BT- 1010 : Life Sciences
Evolution
06/08/2025
Dr. Thomas Paulraj
thpaulraj@iitm.ac.in
IIT Madras
Evolution
Evolution – the process by which living organisms change over time through
changes in the genome
A brief history of evolutionary thought
• Aristotle and Plato
– Archetypes (i.e. no evolution, variation is meaningless)
• Buffon, Cuvier, Smith
– Importance of fossils. Earth may be old.
• Hutton, Lyell
– Current geological processes can explain earth, but only on a long time-scale
• Lamarck and Erasmus
– Transmutationism
– Inheritance of acquired characteristics or ‘use and disuse’
– Universal common ancestor
Inheritance of acquired characters
“I will show in one of the chapters which follow that each species has
received from the influence of the circumstances which it encounters over
a long period the habits which we know about and that these habits have
themselves exerted influences on the parts of each individual of the
species, to the point where they have modified these parts and have
made them appropriate to the acquired habits.” –
Jean-Baptiste Lamarck, Philosophie Zoologique (1809)
Lamarck’s theory of acquired characters
If the body parts that are not being used, such as
the human appendix are gradually disappearing.
If an organism changes during life in order to adapt to its
environment, those changes are passed on to its offspring
Lamarck’s Theory disproved
● Amputation of mouse tails for successive
generations, showing that even after twenty
generations, there was no effect: baby mice
were still born with tails.
● Two people who develop large leg muscles because they
were cyclists will not have a baby born with larger than
normal leg muscles
Charles Darwin
— “Origin of Species” presented later in 1858 (23 years
after then end of his voyage) and published in 1859
— Both men jointly presented their ideas to the London
Philosophical Society
Charles Darwin and Alfred Russel Wallace
— evolution by natural selection
Species evolve over time from common ancestors through a process of natural
selection
The organisms with advantageous traits are more likely to survive and reproduce,
passing those traits to their offspring.
— Widely accepted by the scientific community
What Darwin said
– Individuals in a population vary (there is variation),
– These variations can be passed to offspring (are inherited),
– More offspring are produced than the environment can support, so there is
competition for resources, and
– Those individuals whose characteristics make them best suited to the
environment live and reproduce and have more offspring (survival of the fittest).
Observation 1
• Species have great powers of potential reproduction
• Populations would increase exponentially if all
individuals survived and reproduced
Observation 2
– But populations tend to remain stable over time
– Except for seasonal fluctuations
Observation 3
– Environmental resources are limited
Lynx and hare
Observation 4
– Individuals in a population vary extensively
– features-they also vary in reproductive success
Observation 5
– Much of this variation is heritable
– Darwin did not know the mechanism
Why are there so many kinds (=species) of living Things?
• How do new species arise?
• How do populations of organisms evolve over time?
• What produces the observed bio geographical patterns of extant and extinct life
forms?
• How are species related to one another?
• How do organisms adapt to their environment?
