Darwin’s general observations
The rich diversity of life
▪ The many shared characteristics (unity) of life
▪ Organisms are suited for their environment
Evolution can be
viewed in two related
but different ways:
As a pattern
(observations of
living world) and as a
process
(mechanisms that
cause observed
patterns)
Aristotle 384-322BC
scala naturae (natural ladder)
Used ‘evolution’ in a descriptive sense
- everything in nature having a certain order or purpose
* Classified all living organisms hierarchically
* Great Chain of Being
* Did not address change over time because he saw the
universe as static and species fixed as they were
1700’s conventional view of Earth and its life -
– Most scientists accepted the views of the Greek
philosopher Aristotle, who generally held that species are
fixed, permanent forms that do not evolve.
– Judeo-Christian culture taught a literal interpretation of
the biblical book of Genesis, that each form of life is
individually created in its present-day form.
– Earlier religious scholars estimated the age of Earth at
6,000 years.
– Thus, the idea that all living species came into being
relatively recently and are unchanging in form dominated
the intellectual climate of the Western world at the time.
Carl Linnaeus
Systema Naturae
Botanist who sought to classify life’s diversity “for the
greater glory of God”
* Developed a nested classification system,
grouping similar species into increasingly general
categories
* Did not ascribe relationships among species to
evolution but their pattern of creation
* Interpreted the remarkable match of organisms to
their environment as evidence that the Creator had
designed each species for a particular purposeClass
* Order
* Genus
* Species
* Mammalia
* Primates
* Homo
* Sapiens
Homo sapiens
“Wise man”
Georges Cuvier
“Founding father of Paleontology”
* Geologist studying Paris strata
* Anatomical drawings
Fossils in one stratum were different from
those appearing in older (deeper) strata
* Proponent of catastrophism – sudden
cataclysmic events and repopulation of
sites by different immigrant species
Jean-Baptiste Lamarck
Philosophie Zoologique (1809)
The complexifying force
* Simple organisms were the
product of spontaneous
generation
* Higher life forms were
derived from simple forms
via some kind of force that
drove animals to become
more complex over many
generations
The adaptive force
* life adapts to the changing
conditions at the same time as it
progresses up the chain of
being
* the environment create the
animal’s needs, which in turn
determine how it will use its
body
* characteristics acquired as a
result of effort are
transmitted to the offspringParts of the body that are used extensively become
better – ‘Use and Disuse’ idea
* These efforts during the life the individual would be
passed to offspring - inheritance of acquired
characters
A sea voyage helped Darwin
frame his theory of evolution
Charles Darwin is best known for his book On the
Origin of Species by Means of Natural Selection,
commonly referred to as The Origin of Species,
which launched the era of evolutionary biology.
▪ At the age of 22, Darwin took a position on HMS
Beagle, a survey ship preparing for a long
expedition to chart poorly known stretches of the
South American coast.
As the ship’s naturalist (field biologist), Darwin
– spent most of his time on shore collecting thousands of specimens of
fossils and living plants and animals and
– kept detailed journals of his observationsMany of Darwin’s observations indicated that
geographic proximity is a better predictor of
relationships among organisms than similarity of
environment.Darwin was particularly intrigued by the geographic
distribution of organisms on the Galápagos Islands
He hypothesized that species
from South America had
colonized the Galápagos and
speciated on the islandsWhile on his voyage, Darwin was strongly influenced
by the newly published Principles of Geology, by
Scottish geologist Charles Lyell.
– The book presented the case that changes in Earth’s
surface can result from slow, continuous actions still
operating today
Darwin’s Focus on Adaptation
In reassessing his observations, Darwin perceived
adaptation to the environment and the origin of
new species as closely related processes
▪ Hypothesis: as the descendants of a remote
ancestor spread into new habitats or their habitat
changes, they accumulated diverse modifications, or
adaptations, that fit them to specific ways of life in
their environment
▪ Hypothesis: that present-day species are the
descendants of ancient ancestors that they still
resemble in some ways
One ancestral
species over
time adapted
to the specific
foods on their
home islands
Descent with modification
The phrase descent with
modification summarized
Darwin’s perception of the unity
of life
▪ The phrase refers to the view
that all organisms are related
through descent from an
ancestor that lived in the
remote past
Darwin’s Hypothesis
Hypothesis - proposed explanation made on the basis
of limited evidence as a starting point for further
investigation
▪ Scientists now consider Darwin’s concept of
evolution by means of natural selection as a theory,
a widely accepted explanatory idea that
– is broader in scope than a hypothesis,
– generates new hypotheses, and
– is supported by a large body of evidence.
