Charles Darwin and
Natural Selection
Darwin’s ideas were influenced by:
• Jean Baptiste
Lamarck, who
hypothesized that
acquired traits
were passed onto
offspring
•Charles Lyell, a geologist, who
suggested that the Earth was much
older than 6000 yrs
•Thomas Malthus, who wrote that
human populations grow much faster
than their food supply
•Alfred Wallace, who suggested
natural selection after studying wildlife
in the Malay Archipelago.
Darwin journeyed on the
HMS Beagle as a naturalist
• 5 year journey
• studied and collected many biological specimens
• on Galapagos Islands, off coast of Ecuador, observed
animals such as finches, tortoises, and iguanas
• Thirteen different but similar species of finches, each
with a distinctive bill that is specialized for a particular
food source.
• Suggested that these birds migrated from Ecuador
and changed after they arrived.
Darwin observed differences
among island species.
Marine iguana
Land iguana
Thirteen different but similar
species of finches, each with a
distinctive bill that is specialized for
a particular food.
Suggested that these birds migrated from South
America and changed after they arrived
Principles of Natural Selection
1. Variation. What can cause variation in a
population?
2. Overproduction. What are pros and cons
of overproduction?
3. Adaptation. What determines whether
an adaptation is beneficial or not?
4. Descent with Modification. How does
natural selection change a population
over time?
Elephants in Queen Elizabeth
National Park, Uganda, Africa
Normally, nearly all African
elephants, male and female,
have tusks. In 1930, only one
percent of the elephant population
in Queen Elizabeth Park was
tuskless because of a rare
genetic mutation. Food was
plentiful, and by 1963 there were
3,500 elephants in the park.
In the 1970’s, a civil war began in
Uganda. Much of the wildlife was killed for
food, and poachers killed elephants for
their ivory tusks. By 1992, the elephant
population had dropped to about 200. But
by 1998, the population had increased to
1,200. A survey in 1998 revealed that as
many as 30 percent of the adult elephants
did not have tusks. Ugandan wildlife
officials also noted a decline in poaching.
Natural selection acts on distributions of traits.
• A normal distribution graphs as a bell-shaped
curve.
– highest frequency near
mean value
– frequencies decrease
toward each extreme
value
• Traits not undergoing
natural selection have a
normal distribution.
Types of Natural Selection
• Stabilizing selection –
favors the average
• Small spiders have a hard time capturing
prey
• Large spiders easily spotted by birds
• Medium sized spiders are best suited to
survive in their environment, reproduce
more often, leave more offspring.
Directional selection – favors one
of the extreme variations
• Woodpeckers with long beaks capture the
most insects, as they can reach the
insects deep in the tree trunk.
Disruptive selection - favors both extremes
• On light colored rocks, the light limpets are
camouflaged and survive the best
• On dark rocks, the dark limpets are most
successful
• Tan (intermediate) limpets are visible on
both the light rocks and dark rocks, and
their numbers decline due to predation
Evidence of Evolution
A. Fossils
Fossil links found between
• fish and amphibians
• reptiles and birds
• reptiles and mammals
http://www.pbs.org/wgbh/evolution/library/11
/2/e_s_3.html
Fossil linking fish and amphibians
• 365 million years old
• arm bone with fish fin
characteristics
• found in Pennsylvania
• thought to be from a
lobed-finned fish
Archaeopteryx – links reptiles and
birds
A fossil of Archaeopteryx was discovered at about the same time Darwin published On the Origin of
Species. This pigeon-size creature had a dinosaur like shape, complete with a long bony tail, heavy
jaws with serrated teeth, and three long fingers. It also had feathers like those of modern birds.
Hind leg bones in whales
An amphibious reptile
found in Texas, 2005
Diarthognathus, an animal with
reptile and mammal characteristics
Early mammals may have looked
like this
Evolution
of the
horse
B. Biological Molecules
• Differences in amino acid sequences
and DNA are greater between species
that are distantly related than between
species that are closely related
• phylogenetic trees show how
organisms are related through
evolution
http://www.pbs.org/wgbh/evolution/library/03/4
/l_034_04.html
C. Homologous structures – similar
in structure, with different functions
D. Vestigial Structures
• Structures that are reduced in size and
either have no use or a less
important use than they do in other,
related organisms.
• Examples: wings on flightless birds,
Human ear muscles, human wisdom teeth
human appendix , hind leg bones in
whales
The cassowary, a flightless bird
with wings
Wisdom teeth in human
Human appendix
E. Vertebrate Embryos
• Early in development, vertebrate embryos
have similar characteristics such as a tail,
buds that become limbs, and pharyngeal
pouches that hold the gills of fish and
amphibians.
http://www.pbs.org/wgbh/evolution/library/04/2/l_042_02.html
Examples of Evolution
A. Tuskless elephants becoming more common in
Africa
B. Antibiotic resistance in bacteria
such as those that cause
pneumonia and tuberculosis
C. Pesticide resistance in insects
• Tobacco plants are sprayed
with pesticides as many as
16 times in 3 months.
• The pesticides kill many
insects, but not all.
• Only those insects that can
resist the pesticides survive
to lay eggs.
• These insects pass their
ability to resist pesticides to
future generations of insects
through their genes.
The result is insects that are not
affected by pesticides.
D. Industrial Melanism
• Example is the peppered moth.
• Explained by the concealment hypothesis.
E. Beaks of finches
•
Adaptation
• the changing of a species that results in its
being better suited to its environment.
• Examples: camouflage, mimicry,
echolocation, migration, dormancy
Camouflage
Mimicry: one species resembles
another
Snake mimicry: which is
harmful?
Eastern Coral snake
Highly venomous
King snake
Non-venomous
Echolocation in bats.
Hibernation
Migration
Dormancy: cacti embryos coming
out of dormancy
Patterns of Evolution
A. Divergence – Darwin’s finches.
Dogs evolving from wolves. Can lead to
formation of new species (speciation)
B. Convergent evolution
• distantly related organisms evolve similar
traits.
• Example is seen in the streamlined, finned
bodies of dolphins and sharks.
• The fins would be an example of
analogous structures.
Five Evolutionary forces
1. Natural Selection: certain
traits might be an
advantage for survival
2. Mutation: creates new
genetic variation
3. Sexual selection: certain
traits may improve
mating success; alleles
for these traits increase
in frequency
4. Gene flow: movement of individuals to
or from a population (also known as
migration). Immigrants add alleles,
emigrants take alleles away.
Example: troops of baboons in eastern
Africa. Females remain with the troop,
but younger or less dominant males
leave their birth troop, eventually joining
another troop. This ensures gene flow.
5. Genetic drift: random change in allele
frequency in a population. Causes a loss
in diversity.
Example: In the 1800’s, northern elephant
seals were overhunted. The population
was reduced to about 20 individuals.
Hunting has ended, and there are now
about 100,000 seals. However, the
population has little genetic variation.
Genetic drift
Fitness
the genetic contribution of an
individual to the next generation's
gene pool relative to the average
for the population, usually
measured by the number of
offspring that survive to
reproductive age
Microevolution
• a change in gene frequency in a population —
such as all the individuals of one beetle species
living on a particular mountaintop.
Macroevolution
• generally refers to evolution above the
species level
Evolution of whales from land-
dwelling mammals
Evidence
• transitional fossils between land mammals
and whales
• vestigial structures such as pelvic and leg
bones, and external ear muscles
• nostrils at end of snout in embryos;
nostrils travel to top of head before birth
• DNA for milk protein very similar in hippos
and whales