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Chapter 21
The Evidence for
Evolution
1
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Beaks of Darwin’s Finches
• Darwin collected 31 specimens
from 3 islands in the Galápagos
Islands
– Darwin was not an expert on birds
– Took them back to England for
identification
– Told his collection was a closely
related group of distinct species
– All were similar except for beak
characteristics
– In all, 14 species now recognized
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http://people.rit.edu/rhrsbi/GalapagosPages/Pictures/LandBirds/FinchTypes.jpeg
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Woodpecker finch (Cactospiza pallida)
Large ground finch (Geospiza magnirostris)
Cactus finch (Geospiza scandens)
Note the beak
shape and how
it relates to
feeding style
Warbler finch (Certhidea olivacea)
Vegetarian tree finch (Platyspiza crassirostris)
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• Finches eat a diversity of foods
– Darwin hypothesized that different beak
shapes were related to food gathering
– Darwin wrote “… one might really fancy that …
one [original] species has been taken and
modified for different ends.”
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http://www.pbs.org/wgbh/evolution/library/01/6/image_pop/l_016_02.html
Modern research has verified
Darwin’s selection hypothesis
• 3 conditions necessary for natural selection
– Phenotypic variation must exist in the
population
– This variation must lead to differences among
individuals in differential lifetime
reproductive success
– Phenotypic variation among individuals must
be genetically transmissible (heritability) to
the next generation
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http://evolution.berkeley.edu/evosite/evo101/images/mechan_intro.gif
http://www.princeton.edu/main/images/news/2009/06/20090619_GrantP_RM_008-thumb.jpg
Peter and Rosemary Grant
• Studied medium ground finch on
Daphne Major
• Found beak depth variation among
members of the population
• Average beak depth changed from one
year to the next in a predictable fashion
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http://californiasislands.files.wordpress.com/2011/04/daphnemajor.jpg
Peppered moths
• When the environment changes, natural
selection often favors different traits in a species
• Adult peppered moth, Biston betularia, come in
a range of shades
– Body color is a single gene
– Black individuals have the dominant allele
• Black color was rare in the population until 1850s
• From that time on, frequency increased to near 100%
• WHY?
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http://www.cals.ncsu.edu/course/ent425/tutorial/Ecology/survival.html
• J.W. Tutt hypothesized that light-colored
moths declined because of predation
– Light moths were easily seen by birds on darkened
(sooty) trees
– Soot came from factories starting with Industrial
Revolution
– Confirmed with separate field studies with a variety
of experimental designs
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“Industrial” melanism
• Phenomenon in which darker individuals come
to predominate over lighter ones
– Other moths in other industrialized areas showed
same trend to darken
– Pollution control resulted in bark color being lighter
again
– Light-colored peppered moths now are dominant in
the population
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Artificial Selection
• Change initiated by humans
• Favored individuals with certain phenotypic
traits are bred, passing their genes on to the
next generation
• This directional selection should result in
evolutionary change
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https://sites.google.com/site/selectivebreedingofplants/
http://en.wikipedia.org/wiki/Drosophila_melanogaster
• Experimental selection on
Drosophila melanogaster (fruit
fly)
– Selected fruit flies with many
bristles on abdomen
– At the start, average number of
bristles was 9.5
– Chose only those with most
bristles to reproduce
– 86 generations later, average
number of bristles had
quadrupled to nearly 40
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http://bio.illinoisstate.edu/kaedwar/other/Sexing_Drosophila.shtml
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http://leahlefler.hubpages.com/hub/How-Animals-Are-Domesticated-Domesticated-Foxes-Demonstrate-Genetic-Changes
• Domestication may lead to unintentional selection
for some traits
– Attempt to domesticate silver foxes
– Chose most docile animals only to breed
– Within 40 years, had many of the same behavioral &
physical traits as domestic dogs (curled tails, floppy ears,
color)
– Are traits for behavior linked to other traits?
• Pleiotropy or linkage at work?
