bird or amphibian, and >2 x 10^8 bp to develop a
      mammal."

The figure legend says "DNA content of the haploid genome is not closely
related to morphological complexity of the species"! 

Lewin has a point here although he managed to completely misunderstand 
evolution (and taxonomy). The MINIMUM size of a genome should be related to
the number of genes that an organims needs to survive and develop. It seems
reasonable that multicellular organisms need more genes than unicellular
ones and that multicellular organisms such as hydra are less complex than
lobsters and therefore need fewer genes. The problem is that DNA content
does not correlate very well with number of genes. What this means in
practice is summarized by Futuyma (1987, Evolutionary Biology, p.459),

     "The DNA content per haploid genome varies enormously among 
      organisms, even among closely related species. In itself, the
      amount of DNA has few discernible effects on an organism's
      phenotype, except for its influence on cell size and the rate
      of cell division."

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Slight change of subject.

This is a plea to all scientists who read bionet. Please avoid using the
words "higher" and "lower" to describe living organisms. We are perpetuating
a false image of evolution that general public has come to believe. All 
living organisms have evolved for the same length of time. They are ALL highly
evolved. When you say "higher animals" or "higher plants" you are making
a non-scientific value judgement about the worth of these animals or plants
and you are implying that evolution has produced a hierarchy of living
things. There are plenty of perfectly good scientific terms that can replace
all uses of the adjectives "higher" and "lower". Let's use them. Also, 
please avoid using phrases such as "the sequence of this gene has been
conserved from yeast to humans". It would be just as valid (and just as
wrong) to say that the sequence is conserved from humans to yeast! There is 
no hierarchy among living organisms - the correct statement is "the sequence 
of this gene is similar in yeast and humans which diverged very early in 
eukaryotic evolution".

Laurence A. Moran (Larry)
Dept. of Biochemistry