Fitness, Adaptation, & Natural Selection in real populations

      a phenotype of organisms and populations
      quantifiable relative to other organisms and populations
      related to capacity for survival and reproduction
      variable in space & time; short-term and long-term (see below)

      Short-term measures: "Life Table" parameters

            rate of instantaneous increase (r) of a phenotype

            recall logistic equation: dN/dt = rN = rN (K-N) / K
                                                 where K = carrying capacity

         net reproductive rate: exp(r) = er
                                                 r is "compound interest" on N

         replacement rate (RO): lifetime reproductive output
                                                     ~ er(at low density)

         components of fitness: traits that contribute to survival & reproduction
         Ex.: survivorship (expected survival time)
                 fecundity (# offspring at age x)

Adaptation is the phenotypic consequence for populations of natural selection on individuals
            [cf. adjust / acclimate]

      Phenotypic traits that change as a result of selection
            are sometimes referred to as "adaptations" or "adaptive characters"

Measuring 'fitness' and observing 'adaptation' in natural populations

Life table analysis: survivorship and fecundity vary with age

     lx  = prob. of survival from birth to age x  (cumulative)
           survivorship = probability of survival to age x+1 from age x
     mx = fecundity (# offspring) at age x

     then      (lx)(mx) exp(-rx)  =  1  (in a stable population,
                x=1                                      where L = life expectancy)

      Ro=(lx)(mx  replacement rate  er at low density

      This equation is a discrete solution to the continuous logistic equation

Ro can be calculated for two reproductive 'strategies'
        as a measure of their relative 'fitness'

Consider a population with two demographic phenotypes:
      These phenotypes correspond to two reproductive 'strategies
       iteroparous strategy: offspring produced over several seasons
       semelparous strategy: offspring produced all in one season

      A survivorship and fecundity schedule will compare their life histories
       *=> life table parameters can be measured experimentally <=*

      Under 'typical' environmental conditions, survivorship is 50% / year:
            both strategies produce 2 young / female / lifetime
                  => both phenotypes are equally 'fit' [and N is stable]

      In 'good times', survivorship increases to 75% / year:
            iteroparous strategy produces 4 young / female / lifetime
            semelparous strategy produces 3 young / female / lifetime
                  => iteroparous phenotype is 'more fit' [and N is increasing]

      In 'bad times', survivorship decreases to 25% / year:
            iteroparous Ro = 0.72,   semelparous Ro = 1.00
                  => semelparous phenotype is 'more fit' [and N is decreasing]

=> Population phenotypes will adapt to changing conditions

      In a favourable environment, K increases:
       e.g., productivity of meadow increases
                    iteroparity more advantageous, population density increases

      In an unfavourable environment, r increases:
       e.g., severity of winter highly variable
                  semelparity more advantageous, early reproduction favoured

K-strategy: maintain population size N close to K
      long-lived, reproduce late, smaller # offspring, lots of parental care
       E.g., many bird species, primates (including Homo)

r-strategy: maximize growth potential r
      short-lived, reproduce early, larger # offspring, little parental care
       E.g., most invertebrates, some rodents

Natural Selection on multilocus traits: Quantitative genetics

We can extend single-locus  multilocus  quantitative models

      p2:2pq:q2                       W0,W1,W2          Mendel's Laws & H-W Theorem
 normal distribution     fitness function              high heritability

Variation can be quantified

       mean  standard deviation: 
      variance: 2
      coefficient of variation (CV)  =  (/) x 100

      CV removes size effect when comparing variance:
        Ex.: Suppose  X = whale length     Y =  tail width
                X = 100  1.0 versus Y = 1.0  0.1
                CV of X = 1%        CV of Y = 10%
                Y is more variable, though X is larger

      Quantitative variation follows "normal distribution" (bell-curve) iff
              Multiple loci are involved
              Each locus has about the same effect
              Each locus acts independently
                    [interaction variance (see below) is minimal]

Variation has two sources: genetic (G2) & environmental (E2) variance

      phenotypic variance      P2  = G2E2GxE2
      additive variance            A2  = G2E
      heritability                        h2  = G2/A2  = G2 / (G2E2)

          "heritability in the narrow sense": ignores GxE interaction variance:
           Identical genotypes produce different phenotypes in different environments.
               Ex.: same breed of cows produces different milk yield on different feed

Artificial breeding indicates that organismal variation is highly heritable
      ex.: Darwin's pigeon breeding experiments
             Artificial selection on agricultural species
                  Commercially useful traits can be improved by selective breeding
             IQ scores in Homo: h2 0.7
                   [But: IQ scores improve with education: GxE2 is large]
            Offspring / Mid-parent correlation

            For many traits in many organisms:
            CV   =     5 ~ 10 %
            h2    =     0.5 ~ 0.9

[Read "Suggestions for using the Website" for comments on the examples used in this course]

Fitness function expresses relationship between genotype & fitness
    Function is a continuous variable, rather than discrete values for W0, W1, & W2

=> Most traits vary & are heritable.
          Many traits do respond to 'artificial' selection.
          Many traits should respond to 'natural' selection.

