Evolution BIO 4220-001
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This 5 page Class Notes was uploaded by La-Teisha Allen on Saturday September 10, 2016. The Class Notes belongs to BIO 4220-001 at University of North Carolina at Pembroke taught by John Howard Roe in Fall 2016. Since its upload, it has received 3 views. For similar materials see Evoloution in Biology at University of North Carolina at Pembroke.
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Date Created: 09/10/16
Natural Selection Notes Types, Dimensions, and Constraints Artificial Selection – modern examples • Illinois Long-Term Selection Experiment on Corn (1896 – current) – selectively bred corn for either high (top 20%) or low (bottom 20%) oil content – generated corn with extreme oil content well beyond the initial variation (limits of original population) • Russian Farm-Fox Experiment (1959 – current) – selectively bred silver foxes for tameness (behavior) – numerous other unintended traits (side-effects) became more common Population thinking • both Artificial Selection and Natural Selection – process of “sculpting phenotypes” by “sculpting the gene pool” – systematic increase or decrease in gene/phenotype in population = Evolution Linking Artificial and Natural Selection Scientific Thinking • Artificial Selection is a manipulative experiment • Hypothesis: non-random survival of random geneticàphenotypic variation as cause of evolution – Artificial: human eyes as selective agent – Natural: nature as selecting agent (differential survival and reproduction) Natural Selection – Directions A. Directional Selection (positive or negative) • Favors indiviudals near one end of phenotypic spectrum B. Stablizing Selection • Favors individuals with intermediate phenotypes C. Disruptive (Balancing) Selection • Favors indiviuals with both extreme phenotypes Disruptive (Balancing) Selection • European Garden Snails – variation in shell color – uniform dark morphs favored on forest floor – yellow striped morph favored in grass – habitat patchiness maintains variation • Pacific Salmon spp. males exhibit alternative life history and breeding strategies – Hooknose mature late (3 yr) at larger size (70 cm) • fighting, nest guarding – Jacks mature early (2 yr) at smaller size (40 cm) • hide and sneak fertilizations – Intermediate sizes? – Too small to fight, too large to hide ESS or BBS? Relative Fitness depends on not only own strategy, but strategy of competition • Evolutionarily Stable Strategy (ESS) – Where phenotypic strategy cannot be successfully invaded by variant (all individuals in a population employ the same) – Mixed ESS allows for multiple strategies to coexist at stable equilibrium • can change as Selective Environment shifts • Strategy= genetically hard wired • Best of a Bad Situation (BBS) – Where multiple phenotypic variants coexist in population with hierarchy of success • one best strategy has highest fitness payoff relative to alternate strategies • other lower fitness strategies represent individuals making the most of their constrained situation • Not genetically hard wired Frequency-Dependent Selection • Frequency-dependent selection: when selective advantage of a phenotype depends on its frequency in the population – rare phenotype is favored – adaptive trait shifts with Selective Environment Viability vs. Fecundity Selection • Male birds of paradise showy have plumage and attention-grabbing displays • - increase mating oppurtunities at expense of survival • Female drab coloring and secretive behavior • Increase survival by avoiding predators Phenotypic Variation Tradeoff: balance/compromise achieved among competing, incompatible features No such thing as a free lunch! Tradeoffs in Reproductive Investment • Invest in a few showy flowers and energy-rich nectar to attract pollinator (e.g., hummingbird) – targeted fertilization • Invest in millions of pollen grains and broadcast them into environment, dispersed by wind – non-targeted fertilization Tradeoffs in Survival Investments • Mexican cavefish divert resources from building/maintaining eyes to invest in sensory capacity of jaws, taste buds, teeth • Star-nosed moles divert resources from eyes to improving tactile and chemical sensitivity on nose tubercles Tradeoffs between Survival and Reproduction • Flightlessness common among island birds – diversion of resources from flight (energy, materials to build wings and muscles) – increased investment in larger body size and reproduction – How does the selective environment differ from mainland? Dimensions • Evolution can occur in two primary dimensions – Cladogenesis • temporal (vertical) change in a population – Anagenesis • spatial (horizontal) change between reproductively isolated populations Constraints adaptations are rarely perfect 1) Source of variation is random no preordained directional goals 1) Tradeoffs • Competing allocations/functions within individual • Multiple selective agents in environment 1) Can only “tinker” with existing parts 1) Selective Environments change Source of Variation is Random • Lots of ways for variation to appear in a population (e.g., mutation), but limits on rate of occurrence – genetic mutations are usually repaired • If particular variation never occurs, trait variant never appears Tradeoffs competing allocations and functions in individual • Physical Constraints: • Investing in extra long legs could be beneficial (reaching higher food, running faster) • But they would more easily break ( inc. risk of injury) • Resource Allocation: • Investing more calcium in bones can strengthen them • But there would be less calcium for other uses ( feeding offsprng via lactation) • Owl has eyes that face forward • Pro: binocular vision improves visual acuity in front (catching food) • Con: limited field of view increases vulnerability to predators (mortality) • Ostrich has eyes on side of head • Pro: can see predators approaching from all sides (survival) • Con: harder to catch prey (food acquisition) Natural Selection can only tinker with existing parts • Freshwater mussels modify the mantle to crudely resemble a fish – larva require fish gills to develop – lure predatory fish close enough to infect (prey make false- positive error) • Anglerfish modify dorsal fin spine to vaguely resemble a food item – sit-and-wait predators – lure predators close enough to engulf (prey make false-positive error) • Bird feathers served original function of insulation and signaling – first fossil evidence of feathers in animals that did not fly – feathers later modified for flight as well • Snake venom original function for digestion – venom full of digestive enzymes, many snakes have mild venom for only digestive purposes – venom delivery apparatus later modified in some snakes for defense as well (spitting cobra) Adaptation or Exaptation? • Adaptations: – trait must be shaped by Natural Selection and serve the same primary function in current Selective Environment as it did in past • Exaptation: – trait must be shaped by Natural Selection and serve different primary function in current Selective Environment than it did in past • trait was co-opted, re-worked, and tweaked for new purpose • “Concluding that feathers evolved for flight is like maintaining that digits evolved for playing the piano” (Prum and Brush 2002) Selective Environments Change moving targets • Natural selection favors trait variants that are immediately beneficial, not variants that may be favorable in the future – shaped by past and current Selective Environments, cannot anticipate the future • Abiotic and biotic Selective Environments are constantly changing – abiotic: • climate shifts between ice age and hot periods (thousands to millions of years), flood and drought (years to centuries), seasonal variation in temperature and rainfall – biotic • predator/prey, host/parasite, competitive or mutualistic interactions
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