Monday, June 17, 2013
PRINCIPLES OF PHYLOGENY
Phylogeny, the ancestral tree of species, is often difficult to determine when a species has no closely related species. Molecular phylogeny was thought to be a source of superior answers to the questions of relationships. That would be true if all genetic material evolved at the same rate. But different portions of the genome can have different rates of mutation, so can different species, and the rate can be affected by the longevity of individuals. So other factors have to be considered to improve accuracy of phylogeny.
The path is determined by natural selection. Selection may maintain a stable species composition for long times when it is well-adapted to stable environmental conditions. In a variable environment changes may be relatively rapid over a comparable time period.
Features can be lost more rapidly than they are gained. The formation of a new structure or a biochemical substance is usually due to accumulation of many mutations. Many accumulated factors must interact in proper sequence to produce the feature. But one change may disrupt the entire process. The change may be fatal if it is an essential feature. An example, albino organisms may result from one of many possible disruptions leading to a failure to produce the melanin pigment.
Complex features are present in some way in ancestral species. The eye is an excellent example of this principle. Some one-celled animals had pigment spots near a light sensitive swelling of the basal part of a flagellum. The flagellum is a thread-like structure that can undulate; it has internal fibrils in a unique pattern common to cilia. Variations of the fibril pattern occur in the photo-receptors of eyes of all groups of higher animals with eyes. The eye structure itself shows variation distinguishing major groups.
Genotypic selection is accomplished through phenotypic selection. Because genes interacting with environmental factors determine the phenotype (the physical expression resulting from that interaction) many seem to think natural selection acts directly on the genotype. But survival of the individual is dependent on the success of the phenotype. Consequently, anatomy and other phenotypic expressions are better guides than genes are to evolutionary pathways in many cases.
Factors causing natural selection can be variable or constant. They can be characteristics of (a) the physical environment, (b) the biotic environment, or (c) the interaction of those factors. Ice ages come and go. Solar radiation is relatively constant, but varies greatly by latitude, time of day, and cloud cover. Microhabitats vary, such as the side of a tree favorable for moss growth.
Extinction events are typically followed by accelerated evolutionary changes among survivors in the post-extinction period. The early Cambrian expansion of major phyla is the most remarkable example.
Rapidly evolving species may have greater genetic similarity to descendants of slowly evolving ancestral stock species than to more recently evolved sister groups. The most dramatic example is the pogonophorans that show up in an intermediate position among other more recent deuterostome clusters. This aspect of evolution can probably not be found in the scientific literature, since I appear to be the only one that knows about the extreme age of pogonophorans and their connecting link position between annelids and deuterostomes. Some such suggestion may have been made in a 1978 Copenhagen symposium on the Pogonophora before my 1983* presentation to the Am. Soc. of Zoologists.
Geographic factors are immensely important in affecting the course of evolution. Continents can act as barriers to marine species dispersal and highways for terrestrial species. Isolation of groups allows separate evolutionary paths for related forms.
Isolation of marsupials in Australia enabled them to evolve into forms comparable to many of the placental mammals that were so successful on other continents. The variations in each location are examples of adaptive radiation into new species, while those of opposite groups radiating into forms similar to those in other locations are examples of convergent evolution.
An interesting example of geographic barriers and highways is shown by some terrestrial species found at many different latitudes found at higher locations in the tropics and progressively closer to sea-level as latitudes closer to the poles are reached. Conversely, some marine invertebrates of shallow coastal seas of high latitude regions are found in progressively deeper waters as they transition to tropical locations. Temperature is presumably the major factor determining such distribution.
*Engemann, J. G. 1983. Coelomate animals are monophyletic. American Zoologist, 23(4):1008. Abstract # 753. A submission of the complete paper was rejected by an editor of Nature because he considered it to not be of enough general interest.
Joseph G. Engemann June 17, 2013