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Chapter 3

Chapter 3- Darwinian Natural Selection.docx

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BIOL 359
Jonathan Witt

BIOL 359 Lecture 4 Janice Wong Evolution Introduction to Natural Selection Chapter 3: Darwinian Natural Selection 3.1| Artificial Selection: Domestic Animals and Plants • To increase the frequency of desirable traits in their stocks, plant and animal breeders employ artificial selection 3.2| Evolution by Natural Selection Darwins Postulates Claims 1. Individuals within populations are variable 2. The variations among individuals are, at least in part, passed from parents to offspring 3. In every generation, some individuals are more successful at surviving and reproducing than others 4. The survival and reproduction of individuals are not random; instead they are tied to variation among individuals. The individuals with the most favourable variations, those who are better and reproducing, are naturally selected. • The characteristics of the population will change slightly with each succeeding generation • Darwinian evolution: gradual change in populations over time • Natural selection is a process that produces descent with modification, or evolution • Darwinian fitness: the ability of an individual to survive and reproduce in its environment • Adaptation: a trait or characteristic of an organism that increases its fitness compared to individuals without the trait • The Theory of Evolution by Natural Selection is testable 3.3| The Evolution of Flower Colour in an Experimental Snapdragon Population Postulate 1: There is Variation among Individuals • The snapdragons in Jones and Reithel’s population varied in flower colour Postulate 2: Some of the Variation is Heritable • The variation in colour was due to differences in the plants’ genotypes for a single gene • The gene has two alleles S and s Postulate 3: Do Individuals Vary in Their Success at surviving or Reproducing? • They let free-living bumblebees pollinate the plats • They tracked the number of times bees visited each flower • The plants shower considerable variation in reproductive success, both as pollen and donors and as seed mothers Postulate 4: Is Reproduction Nonrandom • All yellow flowers lack nectar guides and so might be less attractive to bees • Found that white flowers attracted twice as many bee visits as yellow flowers BIOL 359 Lecture 4 Janice Wong Evolution Introduction to Natural Selection • Through both pollen donation and seed production, white plants had more reproductive success Testing Darwin’s Prediction: Did the Population Evolve? • The snapdragon population evolved as predicted • White plants had higher reproductive success than yellow, and since flower color is determined by genes, the next generation of snapdragons should have a higher proportion of white flowers 3.4| The Evolution of Beak Shape in Galápagos Finches • All species of Darwin’s finches are similar in size and coloration • Lengths range from 4 to 6 inches and from brown to black in color • Have remarkable variation in the size and shape of their beaks • Galápagos finches reflects the diversity of the foods they eat • Beak size correlates to the size of the seeds harvested Testing Postulate 1: Is the Finch Population Variable? • Yes there is variability. From wing length, tail length, beak length and especially beak depth Testing Postulate 2: Is Some of the Variation among Individuals Heritable? • Finches could vary in beak depth because the environments or their genotypes are different, or both • Variation in the amount of food that individual birds happened to have receive as chicks can lead to variation in beak depths in adults • Injuries or abrasion against hard seeds or rocks can also affect beak size and depth • The heritability of a trait is defined as the proportion of the variation observed in a population that is due to variation in genes • Parents with shallow beaks tend to have chicks with shallow beaks • Parents with deep beaks tend to have chicks with deep beaks • The beaks depths of parents and offspring are similar • This observation suggests that beak depth is genetically based and can be passed to offspring • Do not know the specific genes responsible for the variation in beak size • Bone morphogenic protein 4, BMP4, is a signalling molecule that helps sculpt the shape of bird beaks • Finches with larger beaks make BMP4 mRNA earlier and in large quantities than finches with smaller beaks Testing Postulate 3: Do Individuals Vary in Their Success at Surviving or Reproducing? • There was an enormous drought in 1977, and plants made few flowers and few seeds • The medium ground finches did not even try to breed • Over the course of 20 moths, 84% of the G.fortis disappeared • In every natural population studied, more offspring are produced each generation than survive to breed • Some of the individuals that survive to breed are