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Transcript
Descent with Modification
Theme:
• Evolutionary change is based on the
interactions between populations & their
environment which results in adaptations
(inherited characteristics) to increase fitness
Evolution = change over time in the genetic
composition of a population
Charles Darwin
(1809-1882)
• English naturalist
• 1831: joined the HMS Beagle for
a 5-year research voyage around
the world (stopped at: Galapagos
Islands)
• 1859 – published Origin of
Species
• Influenced by Lamarck, Hutton &
Lyell, Malthus
Darwin’s Theory of Natural
Selection:
1. Populations produce more offspring than can
possibly survive.
2. Individuals in a population vary extensively from
each other, mostly due to inheritance.
3. Struggle to survive: individuals whose inherited
characteristics best fit to environment leave more
offspring than less fit.
4. Unequal ability of individuals to survive and
reproduce leads to gradual change in pop, with
favorable characteristics accumulating over
generations.
• Populations evolve, not individuals.
• Fitness is determined by the environment.
In summary:
Natural Selection = differential success in
reproduction
Product of natural selection = adaptations of
populations to environment
Natural Selection
Artificial Selection
•Nature decides
•“Man” decides
•Works on individual
•Selective breeding
•Inbreeding occurs
•i.e. beaks
•i.e. dalmations
Therefore, if humans can create substantial
change over short time, nature can over
long time.
Evidence for Evolution
1. Biogeography
▫
▫
Geographic distribution of a species
Geographic, reproductive isolation
2. Fossil Record – transitional forms
3. Comparative Anatomy
1. Homologous structures
2. Vestigial structures
4. Embryonic Development
5. Molecular Biology
▫
DNA, proteins
Human
embryo
Chicken
embryo
Gill
pouches
Postanal tail
Population Genetics = Foundation for
studying evolution
• Darwin’s could not explain how inherited
variations are maintained in populations - not
“trait blending”
• A few years after Darwin’s “Origin of Species”,
Gregor Mendel proposed his hypothesis of
inheritance:
Parents pass on discrete heritable units (genes)
that retain their identities in offspring
Hardy-Weinberg Theorem:
• Frequencies of alleles & genotypes in a
population’s gene pool remain constant from
generation to generation unless acted upon by
agents other than sexual recombination (gene
shuffling in meiosis)
• Equilibrium = allele and genotype frequencies
remain constant
Hardy-Weinberg Equilibrium
Allele Frequencies:
• Gene with 2 alleles : p, q
p = frequency of allele “A” in a population
q = frequency of allele “a” in a population
p+q=1
Note:
1–p=q
1–q=p
Hardy-Weinberg Equilibrium
Genotype Frequencies:
• 3 genotypes (AA, Aa, aa)
2
p
+ 2pq +
2
q
p2 = AA
2pq = Aa
q2 = aa
=1
The Hardy-Weinberg Theorem describes a nonevolving population.
Conditions for Hardy-Weinberg Equilibrium:
1. Extremely large population size (no genetic
drift).
2. No gene flow (isolation from other
populations).
3. No mutations.
4. Random mating (no sexual selection).
5. No natural selection.
• If any of the Hardy-Weinberg conditions are not
met  microevolution occurs
• Microevolution = generation to generation
change in a population’s allele frequencies
Main Causes of Microevolution
1. Mutations – changes in DNA


Point mutations
Gene duplication
Mutation will
alter or create
new alleles in a
population.
Main Causes of Microevolution
2. Sexual Recombination
▫
Rearrange alleles into fresh combinations every
generation
Main Causes of Microevolution
3. Natural selection
Douglas fir trees only release their seeds during fires.
Fire rarely occurs in the river bottom of this valley.
