INTRODUCTION :
Evolutionary Biology is the study of history of life forms on earth.
What exactly is evolution? To understand the
changes in flora and fauna that have occurred over millions
of years on earth, we must have an understanding of the
context of origin of life, i.e., evolution of earth, of stars and
indeed of the universe itself. What follows is the longest of
all the construed and conjectured stories. This is the story
of origin of life and evolution of life forms or biodiversity on
planet earth in the context of evolution of earth and against
the background of evolution of universe itself.
ORIGIN OF LIFE :
When we look at stars on a clear night sky we are, in a
way, looking back in time. Stellar distances are measured
in light years. What we see today is an object whose emitted
light started its journey millions of year back and from
trillions of kilometres away and reaching our eyes now.
However, when we see objects in our immediate
surroundings we see them instantly and hence in the
present time. Therefore, when we see stars we apparently
are peeping into the past.
The origin of life is considered a unique event in the
history of universe. The universe is vast. Relatively speaking. the earth itself is almost only a speck. The universe is very
old – almost 20 billion years old. Huge clusters of galaxies comprise the
universe. Galaxies contain stars and clouds of gas and dust. Considering
the size of universe, earth is indeed a speck. The BIG BANG THEORY attempts
to explain to us the origin of universe. It talks of a singular huge explosion
unimaginable in physical terms. The universe expanded and hence, the
temperature came down. Hydrogen and Helium formed sometime later.
The gases condensed under gravitation and formed the galaxies of the
present day universe. In the solar system of the milky way galaxy, earth
was supposed to have been formed about 4.5 billion years back. There
was no atmosphere on early earth. Water vapour, methane, carbondioxide
and ammonia released from molten mass covered the surface. The UV rays
from the sun brokeup water into Hydrogen and Oxygen and the lighter H2
escaped. Oxygen combined with ammonia and methane to form water,
CO2
and others. The ozone layer was formed. As it cooled, the water vapor
fell as rain, to fill all the depressions and form oceans. Life appeared 500
million years after the formation of earth, i.e., almost four billion years back.
Did life come from outerspace? Some scientists believe that it came
from outside. Early Greek thinkers thought units of life called spores
were transferred to different planets including earth. “PANSPERMIA”still
a favourite idea for some astronomers. For a long time it was also believed
that life came out of decaying and rotting matter like straw, mud, etc.
This was the theory of spontaneous generation. Louis Pasteur by careful
experimentation demonstrated that life comes only from pre-existing life.
He showed that in pre-sterilised flasks, life did not come from killed yeast
while in another flask open to air, new living organisms arose from ‘killed
yeast’. Spontaneous generation theory was dismissed once and for all.
However, this did not answer how the first life form came on earth.
Oparin of Russia and Haldane of England proposed that the first form
of life could have come from pre-existing non-living organic molecules
(e.g. RNA, protein, etc.) and that formation of life was preceded by chemical
evolution, i.e., formation of diverse organic molecules from inorganic
constituents. The conditions on earth were – high temperature, volcanic
storms, reducing atmosphere containing CH4
, NH3
, etc. In 1953, S.L. Miller,
an American scientist created similar conditions in a laboratory scale
(Figure 7.1). He created electric discharge in a closed flask containing
CH4
, H2
, NH3
and water vapour at 8000C. He observed formation of amino
acids. In similar experiments others observed, formation of sugars,
nitrogen bases, pigment and fats. Analysis of meteorite content also
revealed similar compounds indicating that similar processes are
occurring elsewhere in space. With this limited evidence, the first part of
the conjectured story, i.e., chemical evolution was more or less accepted.
We have no idea about how the first self replicating metabolic capsule
of life arose. The first non-cellular forms of life could have originated 3 billion years back. They would have been giant molecules (RNA, Protein,
Polysaccharides, etc.). These capsules reproduced their molecules perhaps.
The first cellular form of life did not possibly originate till about 2000
million years ago. These were probably single-cells. All life forms were in
water environment only. This version of a biogenesis, i.e., the first form of
life arose slowly through evolutionary forces from non-living molecules is
accepted by majority. However, once formed, how the first cellular forms
of life could have evolved into the complex biodiversity of today is the
fascinating story that will be discussed below.
