Transcript Evolution

Modern Biology II
Who Are You?
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Your major
Your year, part time or full time
Your plans
Your objectives
Your interests
Evolution
• Please read Chapter 20
(Genes Within
Populations) if you have
not done so already
• Know who Charles
Darwin is!
Evolution
• Evolution is descent with modification; living
species are descendents of ancestral species
that were different from present-day ones
• Evolution describes the genetic changes in a
population over time
Evolution
• Organisms are not perfectly fit – a good fit,
but not perfect
• As descendents of a remote ancestor spread
into various habitats over millions and
millions of years, they accumulated diverse
modifications (adaptations) that fit them to
specific ways of life; descent with
modification  evolution
Evolution
• Natural variation among individuals is based
on heredity (and mutation). These variations
enable organisms to become adapted to their
environment over time
Natural Selection
• Natural selection is the process by which
favorable, inherited traits become more
numerous in successive generations of a
population of reproducing organisms
Natural Selection
• Over time, natural selection leads to species
that are well adapted (highly evolved) to their
environments
The Principles of Natural Selection
• Struggle for existence/Competition
– More offspring are produced than can be supported
by resources
King Penguin Rookery © Momatiuk - Eastcott/Corbis
The Principles of Natural Selection
• Variation
– Some individuals, due to heredity or mutation,
possess characteristics which make them better
adapted to their environment
The Principles of Natural Selection
• Inheritance of Traits
– Best-suited organisms will survive to produce
more individuals that share same adaptation
Survival of the Fittest
• Organisms are adapted to their environment
through natural selection
• Natural selection is a pessimistic process
1Population with varied inherited traits
2Elimination of individuals with certain traits
Reproduction of survivors
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Certain
individuals with
a distinct,
inherited
characteristic
will be selected
against, while
others with a
(different)
distinct,
inherited trait
will survive
Evidence for Evolution
• Microevolution – can be observed in nature;
small changes in organism, generations
changing over time
• Artificial selection – evolution can be
experimentally produced
Microevolution
• A well-known example of microevolution
involves the peppered Moth, Biston betularia
in England during the industrial revolution
• Prior to the industrial revolution, light variants
of the peppered moth survived better than
dark variants because they blended well with
the light colored trees
– caused by the presence of a
light-colored lichen on the darkcolored bark
Microevolution
• Pre-industrial era – only light variant known
• During the industrial revolution, poor air
quality killed the lichens which covered the
(otherwise dark) trees and camouflaged the
light moths against predation
• In 1848, the first dark variant collected
• By mid-1900’s, the dark variant made up 90%
of population in industrial areas!
Microevolution
Artificial Selection
• Darwin got idea of natural selection by
artificial selection!
• Modern corn looks very different from its
ancestor
• Tumbler pigeons!!!
www.flickr.com/photos/rinalia/
3285371111/
www.flickr.com/photos/
terryandchristine/
2399227035/
Artificial Selection
• All dogs are domesticated breeds of the Gray
wolf, Canis lupus; “Fido” is actually a
subspecies of the wolf!
Artificial Selection
Evidence for Natural Selection
• Darwin’s fishes of the Galapagos
14 species of finch; 1
common ancestor
(from the mainland);
different beaks
Evidence for Evolution
• Homologous similarities: similarities between 2
species that is NOT functionally necessary
• Provide clues to common ancestry
• Constraint is not there, may look the same, but
doesn’t have to
• Example: Tetrapods; do not need 5 digits to
make flying wings, swimming structure, etc.
• Common pentadactyl ancestor; limb adapted
into various ‘means’
Evidence for Evolution
• Homologous structures – similar characteristics
that result from a common ancestry
Humerus
Radius
Ulna
Carpals
Metacarpals
Phalanges
Human
Cat
Whale
Bat
The Panda’s Thumb
• The panda’s thumb is a homologous trait;
modification of the wrist bone, not
anatomically a finger (or thumb) at all
• Constructed from the radial sesamoid,
normally a small component of the wrist
• May have originated from a single genetic
change (mutation)
The Panda’s Thumb
Homologies
• Fossil evidence of
evolution: Whale
‘missing link”
• Vestigial structures –
no apparent function,
but resemble structures
ancestors possessed
Homologies
Vestigial structures of
a whale
http://www.edwardtbabinski.us/mpm/mpm_whale_limb.html
Evidence for Evolution
Evolutionary View of Homology
• Use the starting materials and processes
already available
• Fashion adaptations, rather than starting from
scratch
• Evolution is a tinkerer, not a master engineer
Evolutionary View of Homology
• Adaptations are NOT perfect – it is the
imperfection of adaptation that, instead, gives
evidence for evolution (a tinkerer uses tools
already there to improve)
• Different homologies are correlated – similar
patterns between human, ape, and monkey
for many proteins
• Similarities stem from common ancestor
Anatomical Evidence of Evolution
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Orchid petals - used as pollinator lure
Snake with 2 leg bones
Manatee fingernails
Humans – muscles for wiggling ears
Anatomical Evidence of Evolution
• Developmental similarities
reflect descent from a
common ancestor
• “Ontogeny recapitulates
phylogeny”!
• (Ontogeny = growth and
development of an
organism; Phylogeny =
evolutionary history of a
species)
Evidence for Evolution
• In contrast to homologous structures,
analogous structures also provide evidence
for evolution
• Analogous structures are structures that share
similarities by a way of life, not by a common
ancestry
• Analogous structures arise among unrelated
organisms in response to similar needs or
similar environmental factors
Analogous structures
Examples: wings
of insects (a) and
birds (b); flippers
of seals (c) and
penguins (d)
Origin of New Species
• Individuals do NOT evolve
• Populations are the smallest units that can
evolve
• A population is a group of interbreeding
individuals belonging to a particular species
sharing a common geographic area
Origin of New Species
• What is a species?
