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Evolution
 

Evolution

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    Evolution Evolution Presentation Transcript

    • 5b: Evolution Evolution
    • Darwin
      • Darwin observed several things on his voyage on the Beagle:
        • Geological oddities: marine shells in the mountains, for example
        • Fossils: differed from modern examples
        • Animals: similar to some in England but different as well- the Patagonian hare, for example
    • Darwin
      • Darwin’s observations:
        • Geographic change in species: Variation due to environmental differences
          • Galapagos finches:many species with different beaks, living on different islands, eating different things
          • Galapagos tortoises: One type of tortoise per island
        • Adaptation : a feature that allows the organism to survive better in its environment
    • Natural Selection
      • Darwin’s observations led him to propose a method by which adaptations might arise: Natural Selection
      • The 4 steps, revisited:
        • Heritable variations passed to offspring
        • More offspring produced than environment can support
        • Favorable traits in some offspring lead to those having a higher survival rate
        • Over time, more and more of the population possesses the favorable traits
    • Natural Selection
      • IMPORTANT:
        • Variation arises from RANDOM changes that happen to arise from genetic mutations-- there is no directedness or ability to predict future needs
        • Some mutations are detrimental, and some are neutral
    • Natural Selection
      • IMPORTANT:
        • Natural selection is ongoing continuously because the environment is constantly changing
    • Natural Selection
      • Because resources are always limited, some individuals will fail to survive and reproduce-- thus removing their genes from the population
        • Failing to reproduce is functionally the same as failing to survive
    • Natural Selection
      • Fitness varies among individuals
        • It is the reproductive success of one individual compared to others in the population
        • More fit animal:
          • Uses more resources
          • Avoids death
          • Leaves more offspring
    • Natural vs. Artificial Selection
      • Humans practice artificial selection on domestic animals and plants
        • Individuals with desired traits are bred, while those with undesired traits are not
          • Eventually, offspring change to resemble the predetermined idea the humans had
          • Ex. Any domestic animal, vegetables
    • Natural vs. Artificial Selection
      • In artificial selection, the result is predesired, and the breeding is directed
      • In natural selection, the result is determined by the environment with no direction or desire involved
    • Natural Selection- Terms
      • Selection pressure: What acts on an animal to either increase or decrease its chances to survive and/ or reproduce
        • Selected for: the trait conveys advantage
        • Selected against: the trait conveys disadvantage
    • Adaptations
      • Adaptations may take many generations to evolve
      • Explain why animals are suited to their environment and ‘lifestyle’
      • The more ‘ adaptive ’ a trait is, the greater the advantage that the individual has
    • Convergent Evolution
      • Convergent Evolution : when unrelated species share similar characteristics
        • Ex: Flippers on manatees, penguins and sea turtles
        • Ex: The horny toad of the US, and the thorny devil of Australia
    • Wallace
      • Important to realize that Darwin was not the only guy to come up with Natural Selection- Alfred Wallace came to the same conclusions studying different organisms, and they first published at the same time
    • Evidence for Evolution
      • Fossils:
        • Fossils frequently show the pattern of a succession of species from simple to more complex, though this is not universal
        • Transitional fossils: Archaeopteryx
          • Whales: see p. 226
    • Evidence for Evolution
      • Biogeography: the study of the distribution of organisms
        • Different mix of organisms when geography separates areas
        • Ex: Cacti and Spurges in deserts
        • Ex: Marsupials in Australia
          • Americas and Australia connected at one time, but marsupials able to evolve separately from placental mammals in Australia
    • Evidence for Evolution
      • Anatomy:
        • Vestigial structures: fully developed in one group, but reduced and nonfunctional in others
          • Ex: Appendix, tailbone in humans
          • Ex: Hipbones in snakes
          • Ex: Wings in flightless birds
        • These occur because organisms inherit structures from their ancestors
    • Evidence for Evolution
      • Anatomy:
        • Homologous structures: Anatomically similar structures, frequently used for different purposes, explained by a common ancestor
          • Ex: Horse legs vs. bird wings. vs. bat wings vs. whale flippers vs. human arms
    • Homologous Structures
      • Example from humans:
        • Pharyngeal pouches- seen in all vertebrate embryos
          • Develop into gills in fish and amphibians
          • Develop into tonsils, inner ear canal, thymus and glands in humans
        • It is easier (therefore more likely to arise randomly) to modify an existing structure than create an entirely new one
    • Evidence for Evolution
      • Molecular Evidence:
        • All living organisms use the same biological molecules-- DNA, RNA, ATP, etc.
        • DNA code is the same for all living organisms
        • Many of the same genes have been modified to result in the wide variety we see
        • Gene sequences are more different the further apart organisms are evolutionarily
    • Evolution on a Small Scale
      • Remember: individual organisms can not evolve, populations/ species evolve
      • Microevolution : small measurable changes in a population from generation to generation
    • Hardy-Weinberg Equilibrium
      • Hardy-Weinberg Equilibrium allows us to measure small changes in the frequency of alleles in a population