=> To demonstrate & measure Natural Selection,
           we must show experimentally that heritable variation has consequences for fitness  <=

Fitness & Adaptation on a large scale & in the long term:

       "Form & Function":
             Organisms typically exhibit engineering criteria of "good design"

       Beak variation in Hawai'ian honeycreepers (Drepaninae)
                Beak type matches food type

       Aerodynamics of bat & bird flight
                   Slow fluttering bats versus fast soaring birds
                   Wings match aerodynamic principles

             Assumption: Form & Function affect survival & reproduction

       Persistence: "Estimated time to extinction"
             Are long-lived lineages "better adapted"?

       Multituberculata versus modern mammalian orders (3D animation)
                Order persisted more than twice as long as any extant order
                Ultimately out-competed by Rodentia

       Chondrichthyes (sharks & rays) versus Teleosts
                Body form is unchanged in 400 MY
                Class is about as diverse now as at anytime in last 250 MY

       Agnathan orders [Hagfish & Lampreys] versus gnathostome orders
                Descendants of Ostracoderms, 500 MYBP (million years before present)
                Jawlessness works [ectoparasitism is probably secondary]

      "Adaptive characters" cannot be separated from the organisms that bear them

            Ex.: We typically say "Hair & feathers evolved from scales".
            But: It is more accurate to say:
             "Reptiles (with scales) evolved into mammals (with hair) and birds (with feathers)."
               [and this isn't completely accurate either]
                                                             Agnaths (scaleless)
Chondrichthyes (placoid scales)   Placoderms (denticles)    Teleosts (cycloid scales)
                                              Amphibia (dermal scales)    Lissamphibia (2o loss of scales)
                            Mammalia (hair  reptiles (imbricate scales  Aves (feathers)

        Ex.: In a mammalogy class, we might say
                "The carnassial pair evolved from the P4/M1 combination."
            But: it is more accurate to say
         "Carnivorous mammals evolved from insectivorous ancestors.
                  The carnassial pair is adapted for slicing meat."

Modes of Selection in natural populations

 Quantitative trait distribution can be described as a bell curve
            with a particular mean & variance:

       What happens to this distribution under Selection?

(1) Directional Selection

      Fitness function has constant slope:
      Trait mean shifted towards favored phenotype
            trait variance unaffected

     In single-locus models, the limit of selection is
      Elimination of variation by fixation of favored allele

    In quantitative models, rate is limited by
   substitutional genetic load:
            "cost" of replacing non-favored allele ( "intensity" of selection)

  "Hard" selection
          Mortality is density-independent
          In Lab #1: N(after) < N(before)
        Load is cumulative (N) over time as q 0
               Fitness is more or less absolute: less realistic, easier to model
       Ex.: Exercise #2, in a malarial environment, 50% die before reproduction.
                 Population "after" is much smaller than "before",
                 but rebounds to N only at start of next generation

   "Soft" selection
          Mortality is density-dependent
          In 'real' populations:   N(after) N(before)

          Survivorship is proportional to fitness up to K: more realistic
              Selection will affect recruitment to next generation
         Ex.: If the first-born dies of malaria, s/he will be replaced.
                       More births occur such that N is continually "topped up".
                       Birth of succeeding offspring will maintain N near K

       Lab #1: industrial melanism in pepper moths (Biston betularia):
                  'dark' moths replace 'light' moths in polluted environment

       artificial selection on agricultural species

            Gecko lizard (Aristelliger) has "suction pad" feet:
          lamellar scale counts increase with age

            Darwin's Finch (Geospiza fortis) adapts to drought:
                larger birds survive because of changes in seed size & hardness
                (recall that size is heritable)

      Developmental canalization limits extent of directional selection
              Systems are controlled by multiple epistatic loci:
                    it is difficult to select on all loci simultaneously
              Organisms have mechanical limits:
                   size cannot increase indefinitely
                 Johanssen's bean experiment
                 Skull volume versus birth canal diameter in Homo
                   Phenotypes are not infinitely plastic:
                     [But: Eozostrodon lineage evolved into whales & bats]

(2) Stabilizing Selection (AKA truncation selection)
      Fitness function has a "peak"
      Trait variance reduced around (existing) optimal phenotype,
            trait mean unaffected

      Limits: elimination of  variant alleles
              or, 'weeding out' of disadvantageous variants
              homozygosity at multiple loci:
                    difficult iff variance due to recessive alleles (Lab #1)
             inbreeding depression: loss of 'health' in inbred lines

       Lab #1: Elimination of non-cryptic pepper moths (Biston)
         Dark variants are eliminated rapidly in light environments
         Light variants are reduced (more slowly) in dark environments (why?)
                    [This may look like an example of directional selection: why isn't it?]

