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PSYC 3100 (39)
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Chapter 2

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University of Guelph
PSYC 3100
Hank Davis

Chapter 2 The Mechanism of Natural Selection - Darwin‟s ideas are important to psychology because theory of evolution by natural selection is not just a theory about the past, but the theory details a process that continually shapes organisms - Theory argues that all living things are sculpted not by a conscious designer like a human engineer but by the inanimate force of natural selection - Proposed process, a mechanism, that could cause evolution – natural selection Darwin‟s Argument 1. Natural populations (giraffes, daisies or, people) could grow exponentially – Darwin selected more leisurely reproducer he could think of, elephants. Assumed that female elephant would begin reproducing by age 30 and would produce 1 offspring/ 10 years, up to total 6 offspring, then die of old age. Calculated that after 500 years a female would have 15 million living descendants, and so on. This is exponential growth! 2. Despite this potential for exponential growth, natural populations normally are relatively stable – Some elephants do not produce 6 offspring over their lifetime 3. Many individuals do not leave as many offspring as they might – Failure often due to limited resources. Every time elephant A reproduces, offsprings use food, water and other resources that would otherwise be available for offspring of elephant B. Populations soon grow to point where resources are inadequate to support more reproduction 4. Those best suited to prevailing environment leave the most offspring – Some individuals better than others at finding food, avoiding predators, fending off parasites, or coping with changes in climate. Better-suited individuals tend to leave more offspring – natural selection 5. Because of heredity, offspring are like their partners – Advantages that allowed certain individuals to reproduce in spite of local environmental challenges will be passed on to next generation. Disadvantages that caused some individuals to fail will not be passed on. Results of natural selection in on generation form starting point for the next 6. Over many generations, natural selection builds individuals that are well adapted to their environment – Traits that make species well suited become more common in population over time - “Survival of the fittest” – not coined by Darwin, but by his peer, Herbert Spencer – not an accurate summary of natural selection; requires 2 important modifications o First – Reproduction, not mere survival, is what makes a difference in evolution o Second – “fittest” must not mean “those that reproduce”, but must refer to the fit between organism and the environment; the fittest are the ones best able to cope with local environmental challenges. Unless a trait is reproduced, it will eventually be lost, what is fit depends on environment because each environment poses a unique set of challenges Evidence in Favor of Evolution 1. The existence and pattern of the fossil record – Shows multitude of plausible ancestor-descendant relationships 2. Homology – Fact that the same structural elements are found in very different kinds of creatures, often the function of these elements is the same in different organisms, but sometimes their function is different 3. The universality of genetic code – The genetic code of all living things is structurally the same; genes are made out of the same elements, arranged according to the same rules 4. The analogy with animal breeding – Darwin noted how many different breeds of dogs, sheep, and other domesticated species had been created simply by selective breeding – E.g. a farmer wants denser wool on his sheep, so he breeds the sheep with the densest wool over many generations 5. Direct observation A Mistaken View of Evolution: The Great Chain of Being - Great Chain of being – idea that life forms can be ordered in a ladder of complexity and sophistication - Humans stand between animals and angels in a sequence that rises from inanimate objects to Gods - God  Angels  Humans  Apes  Cats  Clams  Plants  Rocks - Beings share certain similarities with their nearest neighbours and that beings higher on the scale are considered to have „higher‟ natures - Mistaken for 2 reasons o First - Implies evolution proceeds in a linear fashion, with „higher‟ creatures directly related to „lower‟ animals o Second – Implies progress – „higher‟ creatures are better and more sophisticated than those below EVOLUTION DOES NOT MEAN PROGRESS, ONLY CHANGE Natural Selection Crafts Adaptations - Designs for reproduction are transmitted to offspring; other kinds of designs are not and therefore disappear – Called adaptations o Preserved by natural selection because they aid reproduction in some way - Naturally occurring variations among individuals are constantly being tested against the environment - Any variation that aids reproduction, whether it happens to be in the liver, the lung, or the brain, will be passed to the next generation - Improvement in the brain that aids reproduction is just as much a design for reproduction as is an improvement in the ovaries - Adaptations exist because they enhance the fit between the organs, and the environment, and thereby increase the chances of reproduction Adaptations Are Formed Slowly - Natural pace of evolution is measured in generations. In each generation individuals are tested against the prevailing environment, and the fitter ones pass on traits. Traits cannot spread any faster than the population can reproduce - Natural selection can choose only naturally occurring alternatives; it cannot create entirely new variants (e.g. selection cannot favour an organism with 3 eyes unless a 3-eyed individual actually exists) - Evolution can happen quickly when selection is strong, that is, when successful individuals leave many more offspring than unsuccessful ones Cumulative Selection - Selection can add refinements over time to build complex adaptations - Eyes must have been built through the accumulation of small improvements, each one an adaptation in its own right - Natural selection will retain any chance variation that provides a reproductive edge. This way, it can gradually build very complex adaptations form simpler elements Some Traits Are Not Adaptations - Nonadaptive features could arise simply by chance; or they could be incidental side effects of other traits that are adaptations - Adaptations are often structurally complex, typically made up of several integrated parts o Show evidence of design for particular function in that parts work together to efficiently accomplish that function Adaptations Are Specialized - Each adaptation is narrowly suited to solving a particular kind of problem o Each organ is specialized for a narrow function - Selection is constantly refining adaptations to achieve better and more efficient results - Each adaptation tends to be specialized for solving a narrow class of problems simply because less specialized designs did not work so well, and selection tended to weed them out Adaptations Are Costly - Everything has to be paid for one way or another, even if it seems free at first o Same is true for adaptations - Variety of costs – costs associated with assembling the trait during development and costs associated with maintaining it metabolically - Materials and energy used could have been put to some other use and probably would have been put to another use unless the trait gave a reproductive benefit sufficient to justify its costs A General Method for Studying Adaptations - Comparative method – traits the outcomes of evolution as quasi-experiments, experiments in adaptation performed by natural selection - Relies on 2 kinds of comparisons o First – involves closely related forms that differ on the trait of interest. Expect closely related forms to be similar o Second – involves distantly related forms that all share a trait. Distantly related forms are not expected to be very similar, when they share a trait, selection is again implicated Adaptations May Be Out of Date - Of we combine the slow pace of evolution with the reality of changing environments, organisms are often better adapted to the past than to the present - Adaptations we see around us today are here because they have survived the challenges of the past - Some organisms may experience relatively slow environmental change – past and present environments are similar and so past adaptations tend to still be effective - Stable environments selection tends to prevent change in order to maintain previously constructed adaptations - Rapidly changing environments make past adaptations out of date Adaptations Often Fail in Nov
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