Natural.Selection

Natural Selection––#

NATURAL SELECTION

Introduction
1. We examined the Hardy-Weinberg equilibrium and introduced the concept of equilibrium populations
2. We also reviewed factors that upset the Hardy-Weinberg equilibrium and thereby cause microevolutionary change
  1. Mutation
  2. Gene flow (migration)
  3. Sampling error (drift, founder effect, and bottleneck)
3. Here we focus on a fourth process upsetting the Hardy-Weinberg equilibrium––natural selection
  1. Charles Darwin’s postulated mechanism for biological change
  2. Perhaps the most important factor to upset the Hardy-Weinberg equilibrium
The nature of natural selection––What it is and isn’t
1. Review of Darwin’s postulates
  1. Variability of offspring
  2. Some of variability is heritable
  3. More offspring are produced than can survive
  4. Differential reproductive success among offspring––natural selection
2. Misconceptions about natural selection
  1. That natural selection is tautological
    1. If we define natural selection as “survival of the fittest,” and we define fitness as the ability to survive, then the concept of natural selection is tautological
    2. But if natural selection is properly defined as “differential reproductive success”––simply that some offspring with certain traits tend to pass their genes on more frequently than offspring with other traits––then the tautology is avoided
  2. That natural selection creates perfect structures
    1. Natural selection apparently can “create” some amazingly well designed structures
    2. Every structure, however, is a compromise among various possibilities
      1. Human upright posture
      2. Panda’s thumb
      3. Sea otter’s appendages
    3. Natural selection can shape only that variability that is available to it
  3. That natural selection is purposeful
    1. Natural selection never leads “toward” something
    2. Natural selection is mindless and non-teleological
  4. That natural selection can accomplish anything
    1. Again, natural selection is limited in what it can accomplish by the degree of variation present in the population
    2. Natural selection does not create new variation––It only works on existing variability
  5. That natural selection is responsible for all exquisite design in organisms
    1. This remains completely undemonstrated
    2. Processes other than natural selection have probably played important roles in biological change
      1. Natural processes
        
        Mutation
        
        Sampling error
        
        Gene flow
      2. Original creation of diversity
  6. That natural selection should reduce variability to the point at which it would no longer have any variability upon which to work
    1. Often natural selection does, indeed, reduce variability.
    2. Many processes, however, actually promote variability and work against the loss of variability engendered by selection (lecture on variability and its promotion)
  7. That natural selection always leads toward greater complexity in organisms
    1. Clearly, this is not the case.
    2. Natural selection can lead to losses in complexity
      1. Cave animals––loss of eyes
      2. Parasites––loss of digestive and other modalities
  8. That natural selection is cruel and immoral––“Nature red in tooth and claw”
    1. Natural selection is morally neutral
    2. It does not always lead to pain and death––frequently it results in nothing more than differences in reproductive output
    3. Natural selection is automatic––it takes no planning or thought
    4. Processes analogous to natural selection occur at every level of reality, not just at the biological level
      1. Cosmic––Some stars larger than others, and thus last longer
      2. Geologic––Some layers of rock are more resistant to erosion than others and thus last longer
      3. Human behavior––Some things we do work, while others don’t. We reproduce those that work and let others go extinct
Levels of natural selection
1. Typically we think of natural selection as working at the individual level
  1. In individual selection, individual organism is “tested” by the environment
  2. As a result it either passes many of its genes on to the next generation or does not
  3. This is unquestionably an important level of selection
2. Other levels of natural selection
  1. Meiotic drive (genic selection)
    1. Occurs when one allele is transmitted to more than 50% of the gametes produced by a heterozygote
    2. Example: t allele in Mus musculus
      1. Allele t causes sterility in homozygous state
      2. A male heterozygote (Tt), however, produces 80-95% t-bearing sperm
      3. Thus, the frequency of allele t, when introduced into a new population, can rapidly increase, even though it is deleterious
      4. But because it is deleterious, allele t is acted against by individual selection
      5. The result is that the frequency of allele t is eventually fixed at about 0.70
  2. Group selection
    1. Group selection is differential productivity among genetically variant demes (populations)
    2. Promoted by Wynne-Edwards to account for the existence of altruistic behavior in animals
    3. The concept of group selection is generally rejected by biologists because of the probability of “cheaters”
    4. There may be some good examples of group selection
      1. Parasites
        
        Individual parasites are likely to be advantaged by high fecundity
        
        Problem arises if high fecundity leads to death of host––if this happens, parasites may die along with the host
        
        Thus, groups of parasites in which individuals are genetically programmed to limit fecundity (an “altruistic act”) survive better than those in which fecundity is not limited
        
        Possible example: myxoma virus
        
        Introduced into population of European hares in Australia to control their huge numbers
        
