Common Principle of Evolution:
The changes in the given generation may be minute and not enough to cause any obvious change but can collect with each generation and can, over time cause substantial changes in population resulting in emergence of new species. For instance, if we say that mammals evolved from reptiles, which would signify that changes happened in reptiles and collected over several generations and over time (generally millions of years), changes turned into substantial leading to emergence of new organisms in form of mammals. According to theory of evolution, all recognized species of organisms descended from the common ancestor (or ancestral gene pool) and came in being through the procedure of slow change and divergence as we have highlighted above. So, inheritable material that is genes are passed on from generation to generation giving the organism its inherited features. These traits (inherited characteristics) differ within populations, with organisms illustrating heritable differences (variations) in the traits in course of time.
Darwin's Theory of Evolution is extensively held notion that all life is associated and has descended from the common ancestor: birds and bananas, fishes and flowers -- all connected. Darwin's general theory assumes growth of life from non-life and stresses the merely naturalistic (undirected) descent with modification. Complex creatures evolve from more simplistic ancestors naturally over time. In short, as random genetic mutations happen within the organism's genetic code, advantageous mutations are preserved ad they help survival -- the process called as natural selection. These advantageous mutations are passed on to next generation.
Relative significance of natural selection and genetic drift in the population differs depending on strength of selection and efficient population size that is number of individuals able to breed. Natural selection generally predominates in large populations, while genetic drift dominates in little populations. Dominance of genetic drift in small populations can even guide to fixation of somewhat deleterious mutations. Consequently, changing population size can dramatically influence course of evolution. Population bottlenecks (where population shrinks temporarily and thus loses genetic variation) result in more uniform population.
Evolution of Chordates:
It is thought that chordates originated from invertebrates. It has been difficult to find out which invertebrate group chordates developed from, chordate ancestors were soft bodied animals and therefore were not preserved as fossils. Unlike previously held position that chordates evolved from hemichordates, it is now believed that chordates evolved from urochordates. This describes why hemichordates are no longer regarded as chordates.
In attempts to work out evolutionary relationships of chordates, many hypotheses have been generated, but current consensus is that chordates are monophyletic, i.e., they have single common ancestor, that itself is a chordate. In other words, chordates evolved from primitive chordate ancestors. It should be stressed that there is still no universal agreement as to origin of chordates. Though, most celebrated hypothesis on evolution of chordates is that by Garstang. Garstang's Hypothesis noted resemblance between tadpole larva and fish. He planned that sea squirt tadpole larva might have been ancestor of vertebrates, if it never matured or metamorphosed. This procedure is known as pedomorphosis - that is evolutionary retention of juvenile or larval traits in adults. An ancestral tunicate gave rise to higher chordate groups through evolutionary process (pedomorphosis) whereby structural and swimming features of tunicate tadpole larva were retained in adulthood. Through this procedure, tunicate ancestral line is believed to have evolved in larger swimming chordates. Evolution is accompanied by anatomical, morphological and physiological changes. With exclusion of hemichordates from phylum Chordata, most primitive chordates are urochordates (tunicates). In tunicates, chordate characteristics can be observed only by studying complete life cycle. Adult feeds using pharyngeal basket, a kind of pharyngeal gill formed in mesh-like basket. Cilia on gill draw water in mouth, through basket mesh and out through exhalent siphon. Tunicates have the unusual heart that pumps by wringing out. It also reverses direction sometimes. You would recall that tunicates are regularly hermaphroditic, frequently casting eggs and sperm directly in sea. After fertilization, zygote develops in tadpole larva. It is only in this swimming larva that remaining three chordate characters that is notochord, dorsal nerve cord and post-anal tail manifest.
Modern systematic biology tries to arrange groups of organisms in the method which recommends genealogical relationships (branching sequences) and thus presents the epitome of evolutionary history. It also may try to show where there are significant differences among different groups. These goals frequently conflict. In the entirely genealogical system, every group must correspond to single lineage (clade) made up of common ancestor and all its descendants. The group which doesn't meet both of the given requirements is known as grade and may be utilized as informal group. Groups which don't have common ancestor, and thus had 2 separate origins, are said to be polyphyletic. Such polyphyletic grades, that would put whales among fish or birds together with bats, have usually been abandoned as soon as they were identified. Another type of grade, that doesn't comprise all descendants of common ancestor, is said to be paraphyletic and is preserved in more conservative systems. Within vertebrates class Aves is a clade, but class Reptilia is grade, for birds are modified dinosaurs. Few systems don't identify Reptilia as formal group. Similarly, mammals, birds, amphibians and reptiles are all altered fish, and old class of fishes (Pisces) is now hardly ever used. Vertebrata is single clade, but invertebrate is a grade comprising of all animals except vertebrates. Thus there is no formal group known as Invertebrata.
Several differences among systems are fairly subjective. This is frequently the case when the group may be ranked either as class or as subphylum. Organizational limits of some groups are also mainly a matter of opinion. Few authors have placed phylum Hemichordata within Chordata, stating close genealogical relationship. Others wish to keep them as separate phylum as hemichordates lack what are regarded as significant chordate features.
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