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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.
7.2 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
Figure 7.1 Diagrammatic representation of Miller’s
experiment
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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.
7.3 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 (Figure 7.2). 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?
Embryological support for evolution
was also proposed by Ernst
Heckel based upon the observation of certain features during embryonic
stage common to all vertebrates that are absent in adult. For example,
the embryos of all vertebrates including human develop a row of vestigial
gill slit just behind the head but it is a functional organ only in fish and
not found in any other adult vertebrates. However, this proposal was
disapproved on careful study performed by Karl Ernst von Baer. He noted
that embryos never pass through the adult stages of other animals.
Comparative anatomy and morphology shows similarities and
differences among organisms of today and those that existed years ago.
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Figure 7.2 A family tree of dinosaurs and their living modern day counterpart organisms like
crocodiles and birds
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
(Figure 7.3b). 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
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plants also, the thorn and tendrils of
Bougainvillea and Cucurbita represent
homology (Figure 7.3a). 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 Dolphins.
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.
In the same line of argument,
similarities in proteins and genes
performing a given function among diverse
organisms give clues to common ancestry.
These biochemical similarities point to the
same shared ancestry as structural
similarities among diverse organisms.
Man has bred selected plants and
animals for agriculture, horticulture, sport
or security. Man has domesticated many
wild animals and crops. This intensive
breeding programme has created breeds
that differ from other breeds (e.g., dogs) but
still are of the same group. It is argued that
if within hundreds of years, man could create new breeds, could not nature
have done the same over millions of years?
Another interesting observation supporting evolution by natural
selection comes from England. In a collection of moths made in 1850s,
i.e., before industrialisation set in, it was observed that there were more
white-winged moths on trees than dark-winged or melanised moths.
However, in the collection carried out from the same area, but after
industrialisation, i.e., in 1920, there were more dark-winged moths in
the same area, i.e., the proportion was reversed.
(b)
Figure 7.3 Example of homologous organs in
(a) Plants and (b) Animals
(a)
Tendril
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The explanation put forth for this observation was that ‘predators will
spot a moth against a contrasting background’. During post-
industrialisation period, the tree trunks became dark due to industrial
smoke and soots. Under this condition the white-winged moth did not
survive due to predators, dark-winged or melanised moth survived. Before
industrialisation set in, thick growth of almost white-coloured lichen
covered the trees - in that background the white winged moth survived
but the dark-coloured moth were picked out by predators. Do you know
that lichens can be used as industrial pollution indicators? They will
not grow in areas that are polluted. Hence, moths that were able to
camouflage themselves, i.e., hide in the background, survived
(Figure 7.4). This understanding is supported by the fact that in areas
where industrialisation did not occur e.g., in rural areas, the count of
melanic moths was low. This showed that in a mixed population, those
that can better-adapt, survive and increase in population size. Remember
that no variant is completely wiped out.
Similarly, excess use of herbicides, pesticides, etc., has only resulted in
selection of resistant varieties in a much lesser time scale. This is also true for
microbes against which we employ antibiotics or drugs against eukaryotic
organisms/cell. Hence, resistant organisms/cells are appearing in a time
scale of months or years and not centuries. These are examples of evolution
by anthropogenic action. This also tells us that evolution is not a directed
process in the sense of determinism. It is a stochastic process based on
chance events in nature and chance mutation in the organisms.
7.4 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
Figure 7.4 Figure showing white - winged moth and dark - winged moth (melanised)
on a tree trunk (a) In unpolluted area (b) In polluted area
(a)
(b)
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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 (Figure 7.5). 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 (Figure 7.6) 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
Figure 7.6 Adaptive radiation of marsupials of Australia
Figure 7.5
Variety of beaks of finches that Darwin found in Galapagos Island
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different habitats), one can call this convergent
evolution. Placental mammals in Australia also
exhibit adaptive radiation in evolving into
varieties of such placental mammals each of
which appears to be ‘similar’ to a corresponding
marsupial (e.
g., Placental wolf and Tasmanian
wolf-marsupial). (Figure 7.7).
7.5 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 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 of
Figure 7.7 Picture showing convergent evolution
of Australian Marsupials and
placental mammals
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