Phylogeny is the history of the evolution of a species
or groups, especially in reference to lines of decent and relationships among
broad groups of organisms. Fundamental to phylogeny is the preposition,
universally accepted in the scientific community that plants or animals of
different Species descended from common ancestors. The evidence for such
relationships, however, is nearly always incomplete, for the vast majority of
species that have ever lived have become extinct, and relatively few of their
remains have been preserved (www.MerriamWebster.Com).
Most judgments of phyloganicity, then, are based on
indirect evidence and cautions speculation. Even when biologists use the same evidence,
they often hypothesize different phylogenies, though they do agree that life is
the result of organic descent from earlier ancestors and that true phylogenies
are discoverable, at least in principle. Taxonomy, the science of classifying
organisms, is based in phylogeny.
Early taxonomic systems had no theoretical basis;
organisms were grouped according to apparent similarity. Since the publication
of Charles Darwin origin of species in 1859, however, has been based on the
accepted prepositions of evolutionary descent and relationship (stanier etal
1987)
Biologists who postulate a phylogeny derive their most
useful evidence from the field of paleontology, comparative anatomy,
comparative embryology and biochemistry. Studies of fine structure of cells and
geographic distribution of flora and fauna are also useful. The fossil record
is often used to determine the phylogeny of groups containing hard body parts;
soft parts are generally not preserved.
Most of the data used in making phylogenic judgment
have come from comparative anatomy and from embryology. In comparing features
common to different species, anatomist try to distinguish between homologies, or
similarities inherited from a common ancestors, and, analogies or similarities
that arose in response to similar habits and living conditions. Biochemical investigation
carried out in the latter half of 20th century has contributed
variables data to phylogenetic studies. By counting differences in the sequence
of units that make up protein and deoxyribonucleic acid (DNA) molecules,
researchers have devised a tool for measuring the degree to which different
species have diverged since evolving from a common ancestor.
The earliest organisms were probably the result of a
long chemical evolution, in which random reactions in the primeval seas and
atmosphere produced amino acids and then proteins. It is supposed that droplets containing
protein then formed membranes by binding Molecules to their surface and these
membrane-bound proteins are said to have become organisms when they developed
the capacity to reproduce. It is not certain whether these
earliest self-reproducing organisms were proteins, nucleic acid-protein
associations, or viruses.
There is general agreement that they were
heterotrophic organisms i.e. those that required nourishment in the form of
organic matter from early seas. Later, autotrophic forms appeared, having the
ability to make their own food from inorganic matter. These organisms were the
earliest bacteria; they could store energy as food and release energy as needed
through respiration.
Cyanobacteria (sometimes called the blue-green algae)
are thought to have been the next evolutionary step in that they were able to
use photosynthetic pigments to manufacture their own supply of food and
therefore are not totally dependent on their enviroment for nutrition.
After the cyanobacteria there appeared an extensive
array of algae; molds, protozoan, plants and animals. Three groups of algae can
be dismissed with passing mention, as they arose from uncertain ancestors and
have given rise to no further groups. These groups are the Chrysophytes
(yellow-green and golden-brown algae, chiefly diatoms); the pyrrophytes
(cryptomonands and dinoflagellates); and
rhodophytes, or red algae. Three more group, have greater phylogenetic
importance: the chlorophytes (green algae, which almost certainly gave rise to
the land plants i.e the bryophytes (mosses and liverworts) and the tracheophytes,
or vascular plants (including all of the higher plants); the euglenoids,
(unicellular, flagellate organisms), which suggest a broad connection between
plants and animals at this primitives level, and the phaeophytes (broken algae),
which some biologist have probable source of the animal kingdom. Finally, the
protozoans, (unicellular prokaryotic microgarisms) were derives from unknown,
more primitive ancestors, and one or more groups of protozoan have given rise
to metazoan i.e multicellular animals.
Land plant contains two major groups; Bryophytes and Tracheophytes
which differ in many ways but which share distinctive characteristics for
adaptation to dry land. These include the housing of the plant embryo in
maternal tissue.
Bryophytes are descended from green algae and include
mosses, liverworts, and hornworts. Only small quantities of water are needed
for their reproduction, so that the sperm may travel to the eggs. The
fertilized egg matures within the maternal tissue. The plants is protected from
dessication by a waxy cuticle. Bryophytes have apparently not advanced far
beyond their algal predecessors and do not seem to be the evolutionary source
of other groups.
All the dominant plant on earth are included in the
tracheophytes. The tracheophytes development of large plant bodies has been
made possible by having vascular parts that carry water and food inside these
plants, and by a dominant saprophyte stage with microscopic sized gametophytes.
Tracheophytes tissues have differentiated into leaves, stems, and roots, and in
the highest plants, seeds and flowers are featured.
