The Nature of Natural Selection.docx

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Biomedical Engineering
BMEN 515
William Huddleston

The Nature of Natural Selection Although the Theory of Evolution by Natural Selection can be stated concisely, tested rigorously in natural populations, and validated, it can be difficult to understand thoroughly. o One reason: Darwin’s theory descent with modification is essentially a statistical process: a change in the trait distributions of populations.  Natural Selection Acts on Individual, but Its Consequences Occur in Populations o When HIV strains were selected by exposure to AZT, or finch populations were selected by changes in seed availability; none of the selected individuals changed in any way. o They simply lived through the selection event while others died or reproduce more than the competing virions or birds o What changed after the selection process was the characteristics of the populations of virions and finches, not the individuals themselves. o A higher frequency of HIV virions in the population were able to replicate in the presence of AZT and a higher proportion of finches had deep beaks.  Natural Selection Acts on Phenotypes but Evolution Consists of Changes in Allele Frequencies o Selection would have altered the frequencies of the phenotypes in the population, but in the next generation the phenotype distribution might have gone back to what it was before selection occurred. o Only when the survivors of selection pass their successful phenotypes to their offspring, via genotypes that help determine phenotypes, does natural selection cause populations to change from one generation to the next.  Natural Selection Is Not Forward Looking o Natural selection adapts populations to conditions that prevailed in the past, not conditions that might occur in the future o Adapting for the future is impossible and is often a misconception  New Traits Can Evolve, Even Though Natural Selection Acts on Existing Traits o The evolution of new traits is possible for two reasons:  During reproduction in sexual species, meiosis and fertilization recombine existing alleles into new genotypes.  Mutation and recombination yield new suites of traits for selection to sort  Read example on page 91. o Persistent natural selections can lead to the evolution of entirely new functions for existing behaviour (e.g. the giant panda’s thumb; read on page 92). o A trait that is used in a novel way and is eventually elaborated by selection into a completely new structure is known as a preadaption.  Preadaption improves an individual’s fitness fortuitously –not because natural selection is conscious or forward looking.  Natural Selection Does Not Lead to Perfection o Evolution does not result in organisms that are perfect o Each population evolves a phenotypes that strikes a compromise between opposing agents of selection o Natural selection cannot simultaneously optimize; it leads to adaptation, not perfection  Natural Selection is Nonrandom, but It Is Not Progressive o Mutation and recombination are random with respect to the changes they produce in phenotypes.  But natural selection, the mechanism that sorts among var
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