Main Causes of Microevolution
4. Genetic drift: a change in a population’s allele
frequencies due to chance

bottleneck and founder effect
A. Bottleneck Effect – genetic drift due to drastic
reduction in population size
▫ Certain alleles may be over/under represented
Northern elephant seals hunted
nearly to extinction in California
B. Founder effect – few individuals become
isolated from larger population  certain
alleles over/under represented
Polydactyly in Amish population
Main Causes of Microevolution
5. Gene flow – genetic exchange due to migration
of fertile individuals



i.e. wind storm blows pollen to another field
Reduces differences between populations
Gain/lose alleles
Fitness : the contribution an individual makes to
the gene pool of the next generation
Natural selection can alter frequency distribution
of heritable traits in 3 ways:
1.Directional selection
2.Disruptive (diversifying) selection
3.Stabilizing selection
Directional Selection:
eg. beak sizes of birds
during wet/dry
seasons in Galapagos
Diversifying Selection:
eg. small beaks for
small seeds; large
beaks for large seeds
Stabilizing Selection:
eg. average human
birth weight
Preserving Genetic Variation
• Diploidy: inherit 2 alleles
 Recessive alleles less favorable
 Heterozygote protection
• Heterozyote Advantage:
 People hybrid for sickle cell anemia protected
against malaria.
Darwinian Fitness : ability to survive AND
reproduce AND pass on alleles to offspring
• Sexual selection for mating success
▫ Intra (within same sex) – competition for mate
▫ Inter (out) – mate choice
Sexual selection may lead
to pronounced secondary
differences between the
sexes
Evolution of Populations
• Remember:
▫ Individuals are selected
▫ Populations evolve
• Terms:
▫ Population = localized group belonging to same
species
▫ Species = members of a population that can interbreed
and produce fertile viable offspring
▫ Gene pool = total combo of genes in a population at
any one time
▫ Fixed population = all members are homozygous for
trait (usually not the case)
Speciation = origin of species
• Microevolution: changes within a single gene pool
• Macroevolution: evolutionary change above the
species level
▫ cumulative effects of speciation over long periods of
time
Similarity between different species.
Two Patterns of Evolutionary Change
Anagenesis
Cladogenesis
Two Patterns of Evolutionary Change
Anagenesis (“new” “race”)
Cladogenesis (“branch” “race”)
• Phyletic evolution
• A single species gradually
changes into a different
species
• No original group left
• Evolution in single direction
• Branching evolution
• One species stays same, but small
portion leaves and changes to
another species
• Gene pool splits
• Original + new groups
• Increase in diversity/# of species
Biological Species Concept
• Proposed by Ernst Mayr (1942)
• Species = population or group of populations
whose members have the potential to interbreed
in nature and produce viable, fertile offspring
▫ Reproductively compatible
• Reproductive isolation = barriers that prevent
members of 2 species from producing viable,
fertile hybrids
Types of Reproductive Barriers
Prezygotic Barriers:
▫ Impede
mating/fertilization
Types:
▫ Habitat isolation
▫ Temporal isolation
▫ Behavioral isolation
▫ Mechanical isolation
▫ Gametic isolation
Postzygotic Barriers:
▫ Prevent hybrid zygote
from developing into
viable adult
Types:
▫ Reduced hybrid viability
▫ Reduced hybrid fertility
▫ Hybrid breakdown
Types of Reproductive Barriers
Prezygotic barriers impede mating or hinder fertilization if mating does occur
Habitat
isolation
Temporal
isolation
Behavioral
isolation
Individuals
of
different
species
Mechanical
isolation
Gametic
isolation
Mating
attempt
HABITAT ISOLATION
Fertilization
TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION
GAMETIC ISOLATION
Postzygotic barriers prevent a hybrid zygote from
developing into a viable, fertile adult
Reduced
hybrid
viability
Reduced
hybrid
fertility
Hybrid
breakdown
Viable,
fertile
offspring
Fertilization
REDUCED HYBRID
VIABILITY
REDUCED HYBRID
FERTILITY
HYBRID BREAKDOWN
Other definitions of species:
• Morphological – by body shape, size,
and other structural features
• Paleontological – fossil record
• Ecological – niche/role in community
• Phylogenetic – unique genetic history,
branch on tree of life
Two main modes of speciation
Allopatric speciation
Sympatric speciation
Two main modes of speciation:
Allopatric Speciation
Sympatric Speciation
“other” “homeland”
“same” “homeland”
Geographically isolated
Evolves by natural selection
& genetic drift
Eg. Galapagos finches
Overlapping populations
within home range
Subset of population isolated
from parent pop. change
due to:
• chromosomal changes
• nonrandom mating
• habitat differentiation
Eg. polyploidy in plants (oats,
cotton, potatoes, wheat)
Adaptive Radiation
• Emergence of numerous species from a common
ancestor introduced into new environment
• Occurs when:
 A few organisms make way to new, distant areas
(allopatric speciation)
 Environmental change  extinctions  new
niches for survivors
• Eg. Hawaiian archepelago
Founding
Parents
When 2 splintered groups rejoin geographically:
Possibilities:
1. Still one species
2. Two distinct species (no interbreeding)
3. Hybrid zone
A
B
Interbreeding
zone
Tempo of Evolution
Gradualism
• Darwin
• Slow, constant change
• Less likely
Punctuated Equilibium
• Eldridge & Gould
• Long period of minor change
are interrupted by short
bursts of significant change
• More likely
Convergent Evolution
• Independent development of similar features
between 2 unrelated species
• Similar environments
• Analogous structures
• Eg. wings on bees & wings on birds
REMEMBER!!