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http://heatherlindayoung.wordpress.com/2011/01/19/the-one-with-the-waggley-tail/
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Teosinte
Intermediates
Modern
corn
• Agricultural selection
– Differences have resulted from generations
of human selection for desirable traits,
such as greater milk production and larger
corn ear size
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Can selection produce major
evolutionary changes?
• Most scientists think that natural selection is
the process responsible for most evolutionary
changes documented in the fossil record
• Some critics accept that selection can lead to
changes within a species, but not the
substantial changes documented in the fossil
record
– This argument does not fully appreciate the extent
of change produced by artificial selection
– Separate dog breeds would be species if found as
fossils
– All came from domestication of wolves
16
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• Domestication
– Human-imposed
selection has
produced a variety
of cats, dogs,
pigeons, grains
and others
– Breeds may have
been developed for
specific purposes
• Dachshunds for
badger pursuit
Chihuahua
Dachshund
Wolf
Greyhound
Mastiff
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Canid Domestication
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http://www.csus.edu/indiv/l/loom/lect%2036s07.htm
Fossil Evidence of Evolution
• Fossils are the preserved remains of
once-living organisms
– The age of fossils can be estimated by
comparing strata (relative dating) and by
isotopic dating (determines absolute age)
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http://pubs.usgs.gov/gip/fossils/numeric.html
http://www.agiweb.org/news/evolution/figures/dating1.jpg
Fossil Evidence of Evolution
• Process of fossilization is rare event
• Rock fossils are created when three events occur
– Organism buried in sediment
– Calcium in bone or other hard tissue mineralizes
– Surrounding sediment hardens to form rock
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http://jpostema.napsk12.org/blob/full/150186.gif
Evolutionary transitions
• Given the low likelihood of fossil
preservation and recovery, it is not
surprising that there are gaps in the
fossil record
• Many intermediate forms have
been found
• Oldest known bird fossil is
Archaeopteryx
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– Clearly intermediate between bird and
dinosaur
– Possesses some ancestral traits and
some traits of present day birds
http://universe-review.ca/I10-72-Archaeopteryx2.jpg
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Archaeopteryx
http://www.healthstones.com/dinosaurdata/a/archaeopteryx/archaeopteryx.html
• Recent “transitional”
discoveries
– Four-legged aquatic
mammal
• Important link in the
evolution of whales
and dolphins from
land-dwelling, hoofed
ancestors
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http://biologos.org/blog/evidences-for-evolution-part-2b-the-whales-tale
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Modern toothed whales
Rodhocetus kasrani's
reduced hind limbs
could not have aided it
in walking or swimming.
Rodhocetus swam with
an up-and-down motion,
as do modern whales.
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Ambulocetus natans
probably walked on land
(as do modern sea lions)
and swam by flexing
its backbone and paddling
with its hind limbs
(as do modern otters).
Pakicetus attocki lived on land,
but its skull differed from that of
its ancestors and exhibited
many characteristics seen in
whales today.
