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