BIOLOGY 103
      Chapter 22:  VARIATION & SELECTION

 
      most important concept:
When a particular allele increases the survivorship or fecundity or immigration of individuals, the frequency of that allele increases in the population's next generation.
TEXTBOOK, the most important parts

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The main focus is genetic variation.  Keep in mind that genes are the important link connecting the topics before and after fall break.

To measure genetic variation (and even just to think about genetic variation) biologists use frequency distributions.  You are already familiar with other frequency distributions, like bell-shaped curves and the grade distribution for the last test.  When a population is evolving, there is a change in the frequency distribution of its genetic characteristics--     its bell-shaped curves become fatter or skinnier or skewed in one or both directions.  Biologists think about evolving populations this way:  When a particular allele increases survivorship or fecundity or immigration of individuals, the frequency of that allele increases in the population's next generation and the graph of the frequency distribution changes shape.   A change in frequency distribution is evidence of evolution in progress.

MOST IMPORTANT POINTS FROM THE TEXTBOOK

  • Review genetics concepts, especially polymorphic,  mutation, genotype, phenotype
  • Allele frequency is a simple concept--like grade distributions and the graphs of genetic (allele) frequency distributions which are on pages 422, 425, 440, 441, and 442 and many more which may appear in class or on upcoming tests, etc.  Make sure you know what the graphs show and why they are important.

  • The Hardy-Weinberg Law (pages 433 ff.) is a mathematical proof (but you can ignore the math unless you're interested) that frequency distribution of alleles stays in equilibrium; in other words, genetic frequency cannot change from one generation to another unless something "selects" one allele over its alternatives:  these "somethings" which do change the equilibrium are important and listed on page 433.  These five factors which can change the Hardy-Weinberg equilibrium give biologists a method of measuring the rate of evolution and they guide biologists to focus on the possible sources of changes in frequency distribution.

    • connect the four mechanisms on p. 429 to the five assumptions on p. 433 ; then try to imagine replacing the marbles on fig. 22.3 with individual genotypes.
    • Genetic Drift is related to several other concepts:   the founder effect and genetic bottlenecks or sampling errors in that the population size becomes so small that chance causes changes in the frequency distribution.  Small population size should be #1 on the list of conditions which upset Hardy-Weinberg  equilibrium.  Genetic drift and the founder effect and genetic bottlenecks are important because they explain how evolution could occur without natural selection  (=how some populations have genetic differences which are not adaptations).  Note that gene flow has an opposite effect of bottlenecks:  gene flow increases variation within a population and makes the population more like similar populations.  Which of the five factors does "gene flow" violate?
    • Also, just in case you need to be reminded, like most previous chapters, again, we're really talking about sex again. See p. 442 ff.  Non-random mating is when certain genetic characteristics cause some individuals to be more likely to mate.  When genetic characteristics evolve only for attracting mates (and apparently do not contribute to "survival of the fittest") we call this sexual selection.  The sexiest alleles become more frequent in the next generation.  In Hardy-Weinberg equilibrium, all alleles must be equally sexy so that mating is completely random.

    • Frequency distributions are NOT the same as the 3/4:1/4 ratios you find in F2's.  If you keep getting confused, Hardy and/or Weinberg had to explain it to Punnett, too.  Frequency distributions show how common an allele is in a real population (with or without random mating), not just in the offspring of hybrids.

  1.     Hardy-Weinberg Equilibrium =

    • No evolution
    • No natural selection
    • No adapting
    • No genetic change
    • No change in frequency distribution of polymorphic alleles

    (So are any populations actually in Hardy-Weinberg equilibrium?   Biologists say NO.  Hardy-Weinberg lets us calculate a frequency distribution "control" or null model for estimating rates of evolution.)
  • The most important concept = Natural Selection pp. 440 ff.  Adaptations are phenotypes which increase the probability of success in the particular habitat where individuals compete to reproduce, to avoid predators and to capture prey,  maybe to migrate etc.  These adaptations are the focus of most news stories on natural selection.  Again, you must try to imagine that the "selective agents" or factors in a particular habitat would tend to "weed out" individuals with noncompetitive phenotypes, controlled by genes which affect camouflage, speed, visual acuity, agility, intelligence, metabolism, number and sizes of eggs in a clutch, sizes of seeds and hardness of their coats, beak size of seed-eating birds, anti-herbivore toxins in leaves, sexy smells, hairiness, hair color....  The alleles of the most competitive individuals become more frequent in the next generation.  In other words, the population becomes better adapted.  In Hardy-Weinberg equilibrium, all alleles must be equally advantageous so that no "agents of selection" have any effect on relative survival or reproduction.  In Hardy-Weinberg equilibrium, adaptation does NOT occur.
When a particular allele increases the survivorship or fecundity or immigration of individuals, the frequency of that allele increases in the population's next generation.

