Phylogeny of angiosperms phylogeny is the study of evolutionary history and relationship among individuals or group of organism. The aim of phylogenetic method is to develop a classification based on the analysis of phylogenetic data and developing a diagram either a cladogram or a phylogenetic tree. There are many views regarding the origin of angiosperm. Some of the scientist such as Melville considered angiosperm as a monophyletic group. A monophyletic group is a group of organism which form a clade. it consists of an ancestral species and all of its descendants. Many authors considered angiosperm as a polyphyletic group which means that angiosperms are originated from more than one ancestor. So for understanding the evolutionary history of angiosperms some terms are used in evolution.
1. Plesiomorphic and apomorphic character
Plesiomorphic or we can say it as primitive or ancestral characters, apomorphy are advanced or derived characters. The number of primitive and advanced character present in any group is important for determining the phylogenetic position of that group. Earlier there were only two reasons for determining the primitiveness of any group and these reasons were, that these families are primitive: why? because they have primitive characters and second, primitive characters are those characters which are possessed by the primitive family. But now as we know that evolution has taken place at different rate in different group of plants. Some plant group evolved faster as compared to the other groups, so to confirm that which character is primitive and which character is advance it is first required to classify the character into plesiomorphic and apomorphic. Many doctrines or belief were proposed to decide the relative advancement of character. these doctrines are based on circular reason. Doctrine of conservative regions: according to this, the regions or part of plants which are less susceptible to the environmental changes, compared to the other are considered as primitive. example is: flower is considered as more conservative part as compared to the vegetative part because flower is less susceptible. so many classification are based on this assumption. Doctrine of recapitulation or doctrine of repeats: it states that early development stages exhibit primitive features. developmental stages of an organism are studied by ontogeny. ontogeny is the history of the development of an organism through its entire life. Theory given by Ernst Haeckel ontogeny repeats phylogeny states that evolutionary changes occur by successively adding new features on to the end of an un altered ancestral ontogeny. Example is: that in plants Petals originates from polypetalae to polysepalae, because the petals primodia i.e the part of the flower from which petal is formed are initially separated which means they are free and the Tubular portion of the Corolla arise later. another examples are polysepaly to gamosepaly, apocarpy to syncarpy. Doctrine of teratology: teratology is the study of abnormalities. was given by Sahani in 1925. teratology is the study of abnormalities of physiological development. If there is a disturbance in the normal equilibrium, then the organism adjust on the superior character from the past. Doctrine of sequences: when the organisms are arranged in a series to show gradation or changes of a particular character, then one end of the series represents the apomorphy and the other end represents the plesiomorphy.
A-B-C-D (A is plesiomorphic, D is apomorphic)
Doctrine of association: if one structure has evolved from another then the primitive condition of the derived structure will be similar to the general or advanced condition of the ancestral structure. For example if we say that vessels have evolved from the tracheids, then the tracheid like vessels i.e the longer element, small diameter, thin wall and oblique end walls, such vessels represent a more primitive conditions similar to tracheids, as compared to the vessels with broader, shorter and more circular elements with horizontal end wall. so it shows that the primitive state of the earlier character is similar to the advanced stage of another character. Doctrine of common ground plan: it states that the characters which are present in all the members of a group are considered as primitive, because they are supposed to be possesed by the original ancestor of the group and from that original ancestor they passed onto all the members so that is why all the members have that common character. Doctrine of character correlation: certain morphological characters are statistically correlated. Within any taxonomy group primitive characters are expected to show positive correlation. the primitive member of a group retain a relatively high proportion of the ancestral or plesiomorphic character and the advance members have high proportion of Advanced characters either by loss of primitive character or by replacement of the primitive character with different apomorphic character. therefore the distribution of plesiomorphic character is more towards the primitive members. In 1974 sporne classified 24 characters showing positive correlation. He made a list of 24 characters in dicots which are distributed in the member of magnoliales and 14 characters in monocots which are distributed in Amarylloid. primitive characters are large flowers with numerous sepals and petals actinomorphic flowers perfect and complete flower polypetala,e polycephaly indefinite number of parts (stamen and carpel) apocarpous flower entomophilous flower absence of vessels seeds with small embryo and large endosperm fruits formed from polycapellary and apocarpous ovary advanced characters are: fewer number of flower part gamopetaly syncarpous zygomorphic flower
2. Homology and analogy the terms first time used and Defined by owen in 1848. homology is the occurrence of the same organ in different organism under every variety of forms and functions. the resemblance due to homology are real. Simpson in 1961 define homology as the resemblance due to inheritance from the common ancestor. He added phylogenetic interpretation to these terms, homology may either be between two characters or between two organism for a particular character. Two characters are homo logous if one is directly derived from the other. examples of homologous characters are: rhizome of Ginger corm of Colocasia Tuber of potato All these are homologous character and all are stem. When a character is derived from other character they form evolutionary transformation series also called as Morphoclienes or phenoclienes. A B , A B C
in this series the original pre-existing character is called plesiomorphic and the derived one is called apomorphic. for example ovary superior half inferior inferior, in this series there are three or more character maybe homologous if they belong to same evolutionary transformation series, another term is shared homalogue- two or more organism maybe homalogue for a particular character if their immediate common ancestor also had this character. Synapomorphy: if a character is present in the immediate common ancestor but not in earlier ancester i.e the character is derived one, then the situation is called synapomorphy.
