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Natural Selection
-^ Directional Selection^ –^ One trait is favored over others^ –^ Results in directional changes •^ Stabilizing selection^ –^ Promotes phenotypic uniformity •^ Disruptive selection^ –^ Intermediate traits are disfavored,while two extremes are favored^ –^ Directional selection acting in severaldirections.^ –^ A common allelic trait is favored, lesscommon are disfavored
Me a n Frequency^5 10 15 20 255 10 15
25 Me a n Fre quency 5 10 15 Me a n Frequenc y^20 255
Me a n 5 10 15 20 25 Mea n 5 10 15 20 25 30 Me a nMe a n^15 20 Sta bilizing Selection^ Dire ctiona l Selection Disruptive Sele ction
Stabilizing Selection
-^ Reduces genetic variability by selecting out extremes (pulls in the “skirts”of the distribution. •^ Average individuals have higher fitness than individuals at the extremes. •^ The survival of sparrows during the winter of 1898 on sparrows^ –^ Wing length and other characteristics were correlated withsurvivorship.^ –^ Birds with average traits had a higher survival rate that smaller orlarger birds.
5 10 15
20 25 requencyF
Me a n
5 15 10 Mea n^20 Sta bilizing Selection
Stabilizing selection for number of offspring per birth • Clutch Size in Birds ¾ Too few eggs mean reproductive potentialis not met = lowered fitness ¾ Too many eggs means low fledglingsurvivorship = lowered fitness• The fact that humans usually give birth toone child is undoubtedly explained by lowsurvivorship of multiple births in earlierhuman populations.
Directional Selection
-^ Selection that favors one extreme •^ Responsible for evolutionary change •^ Examples:^ ¾^ Artificial selection is a special form of directional selection in whichhumans provide the selective pressure^ ¾^ Penicillin resistant Micrococcus Industrial melanism^ ¾^ Adaptation of humans and horses to environmental change
5 10 15
20 25 (^) Frequenc y
Me a n
5 10 15
Mea n^20 25
Directiona l Selection
The Peppered Moth Story
-^ The peppered moth story is anexample of directional selection •^ Light colored moths were favoredbefore pollution •^ Soot darkened the bark of treesand favored dark colored moths. •^ Later, when pollution was reducedthe reproductive success of lightcolored moths again increased •^ A good example of how the fitnessvalues associated with a trait iscontingent of currentenvironmental conditions.
Horse Evolution
-^ A well-documented example ofdirectional section in response toenvironmental change.•^ Grasslands expanded at the endof the Miocene epoch•^ These environmental changesproduced selective pressures thatfavored fewer toes and higher,longer wearing teeth in horses.
Diversifying Selection
-^ Also know as destabilizing ordisruptive selection •^ Promotes phenotypic differences,favors the presence of multiplealleles •^ Thought to be important in speciesformation •^ The fishing net analogy
Disruptive artificial selection onbristle number in
fruit flies
Balancing Selection
-^ Balancing selection occurswhen the heterozygote hashigher fitness thanhomozygotes •^ This is called heterozygoteadvantage •^ Balanced Polymorphism^ ¾^ The situation in whichheterozygotes have a selectiveadvantage over homozygotes^ ¾^ A balanced polymorphism existswhen an equilibrium amongdifferent alleles at a givenchromosomal locus
Balanced Polymorphism 7 6 5 4 3 2 1 0 AA Aa^
aa Genotype Fitness
The Spread of the S Allele
Sexual Selection
•^ Darwin’s (1871)definition of sexualselection:^ "sexual selection depends noton a struggle for existence inrelation to other organicbeings or to externalconditions, but on a strugglebetween the individuals of onesex, generally the males forpossession of the other sex."
Sexual Selection and Sexual Dimorphism • Sexual dimorphism:morphological differencesbetween the sexes • Over mates tends oftenfavors larger male size incomparison to that offemales as is exemplifiedby the elephant seal • The angler fish is anexception
Sexual Selection and Natural Selection • Sexual selection and naturalselection can reinforce eachother as well as act at cross-purposes • A trait that increases success inattracting mates (a longer tail)may increase vulnerability topredators • Ultimately an equilibrium willbe reaches between thepressures of sexual selection andnatural selection.
Sexual Selection in Humans • How can we explain the physical differences between human males andfemales ¾ To what extent are they a result of natural selection? ¾ To what extent are they a result of sexual selection? • Darwin’s perspective
: "We may conclude that the greater size, strength, courage, pugnacity, and energy man, in comparison with woman, wereacquired during primeval times, and have subsequently been augmented,chiefly through the contests of rival males for the possession of the females."Charles Darwin,
Descent of Man
Sex Differences in the Human Pelvis
-^ Natural selection
favored a pelvic outlet large enough togive birth to large-headedbabies.• Sexual selection
may have reinforced this: males mayhave preferred mates withwider hips• Wide hips:^ present mechanicaldifficulties during running (lossof energy through angularmomentum. This wouldcounterbalance their obstetricaladvantage• Narrow hips:
women may have preferred males withnarrower because of itsassociation with running abilityand hunting prowess
Man^
Woman Chimpa nzee^
Gorilla^
Woma n Bre a dth idthW Shoulde rs