This concept is a re-statement of Natural Selection:  populations evolve because individuals with favorable characteristics have more offspring.  Now that we know more about genetics of individuals and genetic variation in populations, we can develop a more sophisticated understanding of natural selection.  This part of the chapter is about fitness, which biologists define as the ability of an individual to produce more offspring who survive and then reproduce themselves, transmitting these "successful" genes to yet another generation.  Individuals with greater fitness contribute more genes to the gene pool of future generations of their species or at least their population.

  • In this part of the chapter we are concerned with how the genetic composition of a population adapts to a particular ecological niche when individuals compete to reproduce, to avoid predators and to capture prey,  maybe to migrate  etc.  Imagine how an individual's fitness can be changed by genes which control camouflage, speed, visual acuity, agility, intelligence, metabolism, number and sizes of eggs in a clutch, sizes of seeds and hardness of their coats, beak size of seed-eating birds, anti-herbivore toxins in leaves, sexy smells, hairiness, hair color....  In fact, it's hard to think of a gene which does not have some impact on reproduction or at least surviving long enough to reproduce.
  • In a changing world, however, a genetic advantage is only temporary.  A large seed or egg reduces juvenile mortality only until the advent of a large predator.  A sexy smell which attracts mates or pollinators may also attract parasites.  Populations or species without genetic alternatives become extinct when their world changes.  Species which have avoided extinction seem to be those with flexibility in their gene pools; this flexibility is usually called genetic variation or genetic diversity.
  • Make sure that you understand how the examples in the book connect to the information above; and be able to define, compare and contrast, and USE these terms:
Sexual Selection, a continuing controversy.  Darwin invented this term to explain cases in which bright colors and fancy equipment, like a peacock's tail, seem to evolve simply to attract mates despite their probable disadvantages in survival.   Some biologists agree with Darwin on this point, but others think that the "handicaps" are like advertisements for superior genes and resistance to parasitic disease.  Hypotheses about attracting mates and competing for mates is a big field in biology today.  You might want to read Why Is Sex Fun? by Biologist Jared Diamond or check out some of these articles:

 

When a particular allele increases survivorship or fecundity or immigration of individuals, the frequency of that allele increases in the population's next generation.
  • CHECKLIST:  evolution, allele frequencies, natural selection, populations, mutation, gene flow, migration, genetic drift, adaptation, fitness, genetic diversity, resistance to disease, Hardy-Weinberg principle or Law, gene pool, random allele frequency changes, random mating, nonrandom mating, sexual selection, heterozygote advantage, mutation rates, genome, genetic marker, "fixed" alleles, genetic bottleneck, inbreeding, inbreeding depression, "self-incompatibility" loci, directional selection, oscillating directional selection, stabilizing  selection, disruptive selection, sexual dimorphism, energy investment, parental care investment,  null model,
  • SAMPLE QUIZ and TEST QUESTIONS
    1.   A change in the relative proportions of alles in the gene pool is the Hardy-Weinberg definition of
      [a] artificial selection. [b] evolution. [c] fit individual.
      [d] founder or founder effect. [e] species.
    2. Which of the following is NOT a condition of the Hardy-Weinberg Law?
      [a] all mating must be random.    [b]  natural selection must occur.
      [c] the gene pool must be large enough to be unaffected by chance events.

    3. Since the genes for hemoglobin include normal alleles and also alleles which cause sickle-cell disease, geneticists say that the hemoglobin trait or locus is
      [a] hybrid. [b] mutant. [c] pleiotropic. [d] polymorphic

    4. A change in a gene pool simply by chance occurs in the founder effect and also in        [a] artificial selection. [b] disruptive selection. [c] genetic drift. [d] pleiotropy.

    5. Which of these is most likely to be the label on the vertical axis of a gene frequency distribution?
      [a] alleles. [b] genotypes. [c] phenotypes. [d] percentage of population.  you want answers?

      6.

    • And these:   

    1.  Survival of the fittest = "may the best team win"....does the best adapted team always win?            Explain.
    2. Darwin's Theory of Natural Selection explains evolution.    Is evolution itself also a theory?
      • Or is evolution more like a law?
      • Or is it only a belief and not true science, as claimed by a creationist?
      • Or is it the cornerstone of modern biology, as resolved by the NC Academy of Science? 
        •   [To answer this question, consider the science news stories you've studied all semester; do the hypotheses and experiments seem to be
        • direct tests of evolution?   or
        • tests of hypotheses with no connection to evolution at all?       or
        • tests of hypotheses based on a tacit assumption that evolution happens?]
      • By the next test, you should be able to fill an entire blue book with answers to these questions.  Obtw, is it EVER scientifically proper to say, "only a theory"?
      3.

answers 1b   2b  3d   4c  5d   

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