III. A Closer Look at Natural Selection A. Natural selection, adaptation, and evolution B. Patterns of natural selection 1. Directional selection 2. Stabilizing selection 3. Diversifying selection C. Effects of selection on the distribution of phenotypes

Fig

Frequency Phenotype (trait) before Fitness (W) Phenotype (trait) 1 0 Directional selection Pattern Effect

Frequency Phenotype (trait) before after Fitness (W) Phenotype (trait) 1 0 Directional selection Pattern Effect

Mallett, J TREE 4: 336 Pesticide Pesticide application Pesticide % resistant

Fig 22.13

Evolution of antibiotic resistance Source: Otaya, Epidemiological study of erythromycin resistance in Staphylococcus aureus in Japan. In Drug Resistance in Bacteria. 1. Macrolide antibiotics. Edited by S. Mitsuhashi. University Park Press, Baltimore.

Guppies Poecilia reticulata 5 cm

smalllarge Size of predators in pool Size of adult guppies in pool

Adult guppy size in each pool Number of guppies Guppy size is variable

In lab without any predators…. Time (generations) Guppy size Guppy size is heritable (genetic)

Big predator – Pike-Cichlid Crenicichla alta 16 cm Smaller predator – Killifish Rivulus harti 10 cm eats large guppies eats small guppies Guppy size affects survival to reproduce

Large predators eat large guppies, more small size alleles passed on Small predators eat small guppies, more large size alleles passed on B – large size B’ - small size

Time (generations) Adult guppy size Transplant to Small predator pool Control (stay in Large predator pool Mostly small alleles At beginning

Frequency Phenotype (trait) before after Fitness (W) Phenotype (trait) 1 0 Stabilizing selection Pattern Effect

Stabilizing selection in humans

Too many chicks? Too few eggs? Stabilizing selection on number of eggs in starlings

Conflict and Cooperation Parent-offspring conflict Sibling conflict Inclusive fitness and kin selection B r – C > 0 (benefits to recipient * relatedness – costs to giver) Reciprocal Altruism: If costs are small, benefits are large and altruistic acts are reciprocated – altruism can evolve

Frequency Phenotype (trait) before after Fitness (W) Phenotype (trait) 1 0 Disruptive selection Pattern Effect

Male Lazuli Buntings showing variation in the brightness of their breeding plumage Low plumage Brightness score High plumage Brightness score

Plumage brightness From Greene et al 2000, Nature 407:

Diversifying selection on coat color in deer mice Dark color is favored on rich soil Light color is favored on sandy soil

Frequency dependent selection Why are there equal numbers of males and females?

Sexual Selection

Fig Effects of selection on phenotype distributions Location of curve = mean Width of curve = variance

Frequency Phenotype (trait) beforeafter Possible effect of continual directional selection Eventually variance might decrease Frequency Phenotype (trait) beforeafter Phenotype (trait) before after

Frequency Phenotype (trait) before after Possible effect of continual stabilizing selection Variance decreases to none Frequency Phenotype (trait) before after Frequency Phenotype (trait) before after

Electrophoresis - separates proteins based on differences in size and electrical charge Heterozygous Homozygous fast Homozygous slow

Survey of electrophoretic variation in natural populations **

The Paradox of Variation: Evolution requires variation, but natural selection eliminates variation.

Dominant traits can hide recessive traits from selection

Fig Epistasis can hide dominant alleles from natural selection C= pigment c = none B = deposition of lots of pigment (black) b = less deposition (brown) If cc, fur is white regardless of genotype at B locus

Good gene Bad gene Genetic hitchhiking: tight linkage to a favorable gene can protect a less favorable gene from selection.

SusceptibilityRelative GenotypeRBCto malariafitness HB+ Hb+normalhighestintermediate Hb+ Hbsnormal*lowerhighest Hbs Hbssickledlowerlowest Heterozygote advantage in Sickle cell anemia : Hb+ = normal RBC (co-dominant)Hbs = sickled RBC

Diversifying selection in time: Snow goose Nesting habitat

Diversifying selection in space: Deer mice Dark color is favored on rich soil Light color is favored on sandy soil

Diversifying selection over density: sea urchins