more successful at mating and producing offspring than others BIOL 359 Lecture 4 Janice Wong Evolution Introduction to Natural Selection • Just as variation in survival does, variation in reproductive success represents selection Testing Postulate 4: • During the drought, finches with larger, deeper beaks had an advantage in feeding, and thus in surviving (opposite situation if the weather was wet, smaller birds would be favoured) Testing Darwin’s Prediction: Did the Population Evolve? • As a result of the drought, the finch population evolved • Selection occurs within generations; evolution occurs between generations • The chicks that hatched in 1978 had deeper beaks on average compared to the birds hatched in 1976 • Slide: Shows the pattern of change in the population averages for 3 traits • Shows what happens during the drought. The finch population evolved significantly larger avg. beak size in 1977. The population remained at this large mean beak size until the mid-1980s then evolved back to the mean beak size. There it remained until the next drought • The drought in 2003-2004. Large beaks died at a higher rate due to competition for food sources. The average bird in 2011 had a sig. sharper beak and smaller than the avg. bird in 1970s • The population evolves 3.5| The Nature of Natural Selection Natural Selection Acts on Individuals, but what changes is Populations • 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 anyway • They simply lived through the selection even while others died or reproduced more than competing virions or birds • What changed was the characteristics of the population, not the individuals themselves • The average active site sequence in reverse transcriptase changed because certain mutants did a better job of making virions Natural Selection Acts on Phenotypes, but Evolution Consists of Changes in Allele Frequencies • Only when 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 • As a result, the new phenotypic distribution seen among the survivors persisted into the next generation • Populations evolve only if traits are heritable Natural Selection is Not Forward Looking • Natural selection adapts populations to conditions that prevailed in the past, not conditions that might occur in the future • Each generation is descended from the survivors of selection by the environmental conditions that prevailed in the generation before • If the environment changed again during the lifetime of these offspring, they might not be adapted to the new conditions • Evolution is always a generation behind any changes in the environment BIOL 359 Lecture 4 Janice Wong Evolution Introduction to Natural Selection New Traits Can Evolve, Even Though Natural Selection Acts on Existing Traits • Differences in survival or reproduction can only occur among variants that already exist • However new traits can evolve • 1. During reproduction in all species, mutations produce new alleles • During reproduction in sexual species, meiosis and fertilization recombine existing alleles into new genotypes • Mutation and recombination yield new suits of traits for natural selection to sort • Persistent natural selection can lead to the evolution of entirely new functions for existing behaviours, structures or genes o Ex. Giant panda’s thumb o Pandas with bigger radial sesamoids had firmer grips, fed more efficiently, and consequently had more offspring o As a result of continued selection over many generations, the average size of the bone increased in the population • This is called preadaptation: a trait that is used in a novel way and is eventually elaborated on by selection into a completely new structure o Improves an individual’s fitness by chance, not because natural selection is conscious or forward looking Natural Selection Does Not Lead to Perfection • Consider the male mosquito fish, whose anal fin is modified to serve as a copulatory organ • Females like larger ones • But when predators attack, the bigger fins is a drag, and slows a males escape • A perfect male would be irresistible to females and fleet enough to evade any predator • Population evolves a phenotype that strikes a compromise between opposing agents of selection • Natural selection does not optimize all traits, it leads to adaptation not perfection Natural Selection if Nonrandom, but It is Not Progressive • Mutation and recombination, the processes that generation genetic variation, are random with respect to the changes they produce in phenotypes • Natural selection, the mechanism that sorts among variant phenotypes and genotypes, is the opposite of random • It is the non-random superiority at survival and reproduction of some variants over others • Evolution by means of natural selection in non-random, instead it increases adaptation to the environment • Evolution makes populations “better” only in the sense of increasing their average
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