Friday, October 17, 2014


Molecular clues to evolutionary relationships

Before molecular phylogeny, with varied nucleic acid studies became an important part contributing to our understanding of the tree of life, there were many precursor applications guiding our attempt to classify plants and animals.

Molecular evidence guiding our view of evolution, was compatible from the earliest days, with our classification schemes based on the assumption that each species was discrete from others and separately created.  Today, we know that the origin of species from prior species was a gradual process dependent on the accumulation of numerous changes preceding the advent of reproductive isolation of new species from their ancestral species.  The separation of new from old is typically enhanced by other isolating mechanisms of location, time, and changed aspects of biology.

Chlorophyll, or its green color, was an early clue separating plants and animals that must have been known before written records developed.  As molecular biology developed, we found that there are different variants of chlorophyll and photosynthetic pathways.  Green algae and blue-green algae are easily distinguished from one another by the color of their chlorophylls; they also have many other significant differences not involving color or chlorophyll.  There are efficiency differences, between the C-3 or C-4 pathway, and vascular plants abilities to convert solar energy into glucose which can be stored as sugars and starches.  

Cellulose, a polymer of glucose, is a structural material of plant cell walls that is lacking in animals. 
Insect and other arthropod exoskeletons contain chitin.  Chitin is limited to animals known as protostomes, and is not found in the other main branch of animals leading to vertebrates.  It is a polymer that can be broken down into n-acetyl glucosamine by the digestive processes of a few animals.  Surprisingly, cellulose and chitin digesting enzymes are uncommon in most higher animals, so those using them as a food source usually need the aid of microorganisms.

Plants and animals have many basic molecular features in common.  Nucleic acids, with their many functions for inheritance and production of protein, are one of the first shared features common to all cellular organisms.  Adenine and glucose are two of the substances produced when simulations, of the pre-biotic earth atmosphere and physical factors, are conducted in the laboratory.  Adenine is especially notable for its role in formation of one of the four nucleotides making up the genetic code.  It also is essential in adenosine triphosphate, whose high energy phosphates power many biochemical reactions in living organisms as they use the stored energy originally produced as glucose in plants. 

Hidden Origins of Similar Compounds?

Most biochemical molecules we often think of belonging to recent groups may have had origins much earlier than current evidence shows.  The presumption is that two different groups having a unique compound must either represent descent from a common ancestor or they must be a case of convergent evolution.  But compounds active in minute amounts, such as hormones, may have been present in common ancestors in such low levels they have not been detected.

One possible bit of evidence might be the presence of ecdysone, a hormone important in controlling molting of insects, has also been found in bracken ferns.  If it were present at extremely low levels, in ancestral species reaching back to a common ancestor among one-celled organisms and had little use, it might not be detected until it was produced in sufficient amounts as part of a new process to benefit insect molting.  The near ancestors of bracken ferns that developed increased levels of ecdysone sufficient to disrupt fern-eating insect adults and/or larvae would eventually survive better and replace those being killed by insect activity.

The long periods involved in evolution of different forms of similar compounds is likely to allow the demise of intermediate stages of the evolution once a perfected solution is reached.  Until the activity of a substance is useful in larger amounts there is a selective advantage of not making larger or detectable quantities.  So a substance produced in the cell and doing an activity within the cell by inducing gene action within the cell is not needed in the high levels the amounts hormones transported by the blood require.  My attempt to discover "protozoan hormones" as noted in an earlier post was based on this line of reasoning.

The Genetic Code Evolution and Phylogeny

Non-coding regions of DNA are most likely to be more alike in rates of evolution than are the coding regions.  Unless a region codes for a more important function than merely connecting the coding regions of a chromosome there will be little impact on natural selection rates of retention or elimination.  Spacing effects are an example of how non-coding regions could be involved in changing biological function rates.

The example of cytochrome c, a compound required by all higher animals and one of the first successes of molecular phylogeny relating diverse phyla, should be revisited.  The use of the polymerase chain reaction (PCR) for studying nucleotide sequences might be applicable to the regions of DNA coding the cytochrome.  The data could then be used for groups where adequate amounts for comparing cystochromes were not available at the time of the original study*.

*Fitch, Walter M., and Emanuel Margoliash.  1967.  Construction of phylogenetic trees.  Science, 155:279-284.  [20 Jan 1967]

Joseph G. Engemann    Emeritus Professor of Biology  WMU, Kalamazoo    October 17, 2014

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