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http://news.sciencemag.org/sciencenow/2011/02/scienceshot-snake-legsgoing-going.html
• Recent “transitional” discoveries
– Fossil snake with legs
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• Recent “transitional” discoveries
– Tiktaalik: a species that bridged the gap between
fish and the first amphibian
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http://scienceblogs.com/pharyngula/2006/04/05/tiktaalik-makes-another-gap/
• Horse evolution
– Modern Equidae are all large, long-legged, fastrunning animals adapted to life on open
grasslands
– First horse was small with short legs
• Wooded habitats
– Path to modern horse involved…
• Changes in body size
• Toe reduction
• Changes in tooth size and shape
– Adaptations to climate change
Early, overly
• As grasslands became more widespread simplistic
view
•  changed from browsers to grazers
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Modified from http://www.tutorvista.com/content/biology/biology-ii/heredity-and-evolution/palaeontology.php
Oligocene
30 MYA
45 MYA
50 MYA
55 MYA
60 MYA
Hyracotherium
(browsers)
Orohippus
Eocene
Hyracotherium
40 MYA
Epihippus
35 MYA
Mesohippus
(browsers)
Mesohippus
Anchitherium
(browsers)
Merychippus
(mixed feeders)
Neohipparion
(grazers)
Nannippus
(grazers)
– Rates of evolution have varied widely
– Modern horse diversity is relatively limited
– At peak of horse diversity there were 13 genera in
North America alone
Equus
Dinohippus
Onohippidion
Astrohippus
Pliohippus
Calippus
Protohippus
Cormohipparion
Nannippus
Merychippus
Parahippus
Desmatippus
Archaeohippus
25 MYA
Miohippus
20 MYA
Kalobatippus
15 MYA
Hypohippus
Miocene
Anchitherium
10 MYA
Megahippus
5 MYA
Pseudhipparion
browsers
grazers
mixed feeders
Pliocene
Hipparion
Pleistocene
Neohipparion
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Equus
(grazers)
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Anatomical Evidence for Evolution
• Homologous structures
– Structures with different
appearances and functions
that are all derived from the
same body part in a common
ancestor
– The bones in the forelimb of
mammals are homologous
structures
– Different functions, same
ancestor structure
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• Early embryonic
development
– Strongest anatomical
evidence supporting
evolution comes from
comparisons of how
organisms develop
– Embryos of different types
of vertebrates, for example,
often are similar early on,
but become more different
as they develop
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http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/R/Recapitulation.html
• Early embryonic development
– Ex: early vertebrate embryos possess
pharyngeal pouches that develop into
• In humans: glands and ducts
• In fish: gill slits
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Color-coded pharyngeal arches
and pouches indicate origin of
same structure in fetus
http://www.answersingenesis.org/articles/2007/03/14/fishy-gill-slits
• Imperfections – some organisms do not
appear perfectly adapted
– Workable but imperfect solutions
– Natural selection does not produce
perfectly adapted structures
• Selection acts on what is available
• Produces “what works”
• Environment constantly changing
(“moving the bar”)
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– Ex: most animals with long
necks have many vertebrae
for flexibility
• Geese: 25
• Plesiosaurs: 76
• Mammals: 7
– But giraffes have 7 vertebrae,
very large in size, to make up
for the length of the neck
Giraffe cervical
vertebrae (neck bone)
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http://www.boneclones.com/
http://fossil-chick.blogspot.com/2007_11_01_archive.html
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Blind
spot
Vertebrate eye
Photoreceptor cells Interneuron
Molluscan eye
Photopigment
Nerve fibers
Photoreceptor cells
Light
Light
Photopigment
Nerve impulse
a.
To brain via
optic nerve
Nerve fibers
to brain
b.
– Example: eyes of vertebrates
• Photoreceptors face backward
– Nerve fibers slightly obscure light and create a blind
spot
– Mollusks’ eyes are more optimally designed with no
obstruction or blind spot
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• Vestigial structures
– Have no apparent function, but resemble
structures their ancestors possessed
• Human ear wiggling muscles, tail bone (coccyx)
• Hip bones in boa constrictors
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http://science.halleyhosting.com/sci/ibbio/ecology/notes/natselect/evidence.htm
http://explosionsoflife.tumblr.com/post/22639983292/ichthyologist
-antarctic-ice-fish
http://jeb.biologists.org/content/209/10/1791/F1.expansion.html
Antarctic fish lacks
RBCs and a functional
hemoglobin gene, but
still has remnants of
mutated gene. Blood
from typical fish with
hemoglobin (right) and
Antarctic fish (left)
• Vestigial structures
– Genetic relicts  fossil genes (pseudogenes)
• Nonfunctional genes are retained, but mutated
• Example is hemoglobin gene in Antarctic fishes
– Oxygen content is so high in cold waters that they
don’t need hemoglobin to carry oxygen, blood is
colorless
