DIVERSITY AND
EVOLUTION
Chapter 2
Diversity of life
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Approximately 1.5 million living species
described
Likely at least 10 million species today
May represent only 1% of species ever to
have lived on earth
1 billion species presumed to have lived
Diversity of body form
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Tremendous diversity within each group of
plants, animals, fungi, protistans, bacteria
Structural complexity - apparently
purposeful adaptation of many
characteristics to the environment
Reason for this diversity?
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Natural selection
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Physical environment acts on various
characteristics of organisms (variation among
individuals of some species)
Sorts out “harmful” ones, leaving individuals
with “beneficial” or “neutral” characteristics
to produce next generation
Keeps organisms well-suited for survival in
their environment
Natural selection drives evolution
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Broad scale
Development of various “forms” or species
to best match the environment
Can best take advantage of variations
within that environment
History of concept of evolution
by natural selection
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Lamarck - inheritance of acquired
characteristics
Darwin, Wallace - natural selection, but
mechanism really unknown
Mendel - genetic understanding of the
acquisition of inherited traits
Evolution by natural selection established truths
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1) individuals that form a population of a
species are not identical
Evolution by natural selection established truths
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2) some of the variation between
individuals is heritable
Evolution by natural selection established truths
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3) all populations are capable of
exponential growth, but most individuals
die before reproducing, and most others
reproduce at less than their maximum rate
Evolution by natural selection established truths
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4) different ancestors leave different
numbers of descendents; they do not all
contribute equally to subsequent
generations
SPECIATION
Interaction of heritable variation,
natural selection, barriers to gene
flow
Allopatric (Geographic)
Speciation
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Separating single, interbreeding population
into two or more spatially isolated
populations
Geographic barrier, remains long enough
for speciation
Founder effect, genetic drift (random
mutations)
Parapatric Speciation
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No spatial isolation
Portion of population invades new,
adjacent habitat
Little to no movement/interbreeding
Differing natural selection in differing
habitats
Sympatric Speciation
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No spatial isolation
Production of new species within a
population
Rare
Most likely to occur in insect parasites of
plants, animals
Requires stable polymorphism and underor unused resource
Polyploidy
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Abrupt speciation by doubling the number
of chromosomes
Most common in plants
Agricultural-wheat, alfalfa, potatoes
Native-birches, willows
PATTERNS OF SPECIATION
Anagenesis
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One species changes into another species
over time
Original species “evolves” out of existence
and is replaced by new species
Evolutionary extinction
Cladogenesis
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One species gives rise to one or more
additional species while still remaining
Clade-set of species descended from a
particular ancestral species (e.g., Darwin’s
finches)
TEMPO OF SPECIATION
Gradualism
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Steady change in character(s) resulting in
many intermediate forms exhibiting
“gradual” shift
Punctuated equilibrium
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Rapid, abrupt changes that produce quick
shifts in character
No intermediate forms
REDUCTION IN VARIATION
Inbreeding depression
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Mating among close relatives produces an
increase in expression of recessive traits,
many of which are deleterious
Often results from small population size
Mortality may be increased
“Tighter” inbreeding results in more rapid
loss of genetic variation within population
But….
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Not all populations are harmed by
inbreeding
Long-term, small populations (e.g., on
islands) may be adapted to inbreeding and
survive well even in face of it
Outbreeding
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Some degree of outbreeding usually
beneficial in maintaining genetic diversity
But too much can also be harmful
Too many differences may lead to
problems
Smaller populations
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Genetic variation declines faster in smaller
population because of inbreeding
Rule of thumb-50 individuals needed to
prevent inbreeding
Problem for saving California condor
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Only 26 individuals in 1986
Genetic drift
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Larger population not subject to inbreeding
can lose genetic variation at rates similar to
small populations via genetic drift
Some individuals do not mate, not
represented genetically in next generation
Genetic drift-cont.
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Rule of thumb-happens only in populations
<500 in size
Genetic drift can be counteracted by
minimal levels of immigration into the
population
Neighborhoods
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Even big populations may run into problems if
individuals don’t move around much to mate
Some also just don’t reproduce
Effective population size may then be quite small
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E.g., grizzly bear in Yellowstone
Actual population ~200
Effective population ~50 (25%)
Subject to loss of variation
Bottlenecks
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Can also reduce genetic variation
Bottlenecks - periodic reductions in
population size can reduce genetic
variation greatly even if average population
size is much larger
Founder Effects
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Can also reduce genetic variation
Founder effects - developing gene pool of
growing population is limited by what
variation founders had, plus mutation
Pair of founders at most have 4 variations
in a gene
ORIGIN OF VARIATION
Genetic
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Increase or decrease variability within a
population
DNA - mistakes or mutations during
copying of genetic code
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Gene or point mutation - most important for
enriching the gene pool
Chromosome mutation - most important for
rearranging the gene pool
Point Mutations
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Change in nucleotide base at single
location
Change in single amino acid within protein,
or entirely different protein
Frameshift mutation - insertion or deletion
of single base pair
Mutagens and mutations
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Mutations usually produced by mutagens
(e.g., weak cosmic rays)
1 mutation per gene in every 100,000 sex
cells
Higher organisms have ~10,000 genes
1 in 10 individuals has newly created
mutation
Most mutations are harmful, but..
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1 in 1000 mutations may be beneficial
1 in 10,000 individuals per generation has a
useful mutation
Most individuals have at least one mutant
gene (original, or passed down from
ancestors)
Mutations and Speciation
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Estimate - 500 mutations necessary to
produce new species from existing one
Rate of new mutations ~1 million times
greater than needed to account for known
rate of evolution
Chromosomal Mutations
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No change to variability
Rearrange what is there
Deletions, duplications, inversions,
translocations
Other changes
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Polyploidy - e.g., tetraploid
Failure of gametes to reduce to haploid
state during meiosis
2N + 2N = 4N
So…
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Mutations produce the variation, and
natural selection acts upon the changes
Add in: nonrandom mating, changing
environment
End product = EVOLUTION
Amount of Variation
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Results from protein analyses
(electrophoresis)
Within a population - 15-58% of genes
exhibit variation
Within individuals - 3-17% of genes
exhibit variation
Applying this information:
1) Separate populations of organisms with
movement of individuals among
populations generally exhibit most
variation within each population, and very
little between or among populations
Applying this information:
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2) Reduced movement of individuals
among populations produces more
variation between or among populations
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Populations diverge genetically
Applying this information:
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3) Conservation of endangered species
which move around very little will require
protection of many populations in many
different habitats to conserve genetic
diversity within the species