        • This is how we know microevolution is occurring
        • Population genetics is the study of the occurrence and flow of genes in populations
        • We will look at the example of peppered moths
    • Peppered Moths
      • Peppered moths can be either light or dark, and this is controlled by a single set of alleles:
          • D= dark color
          • d= light color
        • We know the frequency of genotypes in the population:
          • 4% DD
          • 32% Dd
          • 64% dd
    • Peppered Moths
      • From genotypes, we can figure out the frequency of each allele in the population:
      • .04 + .16 =.2 D
      • .16 + .64 =.8 d
      • Frequency of each gamete type will be the same as the frequency of occurrence of each allele
      • We can use a Punnett square to figure out the gene frequencies in the next generation
    • Peppered Moths ** This is a punnett square for freqencies, not individuals .2D .8d .2D .04DD .16Dd .8d .16Dd .64dd
    • Peppered Moths
      • Notice: the allele frequencies in the next generation are EXACTLY THE SAME
      • This means that sexual reproduction alone can not change the frequency of gene/ allele frequencies in a population, provided some assumptions are met
        • Notice the dominant allele does not increase in frequency
        • This is Hardy-Weinberg equilibrium
    • The Assumptions
      • No mutations: allelic changes do not occur
      • No gene flow into or out of the population
      • Random mating
      • No genetic drift- large population, changes in frequency due to chance are insignificant
      • No selection: one genotype is not favored over another
    • HW Equilibrium
      • However, those assumptions are basically never met
      • Changes in allele frequency do occur, and we can see those by comparing RL to the numbers predicted by HW equilibrium
      • Also, we know that deviation from the assumptions is what causes evolution
    • Back to the moths
      • After the industrial revolution, peppered moths that were light did not blend into trees with lots of soot on their bark, and they were eaten by birds
      • The frequency of D then increased in the population to around 80%
    • Causes of microevolution
      • Genetic mutations
        • The ultimate source for allele differences
        • Mutations can be harmful in one environment and helpful in another
    • Causes of microevolution
      • Gene flow:
        • Movement of alleles among populations by migrating animals
        • Can increase variation in a population by introducing new mutations
        • Can prevent speciation by making the gene pool the same across populations
    • Causes of microevolution
      • Nonrandom mating:
        • Assortive mating: individuals mate with those that are similar
          • Ex. tall people mating with each other
          • Causes two groups of homozygotes to become more common, and hets to become less common
        • Sexual selection: Favors characteristics that increase the chance of mating
          • Ex. Male birds with bright colors or crazy feathers
          • Animals that compete for mates
    • Causes of microevolution
      • Genetic drift: changes due to chance
        • Allele frequencies ‘drift’ over time
        • More common in small populations, like on islands or other isolated areas
        • Can result in the loss of rare alleles completely, and fixation of others as they are the only ones left
    • Causes of microevolution
      • Bottleneck Effect: Catastrophic loss of most of a population, only a few individuals survive by chance
        • Those few are all that is left to pass on to later generations
          • Ex. cheetas and poor sperm
      • Founder Effect: Rare alleles are more common in a population isolated from the main population
        • Only a few individuals founded the new population, so their alleles are the ones that are represented
          • Ex. Amish have higher rate of two limb mutations
    • Causes: Natural Selection
      • There are several types of selection, so it gets its own category
      • Directional Selection : an extreme phenotype is more successful and population shifts in that direction
        • Ex. the peppered moths
        • Ex. drug resistance in bacteria
        • Ex. Anacondas
    • Natural Selection
      • Stabilizing Selection : an intermediate phenotype is best, extremes are selected against
        • Ex. male anacondas
      • Disruptive Selection : Two or more extremes are favored over the intermediate
        • Favors polymorphism: the occurrence of different forms in the same species
        • Ex. Snail with two different shell colorings, in two different habitats
    • Perfection?
      • NO!! Animals do NOT tend to be perfectly adapted
        • Evolution does not start from scratch, it is modifying what is already there
        • Compromises: there may be costs to the adaptive benefit
        • Sexual selection may not result in adaptive traits- giant feathers may be energetically expensive and make it difficult to fly, for example
    • Maintenance of Variation
      • Populations will always show variation
      • New mutations always arising
      • Gene flow may be occurring
      • Diploidy and heterozygotes:
        • Only alleles that are expressed in phenotype can feel selective pressures
        • Heterozygous animals can protect alleles that might otherwise be selected against
          • The homozygous recessive will still occur occasionally
    • Example of Sickle Cells
      • Individuals with sickle cell are homozygous for S and typically die very young due to change in shape of RBCs
      • Individuals that are heterozygous are OK, because cells are normal shape until in low O2 environment
      • Individuals that are homozygous A are usually the most fit (most of us fall into this group)
    • Sickle Cell
      • However, individuals with African descent have higher frequency of S- more sickle cell disease, but also:
      • Heterozygote is immune to malaria
      • So, in parts of Africa w/ malaria, heterozygote is selected for, but homozygotes continue to exist to do statistical frequency of each genotype in the next generation