A B C
Symplesiomorphy: if the character is present in the immediate common ancestor as well as in the earlier ancester i.e it is an original character, then the situation is called symplesiomorphy.
B C A
Criteria for identifying homology 1. similarity with respect to topographic position in relation to other parts. for example branch- occur in the exil of leaf but may be modified in different ways. 2. similar ontogeny 3. continuation through intermediates, example vessels evolved from tracheids 4. same relatively simple character is found in large number of species so this character is probably homologous in all the species.
Analogy: analogy are generally superficial and is defined by owen in 1848 as the occurrence of a part or an organ in one organism which has same function as another part or organ in a different organism. examples are tuber of potato and tuber of sweet potato, one is a stem and another is a root. Simpson defined analogy as it represents functional similarity. it is not due to inheritance from a common ancestor
3. Parallelism and convergence Before studying this we should know what is homoplasy: Homoplasy: if the character shared by two organism is not traced to a common ancestor then the similarity can be the result of Homoplasy or we can say, the similarity between taxa not by common ancestry but by independent evolution. It can occur in two ways, (i) parallelism and (ii) convergence. Both of these situation represent false synapomorphy which means that a character is similar in two organism but it cannot be traced to a common ancestor. Definition of parallelism given by Simpson: It is independent occurrence of similar changes in groups with a common ancestry or we can say that a group has similar changes because they had a common ancestor. examples are (i) development of vessel in Gnetales and dicots. (ii) Ranunculus tripartitus and Ranunculus hederacea both are aquatic and both have dissected leaves. In parallelism the organism have a common ancestor but the character was not present in their common ancestor.
A B C
In the above tree ancestor of A and B is common. But the character in A and B have evolved independently.
Convergence According to Simpson, convergence is increasing similarity between two distinct phyletic lines either with regard to individual organ or to the whole organism. In convergence similar features arise separately in two or more genetically diverse and not closely related taxa. The similarities have arisen in spite of lack of affinity. It is derived from different system of genes. Convergence can be brought by similar climates and habitats, for example in all water plants there is lack of root hair and presence of air lacuna.
A B C
In the above diagram A and B are similar but but the character is derived from different lineages convergence can also so be because of similar method of pollination or dispersal. examples are occurrence of pollinia in asclepiadaceae and orchidaceae, circular sheath at nodes in Equisetum and Ephedra. convergence commonly occur between relatively advanced member, although the distinction between convergence and parallelism is not always clear. if we do not know the evolutionary history of the group before a particular level of ancestry, then there is no way of telling whether all the species had a common ancestor or not.
3. Monophyly paraphyly and polyphyly Monopoly is derived from a single ancestor Polyphyly derived from more than one ancestor but it depends on how far back we are going in evolutionary history. Simpson definition of monophyly is derivation of a taxon through one or more lineages from one immediate ancestral taxon of the same or lower rank. there are two level of monophyly: (i) minimum monophyly: one Supra specific taxon is derived from another of equal rank. (ii) strict monophyly: one higher taxon is derived from a single evolutionary species.
in the figure I, group AB and CD are monophyletic as each has a common ancestor m and group ABCD is monophyletic as it has a common ancestor n, and it can be defined as a group of species descended from a single species and which include all the descendant from the species. based on cutting rule devised by Dahlgren and Rasmusen in 1983, a monophyletic group can be separated by a single cut below the group.
Paraphyletic: group ABC (Fgure II) is paraphyletic as we are leaving out a descendant D of the common ancestor at level n, according to cutting rule two cuts are required to separate a paraphyletic group, one cut below the group and one cut above. the paraphyletic do not contain all the descendant of the most recent common ancestor of the group.
Polyphyletic: in this the most recent ancestor is not the member of the group. In figure III, group BC is polyphyletic as their respective ancestor at level m do not belong to this group. according to cutting rule they can be separated by more than one cut below the group. A polyphyletic group represents more than one piece of a branch.