Natural Selection and Adaptive Evolution (Lecture 15)

• Natural selection functions as an evolutionary mechanism, evidenced by case studies.
• Three conditions necessary for natural selection.
• Natural selection examples.
• How selection pressures change across time and environments.

Key Concepts

• Natural Selection: Organisms possessing favorable traits exhibit higher survival and reproduction rates, leading to allele frequency changes.
• Heritability: Phenotypic variance proportion that is attributable to genetic variance.
• Adaptive Evolution: Selection pressures favoring traits that enhance survival and reproduction drives allele frequencies changes.

Requirements for Natural Selection

• Natural selection necessitates three key components.
• Variation in phenotype within the population is essential.
• Variation must have a heritable basis (genetic).
• Variants have to differ in fitness- survival and reproduction.
• Selection strength is variable and fluctuates. There are periods of weak and strong selection pressure.

Darwin's Finches on the Galapagos

• Peter and Rosemary Grant's 40+ year study offers direct proof of natural selection in action.
• This research was on Geospiza fortis (medium ground finch) within Daphne Major island.
• A major drought in 1977 brought intense selection pressure when food resources became scarce.
• Birds that had larger beaks had a survival advantage because they could crack open hard Tribulus seeds.
• Average beak size grew by 0.5mm in the next generation.
• Selection pressure varied at different periods, sometimes reversing in favorable conditions.

Beach Mice Color Adaptation

• Hobie Hoekstra's research demonstrates natural selection favoring camouflage.
• There is adaptation to their environments by light-colored beach mice and dark mainland mice.
• Mice that had colors that didn't match their environment were shown 50% higher predation from clay model experiments.
• Light coloration evolved independently on the Gulf and Atlantic coasts (convergent evolution).
• QTL analysis identified MC1R and Agouti genes responsible for coat color variation.
• Matching color and environment resulted in better survival.

Snowshoe Hares and Climate Change

• Climate change is affecting snowshoe hare coat color adaptation due to changing selection pressures.
• Snowshoe hares typically change from brown (summer) to white (winter).
• Coastal populations with less snowfall exposure remain brown year-round.
• The Agouti gene is responsible for seasonal coat color change.
• The "always brown" Agouti allele was acquired by the Washington state population via hybridization with jackrabbits.
• Selection now favors hares that don't turn white due to milder winters and less snow.
• The winter dark Agouti allele was acquired by introgression with jackrabbits.

Diagrams/Visual Aids

• Parent-offspring Regression graphs show beak depth heritability in finches.
• Selection differential plots show selection intensity both during and after the drought.
• FST plots identifies disparities between winter-white and winter-brown hares on Agouti gene regions.
• Phylogenetic tree illustrate relationships between species vs. gene relationships within snowshoe hares.

Q&A/Discussion Points

• Beak morphology responses to selection, and ways to measure it.
• Observed beak change after the drought demonstrates selection, high heritability (0.9 for depth, 0.5 for length) means the trait is responsive to selection.

Summary/Conclusion

• Natural selection needs variation, heritability, and differential fitness.
• Intensity of selection pressures change across both time and space.
• Evolution through natural selection can be seen in case studies (finches, mice, hares).
• Multiple mechanisms give rise to adaptations (direct selection, convergent evolution, introgression).
• Natural selection is a powerful mechanism driving adaptation to local environments