PARASITES CAUSED THE
PHYLOGENETIC TREE TO BRANCH
Post #24
Donald A. Windsor
The phylogenetic
tree, recently popularized by David Quammen (1), is indeed a very
practical way to illustrate phylogenetic relationships. Horizontal
gene transfer is described as connecting separate branches across the
tree and altering evolution.
But without
parasites, the tree would be just a gnarled stump, sparsely branched,
parsimoniously twigged, and starkly leafless. Even though parasites
can transfer genes from a host species on one branch to a host
species on another branch, without parasitism there would be few, if
any, branches to transfer between. Phylogenetic branches indicate
that parasitism must have emerged very early in the evolution of
living organisms.
The reason is that
parasites prevent monopolistic monocultures. When competition and
predation do not control monocultures, parasitic diseases step in.
The result is biodiversity and multiple ecosystems (2).
Host species push
back against parasites by rearranging their genetic material through
sex. Other factors, such as environmental changes and competition,
also cause sex. All of which leads to speciation and phylogenetic
divergence, resulting in branching. Sex might have evolved without
parasites, but parasites are the main drivers of sex because mating
involves a choice of the most fit mates and a selection against
unfit.
The real test of my
hypothesis, stated in the title, will come when extraterrestrial life
is discovered. Parasitism may be a property of life here on Earth,
but it may not be universal. This is why it is so important for
Earthlings not to contaminate other planets.
References cited:
1. Quammen, David.
The Tangled Tree. A Radical New History of Life. New York,
NY: Simon & Schuster. 2018. 462 pages.
2. Windsor, Donald
A. Role of parasites in the Earth’s biosphere.
paraditesdominate.blogspot.com Post#13.
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