Systematics is the study of evolutionary relationships (through comparisons of fossils, preserved specimens, behavior, and and DNA) and diversity of organisms. Reconstruction of the tree of genealogical relationships among species–their phylogeny–provides a foundation for taxonomic classification and a framework for understanding the evolution of behavior, ecology, and morphology.
The science of naming and classifying organisms according to standardized rules is called taxonomy. A taxon is any group of animals that is recognized in classification. The Class Aves is a taxon that includes all species of living birds. All organisms can be classified in a hierarchy (nested sets of groups with increasingly closer relationships). Charles Darwin discovered that this hierarchy was the result of evolutionary history. Related taxa–those sharing a more recent common evolutionary history–constitute a lineage. Ornithologists classify the diverse species of modern birds into 40 or more different major lineages, which are recognized as orders. The name of each bird order ends in “-formes”. The name of each bird family ends in “-idae”.
Theoretically, each taxon is monophyletic; that is, it includes all the descendants of a single common ancestor. A monophyletic group is also called a clade.
Reconstruction of the phylogeny of birds began with the analysis of morphological traits, or shared, derived anatomical characters that are the result of common ancestry. Different characters evolve at different rates. Conservative characters–those that do not easily change in the course of ecological adaption–are of greater value for discovering older branches because they retain evidence of ancient ancestors. However, the possibility of convergence between unrelated species poses a constant challenge to accurate reconstructions. Unique characters define related groups of species–that is, those with a common ancestor.
Molecular systematics involves identifying shared, derived characters in the sequence of the DNA of the organisms. Molecular phylogenies provide insights into the evolution of avian morphology and ecology, particularly the process of adaptive radiation.
The most recent calibrations of the radiation of living birds imply that only three living lineages of birds survived the Cretaceous-Paleogene mass extinction event: the ancestors of the Paleognathes, the Galloanseres, and the Neoaves.
Species are fundamental units of biological classification. Bird species have characteristic sizes, shapes, songs, and colors as well as ecological niches and geographical ranges. Different species may interact ecologically, but they do not freely exchange genes or novel genetic-based adaptations. The Biological Species Concept (BSC) states, “Species are groups of interbreeding natural populations that are reproductively isolated from other such groups”. Alternative concepts are the Phylogenetic Species Concept (PSC) and the Evolutionary Species Concept (ESC), which are based on the phylogenetic history of lineages. The process of speciation occurs through the division of one species into two or more descendant species as a result of the genetic divergence of isolated populations. Most species of birds evolve as geographical isolates. Bird populations become geographically isolated in two principal ways. First, pioneering individual birds may colonize an area, such as an oceanic island, and thus are separated from their main population. Fragmentation of habitats that were once continuous is the second way in which bird populations may become isolated.
—June 2022
Lance Jones 