Evolution of life on Earth is usually DIVERGENT, i.e. one starts from a simple string of genes, A-C-E, which evolves (by gene duplications, inversions, insertions, deletions, rearrangments) into something more complex such as A-B-C-D-E. On the other hand, CONVERGENT evolution occurs when A-B-C-D-E already exists, but then environmental selective pressures alter the existing genome to become A-B-B’-C-D-E. An excellent example is fish antifreeze. Antarctic fish (Dissostichus mawsoni) were discovered to have an antifreeze glycoprotein gene (AFGP) that arose from a trypsinogen gene –– evolving by adding DNA coding for 41 triplet (Thr-Ala-Ala) repeats. This gene is estimated to have arisen around the estimated time [~10-14 million years ago (MYA) ] when the Antarctic Ocean froze over.
By using a nucleotide probe representing the Antarctic AFGP triplet repeat, scientists found the AFGP gene in Arctic cod (Boreogadus saida); amazingly, this protein is derived from a totally different gene, originating ~2.5 MYA –– which coincides with glaciation of the Arctic Ocean. Here we have different fish species, at the two extremes of the planet, needing a protein (at different planetary times) in response to a sudden cold environment, i.e. so that these fish are able to survive water temperatures at, or below, 2°C. This is convergent evolution –– Mother Nature has selected different genes to “evolve sideways” into antifreeze genes.
Convergent adaptations are important evidences of evolution by natural selection and have been reported for different levels of biological organization. Relevant to the [attached] paper, transitions from independent-living to parasitic-life histories have occurred autonomously hundreds of times across unrelated lineages –– converging to a few modes of parasitism, transcending to genomes, and representing convergent strategies of host exploitation and transmission.
In some cases, the behaviors observed in nature are adaptations on the part of parasites that have evolved to infect animals and manipulate their behavior as a transmission strategy. In these circumstances, the behavior of the host (i.e. its phenotype) is an extension of the genotype of the parasite; this phenomenon is called “the extended phenotype”. What has not been examined is whether parasite manipulation of animal behavior responds to changes in environmental conditions which the host experiences. Because environmental changes are known to result in adaptive shifts of phenotypes, such as animal behavior –– it is reasonable to presume that the same environment may act as a selective force on the extended phenotypes of parasites inside those animals.
Authors [see attached] studied ants that are manipulated by parasitic fungi to bite onto vegetation. They analyzed the correlation between “forest type” (tropical vs temperate) and “substrate that the host bites” (leaf vs twigs). They studied the time required for the fungi to reach reproductive maturity, and the phylogenetic relationship among specimens from tropical and temperate forests from different parts of the planet. They showed that fungal development in temperate forests is longer than the period of time that “leaves are present”; thus, the ants are manipulated (by the fungi) to bite twigs. When biting twigs, 90% of the dead ants examined had their legs wrapped around twigs, which appears to provide better attachment to the plant. Authors conclude that leaf-biting is the ancestral trait and that twig-biting is a convergent trait –– in temperate regions of the planet. These three mind-boggling lines of evidence suggest that changes in environmental conditions have shaped the manipulative behavior of the host by its parasite..!!
Evolution (2018) doi:10.1111/evo.13489