BIOC54 - Chapter 1 .docx

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Biological Sciences
Kamini Persaud

BIOC54H Chapter 1 Tharsini Sivananthajothy January 20, 2012 Darwinian Theory and Ultimate Hypotheses - Darwin’s theory of evolution by natural selection is based on the premise that evolutionary change is inevitable if just three conditions are met: o Variation: such that members of a species differ in some of their characteristics o Heredity: with parents able to pass on some of their distinctive characteristics to their offspring o Differences in reproductive success: such that some individuals have more surviving offspring than others in their population, thanks to their distinctive characteristics - if there is hereditary variation within a species and if some of the variation allows some to reproduce more successfully than others, then large number of the successful type will change the makeup of the species - species will evolve as it becomes dominated by individuals that possess the traits associated with successful reproduction - the process that causes evolutionary change is natural so Darwin called it natural selection - although Darwin knew little about heredity, we now know that variation among individuals within a species arises because of differences in their genes, the segments of DNA that faithfully encode the information needed for the synthesis of proteins - genetic variation within a species occurs when a given gene exists in two or more forms or alleles - the different alleles o different alleles affect the nature or abundance of the protein coded for by the gene so genetically different individuals transmit different instructions for protein manufacture to their offspring - if some alleles are superior in their ability to make individuals reproductively successful, then those alleles will get themselves passed on from generation to generation o will become more common over time o genetic variation + differential reproduction = evolutionary change at the genetic level o alleles will spread in proportion to how well they help build bodies that are unusually good at reproducing Darwinian Theory and the Study of Behaviour - the conditions required for natural selection apply to nearly every organism, which means that almost every species has probably been shaped by natural selection in the past - evolution would happen when people created the conditions needed for selection to occur as they domesticated certain useful animals and plants - test Darwinian theory by attempting to generate evolution in the laboratory, starting with a population that exhibits hereditary variation in attributes that affect the reproductive success of individuals - artificial selection experiment done by Carol Lynch with house mice which build nests of soft grasses and other plant materials in natural but will happily accept cotton as nesting material in the laboratory o the amount of cotton a mouse collects can be quantified as the number of grams it pulls into a nest cage over a 4-day period o in the starting generation, individual mice moved 13 – 18 grams of cotton in to their cage from an external supply o Lynch worked with this variation on the assumption that the differences were hereditary o She attempted to evolve a high line by interbreeding males and females who collected large amounts of cotton and a control line which crossed males and females at random BIOC54H Chapter 1 Tharsini Sivananthajothy January 20, 2012 o Offspring produced by these crosses were reared under the same conditions their parents were, eliminating environmental variation as a possible cause for difference in behavior o The new generation was allowed to collect cotton and the most avid cotton collectors were permitted to breed, creating a second selected generation o Least eager collectors were low line o These procedures were repeated over 15 generations and resulted in the high line mice gathering about 50 grams of cotton for their nests on average while low-line mice brought in about 5 grams on average and control brought in 20 grams o Evolution occurred in the lab under the conditions that should have resulted in evolutionary change if the theory of natural selection is correct - Another example includes lactose tolerance o People began rearing milk cows successfully only 5000 years ago in northern Europe because this area had few deadly contagious diseases of cattle o Once dairying originated, a mutant gene spread naturally through this human population, a gene that contributed to the ability of adults to digest milk sugar o Although children can digest lactose, most lose this ability when they are no longer being nursed by their mothers, after which they become lactose intolerant o Natural selection acting on variation in the ability of adults to absorb milk sugar resulted in the rapid spread of lactose tolerance in adults within the northern European cattle- herding culture and also in a number of African groups where dairy cattle provide milk for children and adults - Another example includes bill size of ground finch in the Galapagos who undergo evolutionary change annually o In drought years, when the smaller seeds eaten by medium ground finch were scarce relative to the large seeds, selection favoured relatively large billed individuals o Birds with the hereditary tendency to develop larger than average beaks survived and reproduced because of their ability to crack open big tough seeds while smaller-beaked cousins were starving to death rather than reproducing o In rainy years, the situation reversed as birds with smaller bills handled the then abundant smaller seeds better than larger-beaked finches o Temporary feeding advantage of smaller-billed finches translated into a reproductive advantage, leading to the evolution of somewhat smaller-billed population o Annual changes in selection caused the average bill size of the finches to fluctuate ANNUALLY in response to the changes in the resources available - Other examples include the evolution of bacterial resistance to antibiotics, change in overwintering behavior of some songbirds in the last few decades, the spread and decline of melanic wing coloration in moths, evolutionary of Australian black snakes of an average to a highly poisonous, very recently introduced toad - If we assume that individuals are given alleles that have spread because they were better than the alternative at getting their bearers to reproduce successfully, then every hereditary attribute of almost every species probably has something to do with reproductive success - When biologists wish to understand the ultimate reasons why an animal does something, they almost always try to come up with a working hypothesis that is consistent with natural selection theory - Some traits do not look especially likely to increase an individual’s reproductive success o Male Hanuman langurs expend much energy and time trying to kill the infants of the females that they live with in bands in northern India BIOC54H Chapter 1 Tharsini Sivananthajothy January 20, 2012 o They form groups composed of one or more large adult males and several smaller females and their offspring o Young langurs are likely to be attacked when a new adult male becomes dominant usually after having chased off the previous top male o The new male may have come from a another band o They may try to separate infants from their mothers, harassing mothers and biting their babies viciously o Infanticidal males have to deal with female langurs which join forces to fight back in defense of their infants o A male risk injury when he tries to destroy a youngster  Infanticide may not be a product of natural selection but an abnormal, pathological response to overcrowding  Langurs live in higher density areas than in the past due to peopled feeding them o Another langur watcher felt that the Darwinian theory provided a legitimate solution  Killer males could boost their reproductive chances by leaving the mothers of dead infants with no other adaptive option except to mate with the killers  Female might resume her reproductive cycling sooner than otherwise and so might be impregnated sooner by the infant killer  If this was the reasoning, then proximate bases for infanticide could h
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