Review Evolution by Natural Selection 1 Must exist for given trait 2 Results in differential survival and reproductive fitness 3 Must be heritable genetically based DNA Chromosomes Trait variation is encoded in DNA o DNA is composed of bas pairs of nucleotides A T C G If base pairs differ among individuals for a particular trait may lead to different trait expression of that particular trait Composed of DNA Regions on chromosome that we characterize at genes o Represents variability of groups of base pairs Genes code for how a trait is going to be expressed o However genes can have alternate forms based upon the base pairs they are composed of Alternate forms alleles forms of the same gene with different base pairs Different alleles can lead to different trait expressions o Lamarck s giraffe neck length o Imagine that giraffe neck length is coded by one gene Allele A long neck Allele B short neck Genotype the underlying genetic code for a trait the allele Phenotype the outward expression of a trait whether the neck is long or short Evolution Change in living organisms over time Evolution Change in allele frequency in a given population over time Individuals are selected for against based on their traits o However individuals do NOT evolve Populations Evolve o Trait expression allele frequency change with selective pressures Natural Selection only mechanism that results in adaptive change o Is NOT random Allele frequency change in response to selective pressures o Does NOT generate new trait expressions It can only act on existing variation for a particular trait Mutation individual o The ultimate source of all genetic variation new alleles are produces within an 1 Spontaneous 2 External forces a During egg sperm production spontaneous changes occur in DNA mutagens a b Cause changes in underlying DNA of an individual c Examples chemicals radiation o Very rare o Effect can be negative positive or neutral o Model organism Example Fruit fly Drosophila Increased mutation in environment can signal environment problems Gene Flow Very common populations can exchange alleles through dispersal movement of genetic material that lead to new alleles in a particular population Can occur through organismal movement and pollination Usually not random but it can be Non random Mating Inbreeding o Self fertilization Hermaphrodite has both viable male and female reproductive parts o Mating by closely related organisms siblings o Often results in decrease in genetic allelic diversity within that population o Completely random process o Changes in allele frequency due to accidental events random mating separation of a population o Very common problematic in small populations o Small gene pool Represents all alleles in a population Some individuals don t mate Random Mating Founder Effect Bottleneck Effect Allelic diversity gene pool is determined by individuals that move are moved to a new population then breed Due to some event only a small number of individuals survive and they then represent the population gene pool Example Northern Elephant Seals o Through hunting they ended up with a single population o All allelic diversity now comes from one population o The seals recovered but they have a really small gene pool genetic diversity
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