        Initially, viruses were highly virulent
        
        Later, virulency decreased
  3. Species selection
    1. Species selection is differential success of species with a related species group (clade) over geologic time
    2. Proposed by punctuationists
    3. May lead to trends (e.g., increases or decreases in body size) in species clades
    4. Could reinforce or reverse effect of individual selection.
    5. Example: Fossil record suggests that ungulates of more recent species were capable of running faster than earlier ungulates
Modes of natural selection
1. Evolutionary biologists typically define several modes of evolutionary change
2. Three major modes are recognized
  1. Stabilizing selection
    1. Occurs in all populations all the time
    2. Favors the mode over the extremes
    3. Responsible for reducing genetic variability
    4. Examples
      1. Newborn baby size in humans (Karn and Penrose, 1951)
        
        Optimal newborn size is slightly more than 7 lbs
        
        Babies smaller and larger than this exhibit higher mortalities
        
        Actual mean baby size is very close to optimal mean size
        
        Thus, parents with genes for the production of “average size” babies will pass more genes on than those with genes for larger or smaller newborns
      2. Clutch size in Swiss starlings
        
        Survival rates of young from clutch sizes ranging from 1 to 8 were monitored
        
        Young from clutches with 4 or 5 eggs were more likely to survive than those from smaller or larger clutches
      3. Body size in lizards of genus Aristelliger (Hecht, 1952)
        
        Larger males are advantaged over smaller males in territorial disputes
        
        But larger males are also more susceptible to owl predation
        
        Thus, extremes in body size are selected against in Aristelliger
  2. Disruptive selection
    1. Favors population extremes at expense of individuals expressing average features
    2. Promotes genetic variability
    3. Could lead to speciation by cladogenesis
    4. Example––Development of two genetic types of plants near old lead mines, one type grows on contaminated soil, other kind cannot
  3. Directional selection
    1. Favors one population extreme at the expense of the rest of the population
    2. Reduces genetic variability
    3. Could lead to speciation by anagenesis
    4. Generally cannot push population mean forever
    5. Examples
      1. Industrial melanism in English moths
      2. Increasing beak size in the Galapagos finch, Geospiza fortis, in response to drought
      3. Clinal variation in house sparrows
3. Other ways of classifying natural selection
  1. Frequency-dependent selection
    1. We generally assume that the fitness of a phenotype is independent of its frequency in the population
    2. But this is not the case for frequency-dependent selection
    3. Frequency-dependent selection favors less common phenotypes at the expense of common phenotypes
    4. Examples
      1. Male body form in coho salmon
        
        Hooknose males
        
        3-years old
        
        Large
        
        Jockey aggressively for proximity to spawning females
        
        Jack males
        
        2-years old
        
        Small
        
        Sneak their way, often undetected by hooknoses, into close proximity of spawning females
        
        Both types are favored by natural selection
        
        When one morph begins to become more common, competition among members of that morph becomes more intense opening up opportunities for other morph
      2. Corixid water bugs (Sigara distincta)
        
        Preyed on by fish
        
        Three color morphs of bugs were used in an experiment
        
        When all three morphs were present in equal frequencies, individuals of the least cryptic morph were most likely to be taken by the fish
        
        By contrast, when proportions were present in unequal frequencies, members of the most common morph were most commonly taken by the fish
        
        May result from development of “search image” on part of predator
  2. Sexual selection––To be addressed in the next lecture
What natural selection selects
1. Natural selection works directly on phenotype and only indirectly on genotype.
2. Phenotype can include
  1. Morphological traits (e.g., size, color, shape)
  2. Physiological traits (function)
  3. Behavioral traits (e.g., time of emergence)
3. Natural selection secondarily affects things like
  1. Biochemistry
  2. Fecundity
  3. Longevity
  4. Genes themselves
Rates of natural selection
1. In 1930 R.A. Fisher stated what he called the “fundamental theorem of natural selection”––Rate of increase in mean fitness is approximately equal to genetic variance in fitness
  1. Genetic variance in fitness is directly proportional to the frequency of heterozygosity
  2. Thus, the higher the degree of heterozygosity, the more rapidly natural selection can respond to environmental change
  3. If this is true, then invertebrates should respond more rapidly to selection pressure than vertebrates
2. A second factor influencing the rate at which selection can operate is generation time
  1. Thus, organisms with shorter generation times should respond to natural selection more quickly than organisms with longer generation times
  2. Again, invertebrates fit this bill more than vertebrates
Conclusion
1. Although the concept of natural selection is a very simple one, it is often misunderstood
2. Nonetheless, natural selection as envisioned by Charles Darwin, does seem to play an important role in natural populations
3. This role is much better understood today, now that we understand certain population genetic processes