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Unit 1: Evolution
chapter 22: descent with modification
chapter 25.2 & 3 & 4 : history of life on earth
chapter 23: evolution of populations
chapter 24 and 25.6 : origin of species
chapter 26.1 and 26.3: phylogeny
Chapter 22: Descent with Modification
Observations of the natural world reveal three things:
• how well organisms are suited (adapted) to their particular
environments
• the many shared characteristics of life (unity)
• the vast diversity of life
What scientific explanation accounts for these
observations? Answer: evolution
• broad definition: descent with modification, change over time,
origin of new species and groups of species
• narrow definition: change in genetic composition of a
population between generations
(What’s the biological definition of “population”?)
Biologists study the unity and diversity of life.
Evolutionary
adaptation
Organisms adapt to their environment
over many generations through
natural selection.
In a population of organisms, there will be natural differences
from individual to individual (because of inherited variation in the
population). That means that some individuals will be better
suited to the local environment. Those individuals will have the best
reproductive rates and pass on the beneficial traits to their offspring.
THEME: Biological evolution is the
fundamental organizing principle of biology.
The theory of evolution, as developed by
Darwin and others, accounts for the unity and
diversity of organisms in the biosphere.
Unity – All organisms share similarities because
they are all descendants of a common ancestor.
Diversity – Organisms have different adaptations because they are necessary
for optimal survival and reproduction in different environments.
The traits have been inherited from previous generations, and natural
selection has acted on populations to produce groups of organisms best suited
for their environment.
Descent with modification
(unity)
(diversity)
History of evolutionary thought:
What was the progression of ideas and contemporary
context that led to the formulation of Darwinian evolution
to explain biological unity, diversity, and adaptation?
Aristotle (384-322 B.C.)
• scala naturae
• Species are fixed.
Linnaeus (1707-1778)
• physician, botanist
• binomial classification (nested system)
• rejected evolutionary kinship to explain categorization of
living things
Cuvier (1769-1832)
• developed paleontology
• Fossils in older, lower strata are dissimilar from those found in
newer, upper strata.
• Used catastrophism (not evolution) to explain changes in fossil
record
Hutton (1726-1797)
• Geologic features result from gradual mechanisms.
• Same processes are still operating today.
Lyell (1797-1875)
• Incorporated Hutton’s explanation of
gradual geological change into principle of
uniformitarianism (mechanisms of change
are constant over time)
• Earth had to be much older than a few
thousand years in order for uniformitarianism
to explain current geological features of the planet.
Hutton and Lyell had great impact on Darwin, but Darwin
was not the first to use gradual mechanisms as a way to
explain biological change over time.
Lamarck (1744-1829)
• Compared living species with fossil
record and discovered several lines of
descent
• Explained his observations with two
principles:
– use and disuse
– inheritance of acquired characteristics
• (Mistakenly) thought organisms
were “driven” to evolve
• Vilified at the time by those
who rejected evolution (Cuvier, et al)
• He was on the right track even
though his proposed mechanism of
evolution was ultimately wrong.
Charles Darwin (1809-1882)
• Begins medical school training but
quits to become clergyman
• Traveled on HMS Beagle
• Read Lyell’s Principles of Geology
• Collected specimens (fossils, living species, etc) during stops
along South American coastline and Galapagos islands that
were particularly influential in formulating his theory of
evolutionary change
• Darwin observed multiple examples
of adaptations.
– finches, tortoises, iguanas, etc.
• Darwin explained the occurrence of
adaptations by invoking natural
selection (his mechanism of
evolution).
• Darwin was initially reluctant to
publish his findings and ideas about
natural selection.
• Evolution by natural selection was
independently proposed by a second
scientist: Alfred Russel Wallace.
Wallace “scooped” Darwin.
Darwin’s The Origin of Species is published in
1859. He proposes natural selection as the
mechanism of gradual biological change to
explain life’s unity and diversity.
His explanations and the “avalanche” of evidence
to support them convinced most scientists within
a decade that life’s unity and diversity is the
product of evolution (descent with modification).
The Origin of Species proposed two main ideas:
(1) descent with modification (2) by means of natural
selection
Darwin’s book provided a plausible
explanation for the 3 general
observations about the living world:
•Unity of life results from common descent
(ancestry).
•Diversity of life results from diverse
modifications arising via natural selection.