EVOLUTION OF LIFE FORMS -a theory :
Conventional religious literature tells us about the theory of special
creation. This theory has three connotations. One, that all living organisms
(species or types) that we see today were created as such. Two, that the
diversity was always the same since creation and will be the same in future
also. Three, that earth is about 4000 years old. All these ideas were
strongly challenged during the nineteenth century. Based on observations
made during a sea voyage in a sail ship called H.M.S. Beagle round the
world, CHARLES DARWIN concluded that existing living forms share
similarities to varying degrees not only among themselves but also with
life forms that existed millions of years ago. Many such life forms do not
exist any more. There had been extinctions of different life forms in the years gone by just as new forms of life arose at different periods of history
of earth. There has been gradual evolution of life forms. Any population
has built in variation in characteristics. Those characteristics which enable
some to survive better in natural conditions (climate, food, physical factors,
etc.) would outbreed others that are less-endowed to survive under such
natural conditions. Another word used is fitness of the individual or
population. The fitness, according to Darwin, refers ultimately and only
to reproductive fitness. Hence, those who are better fit in an environment,
leave more progeny than others. These, therefore, will survive more and
hence are selected by nature. He called it natural selection and implied it
as a mechanism of evolution. Let us also remember that Alfred Wallace , a
naturalist who worked in Malay Archipelago had also come to similar
conclusions around the same time. In due course of time, apparently
new types of organisms are recognisable. All the existing life forms share
similarities and share common ancestors. However, these ancestors were
present at different periods in the history of earth (epochs, periods and
eras). The geological history of earth closely correlates with the biological
history of earth. A common permissible conclusion is that earth is very
old, not thousand of years as was thought earlier but billions of years old.
WHAT ARE THE EVIDENCES FOR EVOLUTION?
Evidence that evolution of life forms has indeed taken place on earth has
come from many quarters. Fossils are remains of hard parts of
life-forms found in rocks. Rocks form sediments and a cross-section of
earth’s crust indicates the arrangement of sediments one over the other
during the long history of earth. Different-aged rock sediments contain
fossils of different life-forms who probably died during the formation of
the particular sediment. Some of them appear similar to modern
organisms .They represent extinct organisms (e.g., Dinosaurs).
A study of fossils in different sedimentary layers indicates the geological
period in which they existed. The study showed that life-forms varied
over time and certain life forms are restricted to certain geological time-
spans. Hence, new forms of life have arisen at different times in the history
of earth. All this is called paleontological evidence . Do you remember
how the ages of the fossils are calculated? Do you recollect the method
of radioactive-dating and the principles behind the procedure?
Comparative anatomy and morphology shows similarities and
differences among organisms of today and those that existed years ago.
Such similarities can be interpreted to understand whether common
ancestors were shared or not. For example whales, bats, Cheetah and
human (all mammals) share similarities in the pattern of bones of forelimbs
Though these forelimbs perform different functions in these
animals, they have similar anatomical structure – all of them have humerus, radius, ulna, carpals, metacarpals and phalanges in their
forelimbs. Hence, in these animals, the same structure developed along
different directions due to adaptations to different needs. This is divergent
evolution and these structures are homologous . Homology indicates
common ancestry. Other examples are vertebrate hearts or brains. In
plants also, the thorn and tendrils of Bougainvillea and cucurbita
represent homology . Homology is based on divergent
evolution whereas analogy refers to a situation exactly opposite. Wings of
butterfly and of birds look alike. They are not anatomically similar structures though they perform similar
functions. Hence, analogous structures
are a result of convergent evolution –
different structures evolving for the same
function and hence having similarity.
Other examples of analogy are the eye of
the octopus and of mammals or the
flippers of Penguins and Dolphis. One
can say that it is the similar habitat that
has resulted in selection of similar
adaptive features in different groups of
organisms but toward the same function:
Sweet potato (root modification) and
potato (stem modification) is another example for analogy.
WHAT IS ADAPTIVE RADIATION?:
During his journey Darwin went to Galapagos Islands. There he observed
an amazing diversity of creatures. Of particular interest, small black birds
later called Darwin’s Finches amazed him. He realised that there were many
varieties of finches in the same island. All the varieties, he conjectured,
evolved on the island itself. From the original seed-eating features, many
other forms with altered beaks arose, enabling them to become insectivorous and vegetarian finches This process of evolution of different
species in a given geographical area starting from a point and literally
radiating to other areas of geography (habitats) is called Adaptive radiation.
