DIVERSITY AND EVOLUTION

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DIVERSITY AND EVOLUTION

• Chapter 2

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Diversity of life

• 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

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Diversity of body form

• Tremendous diversity within each group of plants,
  animals, fungi, protistans, bacteria
• Structural complexity - apparently purposeful
  adaptation of many characteristics to the
  environment

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Reason for this diversity?

• Natural selection
• 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

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Natural selection drives evolution

• Broad scale
• Development of various forms or species to best
  match the environment
• Can best take advantage of variations within that
  environment

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History of concept of evolution by natural
selection

• Lamarck - inheritance of acquired characteristics
• Darwin, Wallace - natural selection, but
  mechanism really unknown
• Mendel - genetic understanding of the acquisition
  of inherited traits

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Evolution by natural selection - established
truths

• 1) individuals that form a population of a
  species are not identical

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Evolution by natural selection - established
truths

• 2) some of the variation between individuals is
  heritable

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Evolution by natural selection - established
truths

• 3) all populations are capable of exponential
  growth, but most individuals die before
  reproducing, and most others reproduce at less
  than their maximum rate

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Evolution by natural selection - established
truths

• 4) different ancestors leave different numbers of
  descendents they do not all contribute equally
  to subsequent generations

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SPECIATION

• Interaction of heritable variation, natural
  selection, barriers to gene flow

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Allopatric (Geographic) Speciation

• Separating single, interbreeding population into
  two or more spatially isolated populations
• Geographic barrier, remains long enough for
  speciation
• Founder effect, genetic drift (random mutations)

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Parapatric Speciation

• No spatial isolation
• Portion of population invades new, adjacent
  habitat
• Little to no movement/interbreeding
• Differing natural selection in differing habitats

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Sympatric Speciation

• 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 under- or unused
  resource

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Polyploidy

• Abrupt speciation by doubling the number of
  chromosomes
• Most common in plants
• Agricultural-wheat, alfalfa, potatoes
• Native-birches, willows

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PATTERNS OF SPECIATION
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Anagenesis

• One species changes into another species over
  time
• Original species evolves out of existence and
  is replaced by new species
• Evolutionary extinction

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Cladogenesis

• 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., Darwins finches)

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TEMPO OF SPECIATION
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Gradualism

• Steady change in character(s) resulting in many
  intermediate forms exhibiting gradual shift

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Punctuated equilibrium

• Rapid, abrupt changes that produce quick shifts
  in character
• No intermediate forms

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REDUCTION IN VARIATION
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Inbreeding depression

• 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

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But.

• 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

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Outbreeding

• Some degree of outbreeding usually beneficial in
  maintaining genetic diversity
• But too much can also be harmful
• Too many differences may lead to problems

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Smaller populations

• Genetic variation declines faster in smaller
  population because of inbreeding
• Rule of thumb-50 individuals needed to prevent
  inbreeding
• Problem for saving California condor
• Only 26 individuals in 1986

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Genetic drift

• 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

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Genetic drift-cont.

• Rule of thumb-happens only in populations lt500 in
  size
• Genetic drift can be counteracted by minimal
  levels of immigration into the population

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Neighborhoods

• Even big populations may run into problems if
  individuals dont move around much to mate
• Some also just dont reproduce
• Effective population size may then be quite small
• E.g., grizzly bear in Yellowstone
• Actual population 200
• Effective population 50 (25)
• Subject to loss of variation

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Bottlenecks

• Can also reduce genetic variation
• Bottlenecks - periodic reductions in population
  size can reduce genetic variation greatly even if
  average population size is much larger

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Founder Effects

• 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

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ORIGIN OF VARIATION
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Genetic

• Increase or decrease variability within a
  population
• DNA - mistakes or mutations during copying of
  genetic code
• Gene or point mutation - most important for
  enriching the gene pool
• Chromosome mutation - most important for
  rearranging the gene pool

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Point Mutations

• 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

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Mutagens and mutations

• 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

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Most mutations are harmful, but..

• 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)

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Mutations and Speciation

• 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

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Chromosomal Mutations

• No change to variability
• Rearrange what is there
• Deletions, duplications, inversions,
  translocations

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Other changes

• Polyploidy - e.g., tetraploid
• Failure of gametes to reduce to haploid state
  during meiosis
• 2N 2N 4N

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So

• Mutations produce the variation, and natural
  selection acts upon the changes
• Add in nonrandom mating, changing environment
• End product EVOLUTION

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Amount of Variation

• Results from protein analyses (electrophoresis)
• Within a population - 15-58 of genes exhibit
  variation
• Within individuals - 3-17 of genes exhibit
  variation

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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

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Applying this information

• 2) Reduced movement of individuals among
  populations produces more variation between or
  among populations
• Populations diverge genetically

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Applying this information

• 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