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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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organs. He gave the examples 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.
It is possible that the work of Thomas Malthus on populations
influenced Darwin. Natural selection is based on certain observations
which are factual. For example, natural resources are limited, populations
are stable in size except for seasonal fluctuation, members of a population
vary in characteristics (infact no two individuals are alike) even though
they look superficially similar, most of variations ar
e inherited etc. The
fact that theoretically population size will grow exponentially if everybody
reproduced maximally (this fact can be seen in a growing bacterial
population) and the fact that population sizes in reality are limited, means
that there had been competition for resources. Only some survived and
grew at the cost of others that could not flourish. The novelty and brilliant
insight of Darwin was this: he asserted that variations, which are heritable
and which make resource utilisation better for few (adapted to habitat
better) will enable only those to reproduce and leave more progeny. Hence
for a period of time, over many generations, survivors will leave more
progeny and there would be a change in population characteristic and
hence new forms appear to arise.
7.6 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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Figure 7.8 Diagrammatic representation of the operation of natural selection on different
traits : (a) Stabilising (b) Directional and (c) Disruptive
(a)
(b)
(c)
7.7 HARDY-WEINBERG PRINCIPLE
In a given population one can find out the frequency of occurrence of
alleles of a gene or a locus. This frequency is supposed to remain fixed
and even remain the same through generations. Hardy-Weinberg principle
stated it using algebraic equations.
This principle says that allele frequencies in a population are stable
and is constant from generation to generation. The gene pool (total genes
and their alleles in a population) remains a constant. This is called
genetic equilibrium. Sum total of all the allelic frequencies is 1. Individual
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frequencies, for example, can be named p, q, etc. In a diploid, p and q
represent the frequency of allele A and allele a. The frequency of AA
individuals in a population is simply p
2
. This is simply stated in another
ways, i.e., the probability that an allele A with a frequency of p appear on
both the chromosomes of a diploid individual is simply the product
of the probabilities, i.e., p
2
. Similarly of aa is q
2
, of Aa 2pq. Hence,
p
2
+2pq+q
2
=1. This is a binomial expansion of (p+q)
2
. When frequency
measured, differs from expected values, the difference (direction) indicates
the extent of evolutionary change. Disturbance in genetic equilibrium, or
Hardy- Weinberg equilibrium, i.e., change of frequency of alleles in a
population would then be interpreted as resulting in evolution.
Five factors are known to affect Hardy-Weinberg equilibrium. These
are gene migration or gene flow, genetic drift, mutation, genetic
recombination and natural selection. 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 population. New genes/alleles
are added to the new population and these are lost from the old population.
There would be a gene flow if this gene migration, happens multiple times.
If the same change occurs by chance, it is called genetic drift. 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 founders and the effect is called founder effect.
Microbial experiments show that pre-existing advantageous
mutations when selected will result in observation of new phenotypes.
Over few generations, this would result in Speciation. Natural selection is
a process in which heritable variations enabling better survival are enabled
to reproduce and leave greater number of progeny. A critical analysis
makes us believe that variation due to mutation or variation due to
recombination during gametogenesis, or due to gene flow or genetic drift
results in changed frequency of genes and alleles in future generation.
Coupled to enhance reproductive success, natural selection makes it look
like different population. Natural selection can lead to stabilisation (in
which more individuals acquire mean character value), directional change
(more individuals acquire value other than the mean character value) or
disruption (more individuals acquire peripheral character value at both
ends of the distribution curve) (Figure 7.8).
7.8 A BRIEF A
CCOUNT OF EVOLUTION
About 2000 million years ago (mya) the first cellular forms of life appeared
on earth. The mechanism of how non-cellular aggregates of giant
macromolecules could evolve into cells with membranous envelop is not
known. Some of these cells had the ability to release O
2
. The reaction
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Figure 7.9 A sketch of the evolution of plant forms through geological periods
could have been similar to the light reaction in photosynthesis where water
is split with the help of solar energy captured and channelised by
appropriate light harvesting pigments. Slowly single-celled organisms
became multi-cellular life forms. By the time of 500 mya, invertebrates
were formed and active. Jawless fish probably evolved around 350 mya.
