Chapters 15 and 16: Evolution
Biological evolution – change of populations
of organisms over generations
•People in the 1800’s did not believe in
evolution
•Thought the Earth was a few thousand
years old and did not change
•Thought all life was created by a divine
creator in the span of 1 week – very
religious!!
•Thought that life did not evolve
Charles Darwin
•1831, set sail on HMS Beagle
•Ship’s naturalist -observer/collector of
plants, animals, & fossils
•Left from England, sailed around South
America, across Pacific, around Africa,
and back to England
•Most famous for observations made at
Galapagos Islands
•Darwin was influenced by:
•Charles Lyell – geologist
•Suggested the Earth was several
million years old
•Suggested that Earth changed over
time due to geologic forces (volcanoes,
earthquakes, wind, erosion, etc.)
•Darwin reasoned that if Earth changed,
those who lived on it would have to
adapt/change to survive.
•Thomas Malthus – an economist
•Wrote an essay on overpopulation of
humans.
•Said that the human population was
limited by the supply of food and living
space.
•Could cause increase in death or birth
rates.
•Darwin reasoned that the same limits could
apply to organisms in nature.
•Darwin published his findings in 1859 in a book
entitled The Origin of Species by Means of
Natural Selection.
•He was motivated to publish his book in 1859
because Alfred Wallace had independently come
up with the same conclusions and was ready to
publish his findings.
•Darwin is credited with the theory of natural
selection due to the volume of evidence to
support his theory.
• Populations become modified through
natural selection
• Natural selection - the process by which
environment acts on a population,
determining which organisms are most
“fit.” Those organisms who are most
“fit” survive and reproduce more often
than those who are not.
• Fitness – reproductive success
•Darwin noticed similarities between
species seen on mainland and island
chains
•Thought similarities could be explained
by descent with modification – species
came to new environment, then changed
over time as the species adapted to its
new environment.
•Darwin was not the first to suggest that life
forms could change – about 50 years earlier,
Jean-Baptiste de Lamarck suggested that
organisms acquired traits during their lifetime to
adapt to their environment and passed those
acquired traits onto their offspring.
•WRONG – only traits found on genes can be
passed onto offspring!!
Adaptations vs. Variations
•Variations – differences that exist within a
population that may have no effect on
fitness
•Ex: length of your thumb
•Adaptations – a variation that all members
of a population have inherited because that
trait improves fitness
•Ex: an opposable thumb
•Sources of variation:
•Mutations – individual genes change
•Ex: A  a or ATC  AGC
•Events during meiosis – during meiosis,
chromosomes get mixed up in making eggs
& sperm
•Random fusion of gametes – which sperm
fertilizes which egg – determined by chance
•Three types of adaptations:
•Structural – physical features of an organism
•Ex: long tongue to get food, sharp teeth
•Behavioral – actions an organism takes
•Ex: migration, tracking prey, storing nuts,
growing towards light
•Physiological – functioning/biochemical
processes
•Ex: venom, ink of octopus, protein in web,
respiration rate, digestive enzyme, blood
clotting
• Three types of natural selection are known:
• Stabilizing selection – average form of a
trait is favored.
• Directional selection – one extreme form
of a trait is favored.
• Disruptive selection – either/both
extremes of a trait are favored over an
average form of a trait.
Stabilizing selection
Directional selection
Disruptive selection
• Species - group that can interbreed and produce
fertile offspring.
