PARASITES CAUSED THE PHYLOGENETIC TREE TO BRANCH
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.
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.