natural selection and evolution - west virginia...
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Natural Selection and
Evolution
Geology 230
Fossils and Evolution
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The Study of Evolution
• Evidence from living and fossil organisms
• Biology – study of living organisms
– Genetics: Genotype
– Comparative Anatomy: Phenotype
• Paleontology or Paleobiology – study of
fossilized organisms: Phenotype
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Natural Selection
• The theory of natural selection was
proposed by Charles Darwin in 1859,
in his book: The Origin of Species by
Means of Natural Selection.
• Alfred Russell Wallace independently
developed the same theory as Darwin
in the 1850s.
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One of the most
famous and
influential books of
science.
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Darwin drew an analogy between artificial
selection and natural selection. Here we see
dog breeds produced by artificial selection.
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Natural Selection: Key Points
• 3 facts lead to a conclusion:
• Fact 1. Overproduction of offspring
• Fact 2. Variation within populations
• Fact 3. Competition for resources
• Conclusion: Survival of the fittest
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Sources of Variation
• Random mutations of DNA in the sex cells (Zimmer, p.
105)
– >>99.9% are harmful or neutral
– adaptive mutations are rare
• Sexual recombination
– new variants created by mixing of genes (alleles)
– e.g., you have 4096 direct ancestors going back just
12 generations (212)
• Genetic Drift – random allele changes that happen in
small populations (Zimmer, p. 112-116)
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Variations within Natural
Populations are the Raw
Materials for Natural
Selection, including Sexual
Selection.
Do you think
I’m sexy?
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One
species of
tree snail.
Why so
many
colors?
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Chromosomes contain Genes
composed of DNA
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The Anatomy of
a Gene. Helical
strands of DNA
contain the
blueprints for all
forms of life.
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The DNA molecule
contains
nitrogenous bases
(A,T,C,G) that
code for different
amino acids, which
form proteins.
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Selection
• Genotypes and phenotypes can be ranked
by fitness.
• Negative Selection – a given allele reduces
reproductive success.
• Positive Selection – a given allele increases
reproductive success.
• Stabilizing Selection – extremes are
eliminated. Human baby size is an example.
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Fitness
• The fitness of a population or species is the
product of the heritability of traits times the
strength of selection.
• If a trait’s heritability is high, even weak
selection can lead to evolutionary change.
• Species will climb peaks on adaptive
landscapes.
• But adaptive landscapes change over time.
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Adaptive
Landscapes
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Evidence of Evolution
• Homology, or homologous features:
Evidence of common ancestry.
• Analogous organs/features, or Convergent
Evolution: Independent evolution of adaptive
features.
• Vestigial organs: Evidence for evolutionary
change.
• Adaptive radiation: New adaptations lead to
rapid speciation.
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Homology
• The recognition of common ancestry
of features. e.g. all primates have 5
fingers; apes and humans lack a tail; all
tetrapods have similar limb bones.
• Studied by comparative anatomy of
living and fossil organisms.
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Homologous
features are used
to recognize
common
evolutionary
ancestry.
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Hominoids
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ChimpHuman
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ChimpGorilla
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Homologous
features are used
to recognize
common
evolutionary
ancestry.
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Examples of homologous features in the
forelimbs of tetrapods.
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Homologous
development
between
different
species
indicates
common
ancestry.
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Universal Tree of Life – mapped with homolgies
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Analogous Organs or
Convergent Evolution
• Produced by evolutionary convergence.
Independent origin of similar features is
called convergent evolution.
• Shows natural selection in operation.
• e.g., insects, birds, bats, and pterosaurs
all evolved wings independently
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Examples of
analogous features
indicating
convergent evolution
and not common
ancestry.
Pterosaur
Bat
Bird
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Other examples of convergent evolution:
armadillo and pangolin; monotreme and
placental anteaters; thylacines and canines
Tasmanian
“Wolf”
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Vestigial Organs
• Organs no longer used, but show clear
evidence of evolutionary change.
• e.g., pelvic bones of modern whales,
vestiges of former legs
• e.g., human body hair, a vestige of
former fur
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vestigial
whale
hips
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Goose Bumps
Vestigial Organs in
Humans. Mostly
useless now.
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Adaptive Radiation
• Natural selection can fill a variety of
niches starting with a single species.
• e.g., Darwin’s finches in the Galapagos
Islands, 13 species evolved from one
ancestral species
• e.g., all birds evolved from Archaeopteryx
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The beaks of Darwin’s finches are adapted to the
variety of food types available on the different islands
in the Galapagos Islands.