Sea weeds and few plants existed probably around 320 mya. We are told
that the first organisms that invaded land were plants. They were
widespread on land when animals invaded land. Fish with stout and strong
fins could move on land and go back to water. This was about 350 mya. In
1938, a fish caught in South Africa happened to be a Coelacanth which was
thought to be extinct. These animals called lobefins evolved into the
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first amphibians that lived on both land and water. There are no specimens
of these left with us. However, these were ancestors of modern day frogs
and salamanders. The amphibians evolved into reptiles. They lay thick-
shelled eggs which do not dry up in sun unlike those of amphibians.
Again we only see their modern day descendents, the turtles, tortoises
and crocodiles. In the next 200 millions years or so, reptiles of different
Figure 7.10 Representative evolutionary history of vertebrates through geological periods
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shapes and sizes dominated on earth. Giant ferns (pteridophytes) were
present but they all fell to form coal deposits slowly. Some of these land
reptiles went back into water to evolve into fish like reptiles probably 200
mya (e.g. Ichthyosaurs). The land reptiles were, of course, the dinosaurs.
The biggest of them, i.e., Tyrannosaurus rex was about 20 feet in height
and had huge fearsome dagger like teeth. About 65 mya, the dinosaurs
suddenly disappeared from the earth. We do not know the true reason.
Some say climatic changes killed them. Some say most of them evolved
into birds. The truth may live in between. Small sized reptiles of that era
still exist today.
The first mammals were like shrews. Their fossils are small sized.
Mammals were viviparous and protected their unborn young inside the
mother’s body. Mammals were more intelligent in sensing and avoiding
danger at least. When reptiles came down mammals took over this earth.
There were in South America mammals resembling horse, hippopotamus,
bear, rabbit, etc. Due to continental drift, when South America joined
North America, these animals were overridden by North American fauna.
Due to the same continental drift pouched mammals of Australia survived
because of lack of competition from any other mammal.
Lest we forget, some mammals live wholly in water. Whales, dolphins,
seals and sea cows are some examples. Evolution of horse, elephant, dog,
etc., are special stories of evolution. You will learn about these in higher
classes. The most successful story is the evolution of man with language
skills and self-consciousness.
A rough sketch of the evolution of life forms, their times on a geological
scale are indicated in (Figure 7.9 and 7.10).
7.9 ORIGIN AND EVOLUTION
OF MAN
About 15 mya, primates called Dryopithecus and Ramapithecus were
existing. They were hairy and walked like gorillas and chimpanzees.
Ramapithecus was more man-like while Dryopithecus was more
ape-like. Few fossils of man-like bones have been discovered in Ethiopia
and Tanzania (Figure 7.11). These revealed hominid features leading to
the belief that about 3-4 mya, man-like primates walked in eastern Africa.
They were probably not taller than 4 feet but walked up right. Two mya,
Australopithecines probably lived in East African grasslands. Evidence
shows they hunted with stone weapons but essentially ate fruit. Some of
the bones among the bones discovered were different. This creature was
called the first human-like being the hominid and was called Homo habilis.
The brain capacities were between 650-800cc. They probably did not eat
meat. Fossils discovered in Java in 1891 revealed the next stage, i.e., Homo
erectus about 1.5 mya. Homo erectus had a large brain around 900cc.
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Figure 7.11 A comparison of the skulls of adult modern human being, baby chimpanzee and
adult chimpanzee. The skull of baby chimpanzee is more like adult human skull
than adult chimpanzee skull
Homo erectus probably ate meat. The Neanderthal man with a brain size
of 1400cc lived in near east and central Asia between 1,00,000-40,000
years back. They used hides to protect their body and buried their dead.
Homo sapiens arose in Africa and moved across continents and developed
into distinct races. During ice age between 75,000-10,000 years ago
modern Homo sapiens arose. Pre-historic cave art developed about
18,000 years ago. One such cave paintings by Pre-historic humans can
be seen at Bhimbetka rock shelter in Raisen district of Madhya Pradesh.
Agriculture came around 10,000 years back and human settlements
started. The rest of what happened is part of human history of growth
and decline of civilisations.
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