Convergence of Evolution of Tree Habit (With Diagram)

The below mentioned article provides a short note on Convergence of Evolution of Tree Habit. After reading this article you will learn about: 1. Definition of Convergence 2. Explanation of Parallelism and Convergence 3. Discussion on Convergence of Evolution of Tree Habit 4. Living Fossils of Trees.

Definition of Convergence:

Convergence is a phenomenon in nature found in all organisms. It is observed tendency of living forms that are quite unrelated phylogenetically, to respond to similar contingencies of life by developing similar structures. In the process of evolution this similarity way arises along two evolutionary lines, i.e., parallelism and convergence (Fig. 32.1.).

Influence of environmental factors may also create similarities by structural modification, e.g., in desert habitat most of the plants show cactus like habit and known as cacti. But only on flowering they can be separated in euphorbias, asclepias and cacti. Members of distantly related taxa attained similar structural features which are attained independently by various groups of plants, is called convergent evolution.

Explanation of Parallelism and Convergence:

According to Hey wood (1967) parallelism is the development of similar features separately in two or more genetically similar, fairly closely related lineages. While Dahlgren et al suggested parallelism implies that basically corresponding structures in two or more plant groups have evolved in a similar way so that the derived states show more resemblances to one another than to their ancestral stages.

On the other hand convergence of development of similar features separately in two or more genetically diverse and not closely related lineages and not due to common ancestry (Heywood, 1967).

While Dahlgren et al suggested that convergence denotes the condition where more or less different ancestral structures or different, generally distantly related taxa have attained a similar appearance in the course of their evolution.

Thus, when two or more closely related groups tend to evolve by acquiring similar characters is called parallelism and when distantly related groups acquire similar characters is called convergence.

Thorne (1973) suggested that germination, nutrition, pollination, fertilization, dispersal, etc. are the channels causing many diverse unrelated groups to proceed along the same evolutionary channel leading to convergence.

According to Dahlgren et al. (1985), in homologous structures convergence results in superficial similarity of the whole plant, e.g., succulents; and in non homologous structures it leads to superficially similar organs in different taxa, e.g., presence of spines and tendrils in different groups of plants.

Convergence is generally brought about by adaptation to similar climate or habitats mostly affecting the vegetative parts, or even to similar methods of pollination or dispersal affecting the flower, fruit or seed.

For example, Arenaria and Minuartia form natural groups of species which were earlier placed in genus Arenaria. However, Arenaria leptocladus and Manuartia hybrida show much similarities. If similarity is patristic, i.e., from common ancestry; the two species are most primitive members and show a single monophyletic group.

But on the other hand, if they are most advanced members, they show distinct line of ancestry and similarity between them will be superficial and purely due to convergence.

Thus, similarity between two plants due to a known or informed common ancestry is called patristic similarity in terms of phyletic lines, the plants are directly related. The pathways by which a given similarity is attained or closeness of relationship in terms of phyletic lines is called cladistic similarity.

Parasitic habit in different families, such as Lauraceae (Cassytha), Convolvulaceae (Cuscuta), Scrophulariaceae (Striga), Orobanchaceae (Orobanche) and Balanophoraceae (Balanophora); Carnivorous accessory nutrition in Droseraceae (Drosera), Utriculariaceae (Utricularia), Sarraceniaceae (Sarracenia), Nepenthaceae (Nepenthes) and Lentibulariaceae are some good examples of convergence.

Discussion on Convergence of Evolution of Tree Habit:

Trees are perennials. The groups of trees looking alike are few, e.g., palms (monocots), conifers (gymnosperms), etc. Generally, the convergence in tree habit can be seen in their height. The primitive pteridophytes, gymnosperms and angiosperms, etc., were trees.

Progymnosperms were present in upper Devonian and lower Carboniferous period of Palaeozoic era. Many palaeobotamists believe in origin of gymnosperms from pteridosperms.

The link between fern like Archaeopteris and gymnospermous Callixylon was discovered in 1960. Archaeopteris was a 18 m high tree with crown of leaves. The members of Lyginopteridaceae, e.g., Lyginopteris oldhamia were large fern like trees with well branched weak stems. Medullosa, a member of Medullaceae (fossil gymnosperm) was large fern like but small tree. The fossil gymnosperms had fern like leaves.

However, members of Cycadales are medium sized trees with fern like leaves. Macrozamia hopei is tallest cycad 60 feet in height. Mexican cycad Dioon spinulosum is 50 feet in height while 1000 year old D. edule is only 6 feet high. Ginkgo biloba, a member of Ginkgoales is called living fossil. This is oldest living seed plant and there is great similarity between fossil gymnosperm Ginkgoites and Ginkgo biloba of today.

The members of Coniferales are of great heights, e.g., Cedrus, Cupressus, Abies, Picea, Pinus, Taxodium, Taxus, Podocarpus, etc.

Gnetales include Welwitschia, Gnetum and Ephedra which make link between gymnosperms and angiosperms.

Angiosperms include some families of high woody trees. Angiosperms first appeared in Cretaceous period; they were divided into dicots and monocots. Trees are found in both dicots and monocots. Palms, Dracaenas, Pandanus and Bamboos are included in monocots. Palms show similarity in appearance with cycads.

Some of dicots, such as Polyalhia trees show similarity with conifers. Similarities between Gnetales and Angiosperms on the basis of number of cotyledons is due to convergent evolution. The flowers of Orchidaceae, Zingiberaceae, Polygalaceae and Papilionaceae are convergently adapted in response to similar methods of pollination.

Living Fossils of Trees:

There are several living ancient plants which have undergone little or no change through the ages. Although they may have acquired specialized features later, they have more or less retained their original features. Such survivors are called living fossils.

These include the maiden-hair tree, Ginkgo biloba which has continued to live for over one hundred million years. Cycas revoluta, a cycad, and conifers such as Down-redwood (Metasequoia glyptostroboides, California) Big Tree (Sequoia sempervirens) are also considered living fossils.

Perhaps consistency of living habitat and isolation of these forms (living fossils) from their competitors can be two factors that may account for their survival.