Darwin’s finches represent one of the best examples of this phenomenon.
Another example is Australian marsupials. A number of marsupials, each
different from the other evolved from an ancestral stock, but
all within the Australian island continent. When more than one adaptive
radiation appeared to have occurred in an isolated geographical area
(representing different habitats), one can call this Convergent evolution.
BIOLOGICAL EVOLUTION :
Evolution by natural selection, in a true sense
would have started when cellular forms of life
with differences in metabolic capability
originated on earth.
The essence of Darwinian theory about
evolution is natural selection. The rate of
appearance of new forms is linked to the life
cycle or the life span. Microbes that divide fast
have the ability to multiply and become millions
of individuals within hours. A colony of bacteria
(say A) growing on a given medium has built-
in variation in terms of ability to utilise a feed
component. A change in the medium
composition would bring out only that part of
the population (say B) that can survive under
the new conditions. In due course of time this
variant population outgrows the others and
appears as new species. This would happen
within days. For the same thing to happen in a
fish or fowl would take million of years as life
spans of these animals are in years. Here we
say that fitness of B is better than that of A under
the new conditions. Nature selects for fitness.
One must remember that the so-called fitness
is based on characteristics which are
inherited. Hence, there must be a genetic basis
for getting selected and to evolve. Another way of saying the same thing
is that some organisms are better adapted to survive in an otherwise
hostile environment. Adaptive ability is inherited. It has a genetic basis.
Fitness is the end result of the ability to adapt and get selected by nature.
Branching descent and natural selection Evolution by natural selection, in a true sense
would have started when cellular forms of life
with differences in metabolic capability
originated on earth.
The essence of Darwinian theory about
evolution is natural selection. The rate of
appearance of new forms is linked to the life
cycle or the life span. Microbes that divide fast
have the ability to multiply and become millions
of individuals within hours. A colony of bacteria
(say A) growing on a given medium has built-
in variation in terms of ability to utilise a feed
component. A change in the medium
composition would bring out only that part of
the population (say B) that can survive under
the new conditions. In due course of time this
variant population outgrows the others and
appears as new species. This would happen
within days. For the same thing to happen in a
fish or fowl would take million of years as life
spans of these animals are in years. Here we
say that fitness of B is better than that of A under
the new conditions. Nature selects for fitness.
One must remember that the so-called fitness
is based on characteristics which are
inherited. Hence, there must be a genetic basis
for getting selected and to evolve. Another way of saying the same thing
is that some organisms are better adapted to survive in an otherwise
hostile environment. Adaptive ability is inherited. It has a genetic basis.
Fitness is the end result of the ability to adapt and get selected by nature.
Branching descent and natural selection are the two key concepts
of Darwinian Theory of Evolution (Figures 7.7 and 7.8).
Even before Darwin, a French naturalist Lamarck had said that
evolution of life forms had occurred but driven by use and disuse are the two key concepts
of Darwinian Theory of Evolution
Even before Darwin, a French naturalist Lamarck had said that
evolution of life forms had occurred but driven by use and disuse of organs. He gave the example of giraffes who in an attempt to forage leaves on tall trees had to adapt by elongation of their necks. As they
passed on this acquired character of elongated neck to succeeding
generations, Giraffes, slowly, over the years, came to acquire long
necks. Nobody believes this conjecture any more.
Is evolution a process or the result of a process? The world we see,
inanimate and animate, is only the success stories of evolution. When we
describe the story of this world we describe evolution as a process. On the
other hand when we describe the story of life on earth, we treat evolution
as a consequence of a process called natural selection. We are still not
very clear whether to regard evolution and natural selection as processes
or end result of unknown processes.
MECHANISM OF EVOLUTION :
What is the origin of this variation and how does speciation occur? Even
though Mendel had talked of inheritable ‘factors’ influencing phenotype,
Darwin either ignored these observations or kept silence. In the first decade
of twentieth century, Hugo deVries based on his work on evening primrose
brought forth the idea of mutations – large difference arising suddenly in
a population. He believed that it is mutation which causes evolution and
not the minor variations (heritable) that Darwin talked about. Mutations
are random and directionless while Darwinian variations are small and
directional. Evolution for Darwin was gradual while deVries believed
mutation caused speciation and hence called it saltation (single step large mutation). Studies in population genetics, later, brought out
some clarity.
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