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Natural selection and adaptation What is an adaptation? • An adaptation is a characteristic that enhances the survival or reproduction of organisms that have it, relative to alternative character states – Can also be the process of acquiring an adaptation • Natural selection causes the evolution of adaptations What is an adaptation? • Can include – Form • molecular to anatomy – Processes • molecular, physiological, behavioral – Degree of plasticity • flexibility in development, physiology, behavior Adaptation examples Philodendron • Adaptive response differs with life stage Orchids • Pseudocopulatory pollination Snake skulls Social insects Adaptation? • Polar Bears evolved from a brown ancestor • White fur blends in with snow “The white fur is important camouflage for the bears as they hunt their prey out on the ice pack.” Camouflage hypothesis • Test: Sterling (1974) observed 288 hunts – 1 "sneak and pounce" – 54 "jump and crush" – 233 "sit and wait" at breathing holes • Conclusion: polar bear doesn’t rely on camouflage! UV hypothesis • Fact: Polar bears don’t reflect UV • New Hypothesis: Hairs act like glass fiber optic to channel UV radiation to black skin, absorbed as heat. – Tested 25 yrs later & shown false – Transmits small % of light • But keratin in hairs does seem to absorb UV. Maybe an efficient heat trap. – Needs testing! Natural selection • The differential survival or reproduction of classes of entities (genotypes, alleles, populations, species) that differ in one or more characteristics • Differences in survival must not be due to chance • Sexual selection is natural selection due to competition for mates Levels of selection • Genes (genic selection) • Individual organisms (individual selection) • Populations within species • Species Natural selection vs. genetic drift • Genetic drift occurs by chance – There is no consistent average difference in survival and/or reproduction • There is no chance involved in natural selection – Due to consistent average differences or biases in survival and/or reproduction Selection “of” and selection “for” • Selection “for” advantageous traits • Selection “of” other features correlated with these traits Selection in bacterial populations • Escherichia coli • Competition between strains – Wild type vs. those that had mutations in β-galactosidase – Large population – Many mutations were selectively neutral Selection in bacterial populations • E. coli • His+/his– Selectively neutral if cells supplied with histidine – Selection of neutral alleles by hitchhiking Inversion polymorphism • Drosophila pseudoobscura are polymorphic for inversions • Frequencies changed seasonally, suggesting that inversions affect fitness Inversion experiment • Natural selection is maintaining polymorphism Sexual selection • Long-tailed widowbirds Endler’s guppies • Vary in coloration, size at reproduction and age of reproduction • Why? Coloration • Guppies are less colorful when predators are present • Transplanted guppies to predator free streams Laboratory experiment Trade-offs in sexual selection • Coloration is a trade-off between mating success and avoidance of predation Population size in flour beetles • 48 experimental populations for each of three treatments • Each generation started with 16 beetles Types of selection • Individual selection in control treatment – Selection within a population – Differences in survival among individual beetles within the population • Group or interdemic selection in other treatments – Selection among populations by experimenter • Allowed some populations to proliferate based on phenotypic characteristic of each group – Operates in addition to individual selection among genotypes within a population Why did all decline? • Control population – Inbreeding ruled out – Adults were more likely to cannabalize pupae, development time was more variable, females laid fewer eggs when with other beetles – These features are advantageous to the individual, but not the group Group selection • The group selection treatment for low population size reinforced the same tendencies • The group selection treatment for high population size opposed the consequences of individual selection within populations – Females had higher fecundity when with other females – Development was more rapid – Adults showed less cannabalism Selfish genes • Transmitted at higher rate than the rest of an organisms genome • Not advantageous to the organism – t locus of mouse – psr chromosome in Nasonia Maternal-Effect Dominant Embryonic Arrest in Tribolium • When a heterozygous Medea female (M/+) is crossed to a wild type male (+/+), the M gene and its homolog segregate normally. • However, all progeny that do not inherit the Medea allele die at or shortly after egg hatch. The lethality is maternal, but the "rescue" is zygotic. The rescuing M allele can be inherited from either parent Levels of selection • “For the good of the species” is not a true statement! • Altruistic trait cannot evolve by individual selection Cheaters • Cheaters would increase in frequency and altruistic trait would be lost Group selection • Populations with selfish genotypes would have a higher extinction rate • Groups as a whole might evolve altruism Arguments against group selection • What appears to be unselfish behavior may not be • Individual selection will prevail over group selection since individuals turn over faster than populations • Very few traits have evolved from group selection! Theory of kin or indirect selection • Proposed by William Hamilton • Genes associated with caring for relatives may be favored by selection • Altruists pass on genes indirectly, by helping relatives who share close genetic similarity to survive and reproduce Hamilton’s rule • Altruistic acts are favored when rb > c • • • r = relatedness b = benefit of altruistic act c = cost of altruistic act Examples of Kin Selection • Belding’s ground squirrel – sound alarm calls when spot predators • Females are more likely to call than males due to colony makeup Selfishness can be selected for too! Species selection • Selection among groups of species or higher taxa is called species selection or taxon selection Orchids and Irises • Rate of speciation in orchids faster than in irises due to specialization of pollinators • Species selection for specialized pollination Differential speciation Differential extinction