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Natural Selection

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York University
BIOL 1500
Alexander Mills

TEXTBOOK NOTES 3 BIOL1001 – Evolution by Natural Selection – Pg. 56-58 & Pg. 74-76 & Pg. 76-88 Acclimation  describes changes in an individual’s phenotype that occur in response to changes in environmental conditions  Phenotypic changes due to acclimation are not passed on to offspring because no alleles have changed in composition  ^ As a result, acclimation does not cause evolution Evolution is Not Goal Directed  It is wrong to say that individuals evolve because of their “need” for a trait  The mutation that created the mutant allele occurred randomly, due to an error during DNA synthesis, and it just happened to be advantageous when the environment changed  The mutation that conferred resistance did not occur because of the presence of the drug since it just happened  Adaptations do not occur because organisms want or need them Evolution is Not Progressive  Complex traits are routinely lost or simplified over time as a result of evolution by natural selection  Tapeworms lost their digestive tract as a result of evolution by natural selection, but they did not become bigger or stronger or more complex No Such Thing as a Higher or Lower Organism  Lamarck’s conception of the evolutionary process - organism progress over time to higher and higher levels on a change of being  Under Aristotle’s and Lamarck’s hypothesis, it is sensible to refer to “higher” and “lower” organisms  A human is no higher than its tapeworm parasite; each is well adapted to its environment  All organisms are adapted to their environment, and are related by common ancestry Organisms do not Act for the Good of the Specie  The good-of-the-species hypothesis maintains that the behaviour evolved because the group benefits  It is not possible for traits that lead individuals to sacrifice themselves for the good of the species to evolve by natural selection because their alleles die and do not produce offspring Nonadaptive Traits  Vestigial traits such as the human coccyx (tailbone), goose bumps, and appendix do not increase the fitness of individuals with those traits  The structures are not adaptive  ^ They exist simply because they were present in the ancestral population  Human males have rudimentary mammary glands  ^The structures are not adaptive  They exist only because nipples form in the human embryo before sex hormones begin directing the development of male organs instead of female organs  Evolution by natural selection does not lead to “perfection” Genetic Constraints  Although individuals with deep beaks survived better than individuals with shallow beaks, birds with particularly narrow beaks survived better than individuals with wider beaks  Narrow beaks concentrate the twisting force more efficiently than wider beaks, so they are especially useful for cracking the fruits  Wider beaks evolved anyway due to a type of constraint called a genetic correlation  ^ Genetic correlations occur because of pleiotropy – in which a single allele affects multiple traits  ^ In this case, selection on alleles for one trait (increased beak depth) cause a correlated, though suboptimal, increase in another trait (beak width)  Birds can sense magnetic fields and see ultraviolet light  ^ Cannot exist in humans because the requisite genes are lacking Fitness Trade-Offs  In nature, selection occurs in the context of fitness trade-offs  A fitness trade-off: is a compromise between traits in terms of how those traits perform in the environment  Even if large size is advantageous in an environment, there is always counteracting selection that prevents individuals from getting even bigger  Because selection acts on many traits at once, every adaptation is a compromise Historical Constraints  Adaptations are constrained by history because all traits have evolved from previously existing traits  Not all traits are adaptive, and even adaptive traits are constrained by genetic and historical factors  ^ Natural selection is not the only process that causes evolutionary change TEXTBOOK PAGES: 74-76 Genetic Drift  Instead of each allele being sampled in exactly its original frequency when offspring formed, as the Hardy-Weinberg principle assumes, a chance sampling error occurs  T is random with respect to fitness: the allele frequency changes it produces are not adaptive  It is more pronounced in small populations - In the computer simulation, allele frequencies changed much less in the large population than the small population - In a small population, the alleles frequency will either head off to being fixed or lost whereas in a large population, the allele frequency will be more stable and consistent - Over time, genetic drift can lead to the random loss or fixation of alleles - When random loss or fixation occurs, genetic variation in the population declines Experimental Studies of Genetic Drift  Kerr and Wright started with a large laboratory population of fruit flies that contained a genetic marker which is a specific allele that causes a distinctive phenotype - In this case, the marker was the morphology of bristles - Fruits flies have bent or straight bristles - 2 alleles were in their lab population: normal and (straight) and forked (bent) - 96 cages were in the lab - 4 adults females and four adult males of the fruit flies were placed in each - The allele frequency in these cages were 0.5 for each allele - Since these alleles do not affect the fitness of the fruit flies, Kerr and Wright would know that if changes in frequency of these alleles did occur, it will not be due to natural selection - 4 males and females from the F1 generation of offspring that were produced, were chosen as random to mate - Did this for this cage (96 cages) - The only evolutionary process operating during the experiment was genetic drift - After 16 generations, forked bristles were found on all of the individuals in 29 populations - ^ forked allele is fixed and the normal allele is lost - In 41 other populations, all individuals had normal bristles so the forked allele was lost due to chance - In the rest of the 26 populations, both alleles were present - 70 out of the 96 populations, genetic drift had reduced allelic diversity at his gene to zero  As predicted, genetic drift decreased genetic variation within populations and increased genetic differences between populations Causes of Genetic Drift  It can occur by any process or event that involves sampling, not just the sampling of gametes that occurs during fertilization or the loss of unlucky individuals due to accidents  The smaller the population, the higher the likelihood that genetic drift will result not only in differences in allele frequencies, but also in loss of alleles Founders Effect  A founder event: When a group of individuals immigrates to a new geographic area and establishes a new population  If the group is small enough, the allele frequencies in the new population are almost guaranteed to be different from those in the source population  Founder Effect: When a change is allele frequencies that occurs when a new population is established  Each time a founder event occurs, a founder effect is likely to accompany it, changing allele frequencies through genetic drift Genetic Bottleneck  Population bottleneck: if a large population experiences a sudden reduction in size  ^ comes from the metaphor: few individuals passing through the neck of a bottle by chance  Disease outbreaks, natural catastrophes cause
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