       Cold shock in house sparrows (Passer) (Bumpus 1898)
                Animals that die are at extremes of distribution

       Birth weight in Homo (Karn & Penrose 1951)
                Modal birth weight is optimum for survival

(3) Diversifying Selection (two kinds)
      There is a lot of variation: does selection explain it?

    (A) Balancing Selection:
      Fitness function has more than one peak (multi-modal)

      Trait variance increases
       polymorphic["strict sense"]: variation maintained within populations
                Ex.: cornsnakes, tomatoes, bell peppers, snails, scallops

       polytypic: variation distributed among populations
                Ex.: shell patterns in Cepaea snails
                        fraction of dark / banded shells varies with substrate

      segregational genetic load:
       loss of reproductive potential due to production of less fit homozygotes
            In Lab #1, Exercise #2, about 1/3 of population "dies" in malarial environment

Maintaining heterozygosity (allelic & genotypic variation) by selection

      Overdominance: heterozygotes have superior fitness at a locus
                  because different alleles are favoured in different environments
       sickle-cell hemoglobin in Homo ('Contradictory' selection)
            Leucine Aminopeptidase (LAP) & salinity tolerance in Mytilus mussels
                  multimeric enzymes with polypeptides from different alleles
                     often show wider substrate specificity, kinetic properties (Vmax & KM)
           myoglobin in diving mammals

      Heterosis: heterozygosity at multiple loci improves general fitness
      Hybrid vigour: crossbreeding of inbred lines improves fitness in F1

      Marginal epistasis: high 'Hobs' is 'good for you'
       Ex.: correlation between phenotype & genotype: antler points in Odocoileus deer
       Ex.: fluctuating asymmetry: Acionyx cheetahs are lopsided

Maintaining polymorphic phenotypic variation by selection

      Alternative phenotypes favored in different environments
       crypsis: Cepaea land snails match background (Fig. 13-06)
       Batesian mimicry:
           'Tasty' mimics converge on 'distasteful' models
                     Viceroy butterflies (Limenitis) converge on Monarch (Papilio)  butterflies
       Mullerian mimicry:
                Distasteful models converge on each other,
              different combinations evolve in different parts of range
         Heliconius butterflies (Futuyma 1997)
       aposematic (warning) colouration warns off predators (Mertensian mimicry]
                   Ex.: scarlet kingsnake (nonvenomous) mimics
                          coral snake (highly venomous) [black / red / yellow pattern]

      Frequency-dependent selection:
            Fitness value of phenotype varies with frequency

       apostatic predation: thrush predation on Cepaea
                 'search image' changes when prey type becomes rare

       'rare male' effect: females prefer "different" male
                Male zebra finches with artificial crest get more copulations (Fig. 20-13)

      Sexual Selection (Darwin 1871):
            'exaggerated' phenotypes are disadvantageous somatically
                but are favoured in competition for mates

            secondary sex characteristics:
          Sexual dimorphism in mallards, peafowl, & lions
          Antlers in Cervidae are used in male-male combat
          Tail displays in peacocks attract mates

       'Runaway sexual selection': the Madonna / Ozzy Osborne Effect
                Females choose males on basis of some distinctive trait
                  Offspring have exaggerated trait (males) & preference for trait (females)
                     selection reinforces trait & preference for trait simultaneously
                            New phenotype spreads rapidly in population

(B) Disruptive selection
      Fitness function is a valley
            Trait variance increases (like balancing), BUT polymorphism is unstable

      [Try NatSel with: q = 0.5, N = 9999, W0 = 1.0, W1 = 0.7, W2 = 1.0]

      Polymorphism can usually be maintained only temporarily:
            One of the phenotypes will outcompete the other
       unless different phenotypes choose different niches (Ludwig Effect)
                [and then this becomes Balancing Selection]

      Scutellar bristles in Drosophila (Thoday & Gibson 1962)
            Selection for 'high #' versus 'low #' lines
                  => 'pseudo-populations' with reduced interfertility
          Might disruptive selection contribute to speciation?

