EVOLUTION
AND CLASSIFICATION
Evolutionary relationships of the species can be worked out by identifying hierarchies of their characteristics.
Characteristics are details of appearance or behaviour; i.e., a particular form
or function. E.g. four limbs of animals, photosynthesis in plants etc.
Most organisms share some basic characteristics. Based on
this, a hierarchy of classification is given below:
If two
species have more common characteristics, they are more closely related and
will have a recent common ancestor. E.g. A brother and a sister have immediate common
ancestors (parents). But common ancestors of first cousins are grandparents. Thus
classification of species reflects their evolutionary relationship.
Going
backwards, we reach to a single species (single common ancestor) at the
beginning of evolutionary time.
Tracing Evolutionary Relationships
Evolutionary
relationships between different species are traced by identifying common characteristics.
The
organs having similar basic structure but modified to perform different
functions are called homologous organs. E.g. mammals, birds, reptiles &
amphibians have four limbs. Their basic structure is similar but perform
different functions.
The organs
having same function but different structure and origin are called analogous
organs. E.g. wings of birds & bats.
Squirrels
& lizards have no wings. It does not mean birds & bats more closely
related.
Wings of
bats are skin folds stretched between elongated fingers. But bird’s wings are a
feathery covering along the arm. So
the design, structure & components are different.
But the
arms (forelimbs) of bird & bat can be considered as homologous because they
have nearly same sets of bones. The forelimbs are modified into wings.
Fossils
Fossils
are preserved traces of organisms lived in the
past. They help to understand about extinct species. E.g.
o A dead insect caught in hot mud will not decompose quickly, and
the mud eventually harden and retain the impression of its body parts.
o The dinosaur skull fossil in the Narmada valley.
Age of
fossils can be estimated by two ways:
o Relative: Fossils closer to the surface
are more recent than the fossils in deeper layers.
o Detecting the ratios of different isotopes of the
same element in the fossil.
How do
fossils form layer by layer?
Imagine
some invertebrates on the sea-bed die 100 million years ago and are buried in
the sand. As more sand accumulates, sandstone forms under pressure. Millions of
years later, dinosaurs living in the area die, and are buried in mud. It also becomes
rock above the rock containing the earlier invertebrate fossils.
Millions
of years later, the bodies of horse-like creatures die and are fossilised in
rocks above the earlier rocks.
Evolution by Stages
Evolution
of complex organs such as eye is the result of selection of number of intermediate
stages. E.g. An intermediate stage such as a rudimentary eye (e.g.
eye spots of Planaria), give a fitness advantage.
Also, a
change useful for one property can become useful for quite a different function.
E.g. Feathers start out as providing insulation in cold weather. But later,
they might become useful for flight. Some dinosaurs (reptiles) had feathers, but
they could not fly. Birds later adapted the feathers to flight. This means that
birds are very closely related to reptiles.
All dissimilar
structures are evolved from common ancestral design. This can be analysed by
using fossils.
Such
evolutionary relationship can be understood with the help of wild cabbage.
Humans started
cultivation of wild cabbage more than 2000 years ago, and generated different vegetables
from it by selection. This is artificial selection rather than natural selection.
|
Artificial
selection |
Produced plant |
|
Very short distances between leaves. |
Cabbage |
|
Arrested
flower development. |
Broccoli |
|
Sterile flowers. |
Cauliflower |
|
Swollen parts. |
Kohlrabi |
|
Slightly larger leaves. |
Kale (leafy vegetable) |
All these
structures are descended from same ancestor.
Changes
in DNA during reproduction are
the basic events in evolution. So comparing the DNA of different species
gives a direct estimate of how much the DNA has changed during the speciation and
find out where each change diverged from the other. This method is called Molecular
phylogeny. It is extensively used to define evolutionary relationships.
EVOLUTION
SHOULD NOT BE EQUATED WITH ‘PROGRESS’
There are
multiple branches possible at each stage of tracing the family trees of species.
If a
new species has emerged it does not necessarily mean that the old species
disappear. It depends on the environment. Also, the newly generated species may
not ‘better’ than the older one.
Human beings are not evolved from chimpanzees. Rather,
both human and chimpanzees have a common ancestor. The two resultant species from
that common ancestor evolved in separate ways to give rise to Human and
chimpanzee.
There is
no real ‘progress’ in the idea of evolution.
Evolution
is simply the generation of diversity and the shaping of diversity by environmental selection.
The
only progressive trend in evolution is that more and more complex body designs
have emerged over time. However, many older and simpler designs still survive. E.g.
bacteria inhabit the habitats like hot springs, deep-sea thermal vents and the
ice in Antarctica. i.e., human beings are not the pinnacle of evolution, but
simply another species in the evolutionary process.
Human Evolution
All
humans are a single species.
The
earliest members of the human species, Homo sapiens, evolved in
Africa.
A
couple of hundred thousand years ago, some of human ancestors left Africa.
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