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What Homologies tell us…
• Similarities in structure and
chemistry provide powerful
evidence that all living things
evolved from a common ancestor
• Darwin Concluded:
– Living organisms evolved through
gradual modifications of earlier
forms  descent with modification
What Similarities tell us…
• Two types of evolution can account
for homologous AND analogous
structures
– Convergent evolution
– Divergent evolution
What Similarities tell us…
• Divergent evolution – two species
evolve from a common ancestor
(speciation)
– They share similarities in anatomy,
biochemistry, and embryology due to
common ancestry
– Explains homologous structures
What Similarities tell us…
• Convergent – two species apparently
becoming more similar
– Two species have adapted in similar
ways to similar environmental
conditions
– NOT due to common ancestry
– Explains analogous structures
Convergent Evolution
• Ocotillo from California and allauidi
from Madagascar have evolved
similar mechanisms for protecting
themselves
Convergent Evolution
• Adaptive radiation of anoles has
occurred on the islands of the
Greater Antilles in a convergent
fashion. On each island, different
species of the lizards have adapted
to living in different parts of trees, in
strikingly similar ways.
Convergent Evolution
Convergent Evolution
Diversity of Life
• Fitness:
– Physical traits and behaviors that
enable organisms to survive and
reproduce in their environment arises
from adaptation.
• Adaptation allows species to be
better suited to their environment and
therefore can survive and reproduce.
Evolution on Different Scales
• Microevolution – generation-togeneration change in a population’s
allele frequencies
• Macroevolution – origin of new
taxonomic groups; speciation
4 Driving Forces behind Evol.
1. Mutation
– Any change in the original DNA
– ONLY ultimate source of variation
in a population
2. Gene Flow
– Movement of genes either into or
out of a population
– Migration – Immigration (add
alleles) and Emigration (subtract
alleles)
4 Driving Forces behind Evol.
3. Genetic Drift
– Change in the allele frequency in a
small population by chance alone
•
•
Bottleneck Effect
Founder Effect
4 Driving Forces behind Evol.
3. Genetic Drift
– Bottleneck Effect:
population
undergoes a high
mortality rate;
genetic variation
decreases
dramatically
– Ex: Cheetahs
Genetic Drift: Bottleneck Effect
4 Driving Forces behind Evol.
3. Genetic Drift
– Founder Effect:
few individuals
leave a large
population to
start their own;
gene pool is very
limited
– Ex: polydactyly in
PA Amish
Genetic Drift: Founder Effect
Genetic Drift: Founder Effect
4 Driving Forces behind Evol.
4. Selection
– Natural – differential success in the
reproduction of different phenotypes
resulting from the interaction of
organisms with their environment
•
Nature does the selecting
4 Driving Forces behind Evol.
4. Selection
(Natural)
– Resistance –
overuse of
insecticides
and antibiotics
have bred
resistant
species of
bugs and
germs
4 Driving Forces behind Evol.
4. Selection
– Artificial – breeding of domesticated
plants and animals
• Humans intentionally
do the
selecting
• Cabbage, cauliflower,
Brussels
sprouts, kale,
kohlrabi and
broccoli have
a common
ancestor in one
species of wild
mustard
4 Driving Forces behind Evol.
• Problems with artificial selection –
not enough genetic variation
4 Driving Forces behind Evol.
4. Selection (Sexual)
– Intrasexual selection – selection
within the same sex (competition,
usually between males
–
–
Competition, usually between males
Exaggerated
anatomy
Bighorn Sheep
Rocky
Mountain
Elk
Five-horned
Rhinoceros Beetles
Stagbeetles
4 Driving Forces behind Evol.
4. Selection (Sexual)
– Intersexual selection – one sex
selects mate based on phenotypes
– Exaggerated anatomy
• Selection can influence
populations in three major ways:
– Directional Sel.
– Stabilizing Sel.
– Disruptive (diversifying) Sel.
Directional Selection
• Environment selects
against one
phenotypic extreme,
allowing the other to
become more
prevalent
Disruptive Selection
• Environment selects
against intermediate
phenotype, allowing
both extremes to
become more
prevalent
Stabilizing Selection
• Environment selects
against two extreme
phenotypes, allowing
the intermediates to
become more
prevalent
1.
Key
Points
Natural selection does not cause
genetic changes in individuals.
2. Natural selection acts on individuals;
evolution occurs in populations.
3. Evolution is a change in the allele
frequencies of a population, owing to
unequal success at reproduction
among organisms bearing different
alleles.
4. Evolutionary changes are not “good”
nor “progressive” in any absolute
Evolutionary Theory
• Foundation on which the rest of the
biological science is built.
