Download What was Darwin trying to explain?

What was Darwin trying to
explain?
Observations
Hypotheses
Charles Darwin
• 1809-1882
• Englishman; bound for
medicine…then the
clergy…sidetracked by biology
• 1831-1836 Beagle
• 1839 Journals
• 1859 The Origin of Species!
– Outlined the process of natural
selection
Observations that suggested
change over time
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Layered Fossils
Extinctions
Transitional forms
Environmental/Geological change
Apparent relatedness of species
Fossil record
• Fossils of complex organisms occur closer to the
surface, in rocks of younger age, than those of
simpler organisms. This layering was globally
consistent suggesting they were different ages.
– Darwin had huge collections to examine, even in early
1800’s
Extinctions
• Some fossils clearly belonged to creatures no
longer walking the Earth
– Irish elk
– More famous examples like T. rex
Transitional Forms &
Environmental Change
• “fossil sandwiches”
occurred in these
layers; middle fossils
intermediate in form
between fossils above
and below.
• Documented changes in
traits through time
Vestigial traits
• Presence of tiny,
useless traits on
contemporary
species
• Anal spurs; coccyx
Legless lepidosaurs
Jon Sullivan; wildherps.com
Transitional Forms
• Darwin’s
predecessors
– Shared distribution
of fossil species &
similar looking
contemporaries
– Law of Succession
Apparent Relatedness
• Darwin’s contribution came after and
almost certainly as a result of his voyage
through the Galapagos island archipelago
• 1831 - Charles Darwin (22) joins crew of
HMS Beagle as gentleman’s companion
(ship’s naturalist) & collects & catalogues
everything. (1831-36)
– Plants, insects, fossils & Galapagos
mockingbirds
– After his return he and his naturalist friends
noted some things about his collections:
Different islands have distinct
(but similar) species
Weight of evidence lead to
his heretical proposal:
• H: Species on neighboring islands look
similar because they are all descended
from a common ancestor; The small
differences between them are due to
changes over time.
– A bold assertion & dangerous for the time.
Species are not static!!
• So, he waited ~20 years after developing
the idea to publish it
On the Origin of Species by Means of
Natural Selection, or the Preservation of
Favored Races in the Struggle for Life
-Charles Darwin, 1859
• Darwin’s “blackbirds”, “grossbeaks”, “wrens” and finches
Origin; Figure 1
• What does this
figure explain?
• Diversity
• Unity
Darwin’s hypothesis
• Accounted for phenotypic differences
between species
– Differences are due to changes that accrue
over time
• What about those phenotypic similarities
between species?
– Similarities are what suggests common
ancestry.
– We use similarities to determine which
species are most closely related.
We recognize 3 types of
similarities (homologies)
• Structural/Morphological
– Outward physical appearance
• Developmental
– Sequence & timing of developmental events;
shared germinal tissues
• Genetic
– DNA sequence
• Darwin had these first two at his disposal.
Structural
• Why would a wing, a shovel, a grasping hand, a
paddle, a rudder & a hinged pendulum be built
from same bones, in same orientation & relative
positions, unless the limb components of each
organism evolved from a common ancestor?
Developmental
• Why should human embryos ever have gills and a tail; why
should chick embryos ever have gills; and why should those
structures appear at similar developmental stages & in
same relative positions as those of fish?
• Fish & human jaws look very different, but develop from
same population of embryonic cells. Why, unless…?
Genetic
• Why should the same 64 codons specify the same
building blocks in ALL organisms?
• Why should these strings of code, or genes, be
the same across dissimilar organisms?
What was Darwin’s contribution?
• Others had proposed evolution (of some
variety) as a pattern that required
explanation.
• Darwin provided the process, natural
selection, that explained the pattern of
descent with modification.
Darwin’s postulates
(hypotheses)
1. Traits in a population vary among individuals
2. Some traits are passed on to offspring (i.e.
traits have a genetic basis and are heritable)
3. Due to battles for resources, some individuals
produce more offspring than others
4. The subset of all offspring that survive are
those that possess beneficial traits; these
traits (and the individual carrying them) are
“naturally selected”
Recall the mockingbirds
• Imagine: ancestral mockingbird population
colonizes 4 Galapagos islands
• If all the postulates are satisfied, then
Darwin predicted that mockingbird
populations on each island would become
different from their ancestral population,
and increasingly well adapted to each of
their respective environments, as better
adapted individuals reproduced at
disproportionately high rates.
Darwin introduced “fitness” to
explain natural selection
• Fitness: the ability of an individual to
produce fertile offspring, relative to that
ability in other individuals.
– This is a measurable quantity, and allows us to
define adaptation
• Adaptation: a heritable trait that
increases the fitness of an individual
relative to individuals lacking that trait.
– These are time and location (environmentally)
dependent