• Hybrids - Offspring produced by the
interbreeding between two different species
– Offspring are usually sterile
– Produced through artificial selection –
organisms bred by humans for specific traits –
ex: dog breeding
• Fertile – capable of reproducing
• Sterile – cannot reproduce
• Examples of hybrids:
1. Zebra x horse
2. M. lion & F. tiger
3. F. lion & M. tiger
4. Killer whale & dolphin
5. M. donkey & F. horse
6. F. donkey & M. horse
7. Zebra x donkey
8. Camel x llama
9. Buffalo x cow
10.Sheep x goat
11.Lemon x lime
12.Tangerine x lemon
13.Broccoli x cauliflower
14.Tangerine x grapefruit
15.Grape x apple
Process of Speciation
• Speciation – development of a new
species
– Occurs whenever reproductive isolation
develops (see next slide)
– Various ways it can happen:
• Coevolution
• Convergent evolution
• Divergent evolution
• Microevolution
• Adaptive radiation
•Coevolution – change of two or more
species in close association with each other
•Ex: predator & prey, parasite & host,
insects and plants they pollinate
•Convergent evolution – unrelated species
evolve similar traits because they are
adapting to similar environments
•Ex: shark & dolphin fins; insect, bird &
bat wings
Ex. of convergent evolution:
•Divergent evolution – 2 or more related
species become unalike as they adapt to
different environments
•Ex: various species of finches on
Galapagos
Ex. of microevolution – peppered moth
population changed as trees became covered in
soot due to Industrial Revolution in England
• Adaptive radiation occurs when a few
individuals migrate to a new area, then
natural selection promotes different
feeding habits in different habitats.
– Ex: several species of finches on the
Galapagos Islands - each one has a
different way of life.
The Galapagos finches
How do new species evolve?
1. Founders arrive – A
few finches travel from
South America to one of
the islands. There, they
survive and reproduce.
2. Separation of populations –
Some birds from species A
cross to a second island. The
two populations no longer share
a gene pool.
3. Changes in the Gene
Pool – Seed sizes on the
second island favor birds
with larger beaks. The
population on the second
island evolves into a
population, B, with larger
beaks.
4. Reproductive isolation – If a
few population-B birds cross back
to the first island, they will not
mate with the birds of population
A. The gene pools are now
separate. Populations A and B are
separate species.
5. Ecological Competition –
As species A and B compete
for seeds on the first island,
they continue to evolve. A
new species, C, may evolve.
Some members of the original
species B may travel to a new
island
6. Continued Evolution –
The process continues,
leading to the formation
of all 13 finch species on
the Galapagos.
Evidence for evolution
• Fossils
• Anatomy
• Analogous parts
• Homologous parts
• Vestigial organs
• Embryology
• Biochemistry
• DNA
• Direct observation
•Fossil Evidence
•Fossils - the remains of past life
•Ex: shells, bones, teeth, imprints
•Tell us things such as age, habitat, diet, &
lifestyle of organisms.
•Record is incomplete – many organisms
leave no fossils behind
•Most found embedded in sedimentary rock
•Usually, any given layer is older than the
one above it, and younger than those
below.
Anatomical Evidence
• All vertebrate forelimbs contain the same sets of
bones – this strongly suggests common they
evolved from a common ancestor.
• Homologous structures - structures that are similar
because they develop from same tissues early in
development; may or may not have same jobs
• Ex: frog, lizard, bird, whale, cat, bat, and
human forelimbs
• Analogous structures - used for the same purpose
but are not due to a common ancestor
• Ex: bird wing & insect wing
Bones of vertebrate forelimbs
• Vestigial structures – structures that have no
function in the living organism but may have
been used by its ancestors.
• Ex: human appendix,
python leg bones
•Embryology – the more similar the embryos are
at certain stages of development, the more
closely related they are thought to be.
Significance of developmental
similarities
Biochemical Evidence
• All organisms have certain organic molecules
in common.
• Ex: Hemoglobin to carry oxygen is
identical in humans and chimpanzees.
• All use DNA, ATP, and many identical or
nearly identical enzymes.
• Organisms use the same triplet code and the
same 20 amino acids in proteins.
• This similarity is not necessary, but can be
explained by sharing common ancestors.
Significance of biochemical
differences
•DNA evidence – the more similar the
genes are, the more closely related
organisms are thought to be.
•Direct observation – We’ve seen
evolution occur in cases like:
•Bacteria becoming resistant to
antibiotics
•Insects that become resistant to
pesticides
The Pace of Speciation
• Two hypotheses:
– Gradualism – suggests that change is
slow and steady
– Punctuated equilibrium – suggests that
a period of no change is interrupted by
period of rapid change
• The fossil record supports both theories.
Gradualism versus punctuated
equilibrium