Homology and Homoplasy in Plant Taxonomy

Sanjeet Kumar

Homology and Homoplasy in Plant Taxonomy

Homology is referred as similarity between organisms originating from common ancestry. The term homology was first introduced by Sir Richard Owen in 1843. The word is derived from “Homologia” in Greek which means “agreement”. It denotes structure and organs that have evolutionary correspondence, regardless of their current function. On the other hand, homoplasy can occur from independent evolutionary origin but not by common ancestry. The term homoplasy was coined by Lankester in 1870. It refers to analogous structures which show similarity and may perform the same function, but that are not derived from a structure found in a common ancestor. The wings of bats and insects are analogous because they both function for flight, but evolved from different primitive structure  so we can say this is the example of homoplasy. Homoplasy is due to convergent evolution, parallel evolution or character reversal. Convergence is the independent evolution of similar features in two or more different lineages.  In plant for example, saprophytic leaves of Lycopodium and gametophytic leaves of Porella evolved independently as photosynthetic appendages (In non-vascular plants, photosynthesis usually occurs in the uppermost part of the plant, resulting in an abundance of small stem- or leaf-like appendages). Reversal is the loss of derived feature with the replacement of an original ancestral condition. For example, the reduced flowers of Lemna minor lack the perianth by secondary loss, thus reverting to a condition, prior to the evolution of a reproductive shoot having a perianth like structure.  Evaluation of structure in terms of homology, analogy and homoplasy can be valuable in determining whether apparently similar organisms have evolved in a convergent (Coming closer together, esp. in characteristics or ideas) or parallel manner, and prior knowledge of the evolutionary history of particular species, aids in identifying homologous and analogous structure. Independent information is also important in making such assessments, but it should be obvious (easily seen, recognized, or understood) that parallel evolution following divergence (The degree by which things diverge) of lines tends to result in homologies, while analogies and homoplasy are likely results of convergent evolution.

In constructing a cladogram, a single branching pattern is selected from among many possibilities. The number of possible dichotomous branching of cladogram use to increase with a corresponding increase in the number of taxa for two taxa, there is only one cladogram.  

Homology vs.  Homoplasy

Homology                                                          vs.                                Homoplasy
Homology is any similarity between characters that is due to their shared ancestry	Homoplasy occurs when characters are similar, but are not derived from a common ancestor
Homoplasy often results from convergent evolution

Character : A character is any recognizable attribute of an organism.

Saprophytes: They  are living organisms that feed on dead organic matter.

Gametophytic: the gamete-bearing individual or phase in the life cycle of a plant having alternation of generations

Analogous: Performing a similar function but having a different evolutionary origin, such as the wings of insects and birds.

Perianth: In flowering plants, the perianth consists of the calyx and the corolla, but in liverworts, the perianth is the sterile tubelike tissue that surrounds the female reproductive structure.

Lemna minor : It is also known as Common Duckweed or Lesser Duckweed. It is occurring everywhere that freshwater ponds and slow-moving streams occur, except for arctic and subarctic climates. It has one, two or three leaves each with single root hanging in the water. As more leaves grow, the plant divide and become separate individuals. It is an important food resource for many fish and birds.

Ancestor: Any person from whom one is descended.

Clade: A group of organisms that share a common ancestor / lineage / monophyletic group.

Convergent evolution: The independent development of similar structures in different groups. It is thought to be the result of similar environmental selection pressures on different groups.

Dichotomous branching: A type of branching in which the apical meristem divides into two more or less equal apices.

Cladogram: Cladogram is a diagram used in cladistics which shows relations among organism. It looks like a series of “Y” or forks in a road. At each branch, or “Y” junction, novel characters of evolutionary origin are used to separate off one group from the rest.It is not however an evolutionary tree because it does not show how ancestors are related to descendants or how much they have changed. The branching off points represent a hypothetical ancestor which would have the combined traits of lines above it. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational phylogenetics are now very commonly used in the generation og cladograms.

Cladistics: It is an approach to classification in which items are grouped together based on whether or not they have one or more shared unique characteristics that come from the group’s common ancestor and are not present in more distant ancestors.