Overdominance: heterozygotes have superior fitness
                  because different alleles favored in different environments
                  Ex.: HbS / HbA Standard, Trait, & Sickle-Cell phenotypes

      Clinal variation: Different alleles advantageous in different environments
                   Ex.: Lap cline follows salinity gradient in Mytilus mussels
 
      Heterodimers:
            multimeric enzymes with polypeptides from different alleles
                often show wider substrate specificity, kinetic properties (V_max & K_M)
           Myoglobin in diving mammals

      Heterosis: heterozygosity at multiple loci correlated with overall fitness
           Ex.: correlation between phenotype & H_e: antler points in deer (Odocoileus)
           Ex.: Hybrid vigor: crossbreeding of inbred lines in maize (Zea) improves vigor in F₁

Maintaining polymorphic multi-locus phenotypes by Natural Selection

      Alternative phenotypes favored in different environments
       Batesian mimicry:
           'Tasty' mimics converge on 'distasteful' models
                     Ex.: Viceroy butterflies (Limenitis) converge on Monarch (Papilio) butterflies
                             Mimic remains rare wrt model
       Müllerian mimicry: 
                Distasteful models converge on each other,
                Different combinations evolve in different parts of range
               Ex.: Heliconius spp. butterflies exchange alleles by hybridization
       Mertensian mimicry:
                aposematic (warning) coloration discourages predators
                   Ex.: non-venomous scarlet king snakes mimic
                          venomous coral snakes with black / red / yellow pattern

      Frequency-dependent selection:
           apostatic predation: Turdus thrush predation on Cepaea snails
                 'search image' changes when prey type becomes rare

      Sexual Selection (Darwin 1871):
            'Exaggerated' ornamentation disadvantageous to individual survival phenotype
                but favored in competition for mates (reproductive phenotype)
          Sexual dimorphism in birds & mammals
          Antlers in deer (Cervidae) used in male-male combat

       Runaway sexual selection
                Females choose males on basis of secondary sex ornamentation
                  Offspring have exaggerated trait (males) & preference for trait (females)
                     selection reinforces trait & preference for trait simultaneously
                            New phenotype spreads rapidly in population

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

      Polymorphism usually maintained only temporarily:
            One phenotype out-competes others
       unless different phenotypes choose different niches (Ludwig Effect)
                then this becomes Balancing Selection
        or frequency selection occurs

      Ex.: Scutellar bristles in Drosophila 
            Selection for 'high #' versus 'low #' lines
                  => 'pseudo-populations' with reduced inter-fertility
            Might disruptive selection contribute to speciation?

Darwinian natural selection defined as
    differential survival & reproduction of individuals:
      Can selection operate on more inclusive biological units?
         Group Selection controversy: cf. Kropotkin (1902) "Mutual Aid"

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

      Related individuals share alleles: r = coefficient of relationship [derivation]
            offspring & parents 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 (W_I) of phenotype for individual i
            = direct fitness of i + indirect fitness of relatives j,k,l,...

       W_I = a_i + (r_ij)(b_ij)   summed over all relatives j,k,l,...

            where: a_i    = fitness of i due to own phenotype
                        b_ij  = fitness of j due to i's phenotype
                        r_ij   = coefficient of relationship of i & j

              If i & j are unrelated
                   warn:          W_individual = 0.0 + (0.0)(1.0) = 0.0
                   don't warn: W_individual = 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?
                   W_brothers       = 0.0 + [2](0.5)(1.0)] = 1.0
                   W_cousins        = 0.0 + [8][(0.125)(1.0)] = 1.0
                Such behaviors can evolve among related individuals in (extended) family groups

       JBS Haldane (1892 - 1964):
            "I would lay down my life for two brothers or eight cousins."
            Humans evolved in small kin groups, function today in large, ~unrelated groups:
                 do evolved Primate behaviors persist?

       Parenting behavior:
           'Broken wing' display in mother birds
           Mother sacrifices herself for (at least two) offspring
           [See also active defense of offspring] [Warning: language!]

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

       Eusocial insects (Hymenoptera, Isoptera)
           Haplodiploidy: females diploid, male drones haploid
                        Females workers sterile (W_i = 0): what selective advantage?
                            related to queen by 1/2
                            related to sisters by 3/4
                  Care for sisters, don't have offspring
                  [NB: 'Queen' Bee mates with multiple drones: workers related to half-sisters only by 1/2]

Text material © 2025 by Steven M. Carr