Natural Selection & Adaptation
A population of rabbits is introduced to an island. After several generations, the rabbits become smaller in size. This is an example of evolution by what process?
Natural selection (rabbits adapted to island resources, with smaller individuals having better survival and reproduction)
What is a gene pool?
The sum of all genes (all alleles) in a population
What type of structures are the human arm, whale flipper, and bat wing?
Homologous structures (same evolutionary origin/common ancestor, different functions)
What are the two main requirements for speciation to occur?
Isolation (reproductive/genetic isolation) and genetic divergence
Which type of selection favors intermediate phenotypes and selects against both extremes?
Stabilizing selection
In Darwin's theory, what does 'fitness' actually measure in an organism?
The ability to survive AND leave the most offspring behind (reproductive success, not just survival)
If a population is in Hardy-Weinberg equilibrium, what is happening to its allele frequencies over time?
Nothing - they remain constant from generation to generation (the population is not evolving)
Give an example of a vestigial structure and explain why it provides evidence for evolution.
Examples: pelvic bones in whales, human appendix, molar teeth in vampire bats. These structures were functional in ancestors but serve no obvious purpose now, showing evolutionary history and descent with modification.
A mountain range forms, splitting a population of squirrels into two isolated groups. What type of speciation is this?
Allopatric speciation (speciation due to geographic/physical barrier)
Give a real-world example of directional selection and explain which phenotype is favored.
Antibiotic resistance in bacteria or pesticide resistance in insects. One extreme phenotype (resistant) is strongly favored, intermediate and opposite extreme are selected against. Average phenotype shifts in one consistent direction.
A pesticide is used on a crop. Initially it kills 95% of insects, but after 5 years it only kills 20%. Explain what happened at the population level.
Directional selection occurred: resistant individuals survived and reproduced, passing resistance alleles to offspring. The allele frequency for resistance increased over generations, shifting the population's average phenotype toward resistance.
A small population of 20 deer is isolated on an island. Explain why genetic drift will have a larger effect on this population compared to a mainland population of 2000 deer.
In small populations, random events (who survives/reproduces) have a bigger proportional impact on allele frequencies. Each individual represents 5% of the island gene pool versus 0.05% of the mainland. Random changes can dramatically shift island allele frequencies, while the mainland population buffers against random fluctuations.
Bird wings and insect wings both allow flight but evolved independently. What are these structures called, and what evolutionary process produced them?
Analogous structures, produced by convergent evolution. Similar environmental pressures (need for flight) led to similar solutions in distantly related organisms with no recent common ancestor with wings.
Two populations of fish live in the same lake but breed at different times of year, preventing interbreeding. What type of speciation is occurring and why?
Two populations of fish live in the same lake but breed at different times of year, preventing interbreeding. What type of speciation is occurring and why?
In a population of birds, some have large beaks for cracking large seeds, others have small beaks for small seeds. Medium-sized beaks are inefficient for both. What type of selection is occurring and what will happen over time?
Disruptive selection. Both extreme phenotypes (large and small beaks) are favored, intermediates are selected against. The population will divide into two distinct phenotypic groups, potentially leading to speciation if reproductive isolation develops.
Why do male peacocks have elaborate tail feathers even though these make them more vulnerable to predators? Explain the evolutionary mechanism.
Sexual selection: females preferentially mate with males having elaborate displays, as these traits indicate health and vigor. The reproductive advantage outweighs the survival disadvantage, so the trait persists through female mate choice.
A hurricane carries seeds from a mainland plant population to a distant island. The island population establishes and evolves differently. Name this mechanism and explain how it changes allele frequencies.
Gene flow (specifically the founder effect). The founding seeds carried only a subset of mainland alleles, immediately changing island allele frequencies. Immigration added alleles to the island; emigration removed them from mainland. This movement of alleles between populations changes frequencies in both.
The cytochrome c gene in humans and mice has very similar DNA sequences. How does this biochemical evidence support evolutionary theory, and what does it tell us about common ancestry?
Similar gene sequences indicate shared ancestry - both inherited the gene from a common mammalian ancestor. The high similarity suggests relatively recent evolutionary divergence. The more similar the DNA sequence, the more recently two species shared a common ancestor. This molecular evidence corroborates anatomical and fossil evidence.
