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Chapters 16
Darwin’s Theory of Evolution
Chapter 16
Darwin’s Theory of Evolution
Evolution- The process
by which organisms
have changed over
time.
Ideas that Led to Darwin’s Theory
of Evolution
1. Jean-Baptiste Lamarck
(1809)
– Said organisms could
change in their lifetime by
either using or not using
various parts of their body
– Said these acquired
changes can be passed
on to offspring
– Example: A bird’s legs
would grow longer if it
waded in deeper water
What would Lamarck say about me?
Why Lamarck’s Ideas Were
Important
• He was the first to suggest that species
can change
• He was one of the first to try to explain
evolution scientifically
• He recognized that there was a link
between an organism’s environment and
its body structure
• His work got people like Darwin thinking
about evolution.
Ideas that Led to Darwin’s Theory
of Evolution
2. Thomas Malthus (1798)
- Realized that more humans were being
born than were dying
- Reasoned that eventually there would
not be enough living space
- Thus, events must occur to control the
population (famine, war, disease)
Why Malthus’s Ideas Were
Important
• Organisms can produce
thousands to millions of
seeds/eggs a year and
only a fraction survive
• Only the ones that
survive can reproduce
• Darwin asked why
certain offspring survive
to reproduce and others
do not
Ideas that Led to Darwin’s Theory
of Evolution
3. Artificial Selection- Humans choose certain organisms to
breed in order to make offspring with a
certain trait
- Allowed Darwin to see that variations in
traits can be passed from parent to
offspring (he knew nothing about genetics)
Darwin’s Voyage on the Beagle
• 1831- Darwin sailed around the world on a
boat called the Beagle
• He collected plants and animals to study
The
Galapagos
Islandslocation of
Darwin’s
most
famous
findings
Darwin’s Ideas About Evolution
A. “The Struggle For Existence”
- If more individuals are born than can
survive, there must be competition for
food, living space, etc.
- He wanted to know which individuals
would come out on top, who would win.
Overproduction  Competition
Darwin’s Ideas About Evolution
B. “Variation and Adaptation”
- Some variations in individuals are better
suited for a certain environment. These
are called adaptations.
- Adaptation- any heritable characteristic
that increases an organism’s ability to
survive in a certain environment
Darwin’s Ideas About Evolution
C. “Survival of the Fittest”
- Fitness- how well an organism can
survive and reproduce in its environment
- If an organism is well adapted to its
environment and successfully reproduces
it has “high fitness”
Darwin’s Ideas About Evolution
D. “Natural Selection”
- The process by which organisms with
variations most suited to their environment
survive and leave more offspring.
NATURE SELECTS WHO SURVIVES
- Well adapted individuals survive and
reproduce
- Over time, the traits of the well adapted
individuals become the traits most
commonly observed in the population
Example of Natural Selection
1. Grasshopper lays eggs
3.Predator eats the
grasshoppers that are
easy to see
2. Yellow and green
grasshoppers hatch
4.Green grasshoppers
are left to reproduce
After many
generations,
almost all
grasshoppers
will be
__________.
Darwin’s Ideas About Evolution
E. “Common Descent”
- To reproduce is to leave
descendents
- Darwin suggested that over
many generations, successful
species evolve into new
species
- Also, living species are
descended, with modification,
from a common ancestor
Evidence for Common Descent
“Descent with Modification”
1. Homologous Structures- structures shared by related
species and have been inherited by a common ancestor
• The same structure adapted in different ways, depending
on the environment, to produce different species
• These structures have different functions.
Common ancestor of
alligator and chicken
Evidence for A Common Ancestor
“Descent with Modification”
2. Analogous Structures- body parts that
share common function, but not structure
Example: the wing of a bee and the wing of a bird
- both are wings (same function)
- structure is completely different
Evidence for A Common Ancestor
“Descent with Modification”
3. Vestigial Structures- structures inherited
from ancestors but have lost much or all of
their function due to different needs of the
descendents
Examples: The wings of an ostrich,
the appendix in humans, the coccyx
bone (remnant of a tail!), the
formation of goose bumps (a
vestigial reflex!)
Coccyx
bone
Genetic Evidence for Evolution
• Darwin had no idea how heredity worked
• We now know that genetic information
comes from DNA, mutations cause
changes in DNA, crossing over causes
variation in offspring, etc.
• All living cells use DNA and RNA to direct
the production of proteins!!
– Suggests that all organisms evolved from a
common ancestor with this genetic code
Genetic Evidence for Evolution
• Homologous Molecules- molecules with
shared structure and/or function
– Example: homologous proteins perform
similar functions in very different cells
• “Cytochrome C”- a protein that functions in cellular
respiration.
• Similar versions are found in cells from yeast to
humans
Genetic Evidence for Evolution
• Homologous Genes- genes with the same
or similar sequences, controlling the same
traits, in different organisms
Example: Hox genes- determine the
head to tail development of embryos
(which end is up)
• Minor changes in these genes
produce variations among
organisms
• Are present in almost all
multicellular organisms (from fruit
flies to humans)