– However, mutated hemoglobin gene is still found in
their DNA
http://science.halleyhosting.com/sci/ibbio/ecology/notes/natselect/evidence.htm
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Biogeography
• Study of the geographic distribution of species
• Reveals that different geographical areas sometimes
exhibit groups of plants and animals of strikingly
similar appearance, even though the organisms may
be only distantly related
• Natural selection appears to have favored parallel
evolutionary adaptations in similar environments
Note the similar forms
of unrelated ant-eating
mammals, an example
of convergence of form
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http://www.pbs.org/wgbh/evolution/library/01/4/image_pop/l_014_01.html
Convergent evolution
• Similar forms having evolved
in different, isolated areas
because of similar selective
pressures in similar
environments
• Marsupial and placental
mammals
– Only marsupials found in
Australia
– Australian marsupials resemble
placental mammals on other
continents
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http://carolguze.com/text/102-15-evolution1.shtml
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Niche
Placental
Mammals
Burrower
Anteater
Mole
Nocturnal
Insectivore
Climber
Glider
Stalking
Predator
Chasing
Predator
Flying squirrel
Grasshopper
mouse
Wolf
Australian
Marsupials
Ocelot
Ring-tailed lemur
Lesser anteater
Numbat
Thylacine
Spotted cuscus
Marsupial mole
Marsupial
mouse
Flying phalanger
Tasmanian
quoll
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• Convergence among fast-moving
marine predators
– Hydrodynamics of moving through water
require a streamlined body shape to
minimize friction
– Sharks, tuna, icthyosaurs, and dolphins
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Biogeographical studies
• Darwin noted on his voyage that
– Islands are often missing plants &
animals common on continents
• Can live there if introduced
– Species present on islands often
diverged from continental relatives
• Occupy niches used by other species
on continents
• Island species usually are more closely
related to species on nearby continents
http://squamates.blogspot.com/2011/01/attempt-to-reduceinvasive-predators-in.html
Adult Key deer
in the Florida
Keys are smaller
than whitetail
deer
43
• Darwin concluded:
– Many islands have never been
connected to the mainland
– Species arrive on islands by
dispersing across the water
– Dispersal from nearby areas is
more likely than distant sources
– Species that can fly, float or
swim are more likely to inhabit
islands
– Colonizers often evolve into
many species
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http://www.guardian.co.uk/science/punctuated-equilibrium/2011/nov/02/hawaiian-honeycreepers-tangled-evolutionary-tree
Darwin’s critics
• Evolution and natural selection are nearly
universally accepted by biologists
• Source of controversy for some in the general
public
• Seven principle objections
1. Evolution is not solidly demonstrated
• “Just a theory”
• In science, theories have a lot of supporting evidence
• Theories of gravity, relativity, plate tectonics, evolution
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Darwin’s critics
2.
There are no fossil intermediates
• Actually many intermediates have been found since Darwin’s
time
• See examples above &
http://www.talkorigins.org/indexcc/CC/CC200.html
3.
The intelligent design argument
• Too complex for a random process
• Natural selection is not random but it is not directed
• Vestigial structures refute an “intelligent designer”
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Darwin’s critics
4.
Evolution violates Second Law of Thermodynamics
•
Things become more disorganized
•
Earth is not a closed system and energy is constantly added from the
Sun
•
Biology does have organization
5.
Proteins are too improbable
•
Probability of hemoglobin as random event = (1/20)141
•
Can’t argue backwards – what are the odds of students having the
birthdates they do in class
–
Statistics might predict the likelihood of having another class
with the same birthdays (looking forward)
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Darwin’s critics
6. Natural selection does not account for major
evolutionary changes
•
•
•
•
No scientist has evolved a fish into a frog
Artificial selection has produced differences more
distinctive than those between wild species
Fossil transitions, over millions of years, is strong
evidence that small changes accumulate
Evolutionary development (“evodevo”) studies
show changes to a few regulatory genes can
have major impact on body form
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Darwin’s critics
7. The irreducible complexity argument
• Intricate machinery of cell cannot be explained by
evolution from simpler stages
• Natural selection can act on a complex system
because at every stage of its evolution, the system
functions
• Parts are added to enhance function, some may
be lost in the process…but still a product of
selection over long periods of time
• With careful study, the pathway of complex
structures may be found
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