Two species of flycatcher birds look almost identical and live in the same forest. Scientists discover they cannot produce fertile offspring together. Explain what mechanisms might be maintaining their reproductive isolation despite overlap.
Multiple pre- and post-zygotic barriers: Different mating songs/behaviors (behavioral isolation), breeding in different areas of forest (ecological isolation), mating at different times (temporal isolation), or if they do mate, offspring could be sterile (hybrid inviability/sterility). These mechanisms prevent gene flow between species.
Human babies born with very low or very high birth weights have lower survival rates than those with average weight. What type of selection is this? Explain what happens to phenotypic variability over time.
Stabilizing selection. Both extreme phenotypes have reduced fitness, intermediates survive best. Phenotypic variability decreases over time as the population converges on the optimal intermediate value. This occurs when environmental conditions are stable and the current average is already well-adapted.
In a population of beetles, green beetles are camouflaged on green leaves while brown beetles are visible to birds. After a forest fire, the area becomes mostly brown soil. Predict and explain the evolutionary outcome over the next 50 generations.
The brown beetle frequency will increase dramatically through directional selection. Birds will preferentially eat visible green beetles, giving brown beetles higher survival and reproduction. The allele for brown coloration will increase in frequency, potentially reaching fixation if selection pressure is strong enough. This demonstrates how environmental change drives evolution.
The northern elephant seal was hunted to ~20 individuals in the 1890s. Today there are 30,000 seals, but they show almost no genetic variation. Explain this paradox using population genetics principles and discuss the long-term evolutionary implications.
This is a population bottleneck - genetic drift in the extremely small population eliminated most alleles through random chance. Only alleles present in the 20 survivors could be passed on. Despite population recovery, genetic diversity cannot be restored without mutation or gene flow. This limits the population's ability to adapt to future environmental changes, as there's less variation for natural selection to act upon. The population is vulnerable because it lacks genetic diversity to respond to new diseases or climate changes.
Fossil series show that modern whales evolved from land-dwelling ancestors. Describe three specific types of evidence (fossil, anatomical, and embryological) that support this evolutionary transition, and explain how each piece of evidence demonstrates descent with modification.
Fossil evidence: transitional fossils showing gradual changes (Pakicetus with legs and ear bones adapted for underwater hearing → Ambulocetus with webbed feet → Dorudon with reduced hind limbs). Anatomical: vestigial pelvic bones in modern whales. Embryological: whale embryos temporarily develop hind limb buds that are reabsorbed. Together, these show a clear evolutionary pathway from land to sea, with structures being modified (limbs to flippers) or reduced (pelvis) over time.
The Cordilleran flycatcher and Pacific-slope flycatcher appear nearly identical despite being separate species (convergent evolution example). Design an experiment to test whether they're truly separate species. What evidence would you collect, and what results would confirm they are distinct species?
Collect multiple lines of evidence: 1) Genetic analysis - look for significant DNA sequence divergence in nuclear genes. 2) Breeding experiments - test if hybrids are fertile (biological species concept). 3) Field observations - document mate selection preferences and breeding territories. 4) Acoustic analysis - compare mating calls for differences. Results confirming separate species: >2-3% genetic divergence, sterile or inviable hybrids, no natural interbreeding, distinct mating calls, and different ecological preferences. All together demonstrate reproductive isolation.
A population of Darwin's finches lives on an island with two abundant food sources: large hard seeds and small soft seeds, but few medium-sized seeds. Predict the evolutionary trajectory over 100 generations. What type of selection will occur, how will beak size distribution change, and under what conditions might this lead to speciation?
Disruptive selection will favor both large-beaked (for hard seeds) and small-beaked (for soft seeds) finches. Medium beaks will be selected against due to feeding inefficiency. The beak size distribution will become bimodal (two peaks). Speciation could occur if: 1) Assortative mating develops (large-beaked birds preferring large-beaked mates), 2) Different feeding strategies lead to spatial/temporal isolation, 3) Genetic divergence accumulates in isolated breeding groups. This is sympatric speciation - reproductive isolation without geographic separation, driven by ecological resource partitioning.