Biogeography – the
geographical distribution of species
All of the 500 or so endemic species of
Drosophila in the Hawaiian archipelago
descended from a common ancestor that
reached oldest island, Kauai, over 5
million years ago, and then colonized
younger islandsSpecies tend to be more closely related to other species
from the same area than to species with the same way of
life from different areas
* The sugar glider from Australia, for example, is more closely related to
other Australian marsupials than to
the flying squirrel, a
placental mammal
from North America
* The resemblance
between them, an
example of
convergent
evolution
Evidence for evolution: Fossils
Fossils
– are the imprints
or remains of
organisms that
lived in the past,
– document
differences
between past and
present
organisms, and
– reveal that many
species have
become extinct.
The fossil record is incomplete because
– many of Earth’s organisms did not live in areas that favor
fossilization,
– fossils that did form were in rocks later distorted or
destroyed by geologic processes, and
– not all fossils that have been preserved are accessible to
paleontologists.
Vestigial structures
remnants of features
that served a function in the organism’s ancestors
Evidence for evolution: Homologous sequences
Darwin’s boldest hypothesis was that all life-forms
are related. Molecular biology provides strong
evidence for this claim.
* All forms of life use the
same genetic language
of DNA and RNA.
* The genetic code—how
RNA triplets are
translated into amino
acids—is essentially
universal.
Evidence for evolution: Homologous sequences
Homologies indicate patterns of descent that
can be shown on an evolutionary tree
▪ Darwin was the first to view the history of life as an
evolutionary tree, with multiple branches from a common
ancestral trunk to the descendant species at the tips of the
twigsHomologous
structures can be
used to determine
the branching
sequence of an
evolutionary tree.
▪ These homologies
can include
– anatomical
structure and/or
– molecular
sequence
Evidence for Evolution Pervades Biology
Homology of proteins
evident from conservation
of critical amino acids
* Permits phylogenetic
relationships of
organisms without
common anatomy or
evident morphology to be
determined
Evidence for evolution: Direct observation of
evolutionary change
Industrial melanism in moths
Emergence of pesticide resistance in insects
Appearance and then spread of new SARS-COV-2 variants
Darwin proposed natural selection as the mechanism of evolution
Darwin’s greatest contribution to biology was his explanation
of how life evolves.
▪ His inspiration came from his observations of artificial
selection, in which humans have modified species through
selective breeding
Artificial selection
depended upon:
Variation &
heritability
Observation #1: Members of a population often
vary in their inherited traits
Observation # 2: All species can produce more
offspring than the environment can support, and
many of these offspring fail to survive and reproduce
1. Individuals in a population vary
in their traits, many of which
are passed on from parents to
offspring.
2. A population can produce
more offspring than the
environment can support – ie.
some die
3. Those individuals with traits
best suited to the environment
are more likely to survive -
thus their proportion in the
population will increase over
generations
Natural Selection
Darwin deduced that the production of more
individuals than the limited resources can support
leads to a struggle for existence, with only some
offspring surviving in each generation.
▪ The essence of natural selection is this unequal
reproduction.
– Individuals whose traits better enable them to obtain food
or escape predators or tolerate physical conditions will
survive and reproduce more successfully, passing these
adaptive traits to their offspring.
1. Natural selection is more of an editing process than a
creative mechanism.
2. Natural selection is contingent on time and place,
favoring those heritable traits in a varying population that
fit the current, local environment.
3. An adaptation is often a compromise – an advantage in
one context might be a handicap in another
It is important to emphasize three key points about
evolution by natural selection.
1. Although natural selection occurs through interactions
between individual organisms and the environment,
individuals do not evolve. Rather, it is the population, the
group of organisms, that evolves over time.
2. Natural selection can amplify or diminish only heritable
traits.
3. Evolution is not goal directed (unlike artificial selection) –
results from environmental factors that vary temporally
and spatially – trait that is beneficial one season may be
detrimental the next
