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Adaptive Evolution
Natural selection is the
mechanism of adaptive evolution
Adaptive Evolution
• Organisms are adapted to their
environment
Converging Evolution
• Some very different species are subject to
the same environmental pressures
Genetic Variation is the starting
material
• Genetic variation leads to phenotypic
variation differential success
• Discrete characters
– Single gene locus
• Quantitative characters
– Polygenic, vary along continuum
Phenotypic Polymorphism
• 2 or more distinct phenotypes exist in high
enough frequencies to be noticeable
• Does not apply to
traits that vary along
a continuum
These traits are
polymorphic at genetic
level but not at
phenotypic level
Variation Between Populations
• Geographic variation
– Differences between geographically
separated populations of the same species
• Environmental conditions vary, natural
selection acts differently
Evolutionary Fitness
• Fitness= contribution that an individual
makes to the gene pool of the next
generation
• Relative fitness= contribution of one
phenotype relative to the alternative
phenotype
– Highest success = 1
– Relative success is compared to 1
• 80% =.8
Selection
• Depends on relative fitness
– Directional
– Disruptive
– Stabilizing
Directional Selection
• Shifts bell curve in one direction
• Population arrives in new environment
• Environment changes
– Black bears during ice ages
Disruptive Selection
• Extremes are favored over intermediate
phenotype
• Can eventually lead to speciation.
– Galapagos finches
Stabilizing Selection
• Intermediate is favored over extremes
• Maintains status quo
• Reduces variation
– Birth weight
Preserving Variation
• Stabilizing and Directional selection
reduce variation
• Variation needs to be maintained for
species to be able to evolve to changing
environment
– Diploidy
– Balancing selection
– Neutral Variation
Diploidy
• Diploid= two copies of each chromosome
• Hidden genetic information
• Harmful, recessive alleles can be hidden
– Only show if individual is homozygous
• Hidden alleles could become
advantageous if environment
changes
Balancing Selection
• Heterozygote advantage
– Negative alleles are maintained if
heterozygote has an advantage over
homozygotes
• Sickle cell trait
Frequency Dependant Selection
– Fitness declines when one morph becomes
too frequent
– Advantage is only an advantage if a few have
it and the rest are vulnerable
• Blue Jays eating camouflaged moths
Neutral Variation
• Some traits have no impact on fitness
– Un-translated genes
– Silent Mutations
Sexual Selection
• Sexual dimorphism
• Intrasexual selection
– Within the same sex
– Competition for mates
• Intersexual selection
– Between different sexes
– Mate selection
Mate Choice
• Male fitness depends on number of
offspring
• Female fitness depends on quality of
offspring
– Jacana=role reversal
• Showy characteristics are an indicator of
overall health??
Why Sexual Reproduction?
• Requires larger population
• Slower population growth rate
– Only half of total population produces
offspring
• Risks associated with finding mates
• What is the advantage???
Some ideas
• Natural selection acts on variation
• Recombination increases variation
– Resistance to disease
– Co-evolution with pathogens
Why Can’t Natural Selection Create
Perfect Organisms?
• Limited by historical constraints
– Can only act on what already exists
• Birds with 4 limbs and wings…
• Adaptations are compromises
– Multiple activities in various environments
• Seals
• Chance and natural selection interact
– Chance events make a difference
• Storm blows birds off course
• Selection can only edit existing variation
– New alleles don’t arise on command/necessity
• HIV resistance