• Biological species concept: a population or
group whose members have the potential to
interbreed with one another in nature to
produce viable, fertile offspring, but who
cannot interbreed with other such groups
• Species are based on ability to interbreed –
NOT on physical similarities
Origin of New Species
• Example: Eastern and Western Meadowlarks – 2
different species with similar shape and
coloration, but differences in song help prevent
interbreeding
• Barriers to breeding can be behavioral
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Origin of New Species
• In contrast,
humans have
considerable
diversity, but we
all belong to the
same species
because of our
ability to
interbreed
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Speciation happens
• It takes a reproductive barrier to keep
individuals of closely-related species from
interbreeding
• Reproductive barriers may be behavioral,
geographical, anatomical, or temporal
Allopatric Speciation
• How do reproductive barriers arise?
• One of the clearest forms of species is
allopatric speciation caused by a geographic
barrier
• When a geographic barrier occurs, the
isolated populations each become adapted to
their own environment, such that over time,
they may no longer interbreed, even if they
were to come into contact with one another
again
Speciation
• Reasons for geographic
(allopatric) isolation
• Example of allopatric isolation:
antelope squirrels on south and
north rim of Grand Canyon
Speciation
Speciation
• Prezygotic vs. Postzygotic
• Prezygotic – mechanisms preventing
formation of a zygote (ecological, behavioral,
temporal, mechanical)
• Postzygotic – mechanisms preventing
organisms from developing into a reproducing
adult
Prezygotic examples
• Ecological – Lions and
Tigers
• Behavioral – bluefooted boobies
• Temporal – Wild
lettuce (different
blooming periods)
• Mechanical - insects
Postzygotic example
• Hybrid inviability or infertility – mule
• Mules are the reproductive result of a horse
and a donkey breeding
• Mules are sterile
• Therefore, a horse and
a
donkey must represent
2
distinct species
Sympatric Speciation
• Sympatric speciation – the process by which
new species arise within the range of another
species
• More controversial
• In this case, a new species does not arise from
geographic isolation
• Instead, a new species may arise by accident
when errors during cell division resulted in
organisms with extra sets of chromosomes
(very common in plants)
Sympatric Speciation
• New species formed by having extra sets of
chromosomes are considered to be polyploid
• Polyploid organisms have more than 2
complete sets of chromosomes
• For example, a polyploid containing 4 sets of
chromosomes will produce diploid (2n)
gametes! This species would be unable to
mate with normal diploid species (which
produce haploid gametes)
Error in
cell
division
Polyploid
cells undergo
meiosis
Selffertilization
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2
1
Parent species
Diploid
Polyploid
(“tetraploid”)
Viable, fertile
tetraploid
species
Diploid
gametes
Polyploidy by error in cell division and selffertilization
Polyploidy by errors in cell division of a ‘sterile’ hybrid
Chromosomes not
homologous
(cannot pair)
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Species A
2n = 4
2
Gamete
n=2
3
Sterile hybrid
n=5
Species B
2n = 6
Gamete
n=3
Viable, fertile
hybrid species
2n = 10
3. However, ‘sterile’ hybrid
can reproduce asexually (as
many plants do), and if subsequent
errors in cell division occur,
chromosome duplications can result
in a fertile polyploid species!
Polyploid speciation
• Remember, polyploidy is a type of sympatric
speciation
• As many as 80% of all living plants today are
believed to have arisen by polyploidy!
• A polyploid contains twice (or sometimes
more) the genetic diversity as its diploid
predecessors, which provides an adaptive
advantage!
Polyploid speciation
• Many of the plants grown for food are
polyploids
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Oats and Barley
Potatoes
Bananas
Peanuts
Plums and Apples
Wheat
Coffee!
Speciation
Macroevolution
• Origin of taxonomic groups higher than the
species level
• Evolutionary change substantial enough to
view its products as new genera, families or
phyla
• Has a random component
• Considers major evolutionary innovations –
bird feathers, insect wings
• Extinctions and Radiations
Macroevolution
Adaptive Radiation
• The evolution of many diverse species from a
common ancestor is called adaptive radiation
• The adaptations of these species allow them
to fill new habitats or roles in their
communities (“niches”)
• New phenotypes arise in response to the
environment, driven by natural selection
Adaptive Radiation
• Example: Development of a fourth cusp in
mammalian tooth – increases range of food
which can be utilized
• Adaptive radiation typically occurs when a few
organisms colonize new, unexploited habitats,
or when environmental changes open up new
opportunities for the survivors
Adaptive Evolution
• The Galapagos Islands is one of the world’s
greatest showcases of adaptive radiation
• Each island arose ‘naked’ from underwater
volcanoes and were gradually clothed by
plants, animals and micro-organisms which
strayed from the South American mainland
Darwin’s finches are a prime
example of adaptive evolution
Rise of the mammals
• The extinction of the dinosaurs provided a
tremendous evolutionary opportunity to
mammals, who once lived in their shadows
Macroevolution
• Mass extinctions
Macroevolution
• Fossil record provides clues for the outline of
macroevolution
• Extant (living) species also supply clues
• What about soft-bodied organisms which do
not leave a fossil record?
• Does evolution occur in ‘fits’ and ‘starts’ or is
our understanding of historical accounts
skewed?
Suggested Readings
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The Selfish Gene, Richard Dawkins
The Blind Watchmaker, Richard Dawkins
Unweaving the Rainbow, Richard Dawkins
The Panda’s Thumb (and any other collection
of stories by), Stephen J. Gould