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Vestigial structures- genes from the past influence present phenotypes, so even though animals might not need afeature they may still have it. (e.g. Cave salamanders continue to make eye sockets although they live in the dark and have no need for eyes).2. Imperfect structures- Evolution can only modify existing structures and functions (can produce less than perfectresults). Vertebrae eyes Vs. Cephalopod eye[Pick the date] Page 17
-Vertebrates and cephalopods developed eyes in separate phylogenies-In vertebrates, the nerve fibers route before the retina, blocking some light and creating a blind spot wherethe fibers pass through the retina and out of the eye. -In Cephalopods, the nerve fibers route behind the retina, and do not block light or disrupt the retina.3. Other imperfect solutions4. Deleterious alleles that evade selection--If a disease is caused by a dominant allele, it is always exposed to selection-FF will be selected against-Ff will be selected against-Ff will not be selected against (it is the genotype for the “normal” phenotype)-Diseases from dominant alleles are relatively rare.-Contrast: if a disease is caused by a recessive allele, it is not always exposed to selectionHuntington’s disease: -A neurological degenerative disease caused by a dominant, disease-causing allele-Most diseases associated with dominant alleles are selected against (because a dominant allele is expressed, and if it causes a disease it will be selected against).-In Huntington’s disease, the onset is at about age 40-So, it is a dominant allele that at first is not exposed to selection5. PleiotropyMechanism- One gene codes for a protein that is used in different cell types (or codes for a signal, with effectson different cell types). -Selection for one trait causes a correlated but perhaps suboptimal (or deleterious) change in another trait.-E.g. pigmentation and deafness in cats: 40% of cats with white fur and blue eyes are deafBecause an allele that commonly is responsible for producing white fur with blue eyes also causes deafnessIf white fur were advantageous, natural selection for it might not succeed because of the huge pleiotropic cost (e.g. deafness).[Pick the date] Page 18
6.Fitness trade-offs-Selection for one phenotype often precludes selection for an alternative phenotype-Or, selection for a compromise precludes attaining the efficiencies that a specialist can experience-E.g. mountain bike vs street bike, -Most fundamental tradeoff: investment in survival- growth, maintenance, competitiveness Vs. Investmentdirectly in reproductionSummary for two ways that make it difficult for selection to eliminate disease 1. Diseases caused by recessive alleles (e.g. must be homozygous recessive) are hard to eliminate because they can persist in heterozygotes who suffer no disadvantage2. Time-delayed diseases caused by dominant alleles are hard to eliminate because their onset occurs after breedingFebruary, 12, 2020Misunderstanding Natural SelectionWhat’s a P-prim?P-prims: “phenomenological primitives”: -