Collection of carefully reasoned and
tested hypotheses about how
evolutionary change occurs.
Speciation
• What is a species?
–Biological definition: a group of
closely related organisms
(population) that can interbreed to
produce fertile, viable offspring
Speciation
• Why can’t/don’t populations
interbreed?
–Prezygotic barriers
–Postzygotic barriers
Prezygotic Barriers
• Ecological (habitat) isolation – pops
live in different habitats and do not
meet
– Parasites generally don’t transfer
hosts
• Temporal isolation – active or fertile
at different times
– Flowering plants pollinate on
different days or different times of
the day
Prezygotic Barriers
• Behavioral isolation – differences in
activities
– Mating calls or actions are different
Prezygotic Barriers
• Mechanical isolation – mating
organs do not fit or match
– Enough said
• Gametic isolation – gametes cannot
combine
– Sperm destroyed in “different” vaginal
cavity
– Sperm and egg don’t fuse due to
different membrane proteins
Postzygotic Barriers
• Hybrid inviability – hybrid zygotes
fail to develop or reach sexual
maturity
• Hybrid infertility – hybrids fail to
produce functional gametes
Summary
• 2 or more mechanisms
may occur at once
• Ex: Bufo americanus and
Bufo fowleri are
ecologically, temporally,
and behaviorally isolated
• Bufo americanus breeds in
early spring in small,
shallow puddles or nearby
dry creeks
• Bufo fowleri breeds in late
spring in large pools and
streams
Limitations of Biological Species
Concept
• How do you classify organisms that:
– have the potential to interbreed, but
do not do so in nature?
– do not reproduce sexually?
– exist only as fossils?
• Alternative species concepts
(ecological, pluralistic,
morphological, genealogical) help
address limitations
Modes of Speciation
• Allopatric (Greek, allos = other;
Latin, patria = homeland)
• Speciation due to geographic
separation
– Barrier stops gene flow
between populations
– Evolutionary change acts
independently on each pop to
establish reproductive barriers
• Mitochondrial DNA
analysis has shown
that certain tamarin
monkey pops (those
separated by wide
rivers) are diverging
toward speciation
Where the Amazon is very wide, tamarins on
one side are brown, but on the other side
are white. Where the Amazon is narrow,
tamarins of both colors are found on either
side
Allopatric Speciation
• Birds can move freely across the
gorge of the Grand Canyon;
squirrels cannot
A. leucurus
A. harrisi
• Two species arose when their
original pop was disrupted by the
A. harrisi
A. leucurus
Allopatric Speciation
• If not given enough time, speciation
will not occur
• Also, even
if they do
come
back
together,
they need
to interbreed to be the same species
Allopatric Speciation
• Figure 24.11
• Adaptive
Radiation:
evolution of many
diversely-adapted
species from a
common ancestor
• Ex: Hawaiian
archipelago
Sympatric Speciation
• Sympatric (Greek, sym = together;
Latin, patria = homeland)
• Speciation occurs in populations
that share a habitat
• Results from:
– Ecological isolation
– Polyploidy (number of sets of
chromosomes increases)
Sympatric Speciation
• Polyploidy (number of sets of
chromosomes increases)
• A result of accidents in meiosis
Will Speciation Occur?
• p+q=1
• p2 + 2pq + q2 = 1
• Will speciation occur? You tell me!
• Hardy-Weinberg PPT 1
• Hardy-Weinberg PPT 2
Evolutionary Time Scales
• Evolution can take a long time or can
occur relatively quickly
– Gradualism
– Punctuated Equilibrium
Evolutionary Time Scales
• Gradualism –
big evolutionary
changes are the
result of many
small ones over
a long period of
time
Evolutionary Time Scales
• Punctuated
Equilibrium –
speciation
occurs fairly
rapidly then
remain constant
Evolutionary Novelties
• Unique and highly specialized
organs seem to complicated to have
been naturally selected
• Ex: eyes are really just
photoreceptors; some are more
developed, but all do the basic
function: receive light
Evolutionary Novelties
Evo-devo
• Evolutionary development
• A field of interdisciplinary research
that examines how slight genetic
divergences can become magnified
into major morphological differences
between species
Evo-devo
• By blocking expression of one gene,
researchers forced a chicken’s foot
to develop to resemble a duck’s foot
• Two embryos from the same animal
Evo-devo
•
•
•
•
Left, a normal
chicken leg will
develop
Right, a normal duck leg will
develop… from a chicken embryo
Chicken leg: scaled with 4 digits
Duck leg: smooth and webbed
Duck legs, due to one genetic
evolutionary difference, help ducks
do many things chickens cannot,