Download Natural selection

Darwin proposed natural selection as the
mechanism of evolution
 Darwin observed that
– Organisms vary in many traits
– traits that increase survival or reproduction
allow those organisms to leave more offspring
than others
– Result  favorable traits accumulate in a
population over generations
 important points
– Individuals do not evolve: populations evolve
– Evolution is not goal directed and does not
lead to perfect adaptation; favorable traits
vary as environments change
Evidence for evolution
1. fossil record - shows that organisms have evolved
in a historical sequence
 Many fossils link
early extinct
species with
species living
today
Pakicetus (terrestrial)
– Ex.
Rhodocetus (predominantly aquatic)
Pelvis and Dorudon (fully aquatic)
hind limb
Pelvis and
hind limb
Balaena (recent whale ancestor)
2. Comparative anatomy
Homology = similarity in characteristics that result
from common ancestry
– Ex. Vertebrate forelimbs
Humerus
Radius
Ulna
Carpals
Metacarpals
Phalanges
Human
Cat
Whale
Bat
3. Comparative embryology =
– Many vertebrates have common embryonic
structures, revealing homologies
– When you were an embryo, you had a tail and
pharyngeal pouches (just like an embryonic fish)
4. Molecular biology: Comparisons of DNA and
amino acid sequences between different
organisms reveal evolutionary relationships
Populations are the units of evolution
 A population = group of individuals of the same
species living in the same place at the same time
 Evolution = change in heritable traits in a
population over generations
 A gene pool
 Microevolution is a change in the relative
frequencies of alleles in a gene pool over time
Mutation and sexual reproduction produce
genetic variation, making evolution possible
 Four sources of genetic variation:
1. Mutation
2. Independent assortment
3. Crossing over
4. Random fertilization
How do we know if a population is evolving?
 Hardy Weinberg principle = allele and
genotype frequencies within population will not
change unless outside forces act to change those
frequencies
 For a population to remain in Hardy-Weinberg
equilibrium for a specific trait, it must satisfy five
conditions:
1. .
2. .
3. No mutations
4. Random mating
5. No natural selection
Is Hardy-Weinberg equilibrium even possible?
Natural selection, genetic drift, and gene flow can
alter allele frequencies in a population
 The three main causes of microevolutionary
change are
 Natural selection
– If individuals differ in their
survival and reproductive
success, natural selection
will alter allele frequencies
 Genetic drift
– Genetic drift
– Ex. In a small population, chance events may
lead to the loss of genetic diversity
 Genetic drift
– The bottleneck effect
– Ex. the northern elephant seal was hunted to
near extinction in the 1700s and 1800s
– A remnant population of fewer than 100
seals was discovered and protected; the
current population of 175,000 descended
from those few seals and has virtually no
genetic diversity
Original
population
Bottlenecking
event
Surviving
population
 founder effect = when a few individuals
colonize a new habitat
– Ex. Four moose were taken from the
Canadian mainland to Newfoundland in
1904. These two males and two females
rapidly formed a large population of
moose that now flourishes in
Newfoundland.
 Gene flow is the movement of individuals or
gametes/spores between populations and can
alter allele frequencies in a population
 The fossil remains of pygmy (or dwarf) mammoths
(1.5 m to 2 m tall) have been found on Santa Rosa
and San Miguel Islands off the coast of California.
This population of pygmy mammoths is descended
from a population of mammoths of normal size (4 m
tall). Dwarfing is common in island populations and
is not the result of chance events. What
mechanism do you think best accounts for the
decrease in mammoth size on these islands?
– Gene flow
– Genetic drift
– Natural selection
3 types of natural selection:
1. Stabilizing selection
2. 2. Directional selection acts against individuals
at one of the phenotypic extremes- common during
periods of environmental change
3. Disruptive selection favors individuals at both
extremes of the phenotypic range- may occur in
patchy habitats
Frequency of
individuals
Types of
selection
Original
population
Phenotypes (fur color)
Original
Evolved
population population
Stabilizing selection
Directional selection
Disruptive selection
 Why doesn’t natural selection act to eliminate
genetic variation in populations, retaining only the
most favorable alleles?
