Genetic Variation - The fuel of natural selection Campbell et al, chapter 23

Populations are polymorphic Nature vs. Nurture Source of variation Maintenance of variation

Populations are polymorphic Nature vs. Nurture Source of variation Maintenance of variation

Polymorphic populations Example: Darwin finches on Galapagos

Polymorphic populations Example: Lazuli bunting

Polymorphic populations Example: Swallowtail butterfly

Populations are polymorphic Nature vs. Nurture Source of variation Maintenance of variation

Nature vs. Nurture Genome (blueprint) Nature Phenotypic Expression Environment Nurture Protein I found the gene that makes us believe all traits are based on genes

Nature vs. Nurture Gene Chromosome Genome Genepool

Nature vs. Nurture Diploidy There are 2 copies of each gene Father Mother Offspring 2 identical copies = homozygot Father Mother Offspring 2 different copies = heterozygot

Nature vs. Nurture Genotype is more variable than phenotype Only genetic variation counts for evolution

Nature vs. Nurture How to separate the two? Example: Altitudinal gradient Common Garden Exp. Gene Environment Mixture of both

Populations are polymorphic Nature vs. Nurture Source of variation Mutation Recombination Maintenance of variation

Source of Genetic Variation 1. Mutation Change in: DNA sequence Chromosome structure Number of Chromosomes Due to: Copying Errors Environmental factors

Source of Genetic Variation 2. Recombination Reshuffling of chromosomes during reproduction Crossing over A B C A B C

Populations are polymorphic Nature vs. Nurture Source of variation Maintenance of variation Selection Heterosis

Maintenance of Variation 1. Stabilising selection Favours mean over tail Loss of variation

Maintenance of Variation 1. Stabilising selection Example: Darwin Finches Bill size Foraging efficiency

Maintenance of Variation 2. Directional selection One tail is favoured over the other Trait is changing over time

Maintenance of Variation 2. Directional selection Example: Heliconius butterfly on passion flower vines Plant: Toxin Insect: Enzymes Plant: False eggs Leads to an arms race between plant and insect

Maintenance of Variation 2. Directional selection The arms race idea lead to a more general hypothesis: Red Queen Hypothesis ‘It takes you all the running to stay in place’

Maintenance of Variation 2. Directional selection Selection may change in time Example: Darwin Finches on Galapagos El Nino Bill size wet dry small large seeds Time

Maintenance of Variation 2. Directional selection Selection may change in space Example: African Firefinches

Maintenance of Variation 3. Disruptive selection Selection favours the two tails over the mean There are two forms

Maintenance of Variation 3. Disruptive selection Example: Lazuli buntings Showy male Average male Drab male Sneak in attracted

Maintenance of Variation 4. Frequency-dependent selection A mode of selection where a phenotype is only favoured when it is either rare or common.

Maintenance of Variation 4. Frequency-dependent selection Example: Swallowtail butterfly (Papillo dardanus) Males Swallowtail females mimic these toxic species without being toxic themselves Toxic species Only works if the cheats are rare

Maintenance of Variation 5. Heterozygote advantage Heterozygote individuals have higher fitness than either homozygote individuals. This is a common principle in plant and animal breeding

Maintenance of Variation 5. Heterozygote advantage Example: Red blood cells - sickle cell disease Homozygot (normal cells): vulnerable to malaria Homozygote (sickled cells): lethal Heterozygote: non-lethal & resistant to malaria

A large number of processes create and maintain genetic variation that is the base for evolution... …but does this lead to new species? - next here