Unit 10: Evolution and Natural Selection
Why populations change over generations, and the fossil, anatomical, and molecular evidence that it happened.
10 topics · 35 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Darwin and the Idea of Natural Selection
- How it worksHow do variation, inheritance, and differential survival combine into Darwin's mechanism of natural selection?
- Common mistakeWhy isn't natural selection organisms trying to adapt to their environment on purpose?
- Common mistakeWhy doesn't a blacksmith's muscular arms built up over years of hammering get passed on to his children?
- Going furtherWhat happens to a trait's frequency across generations when it consistently helps survival?
Your turn1 of 2
Darwin noticed that finches on different Galapagos islands had beak shapes suited to each island's specific food source, from cracking seeds to probing cactus flowers. Why did this pattern convince him that species change over time?Variation and the Struggle to Survive
- How it worksHow does limited food and space create a struggle for survival among individuals with different traits?
- Going furtherWhat happens to the amount of variation in a population if every individual were genetically identical?
- Common mistakeWhy doesn't every individual in a struggling population have an equal chance of surviving?
Your turn1 of 2
A single plant can produce a thousand seeds, and those seeds grow into seedlings that are slightly different from each other in height, leaf size, and root depth. Why do genetically related seeds from one plant still grow into slightly different seedlings?How Natural Selection Changes a Population
- How it worksWalk me through how a population's color distribution shifts generation by generation as predators pick off the easiest moths to see.
- Worked exampleFind the new percentage of dark moths after three generations if the dark allele's frequency rises from 10 to 40 to 70 percent.
- Common mistakeWhy didn't individual moths change color during their own lifetimes to survive better?
- Going furtherWhat happens to the moth population's colors again once pollution is cleaned up and tree bark lightens?
Your turn1 of 2
In a population of peppered moths, the dark allele's frequency rises from 29 percent to 74 percent over one generation as predators pick off more of the easy-to-see light moths. By how many percentage points did the dark allele's frequency increase?Directional, Stabilizing, and Disruptive Selection
- How it worksHow does directional selection push a population's trait distribution toward one extreme over time?
- How it worksHow does stabilizing selection squeeze a population's trait distribution toward the middle instead?
- Going furtherWhat happens to a population's trait distribution under disruptive selection favoring both extremes?
- Common mistakeWhy doesn't every kind of natural selection push a population's traits in the same direction?
Your turn1 of 2
Human birth weight clusters tightly around one average value, with both very low and very high birth weights linked to worse survival odds. Why did birth weight settle around one average value instead of drifting toward an extreme?Genetic Drift, Bottlenecks, and Founder Effects
- How it worksHow does random chance in which individuals survive and reproduce shift allele frequencies in a small population?
- How it worksHow does a founder effect set a new population's genetics based on just a few original colonizers?
- Going furtherWhat happens to a population's genetic diversity after a bottleneck event kills off most individuals?
- Common mistakeWhy doesn't genetic drift happen based on which traits are more useful for survival?
Your turn1 of 2
A population is at genetic equilibrium with a recessive allele frequency q equal to 0.657. According to the Hardy-Weinberg principle, the expected proportion of the population that is homozygous recessive is q squared. What fraction of this population is expected to be homozygous recessive?Evidence for Evolution: Fossils and Biogeography
- How it worksHow does a transitional fossil like Tiktaalik show intermediate traits between two different groups?
- How it worksHow does the distribution of similar species across separated continents support the idea of common ancestry?
- Common mistakeWhy doesn't the fossil record need to be complete with no gaps for evolution to be well supported?
Your turn1 of 2
Geologists find simpler, more ancient-looking organisms in deeper rock layers and more complex, modern-looking organisms in shallower layers. Why do deeper rock layers consistently contain older and simpler fossilized organisms?Evidence for Evolution: Anatomy, Embryology, and Molecules
- How it worksHow does comparing homologous structures across species reveal a shared evolutionary ancestor?
- How it worksHow does comparing DNA sequences between two species measure how recently they shared a common ancestor?
- Common mistakeWhy isn't a bird's wing and a butterfly's wing evidence of shared ancestry the way a bat's wing and a human's arm are?
Your turn1 of 2
A whale's flipper, a bat's wing, and a human's arm look very different on the outside, yet an x-ray reveals the same basic arrangement of one upper bone, two lower bones, and a set of digit bones in all three. Why do these three limbs share the same underlying bone layout?Speciation: How One Species Becomes Two
- How it worksHow does geographic isolation let two separated populations accumulate enough differences to become separate species?
- How it worksHow can a new species form without any physical separation, through sympatric speciation instead?
- Common mistakeWhy isn't two populations simply looking different enough proof that they've become separate species?
- Going furtherWhat happens if the river separating two populations dries up before they've fully become separate species?
Your turn1 of 2
A river cutting through a habitat can split one population of animals into two groups that no longer interbreed, and over enough time those two groups can become separate species. Why can this kind of physical separation eventually produce two distinct species from one?Coevolution
- How it worksHow does a predator and its prey drive changes in each other's traits across generations?
- Common mistakeWhy isn't coevolution one species evolving to deliberately help another species survive?
- Ties togetherHow does coevolution between a flower and its pollinator connect natural selection acting on two species at once?
Your turn1 of 2
One orchid species has a nectar tube so deep that only one specific moth species, with an unusually long tongue, can reach the nectar and pollinate it. Why does this flower's tube depth match exactly the tongue length of the one moth species that pollinates it?Human Evolution and Common Ancestry
- How it worksHow does a shared common ancestor branching into two lineages explain human and chimpanzee similarities?
- Common mistakeWhy didn't humans evolve directly from modern chimpanzees?
- Going furtherWhat happens to the family tree of primates when a new hominid fossil fills in a missing branch?
Your turn1 of 2
Humans and chimpanzees share roughly 98 percent of their DNA sequence, an unusually high similarity compared to more distantly related species. Why do humans and chimpanzees share such a high percentage of their DNA?