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Type specimens of finches collected by
Darwin in the Galapagos Islands
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Archaeopteryx,
the first bird,
Jurassic age.
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9700 species of
living birds
Adaptive Radiation
from Archaeopteryx
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The Origin of New Species:
Speciation
• Natural selection by itself will not increase
the number of species on the planet. How
does this happen?
• Biological species definition: Interbreeding
populations reproductively isolated from
other such populations.
• New species must be reproductively isolated
from their ancestral species to be distinct.
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Lions and tigers
(Panthera) share
a close common
ancestor (within
last million years)
as shown by
ability to
hybridize. But the
hybrids are
usually sterile,
indicating distinct
species.
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Liger
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Horses and
donkeys (Equus)
share a common
ancestor. Their
hybrid, the mule, is
sterile.
Embryo
Equus caballusEquus asinus
62 chromosomes
64 chromosomes
63 chromosomes
31 + 32 = 63
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Zonkey – Zebra and
donkey hybridZebroid or Zorse –
Zebra and horse
hybrid
Messing around with species.
Where are the boundaries?
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Speciation, cont’d
• Allopatric speciation: produced by
geographic isolation of populations.
• Natural selection causes an isolated
population to adapt to its local
environment.
• Given enough time, and no outside
interbreeding, a new species will evolve.
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Rates of Evolution
• Phyletic gradualism - continuous and
gradual change over time
• Punctuated Equilibrium - long periods
of stasis punctuated by rapid change,
probably associated with a bottleneck
in population size.
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Geographic Separation and Isolation
Stasis Stasis
Punctuated
Change
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Example of Stasis
• Medium ground finch (Geospiza fortis) in
the Galapagos Islands (Zimmer, p. 123-125).
• During drought, large beaks adaptive for
opening hard, woody seeds.
• During wet times (El Niño), small beaks
adaptive for abundant small seeds.
• Alternating wet-dry seasons produce no net
change in beak size.
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Rates of Evolution
• Large populations evolve very slowly
or almost not at all. Advantageous
mutations pass very slowly through a
population.
• Small populations can evolve very
rapidly. Advantageous mutations can
be passed very quickly through the
population.
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Natural Selection in Action
• See the example of evolutionary change in
lizards on islands in the Adriatic Sea, p.
103-104 in Zimmer. A new variety, if not a
new species, evolved in just 33 years.
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Evolution in Action
• Passing through a bottleneck:
– Pesticide resistant insects
– Antibiotic resistant bacteria
– New Drug-Resistant Superbugs
Found in 3 States 9-13-2010
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An extreme selection event
severely reduces population
size and changes the
composition of the gene pool
of the species.
Bottleneck Effect
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Microevolution vs
Macroevolution
• Small changes, such as seen in modern
bacteria or beaks in finches, are examples of
microevolution. Microevolution leads to
new species over time.
• Large changes, such as the evolution of
major features, like wings in birds, or legs
in fish, are examples of macroevolution.
Macroevolution leads to significant
evolutionary change.
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Macroevolution
• How did eyes evolve? See this video:
• http://www.pbs.org/wgbh/evolution/library/01/1/l_011_01.html
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Role of the Fossil Record
• Provides the only actual record of
evolutionary change over geologic time.
• Provides data on the timing of evolutionary
origins.
• Shows mass extinctions to be a major cause
of evolutionary change, although strictly
speaking, it is not an evolutionary process.
• Provides information on rates of evolution.
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Evolutionary Origins
• Oldest life: 3.5 BY
• Oldest nucleated cells: 2.1 BY
• First animals: 600 MY
• First tetrapods: 375 MY
• First dinosaurs: 220 MY
• First mammals: 220 MY
• First hominids: 4 MY
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Evolution of
Tetrapods
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Acanthostega and Ichthyostega
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Evolution of
Whales
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Mass Extinctions
• Survival of the luckiest?
• Over geologic time, mass extinctions
may subvert the notion of survival of
the fittest.
• Is evolutionary history predictable?
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The Record of
Mass Extinctions
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Rates of Evolution
• Biologists cannot measure evolutionary
rates. They have only the present.
Darwin assumed gradual rates.
• Paleontologists can measure evolutionary
rates. They have found punctuated
equilibria to be the major pattern in fossil
species rather than gradual change.