Parallelism refers to the evolution of two related species in the same direction so that they resemble each other more than their common ancestor.
Sometimes evolutionary change follows a common pathway in two or more unrelated or distantly-related organisms because of similar environmental pressures. It culminates in unrelated organisms with similar morphological characteristics even though they did not have a common ancestor. This phenomenon is called parallel evolution.
That means in parallelism two related species independently take comparable ways of life. So, they evolve in the same direction and resemble one another in many ways. In fact, they resemble one another more than their common ancestor.
Their initial similarity is due to shared ancestral traits but their continued similarity is due to adaptation to similar conditions.
It’s a homologous evolution. Homologous structures are those that are related by evolutionary descent and divergence.
The presence of homology is evidence that organisms are related.
The wing of a bat and the forelimbs of a monkey are homologous because they descended from the same ancestral structure.
Convergence or convergent evolution refers to the evolution of superficially similar traits by unrelated species as a result of adaptation to similar ways of life.
When parallel evolution under similar environmental conditions in distantly-related organisms results in plants and animals that are morphologically very similar in overall appearance, this is called convergent evolution
It is the process in which two unrelated groups of organisms, living in similar but separate environmental conditions, develop a similar appearance and life style. Similar environmental demands make for similar phenotypic responses.
The requirements of adapting to a modification of body traits. Two similar environments would result in similar characters. These similarities are superficial but in their details they differ in many respects.
It is analogous evolution. Analogous structures are similarity in form and purpose even though not descended from the same ancestor.
The presence of analogy is evidence that organisms are not-related.
The wings of bats and the wings of butterflies are convergent structures as seen from their morphological differences and evolutionary origins..