Natural Selection at other levels: Genic & Kin Selection

Natural selection is ordinarily defined as
    differential survival & reproduction of individuals:
      Can selection operate on other biological units?
      Can such selection 'oppose' individual selection?

Genic (Gametic) Selection
           Differential survival & 'reproduction' of alleles

      Meiotic Drive: t-alleles in Mus
       tt is sterile (W = 0)
       Tt is 'tail-less' (cf. Manx cats) (W < 1)
           t alleles are preferentially segregated into gametes (80~90%)
                  => f(t) is high in natural populations (40~70%)
                       even though it is deleterious to individuals

Kin (Interdemic) Selection
           Differential survival & reproduction of related (kin) groups (families)

      Related individuals share alleles:r = coefficient of relationship [see derivation]
            offspring & parents are related by r = 0.50   [They share half their alleles]
            full-sibs                 "    "                     r = 0.50
            half-sibs                "    "                    r = 0.25
            first-cousins           "    "                   r = 0.125

     Inclusive fitness (Wi) of phenotype for individual i
            = direct fitness of i + indirect fitness of relatives j,k,l,...

       Wi = ai(rij)(bij)   summed over all relatives j,k,l,...

            where: ai    = fitness of i due to own phenotype
                        bij  = fitness of j due to i's phenotype
                        rij   = coefficient of relationship of i & j

Example What is the fitness value of an alarm call?
      When a predator approaches, should i warn j , or keep silent?

              If i & j are unrelated
                   warn:          Windividual = 0.0 + (0.0)(1.0) = 0.0
                   don't warn: Windividual = 1.0 + (0.0)(0.0) = 1.0
                Such behaviors should not evolve among unrelated individuals

            What is the fitness value in a kin group?
                   Wbrothers       = 0.0 + [(0.5)(1.0) + (0.5)(1.0)] = 1.0
                   Wcousins       = 0.0 + [8][(0.125)(1.0)] = 1.0
                Such behaviors can evolve among related individuals in (extended) family groups

       J.B.S. Haldane (1892-1964):
            "I would lay down my life for two brothers or eight cousins."

 Evolution of social & group behaviours

       Parenting behaviour:
           'Broken wing' display in mother birds
           Mother sacrifices herself for (at least two) offspring

       Altruistic behaviour ( "unselfish concern for others")
           'Alarm calls' in Belding ground squirrels (Spermophilus)
                        females warn more in related groups
                  Can behaviours to help unrelated individuals evolve?

       Eusocial insects (Hymenoptera, Isoptera)
           Haplodiploidy: females diploid, male drones haploid
                        Females workers are sterile (Wi = 0): what is the selective advantage?
                            related to queen or offspring by 1/2
                            related to sisters by 3/4
                  Care for sisters, don't have offspring

Last thoughts on Natural Selection

Natural Selection may be the most misunderstood concept in biology.
    It is ...

(1) Not "Survival of the Fittest"
      Herbert Spencer (1820 - 1903) "Social Darwinism"
            the "naturalistic fallacy": 'is'  = 'ought'
            [Darwinian theory was accepted in part because
              it could be read to support British imperial ambition]

      not phenotype-specific mortality
      not predation (nor inter-species competition, usually)
      not "Nature red in tooth and claw"
              Darwin: plants in desert 'struggle' for water
      not equivalent to population growth:
           population declined in semelparous example

(2) Not equivalent to evolution

      Natural Selection may conserve existing types (stabilizing selection).
      Evolutionary change ultimately requires new variation (mutation).
      Migration, population structure, genetic drift are important.

(3) Not a tautology (a self-evident statement; a circular argument)

      "Why do they survive? Cuz they're fit.
            How do you know they're fit? Cuz they survive..."etc.

      More like a syllogism (an if / then statement; a logical consequence):
            (2 W  &  h2) => q
            [cf. physics:  F = M A  depending on how Force, Mass, & Acceleration are defined
                 arithmetic:  1 + 2 = 3  because I and II make III]

(4) Not "Mother Nature"

      not a force, not a thing that acts
           [We don't say, "Arithmetic causes one plus two to equal three."
            We might say, "One plus two equals three. That's arithmetic.]
      not good or bad (amoral)

      no noun / verb / object distinctions
            [In most languages, "nouns verb objects"
                i.e., objects perform actions on other objects. Not.]

(5) Not teleological (goal-directed):

      Evolution does not have "goal", "direction", or "purpose"
            (Homo sapiens are not the endpoint of evolution!)

      Avoid such phrases as "Natural Selection acts ..."
       "in order to ...",
           "for the purpose of ...",
           "so that ...",
           "because its trying to ..."

Text material © 2012 by Steven M. Carr