Download Natural selection and adaptation

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