EVOLUTION: THE UNTOLD STORY

EVOLUTION: THE UNTOLD STORY

‘Evolution’ means the development of life in geological time. The term came from the Latin word ‘evolvere’, meaning ‘to unroll’. In biology, the term is used specifically for the processes that have transformed life on Earth from its earliest beginnings to the vast diversity of fossilized and living forms. In this article, we will explore different evidences in the favour of evolution.

ORIGIN OF EARTH

Humans have long speculated about the origin of the universe. The universe began 13.73 Bya in a sequence of events known as the BIG BANG. The table given below provides the sequence of events occurred in the first 10 billion years after the origin of universe.

THE FIRST TEN BILLION YEARS AFTER THE ORIGIN OF EARTH

TIME(BYA)	EVENT
13.7	Origin of the universe
13.6	First stars form(composed of hydrogen and helium)
12.8	Hundreds of galaxies exists
4.6	Origin of solar system and sun
4.56	Origin of earth
4.53	Origin of earth’s moon
4.2-3.8	Secondary atmosphere forms around the earth

Since its origin, the universe has continued to expand, albeit at a far, far slower place. After the initial inflation, the universe entered a phase during which a variety of elementary particles first appeared, followed by electrons, protons and neutrons. Finally the atomic nuclei of helium and a few other low molecular weight elements were formed.

THE HISTORY OF EVOLUTIONARY IDEAS

Earlier in 17^th century the term ‘evolution’ was used to describe the development of an organism. It was believed that a caterpillar ‘unfolds’ into a butterfly. It emerges from the chrysalis, much as the shape of the umbrella is revealed when it is unfolded. However, the butterfly that appears to unfold form chrysalis has, infact, undergone a metamorphosis from a pupil stage.

Only in the 19^th century, evolution come to mean transformation of a species or transformation of the features of organisms between generations. Various evolutionary ideas of different workers can be summarized.

MODERN THEORY OF ORIGIN OF LIFE

Compounds necessary for the origin of life were present before and during the formation of our solar system. They originated and synthesized from abiotic chemical interactions, independent of living systems.

As a consequence of the high osmic abundance of H2 and H-based compounds, H containing gases existed early in Earth’s history. Sparked by energy from solar and ultraviolet radiations, the origin of a variety of H containing and organic molecules provided the fundamental molecular structures from which life arose.

MILLER’S EXPERIMENT

Oparin of Russia and Haldane of England proposed that first form of life could have come from pre-existing non-living organic molecules. They said that formation of life was preceded by chemical evolution, i.e. formation of diverse organic molecules from inorganic constituents. In 1953, SL Miller, created atmosphere of primitive earth in a laboratory scale. Other scientists in similar experiments observed, formation of sugars, nitrogen bases, pigments and fats.

EVIDENCES FROM COMPARITIVE MORPHOLOGY AND ANATOMY

In general, the more similar features shared by a group, the more likely the group descended from a common ancestor. Greater difficulties in interpretation occurs when similar characters arise in different lineages. It can be seen in organisms that evolve to mimic another species in environment.

EVIDENCES FROM VESTIGIAL ORGANS

From an evolutionary viewpoint, rudimentary or vestigial organs occur when an organism adapts to a new environment without losing some previously evolved structures. As evolution continues, that are no longer used diminish, showing only traces of their former size and function.

EVIDENCES FROM COMPARATIVE EMBRYOLOGY

Comparative embryology is the study of relationships among anatomical structures in the embryos of different species. Early in 19^th century the Estonian comparative embryologist Karl von Baer discovered remarkable similarities among the embryos of vertebrates whose adult forms were quite different from each other. Karl von Baer generalized his finding into a ‘law’, i.e. early embryos of related species bear more common features than do later (more specialized development stages).

Later on, this law was modified as the biogenetic law by Ernest Haeckel. This law stages ontogeny repeats phylogeny.

FOSSILS AS THE PHISICAL EVIDENCE OF EVOLUTION

Discovery of the fossil record of life gave a reach source of data to understand relationship among organisms. It led to the hypothesis that earth surface and organisms on it has existed for a long time and organism succeeded one another through time.

EVIDENCES BY CONNECTING LINKS

Some organisms possesses the characters of two different groups. These organism are known as connecting links.

