Deoxyribonucleic Acid DNA Trait variation is encoded in DNA o DNA is composed of base pairs of nucleotides A T G C If base pairs differ among individuals for a particular trait may lead to different trait expressions Chromosomes Composed of DNA Regions on chromosome that we charactertize are genes with each gene representating different base pairs composed of o Genes code for how a particular trait is going to be expressed o However genes cannot have alternate forms based upon base pairs thay are Known as alleles forms of the same gene with different base pairs Different alleles can lead to different trait expressions o Example Giraffe neck length Imagine the giraffe neck length coded by one gene allele A and allele B Allele A if it is present then the giraffe has Allele B if it is present then the giraffe has Genotype the underlying genetic code for a a long neck a short neck trait Allele A and allele B are a phenotype outward expressions for the trait Evolution Definition change in living organisms over time can also mean change in allele frequency in a given population over time Individuals are selected for against based on their traits However individuals do not evolve Populations evolve Natural Selection Trait expressions allele frequency change with selective pressures Only mechanism to result in adaptive change Populations change is not random o Allele frequency changes in response to selective pressures Natural selection does not generate NEW trait expressions o It can only act on existing variation in a particular trait Mutation Very rare can be negative positive or neutral Ultimate source for all genetic variation new alleles are produced within an individual o Can be spontaneous no direct cause During egg sperm production spontaneous changes occur in DNA o Can be caused by external forces mutagens Cause changes in underlying DNA of an individual Chemical radiations An increase in mutations in an environment means that there is something wrong in the environment itself usually mutagens Gene Flow a mechanism of evolution 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 pollination human movement o Usually not random but can be such as wind pollination Nonrandom mating Can be inbreeding o Can occur from self fertilization Common in hermaphrodites an organism that posses both male and female reproductive parts o Can occur through mating by closely related organisms such as siblings o Results in reduced genetic allelic diversity within that population Genetic Drift Completely random process o Changes in allele frequency due to accidental events in many cases random mating separating of the population o Very common potentially problematic in small populations because there isn t much diversity Small gene pool Represents all alleles in a population o Can happen through random mating Some individuals don t mate Founder Effect allelic diversity gene pool Determined by individuals that move are moved to a new population and then breed The Bottleneck Effect Due to some event only a small number of individuals survive and they better represent the gene pool for their population Small group small gene pool Northern elephant seal Through hunting ended up with one population Protected over time All allelic diversity comes from one population o They have recovered Although there is a large population they have a small gene pool and a low genetic diversity because it doesn t allow for variation upon reproducing
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