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Chapter 3

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BIOL 359
Jonathan Witt

BIOL359 - Evolution Winter 2013 Chapter 3: Darwinian Natural Selection 3.1. Artificial Selection: Domestic Animals and Plants - Darwin’s pigeons - Domestic tomato “huge” that can interbreed with several species of wild tomatoes “tiny” o Fw2.2 gene on chromosome 2 encodes protein during early fruit development (job is to repress cell division)  More protein = smaller fruit (Fw2.2 allele is carried by every wild tomato)  Insert wild Fw2.2 allele into 1 fruit = 30% reduction in size  Large fruits were at first rare mutations, but with years of artificial selection, farmers eventually eliminated small-fruited alleles from their stocks 3.2. Evolution by Natural Selection - Four postulates (Darwin and Wallace) o Individuals within populations are variable o The variations among individuals are, at least in part, passed from parents to off-springs o In every generation, some individuals are more successful at surviving and reproducing than others o The survival and reproduction of individuals are not random; instead they are tied to the variation among individuals. The individuals with the most favorable variations, those who are better at surviving and reproducing, are naturally selected  E.g. Mild chili are eaten by packrats  Natural selection of hot chili  E.g. HIV and the ability of the reverse transcriptase to bind AZT  Each theory of the 4 postulates are testable - Darwinian Fitness: The ability of an individual to survive and reproduce in its environment - Adaptation: Trait or characteristic that increases the fitness of an individual compared to individuals without the trait 3.3. The Evolution of Flower Color in an Experimental Snapdragon Population - Postulate I: There is Variation among Individuals o Snapdragon population varied in flower color (3/4 white w/ 2 yellow spots on the lips & 1/4 yellow) - Postulate II: Some of the Variation Is Heritable o Variation in snapdragon color is due to different genotypes of a single gene (2 alleles S & s) o SS or Ss: White with 2 yellow spots on the lips o Ss: Pure yellow - Postulate III: Do Individuals Vary in Their Success at Surviving or Reproducing? o Let free-living bumblebees to pollinate the plants o Plants showed variations in reproductive success, both as pollen donors & as seed mothers - Postulate IV: Is Reproduction Non-random? o White flowers attracted 2x as much bumblebees as yellow flowers o White plants were slightly more robust than yellow plants (produced more seeds per fruit on average) - Next Generation of snapdragons have a higher proportion of white flowers (75%  77%) 3.4. The Evolution of Beak Shape in Galápagos Finches - Ancestor: Small flock of grassquits that invaded 2.3 million years ago - Descendants: 13 species living in Galápagos today o Beak: The primary tool used by birds for feeding (enormous range of beak morphologies)  Insects, spiders, nectar, seeds, leaves & fruits etc. o E.g. Medium ground finches – Seed eaters  Beak size correlates with seed size (bigger beaks handles bigger seeds more efficiently) - Postulate I: Is the Finch Population Variable? o Capture finch and place aluminum band & colored plastic bands on its legs o Depth of beaks varies considerably BIOL359 - Evolution Winter 2013 - Postulate II: Is Some of the Variation among Individuals Heritable? o Environmental variations:  Amount of food individual birds received as chicks  Injuries or abrasions against hard seeds or rocks o Heritability: The proportion of variation observed in a population that is due to variation in genes (0% to 100%)  offspring will resemble parents if inherited alleles  The beak depths of parents and offspring are similar. Some alleles tend to produce shallow beaks, while other alleles tend to produce deeper beaks  BMP4 (Bone Morphogenesis Protein 4) is a signalling molecule that sculpts the shape of bird beaks during embryonic development  Ground species with larger beaks makes BKMP4 mRNA earlier & in larger quantities than ground species with smaller beaks - Postulate III: Do Individuals Vary in Their Success at Surviving or Reproducing? o Finches with deeper beaks survives more than those with smaller beaks o Individuals that survive to breed are more successful at mating and producing offspring than others - Postulate IV: Are Survival and Reproduction Non-random? o Drought changed the number & type of seed available dramatically  Larger birds with deep beaks were favored in drought conditions  Smaller birds with shallow beaks were favored in wet years o The finches hatched after the drought had deeper beaks “population evolved due to drought”  Selection occurs within generations & evolution occurs between generations - Small evolutionary changes “microevolution” over short time spans can accumulate into larger changes over longer time spans “macroevolution” 3.5. The Nature of Natural Selection - Natural Selection Acts on Individuals, but Its Consequences Occur in Populations o The selected individuals lived through the selection event while others died o However, the characteristics of the entire populations changed - Natural Selection Acts on Phenotypes, but Evolution Consists of Changes in Allele Frequencies o Environment selects & alters the frequencies of the phenotypes. If variation in survival is due to differences in genotype then survivors can pass their successful phenotype to offspring (evolution) - Natural Selection is Not Forward Looking o The offspring are better adapted to environments dominated than their parents’ generation. However, if the environment changes again, they might not be adapted to new conditions.  Natural selection adapts populations to conditions that prevailed in the past, not conditions that might occur in the future o Evolution is always a generation behind any changes in the environment - New Traits Can Evolve, Even Though Natural Selection Acts on Existing Traits o Mutations or Recombination in alleles during meiosis  New alleles o Preadaptation: Novel trait that is eventually elaborated by selection into a completely new structure  Panda’s thumb “part of the wrist bone” can help it to get a firm grip on the stalks of bamboo “Improves an individual’s fitness fortuitously” - Natural Selection Does Not Lead to Perfection o E.g. Male mosquito fish, anal fin = copulatory organ, female prefer males with larger gonopodia, but the presence of it is a drag under predatory conditions “No guy is perfect” - Natural Selection Is Non-random, but It Is Not Progressive
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