SEP 16 , 2010
Lecture 2: Introduction to Darwinian Evolution
1) Big questions in biology
- These questions arise from our observations about the natural world
- The diversity of different life forms on earth
- And the fact that within that diversity there is clusters of similarity
- So, similarity in diversity !
- “So they are diverse within a certain level of similarity that is different from other
organisms” – prof
ADAPTATION [= “GOOD FIT”]
– Organisms tend to be very well suited to their environments. [In some cases the
environment is static ( land), in other cases – it‟s changing (water) ]
- They can do what they need to do – to survive & reproduce in those environments.
Even if those environments are incredibly challenging to live in.
e.g. Crypsis - the ability of an organism to avoid observation or detection by other
organisms. [In sophisticated ways]
for example – a dead-leaf butterfly (Kalima inchos) & flatfish.
- SO these are the types of observations that led ppl to ask questions about how we
could understand biodiversity
So big questions in biology:
How did life arise?
What accounts for the diversity of species?
Why are organisms so well suited to their environments (How do we explain
BEFORE DARWIN‟S TIME:
- The dominant idea about explaining life on earth [that is the existence of life in the
first place, biodiversity & how it was organized as well as adaption – was the idea of
SPECIAL CREATION] BIOB51
SEP 16 , 2010
- But evidence was accumulating that maybe „special creation‟ wasn‟t the best
explanation of biodiversity!
- Evidence suggests that organisms had in fact changed over time. And that all the
lineages were not necessarily the same as ones that existed in the past. Not only had
some changed but some had also gone extinct.
- Extinction was not part of the special creation idea!
- Environment of the earth wasn‟t static since the creation of earth – it had also
changed over time!
The reason why changes in the environment overtime were important to thinking
about biodiversity is that „special creation‟ suggested that organisms had been
built/created in the beginning to fit their environments (thus adaptation). So, they
were created to be adapted to their environment and then had not changed over
So how could that be reconciled to the idea that environment‟s itself had changed
over time? So we have evidence to suggest that environment has changed over
time – so how can organisms that haven‟t ever changed since their creation be
adapted to their current environment?
Therefore, this insight from geology [changing environment of Earth] was
important for biologists!
So accumulation of more and more
evidence that there was suggestions
that species were more or less
related to each other as opposed to
all having being created
independently – there was this
clustering of similarity! – not just
in terms of the way they looked
but also in terms of how they
developed. BIOB51 TH
SEP 16 , 2010
So basically relatedness of species as opposed to independent creation!
- This became more and more of a prominent thought among biologists.
THE CLASSIFICATION SYSTEM AROSE BEFORE DARWIN‟S
This system essentially recognizes the
fact that characteristics of organisms shows natural
This clustering was recognized in
classification schemes even though at that time when
they developed the classification scheme – they
weren‟t yet talking about evolution!
THE NEW IDEA.
So, the new idea that
appeared even before Darwin was that
in fact species had changed over time
and that species derive from common
ancestors as a result of divergence of
lineages over time.
Note: think of lineage
as line of inheritance
So in this case – it is
the diversity we see now requires that
the earth is very ancient – b/c it has to
have been a lot of time for these
organisms to have diversified from
one or few original ancestors.
So this idea of evolution is actually quite an old idea and rests on evidence from
wide range of diff. Sources. BIOB51 TH
SEP 16 , 2010
NOT THE BEST EXPLANATION:
- So, looking at all this evidence - it suggested to
many people at the time that the historical „special creation‟
was not the best explanation for existence of life and diversity
- This evidence – these early ideas
about evolution could still NOT
They knew that species had
changed over time
-> They knew that environment‟s had
But they could NOT explain why it
was that organisms were than adapted to their current environments!
EARLY DEFINITION OF EVOLUTION
(IDEA OF CHANGE)
- Now it might not seem surprising but it was a
radical idea at that time
- This puts the idea of evolution as a possible
central idea that would explain these big questions in biology - but again at the time
they could not explain adaption – so this is where Darwin came in!
- Darwin was not the first to propose the theory of evolution – many famous ppl
that did this - including his own grandfather.
- In addition, Lamarck who proposed the theory of evolution and a mechanism that has
since been widely discredited - that is the inheritance of acquired characteristics.
2) What was Darwin‟s key contribution to
- Darwin proposed a plausible mechanism
- A mechanism by which evolution could work
- And a mechanism that could lead to
- Thus making it plausible that evolution could
explain all of the big questions in biology. BIOB51 TH
SEP 16 , 2010
- So his book – “On the Origin of Species by Means of Natural Selection”
- So it‟s the „Means of Natural Selection‟ that makes his contribution so central to
SO DARWIN IS CONSIDERED THE PARENT OF MODERN
- On the slides
- So, he showed that evolution by natural selection had occurred!
EVOLUTION – PROPOSED BY CHARLES DARWIN
- NO supernatural intervention
required to explain adaptation!
Descent with modification
- Descent = you descent from your
- Modification = b/c there is changes
along the way that eventually leads to splitting of
lineages into new types of organisms
[So this is very diff. to the ideas that were
dominant before him] BIOB51 TH
SEP 16 , 2010
3) Evolution by natural selection
[KNOW THIS ONE!*ON
1) Even zebra‟s that look all the same to us
are different from each other! ( stripe
patterns) This includes flowers and other
Ex: face patterns on wasps – diff. from individual to individual.
So there can be differences in shape, colour, and size!
2) Some of the variations passed on –
offspring tend to look like parents. So you get
traits from your parents/ancestors.
3) Deduction = recognizing the
reproductive potential of individuals and then
calculating how many of them we should see
around us if all of them or most of them were
surviving and reproducing themselves
[Darwin talked about this] –on slides
Observation = we also know the processes that lead to
differences in survival. Because of these
BIOLOGICAL FACTORS, some individuals don‟t
get enough food or the shelter they need – or they are
just outright killed by other individuals.
There are also PHYSICAL FACTORS that can kill
individuals – like clim