– A recessive allele is only subject to natural
selection when it influences the phenotype in
homozygous recessive individuals
– Ex. cystic fibrosis
Endangered species often have reduced variation
 Low genetic variability
Natural selection cannot fashion perfect
organisms
1. Selection can only act on existing variation
2. Evolution is limited by historical constraints
– Birds arose as the forelimb of a small dinosaur
evolved into a wing
Wing claw
(like dinosaur)
Long tail with
many vertebrae
(like dinosaur)
Teeth
(like dinosaur)
Feathers
Natural selection cannot fashion perfect
organisms
3. Adaptations are often compromises
4. Chance, natural selection and the environment
interact
Ch. 16: Evolution of Single-celled organisms
have inhabited Earth for billions of years
are the oldest life-forms and remain the most
numerous and widespread organisms
 How did life arise on
earth?
Water vapor
CH4
“Atmosphere”
Electrode
Condenser
 Simulations of such
conditions have
produced amino acids,
sugars, lipids, and the
nitrogenous bases found
Figure 16.3B
in DNA and RNA
Cold water
H2O
“Sea”
Cooled water
containing
organic
molecules
Sample for
chemical analysis
The first genes may have been RNA molecules that catalyzed
their own replication
C
A
G
U
G
G
G
G
C
U
C
A
U
U
G
C
A
U
U
A
C
C
A C
G U
U G
A
A
U A
A
G
U
C
G
A
Monomers
G G C
U U
U
1 Formation of short RNA
polymers: simple “genes”
2 Assembly of a
complementary RNA
chain, the first step in
replication of the
original “gene”
 RNA might have acted as templates for the formation of
polypeptides which in turn assisted in RNA replication
Self-replication of RNA
RNA
Self-replicating RNA acts as
template on which polypeptide forms.
Polypeptide
Polypeptide acts as primitive
enzyme that aids RNA
replication.
Figure 16.6A
 Membranes may have separated various aggregates of selfreplicating molecules which could be acted on by natural
selection
Membrane
LM 650
RNA
Figure 16.6B, C
Polypeptide
Prokaryote fun facts
 Prokaryotes lived alone on Earth for over 1 billion years
 They remain the most numerous and widespread
organisms on Earth.
 The total biomass of prokaryotes is ten times that of
eukaryotes
 There are ten times as many prokaryotes living in and on
your body as the number of cells in your body
Bacteria and Archaea are the two main branches of
prokaryotic evolution
 Domains Bacteria and Archaea are distinguished on the
basis of nucleotide sequences and other molecular and
cellular features
Types of Nutrition
 Autotrophs make their own organic compounds
from inorganic sources
 Photoautotrophs
 Chemoautotrophs
 Heterotrophs obtain their carbon atoms from
organic compounds
 Photoheterotrophs obtain energy from
sunlight
 Chemoheterotrophs almost any organic
molecule can serve as food for some species
Energy source
CO2
Light
Chemical
Photoautotrophs
Chemoautotrophs
Carbon
source
Organic
Photoheterotrophs Chemoheterotrophs
compounds
Archaea thrive in extreme environments—
and in other habitats
Figure 16.12A, B
SEM 12,000 
Spirochete
that causes
Lyme disease
Figure 16.14A, B
“Bull’s-eye”rash
SEM 2,800
Some bacteria cause disease
Tick that
carries
the Lyme
disease
bacterium
Bacteria can be used as biological weapons
 Bacteria, such as the species that causes anthrax can be used
as biological weapons
Figure 16.15
Prokaryotes help recycle chemicals and clean up the
environment
 Bioremediation
 Prokaryotes are decomposers in Sewage treatment
and can clean up oil spills and toxic mine wastes
Rotating
spray arm
Rock bed
coated with
aerobic
bacteria
and fungi
Figure 16.16A, B
Liquid wastes
Outflow
A model of the origin of eukaryotes
Cytoplasm
Plasma
membrane
Ancestral prokaryote
Endoplasmic
reticulum
Nucleus
Membrane infolding
Aerobic heterotrophic
prokaryote
Cell with nucleus and
endomembrane system
Some
cells
Ancestral host cell
Photosynthetic
prokaryote
Endosymbiosis
Mitochondrion
Chloroplast
Mitochondrion
Figure 16.17
Photosynthetic
eukaryotic cell
Nuclear
envelope
Protists are an extremely diverse assortment of mostly
unicellular eukaryotes
The parasitic Giardia
SEM 2,300
Single celled algae
Apex
Red blood cell
TEM 26,000
Colorized SEM 4,000 
Plasmodium causes malaria
Multicellularity evolved in several different lineages
probably by specialization of the cells of colonial protists
Gamete
Locomotor
cells
2
1
Somatic
cells
3
Foodsynthesizing
cells
Unicellular protist
Colony
Early multicellular organism
with specialized, interdependent cells
Later organism that
produces gametes