Descent with Modification
Chapter 22
Descent with modification – Evolution
The earth is a dynamic place (ever-changing).
In order for life to survive, it too must
change.
This represents an interesting parody.
Evolutionary change can not be seen at the
individual level, it is only noticed over time
as organisms develop adaptations (traits
which enhance survival/reproduction).
This occurs very slowly and may take many,
many generations to develop.
• Evolution is the primary
unifying concept in biology.
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History of evolutionary thought
• Most are content to let Darwin have all the credit
for the concepts and ideology behind evolution;
however we need to look beyond (or perhaps
before) Darwin to realize the true impact of the
intellectual revolution that is evolution.
Early Philosophy on “Changing” Lifeforms
• Aristotle (384-322 BC) - viewed lifeforms as
unchanging (fixed); however, he did understand
that life could be arranged (or placed on a scale)
based upon increasing complexity – scala
naturae
• Not much advancement in evolutionary thought
progressed until the 1700’s
– Why?? Consider the science of biology – lots of
other things were happening – cells, anatomy,
– Any other reasons???
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• Carolus Linneaus (1707-1778) Sweden
• Founder of taxonomy – science of naming and
classifying organisms
• Developed a hierarchical system of classification
– KingdomÆPhylumÆClassÆOrderÆFamilyÆGenusÆSpecies
• Developed system of binomial nomenclature – still
used today – i.e. Aix sponsa – wood duck
• This system did not infer evolutionary kinship
James Hutton (1726-1797)
• Geologist
• Concept of gradualism
– Idea that profound change
can take place through slow
but cumulative effect of
slow but continuous
processes
– Mostly dealt with geologic
change -i.e formation of
valleys from rivers
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Jean-Baptiste de Lamarck (1744-1829)
• Introduced to key principles
– Use and disuse – idea that parts of the
body that are used extensively are larger
and stronger while lesser utilized parts
deteriorate
– Inheritance of acquired characteristics –
organisms could pass along these traits to
offspring
– Example – a giraffe’s neck is long
because it continually stretched out to
reach food. (not the case we now know)
Georges Cuvier (1769-1832)
• Father of paleontology – the study
of fossils
– Fossil – remains or traces of
organisms from the past
• Cuvier studied fossils and began to
understand the deeper the strata
(layers of sedimentary rock) the
more dissimilar the fossils are to
current life
• Made references to extinctions catastrophism
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• Each of these individuals had
influence in the life of a young
scientist from Shrewsbury, England
• Charles Darwin (1809-1882)
• Trained at Cambridge as a botanist
• In 1831, Darwin (22 yrs old) set
sail on the HMS Beagle to
document the and chart poorly
known stretches of the South
American Coastline.
• A key stop on the voyage was the
Galapagos Islands
Voyage of the HMS Beagle – approx 5 year voyage
Why were voyages like this done??
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Darwin and the Galapagos Islands (GI)
• Darwin noted that most of the animals that he found
on the GI resembled animals he had seen on the
mainland, they were in fact different species.
• He even noted that some species on the GI very
closely resembled each other however there were
subtle differences in their appearance or behaviors
from island to island or even habitat to habitat.
• Darwin kept detailed records of his observations
Putting it all together…
• After his return to England (1836-1840),
Darwin began to compile his notes and
formulated an idea that adaptation (traits) to
the environment and the process of forming
new species might be closely related
• He also drew upon the works of others to
substantiate his findings
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• By 1840, Darwin had formulated his theory of
natural selection as the mechanism of evolution
(descent with modification); where new species
are formed based upon the premise that the
environment selects individual members of a
species based upon adaptations they possess and
allows them to produce more offspring.
• He was hesitant to publish these findings due to
the controversy he knew this could create and he
was consulted by several other scientists like
Charles Lyell
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Charles Lyell (1797-1875)
• A geologist
• Took the work of Hutton a step further
• Uniformitarianism – proposed that
geologic change occurs today as in the
past; at the same rate
• Both Lyell and Hutton’s concepts
influenced Darwin as their ideas both
supported slow change of the Earth
• Darwin surmised that the age of the
Earth was in fact greater than 6,000
years and that living things might be
governed by a similar process of slow
change
Darwin was not alone in his ideas that slow
changes to the Earth, coupled with living
things ability to change slowly over time as
well could produce different species…
• We have already talked about Lamarck
– use-disuse and inheritance of acquired characteristics
– Provide insight to living things’ ability to change and
pass on these changes somehow to offspring
• But it was another scientist, Wallace, who had
developed his own idea of “natural selection”
while working in the West Indes
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Alfred Russel Wallace (1823 – 1913)
• Wallace knew of Darwin and in fact,
sent a draft manuscript of his theory to
Darwin to review
• Lyell had been urging Darwin to
publish his findings before someone
else did…now Wallace would be
first!!! (1858)
• Darwin quickly finished his work and
published The Origin of Species the
next year
• Wallace was a great admirer of Darwin
and agreed that Darwin should get the
credit as the architect of natural
selection
• Two main themes to the
book:
– Evolution explains life’s
unity and diversity
– Natural selection is the
cause of descent with
modification
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Darwin’s views portray the history of life as a tree – many
branches arising from a common trunk and then continuing to
branch until the young twigs appear – we know this to be true
today based upon fossil evidence, comparative anatomy and
now biochemistry
Natural selection and the development of adaptations are the
mechanisms that drive this process. Darwin based natural
selection on five observations :
1. Overproduction of offspring
occurs for all species – if all of the
offspring of a given species
survived, the population’s growth
would be exponential. Yet Darwin
observed that this does not occur.
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2. Populations tend to remain stable in size
(relatively stable – over time)
There may be seasonal fluctuations or natural cycles
where the population hovers around a stable
point
Pop. size
time
3. Resources are limited and competition occurs.
overproduction leads to a struggle for resources
(food, water, shelter, nutrients, sunlight, etc…)
Only a fraction of the population is given the
opportunity to reproduce
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4. Variation occurs - Members of a population vary
extensively in their characteristics; no two
individuals are exactly alike
5. This variation is passed from
one generation to the next;
variation is heritable.
heritable
Survival depends on inherited
traits (adaptations). Those
individuals that possess
adaptations that increase
survival are more likely to
reproduce.
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Darwin also based his theory on familiar
examples of how humans had modified living
things – Artificial Selection
• Breeds of dogs, livestock, and plants are created to
produce desired traits
Summary of Natural Selection
• Natural Selection is the differential success in
reproduction that results from the interactions
between individuals that vary in heritable traits
and their environment.
• Natural Selection can increase the adaptation of
living things to their environment
• If the environment changes over time, or if living
things move to a new environment, natural
selection may result in adaptations to these new
conditions
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Example (experimental) of natural selection in action
Evidences of Evolution
• Fossil Record
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• Comparative Anatomy
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Biochemistry
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