•Descendants of ancestral organisms
accumulate adaptations that fit specific
ways of life (producing good match between
organism and environment).
The success of Darwin’s work attributed to:
•Use of scientific method
•Proposal of logical mechanism
•Mountain of supporting evidence
Darwin
illustrated his
concepts using
branching tree
diagrams.
Darwin’s reasoning and explanation used artificial selection
as a framework for understanding natural selection.
Darwin made two key observations and formulated
two key inferences in explaining that natural
selection operated like artificial selection.
• observation #1
– Members of a population
vary in their inherited traits.
• observation #2
– All species produce more
offspring than their
environment can support.
– Malthus (1798), An Essay on
the Principle of Population
Darwin made two key observations and formulated
two key inferences in explaining that natural
selection operated like artificial selection.
What’s an inference?
•Inference #1
– Individuals whose inherited traits give them a higher probability of
surviving/reproducing leave more offspring.
•Inference #2
– Favorable traits accumulate over generations.
Natural selection leads to a better fit between organisms
and their environment and, given enough time, can
produce new species and groups of species.
• Natural selection is a process: beneficial adaptations are passed on at
higher rate
– Natural selection requires heritable variation to be present in a population.
Why?
– Organisms produce more offspring than the environment can possibly
support. Individuals with certain (advantageous) traits are likely to have more
offspring and pass along those traits to their offspring. (And the process
continues in an iterative manner.)
• Over time, natural selection increases the match between organisms and
their environment.
• Can ultimately give rise to entirely new species
• IMPORTANT: Natural selection acts on populations. Individual organisms
do not evolve!
Natural selection leads to a better fit between organisms
and their environment and, given enough time, can
produce new species and groups of species.
Multiple lines of evidence support
evolution by natural selection.
(Chapter 22.3 and 25.2, 3, 4)
1. Direct observations of evolutionary
change
Example: MRSA
methicillin-resistant Staphylococcus aureus
Example: Biston betularia (peppered moths) exhibiting industrial melanism in
pre- and post-Industrial Revolution England
Example: Glucose-averse insect populations
2. Comparative anatomy
homology = similarity resulting from common ancestry
– Homologous versus analogous structures
– Vestigial structures
Vertebrate forelimbs are
homologous structures.
All vertebrate embryos display homologous
structures at some point during development.
3. Biochemical evidence (molecular homologies)
–
All living organisms use same basic molecules and mechanisms:
• Universal genetic code
• Use of DNA and RNA, similar genetic sequences
• ATP as energy “currency” in metabolism
• 20 amino acids
• Common metabolic pathways (glycolysis, etc)
4. Fossil record
–
Fossils are remains or evidence
of some organism that lived long ago.
–
Often found in sedimentary rock
layers called strata.
–
Shows great changes over time
Fossil record documents how new groups of organisms arose
from previous ones.
Fossilization has a limited scope. Why?
Gaps in the fossil record continue to be filled.
How do you determine age of a fossil?
•order tells us relative ages
•radiometric dating tells us
absolute ages
What we can learn from
fossil evidence:
evolution of sea mammals
astragalus, an ankle bone:
= deer, pig, camels, cattle
= whales, dolphins, porpoises
Clock analogy – if Earth’s
history occurred in
12 hours…
Era
Period
(mya)
important events in the history of life
450 - 470
535 - 525
Era
Period
(mya)
6
225
important events in the history of life
Fossil record also shows five major mass extinction events in the
last 500 million years.
Permian extinction
Cretaceous extinction
Cretaceous mass extinction event, 65.5 mya
All five mass extinctions
have been followed by
adaptive radiations.
Era
Period
(mya)
adaptive radiations following mass extinctions
important events in the history of life
5. Biogeography
–
–
geographic distribution of species
influenced by continental drift: movement of continents on tectonic plates
• forms mountains, islands, earthquakes; alters habitats, climate
Plates move few cm per year.
Cumulative effects of shifting land
masses over millions of years can be
dramatic.
Examples of biological effects of
tectonic plate movement:
Island species vs mainland species
Australian marsupials
Same fossil species found in Brazil
and Ghana
Final sentence of The Origin of Species:
“There is grandeur in this view of life…. [in which] endless
forms most beautiful and most wonderful have been,
and are being, evolved.”
-Charles Darwin
One of the founders of modern evolutionary theory:
“Nothing in biology makes sense except in the light of
evolution.”
-Theodosius Dobzhansky