Paraphyletic is derived from the ancient greek words where para means ‘beside or near’ and phylon means genus or species. In phylogenetic studies of biological classification, the paraphyletic term is used to group together on common grounds. In a simple definition, we can state that paraphyletic is a group formed of a collection of organisms which includes the recent common ancestor with most of its descendant organisms. A paraphyletic taxon doesn’t include all descendants of the common ancestor except a few. A group is considered to be paraphyletic with regard to the excluded groups.
The term paraphyly was first used by the German biologist and zoologist, will Henning in the early 1960s. Paraphyletic groups are beneficial in analyzing significant traits causing a subclade to take an evolutionary divergent trait. A paraphyletic group is a collection of organisms based on ancestral similarities as opposed to similarities among the individual group. The use of paraphyletic groups in cladistic taxonomy is only applied when the application of paraphyletic taxon is reasonable for taxonomic study and when incorporating comprehensive classification of the extinct groups because every species originated from the parts of another species.
We can look at the class Reptilia as a paraphyletic group because it excludes its two descendant groups, the class Mammalia and class Aves.
Cladistic Classification Neglects Paraphyly
Cladistics does not allow reference of paraphyletic assemblage as ‘groups’ because they present non-evolutionary events, and the character traits derived from the common ancestors are mere inferences and not observations. Paraphyletic groups also appear problematic in taxonomic studies as they do not precisely distinguish between phylogenetic relationships, character traits, and extinction among species. Hence paraphyletic classifications are sometimes ruled out due to the rise of the clade system of classification.
The only negative aspect of using the cladistic approach is the disregard for evolutionary data in constructing branching patterns. Cladistic principles impose on a restrictive hypothesis, with data from synapomorphy being given importance while data from autapomorphies are discarded as non-informative.
Criteria for Paraphyletic Classification
To establish a paraphyletic group two things must be kept in mind which include the evolutionary significance and the construction of formal classifications. The recognition of evolutionary patterns is necessary to allow discrimination of paraphyletic ones from the polyphyletic category. Hence, establishing detailed criteria for formal classification is required. The following procedural steps must be applied for appropriate adherence to evolutionary classification principles.
- Reconstruction of phylogenetic relationships using appropriate markers such as molecular, morphological, etc, and employing various analytical tools. For instance, distinguishing between paraphyletic and holophyletic species is seen as an acceptable criterion for classification due to their emphasis on the common ancestry.
- Using phenotypic characters and their combinations from the observation of monophyletic and paraphyletic species. Combining phenotypic characters with common descent helps in distinguishing similarities arising from convergent or parallel evolution as observed in polyphyly.
- Using patrocladistic methods to measure the degree of genetic and morphological divergence is a helpful criterion that excludes overestimation of isolated characters and subjective judgement.
- Checking for meaningful evolutionary trait combinations of paraphyletic groups with emphasis on structure and its function plays a significant role in evolutionary success.
- Paraphyletic status should be applied as features of taxa that describe the evolutionary status of the group rather than a criterion for acceptance in classification.
- At last, choosing an appropriate rank as hierarchical systems help in gaining essential information on storage and retrieval and is beneficial for effective communication.
Examples of Paraphyletic
- Prokaryotes are one of the paraphyletic groupings as it excludes the eukaryotes being a distant group. This distinction was given by the french biologist Edouard Chatton in 1937.
- In the plant kingdom of classification, dicotyledons are referred to as a paraphyletic group which excludes the monocotyledons. Phylogenetic studies have shown that monocots are developed from the dicot ancestor, thus excluding monocots from dicots making it a paraphyletic group.
- In the animal kingdom, the order Artiodactyla (animals with even-toed ungulates like camels, pigs, hippopotamuses, etc.) is considered a paraphyletic group as it does not include Cetaceans (dolphins, whales, etc).
- Bony fishes (Osteichthyes) can be referred to as paraphyletic when ray-finned fishes (Actinopterygii) and lungfishes (Sarcoptrygii) are only included in the group.
- The ‘wasps’ are put into the paraphyletic category which includes narrow-waisted organisms from the Apocrita superorder without including families of ants and bees.
Paraphyly in Species
Paraphyly is a common event in speciation, where a mother species (paraspecies) gives rise to daughter species without itself becoming extinct. Research has shown that around 20% of animal species and 20-50 % of plant species are considered to be paraphyletic. Some examples include:
- Daviesia ulicifolia – It is a complex plant species belonging to the Fabaceae family under the division of Mirbelieae. They are found in the region extending from 23 degrees in latitude to 23 degrees in longitude.
- Banksia integrifolia – This plant species belong to the family of Proteaceae under the division of Banksieae, which are widely known as Australian wildflowers with characteristic flower spikes and fruiting cones.
- Eucalyptus caesia – This plant species belong to the Myrtaceae family under the division of Leptospermioideae. They are an endemic species of western Australia, with the characteristic features of mallee species of several stems arising from the lignotuber.
- Corallorhiza maculata – Also known as spotted coralroot, is a species of the family Orchidaceae, found in the regions of the American continents and Canada. They lack chlorophyll, hence parasitizing the mycelium of fungal species of the Russulaceae family to obtain energy, thus called myco-heterotrophs.
Paraphyly or paraphyletic is a term used in phylogenetics which is a study of evolutionary patterns and relationships among organisms. For a group of organisms to be considered paraphyletic, it must have the most recent common ancestry of that group but not all of its descendants. This suggests that some of the descendants of common ancestry are excluded from the paraphyletic group on the basis of certain characteristics or traits they possess that the included members of the group do not.
This form of classification raised a problematic approach in taxonomy as it may lead to confusion about the evolutionary relationship among included organisms.
- Graphical explanation of basic phylogenetic terms – https://ucmp.berkeley.edu/glossary/gloss1/phyly.html
- Paraphyletic – https://www.biologyonline.com/dictionary/paraphyletic
- Paraphyly – https://en.wikipedia.org/wiki/Paraphyly
- Hörandl, Elvira, and Tod F. Stuessy. “Paraphyletic groups as natural units of biological classification.” Taxon 59.6 (2010): 1641-1653.
- Concepts of monopoly, polyphyly, & paraphyly – https://www.mun.ca/biology/scarr/Taxon_types.htm
- Crisp, Michael D., and Greg T. Chandler. “Paraphyletic species.” Telopea 6.4 (1996): 813-844.
- Even-toed ungulate – https://en.wikipedia.org/wiki/Even-toed_ungulate
- Apocrita – https://en.wikipedia.org/wiki/Apocrita
- Banksia – https://en.wikipedia.org/wiki/Banksia
- Corallorhiza maculata – https://en.wikipedia.org/wiki/Corallorhiza_maculata