EVIDENCES BY COMPARATIVE BIOCHEMISTRY

Biochemical analysis suggests a common origin for living things. The biochemical differences between the living organisms of today are limited. This makes it possible to measure the relatedness of different groups of organisms by the amount of differences between specific molecules such as DNA, proteins and enzymes.

EVIDENCES BY GEOGRAPHICAL DISTRIBUTION

South America and Africa both have certain common fossil remains. However, the floras and faunas of these land masses now differ, e.g. South Americas now supports new world monkeys, pumas, jaguars while Africa includes Old World monkeys, antelopes, giraffes, etc. The explanation for this is that these land masses were once joined. Species originated in a given area and dispersed out from that point to occupy favourable habitats.

EVIDENCES BY ARTIFICIAL SELECTIONS

The breeding of domesticated plants and animals has created varieties with little external resemblance to their wild ancestors, e.g. when pegions with slightly fanned tail feathers are inbred for generations, pegions with more widely fanned tail feathers are obtained. This artificial selection results only after a few generations. Darwin argued that if so much change can be introduced in so few generations, then species must be able to evolve into other species by the gradual accumulation of minute changes. Form his breeding of pegions, Darwin noted that there were more than a dozen varieties that had been presented as wild birds. He said that they should be classified as separate species.

EVIDENCES BY SYSTEMATIC

A portion of a multi-limbed phylogenetic tree whose branches connect all extant and ancestral populations of an organism, provide the most informative phylogenetic picture of that particular population of organisms. Such connections would indicate the degree of relationship among organisms of other population of the tree.

THE MECHANISM OF EVOLUTION

Evolution, in true sense would have started when cellular forms of life with differences in metabolic capability originated on Earth.

EVOLUTION BY NATURAL SELECTION

The evolutionary process proposed by Darwin and Wallace is two fold.

     i.          Excess reproduction and limited resources lead to competition, which because of natural variation and the action of natural selection, allows those best adapted to pass their characters to the next generations.

      ii.        Changing environment, hereditary variation and natural selection together can result in the modification of existing characters or the origin of new characters that become established and spread throughout a population of species.

TYPES OF NATURAL SELECTIONS

Three types of natural selection that affect the phenotypes of individuals are recognized.

EVOLUTION OF MUTATION

Hugo de Vries based on his work on evening primrose brought forth the idea of mutation. He believed that it is mutation which causes evolution and not the minor heritable variations, as proposed by Darwin. Mutations are random and directionless while Darwinian variations are small and directional. Hugo de Vries believed mutation caused speciation and hence, called it saltation.

HARDY-WEINBERG EQUILIBRIUM

According of Hardy-Weinberg principle, in a population of randomly mating individuals (a deme), allele frequencies are conserved and in equilibrium unless external forces act on them.

ESSENTIAL CONDITION OF HARDY-WENBERG PRINCIPLE

The Hardy-Weinberg principle explain the stability of population and species over a no. of generations and is applicable only under the following conditions:-

·        No mutation

·        No genetic drift

·        No natural selection pressure

·        No gene flow (gene migration)

·        No genetic recombination

STEPS TO SPECIFICATION

When migration of a section of population to another place and population occurs, gene frequencies change in the original as well as in the new populations. Sometimes, the change in allele frequency is so different in the new sample of population that they become a different species. The original drifted population becomes founder. This effect is called founder effect.

MODES OF SPECIATION

There are two modern modes of speciation

     i.        Allopatric speciation- It is initiated between populations following geographical isolation.

   ii.        Sympatric speciation- It is initiated within a population.

A BRIEF ACCOUNT OF EVOLUTION

About 2000 million years ago, the first cellular forms of life appeared on Earth. Some of these cells had pigments to capture solar energy. They released oxygen by employing water as hydrogen donor in the process of photosynthesis. Prokaryotes originated in proterozoic era. Slowly prokaryotes became eukaryotes. Thus, cellular form gave rise to different organisms present on the Earth, including plants and animals. Thus, it can be concluded that evolution is an on-going process. After the origin of Earth, organic compounds formed in the primitive environment of earth, gave rise to first cellular form and ultimately to different living organisms.

Author- Sayeem Sadik.