In explaining, the evolution of tracheophytes, it has
been suggested that a mutant form of green algae developed a primitive rot like
function with which to supply itself with water and minerals. The progeny of
this organism eventually developed bundles of vascular tissues a stem and
leaves and a cuticle for protection. The earlier vascular plants are called
psilophytes. The development of seeds arose from the retention of the embryo
inside maternal tissues. Early seed ferns gave rise to the gymnosperm group,
including pines, spruces, and firs. Flowering plants, the angiosperms, probably
came from the gymnosperm phase and have two subgroups. The dicotyledons and the
monocotyledons.
The problems of the origin of multicellular animals
(metazoans) was long dominated by the German embryologist Ernst Hackle’s theory
that the original metazoan ancestor was a spherical protozoa that was
structurally similar to the coelentrates (eg. Jellyfishes, corals. Today there
are two alternative explanation. The first traces metazoan back to flagellates,
the presumed ancestor of flattened, ciliated animals (planulas) that eventually
led to coelenterates and flatworms. Another theory hypothesies that
multinucleated protzoans, dividing into subcells, were the original metazoans,
which developed into simple flatworms. No decisive information, however, yet
exist to sustain either contention.
Lower metazoan forms developed the first symmetrical
arrangement of body parts about a main axis, thus establishing the bilateral
symmetry that characterizes most animals; major exceptions are the echinoderm
(eg starfishes, seas cucumbers). The development of tissues into an outer
ectoderm, which provides protection and carries sense apparatus, and an inner
endoderm, serving digesting and reproduction needs, was an important phase.
Another important trend was cephalization (head formation) the anterior end of
the body generally holds the central nervous system, sense organs, and mouth.
The current theories postulate the lineage of the
higher metazoans. The monophyletic sequence suggest that four groups evolved from lower forms to
higher: Ameria (unsegmented animals), which includes flatworms,
coelenterates, and mollusks; polymeria (segmented animals), which includes annelids
and arthropos.; Oligomeria (reduced segmentation), which includes insects and echinoderms;
and chordonia (chordates). (The alternative) diphyletic theory has been
proposed by many zoologist. It contends that the higher metazoan had two lines
of descent, one of which led to annelids, arthropods and mollusks and the other
of which led to echinoderms and chordates.
Humans are included in the chordates. Three basic
structures are shared by all chordates: a dorsal nerve tube (brain and spinal
cord in vertebrates); a notochord (supporting rod under the nerve tube); and a
pharynx perforated by gill slits, at least during the embryonic stage.
The history of evolution is full of examples of
primitive groups giving rise to more advanced groups, but it should be noted
that it is the more primitives and less specialized members of a group – not the
advance member- that produce new groups. For example, birds and mammals arose
not form advanced reptiles but from primitive, unspecialized reptiles.
Phylogenetic relationships are illustrated in the form
of branched diagrams or trees. A Phylogenic tree is a graph made of branches
that connect nodes.
Figure 1. Examples of phylogenetic tree (Prescott et
al 1999)
(a) Unrooted tree joining four taxonomic units
(b) Rooted tree
The nodes represent taxonomic units such as species or
genes; the external nodes, those at the end of the branches, represent living
organisms. The length of the branches may represent the number of molecular
changes that have taken place between the two nodes. An unrooted tree simply
represents phylogenetic relationship but does not provide evolutionary path. The
figure1 (a) shows that A is more closely related to C than it is to either B or
D, but does not specify the common ancestors for the four species or the direction
of change in contrast, the rooted tree (fig 1 (b) ) does give a node that
serves as the common ancestors and shows the development of the four species
from this root. Pylogenetic tree are developed by comparing molecular sequences.
To compare two molecules, their sequences must first be aligned so that similar
parts match up. Once the molecule has been aligned, the number of position that
varies in the sequences can be determined. These data are used to calculate a
measure of the differences between the sequences.Often the difference is expressed
as the evolutionary distance. Organisms are than clustered together base on
singularity in the sequences. The most similar organisms are clustered
together, and then compared with the remaining organism to form a lager cluster
associated together at a lower level of similarity or evolutionary distance.
The process continue until all organisms are included in the tree.(prescot et al1999)
phylogenetic relationship also can be
estimated by techniques such as parsimony analysis
The data and conclusion of phylogeny show clearly that
the world of life is the product of a historical process of evolution and that
degrees of resemblance within and between groups correspond to degrees of
relationship by descent from common ancestors. A fully developed phylogeny is
essential for the devising of a taxonomy that reflects the natural relationship
within the world of living things.
REFERENCE
Lansing
M. Prescott, John P. Harley, Donald A. Klein (1999) General Microbiology Mc Graw – Hill companies,
Inc. 4th Edition pg 400, 405- 407
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Michael
J. Pelczar, JR., E. C. S. Chan, Noel R. krieg.(1999) general Microbiology Graw – Hill companies, Inc 5th
Edition.. pg 314, 324 .
Roger
Y. strainer John L. Ingraham mark L . wheels page R. painter .(1987) General microbiology, macmiikan press
LTD hound mils, basingstoka,
hampshare 5th edition .pg 895
WWW.
UCMP . Berkeley edu/ exhibid/phylogeny
(WWW.
Merriam – Wlebster. Com /../ Phylogeny)