•Dear King Philip Came Over For Good
Spaghetti
•Dear King Philip Crossed Over Five
Great Seas
•Dear King Philip Came Over From
Germany Stoned
•Your own???
• Phylogeny = evolutionary
history of a species or group
of species
Phylogram: the length of a branch
reflects the number of changes that
have taken place in a particular DNA
sequence in that lineage
Cladistics : a form of systematics
• Cladogram: diagram of evolutionary relationship of
organisms
▫ Shared characteristics due to common ancestry
▫ Uses parsimony – simplest explanation, fewest DNA
base changes for tree (“keep it simple”)
Turtle
Leopard
Hair
Salamander
Amniotic egg
Tuna
LE
25- Lamprey
11b
Lancelet (outgroup)
Cladogram
Four walking legs
Hinged jaws
Vertebral column
Comparison of Structures
Homology
Analogy
• Results from:
▫ Adaptive radiation
▫ Common ancestor
▫ Similar origin
• Different functions
• Eg. wing of bat, human arm,
dolphin flipper
• Results from:
▫ Convergent evolution
▫ Different ancestors
▫ Different origin
• Similar functions
• Eg. wings of bird, wings of
insect
Remember:
•Adaptive radiation – emergence of many species from common ancestor
•Convergent evolution – unrelated species independently evolve
similarities when adapting to similar environments
Major events during each Era
• Precambrian: microscopic fossils (stromatolites)
▫ Photosynthesis, atmospheric O2
▫ Eukaryotes (endosymbiont theory)
• Paleozoic: Cambrian Explosion
▫ Plants invade land, many animals appear
▫ Permian Extinction (-96% species)
• Mesozoic: “Age of Reptiles”, dinosaur, plants
▫ Formation of Pangaea supercontinent
▫ Cretaceous Extinction – asteroid off Mexico’s coast
• Cenozoic: primates
Note: All end with major extinction & start with
adaptive radiation
Cenozoic
Humans
Land plants
Animals
Clock Analogy
of Earth’s
History
Origin of solar
system and
Earth
1
4
Proterozoic Archaean
Eon
Eon
Multicellular
eukaryotes
Billions of years ago
2
3
Prokaryotes
Single-celled
eukaryotes
Atmospheric
oxygen
Evolution of Plants
• Non-Vascular (liverworts, hornworts, mosses)
Seedless Vascular (ferns)  Seed Vascular
(gymnosperms, angiosperms)
• Mosses: Gametophytes = dominant form
• Ferns: 1st with vascular tissue (xylem, phloem
▫ wet environment (fertilization in water)
▫ Sporophyte = dominant form
• Gymnosperms: “naked” seeds on cones
▫ Conifers
• Angiosperms: flowering plants
Evolution of Animals – Body Plan
Evolution of Animals – Body Cavities
Evolution of Animals – Development
Evolution of Animals
Evolution of Animals
•
•
•
•
Porifera (sponge)
Cnidarian (jellyfish, hydra)
Flatworms (planaria)
Mollusc
▫ Gastropod (snail), bivalve (clams), cephalopod
(octopus)
• Annelid (earthworm)
• Arthropods (insects, crustaceans)
• Echinoderms (“spiny skin” = starfish, sea
urchins)
• Chordates (vertebrates)
Chordate Characteristics
Phylogeny of living chordates