evolution of populations
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Evolution of Populations
Doonesbury - Sunday February 8, Doonesbury - Sunday February 8, 20042004
Populations evolveNatural selection acts on individuals
differential survival due to various adaptations “survival of the fittest” Results in differential reproductive success
who bears more offspring
Populations evolvegenetic makeup of population changes over
time favorable traits (greater fitness) become
more commonGene pool within a population changes
Changes in populationsPesticidemolecule
Insect cellmembrane
Target site
Resistanttarget site
Insecticide resistance
Target site
Decreased number of target sites
Insects that have resistant target sites will survive, making that trait more common in the population’s gene pool
Individuals survive or don’t survive…
Populations evolve
Individuals are selected
Individuals reproduce or don’t…
Fitness
Survival & Reproductive successindividuals with a more
advantageous phenotype leave more surviving offspring
Body size & egg laying in water striders
Variation & natural selection Variation is the raw material for natural selection
there have to be differences within populations or else gene frequency could not change
some individuals must be more fit than others
Mean beak depth of parents (mm)
Medium ground finch8
8 9 10 11
9
10
11
1977 1980 1982 1984
Dry yearDry year
Dry year
Wet year
Beak
depth
Beak
depth
of
off
spri
ng (
mm
)
Where does Variation come from?Mutation
random changes to DNA errors in mitosis & meiosis environmental damage
Sex mixing of alleles
recombination of alleles new arrangements in every offspring
new combinations = new phenotypes
spreads variation offspring inherit traits from both parents
5 Agents of evolutionary changeMutation Gene Flow
Genetic Drift Selection
Non-random mating
1. Mutation & Variation Mutation creates variation
new mutations are constantly appearingMutation changes DNA sequence
changes amino acid sequence?changes protein?
changes structure? changes function?
changes in protein may change phenotype & therefore change fitness Changing survival and reproductive rates Changing gene frequencies
2. Gene FlowMovement of individuals &
alleles in & out of populationsseed & pollen distribution by
wind & insectmigration of animals
causes genetic mixing across regions
reduce differences between populations
3. Non-random mating Living things do not mate at random Sexual selection – select for advantageous traits in a mate
What is the best combination of traits to pass on to offspring to ensure survival?
Warbler
finch
Tre
e finch
es
Grou
nd fi
nch
es
4. Genetic driftEffect of chance events
founder effect small group splinters off & starts a new colony
bottleneck some factor (disaster) reduces population to
small number & then population recovers & expands again
Conservation issuesBottlenecking is an important
concept in conservation biology of endangered speciesloss of alleles from gene poolreduces variationreduces adaptability
Breeding programs must consciously outcross
Breeding programs must consciously outcross
Peregrine Falcon
Golden Lion Tamarin
5. Natural selectionDifferential survival & reproduction due to
changing environmental conditions climate change food source availability predators, parasites, diseases toxins
Nature selects for the combinations of alleles that provide “fitness”
increase in the population adaptive evolutionary change
5 Agents of evolutionary changeMutation Gene Flow
Genetic Drift Selection
Non-random mating
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