Campbell Biology · Chapter 22
Descent with Modification: A Darwinian View of Life
pp. 452–468 · 3 sections
This chapter introduces evolution: how older ideas about a young Earth and unchanging species gave way to Darwin's theory, how natural selection works, and the main kinds of evidence that life has changed and is still changing. It's the foundation for AP Unit 7, especially Topics 7.1–7.3 and 7.6–7.9, and the exam expects you to explain natural selection precisely, without saying that organisms change because they need to.
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22.1 Ideas about Earth and life before Darwin
pp. 453–455
The exam won't ask about Aristotle, Linnaeus, Cuvier, Hutton, Lyell or Lamarck by name, but it does expect you to avoid Lamarck-style wording when you explain natural selection (Topic 7.1) and to use rock layers as evidence of age (Topic 7.6).
In the course: Topic 7.1 Introduction to Natural Selection, Topic 7.6 Evidence of Evolution, Topic 7.9 Phylogeny (notes, videos and more questions)
Key points
- For a long time, most Western scholars saw species as fixed: each kind of living thing was made once, in its final form, and never changed. Some ranked all life on a single ladder from simplest to most complex.
- Linnaeus gave every species a two-part Latin name, like Homo sapiens, and sorted species into groups inside bigger groups (genus, family and so on). He saw the pattern as a plan of creation. Darwin later read the same nested pattern as a family tree.
- A fossil is any preserved bit of a once-living thing, from a bone to a footprint, and nearly all of them turn up in sedimentary rock. That rock builds up in layers called strata, each new layer settling on top of the last, so in undisturbed rock, deeper means older.
- Cuvier, a pioneer of fossil science, saw that the fossil species in a series of layers kept changing, with the deepest ones looking least like anything alive now. He agreed that whole species had died out, but he insisted species never change, and he blamed the turnover on sudden local disasters (catastrophism).
- Hutton and Lyell argued that ordinary, slow forces we can still watch today, like rivers cutting rock and mud piling up, built Earth's landscapes, and that they acted no faster in the past (uniformitarianism). Slow forces need enormous spans of time, so Earth had to be very old, and that gave Darwin the time that gradual biological change needs.
- Lamarck was right that species change over time and come to fit their surroundings, but his mechanism was wrong. He thought body parts grow with use and shrink with disuse, and that parents pass these changes on. Changes your body picks up during your life, like bigger muscles, don't alter the DNA in your eggs or sperm, so they aren't inherited that way.
- On the exam, a 'Lamarckian' answer says individuals changed because they needed to. A strong answer says individuals already varied, and those with helpful inherited traits left more offspring.
Key terms (11)
- fossil
- Any preserved trace of something that lived long ago, such as a bone, shell, leaf print or footprint.
- sedimentary rock
- Rock formed when sand, mud or other particles settle out of water or air and get pressed and cemented together over time. Most fossils are found in it.
- strata
- The stacked layers of sedimentary rock (one layer is a stratum). In undisturbed rock, lower layers formed earlier than the ones above them.
- paleontology
- The science of studying fossils to learn about life in the past.
- extinction
- The permanent end of a species, when its last member dies.
- catastrophism
- Cuvier's idea that Earth's past was shaped mainly by sudden, violent events, like floods, that wiped out the species living in an area.
- uniformitarianism
- Lyell's principle that slow, everyday forces like erosion shaped Earth in the past just as they do now, at about the same pace.
- binomial name
- A species' two-part scientific name: the genus first, then a word for the species, as in Canis lupus for the gray wolf.
- taxonomy
- The branch of biology that names organisms and sorts them into groups.
- use and disuse
- Lamarck's mistaken idea that body parts get bigger and stronger when used a lot and wither when not used.
- inheritance of acquired characteristics
- Lamarck's mistaken idea that changes an organism picks up during its life get passed to its offspring. Genetics shows this isn't how traits are inherited.
Check yourself: 22.1 Ideas about Earth and life before Darwin
4 questions on 22.1 Ideas about Earth and life before Darwin. Pick an answer to see if you got it, and why.
A student writes four explanations for why a fast-running desert lizard species has long hind legs. Which explanation is Lamarckian?
A cliff exposes four undisturbed layers of sedimentary rock, numbered 1 (bottom) to 4 (top). Fossils of snail species S appear in layers 1 and 2. Fossils of clam species C appear in layers 2, 3 and 4. Fossils of crab species K appear only in layer 4. Which conclusion is best supported?
Today, mud at the bottom of a lake builds up by about 2 mm each year. Under the lake bed lies 300 m of similar layered mud rock. If the mud built up in the past at the same rate as it does today, about how long did the 300 m take to form?
For 15 generations, researchers make lab rats run on a treadmill every day, and every trained rat develops strong leg muscles. Each generation is bred only from trained parents. A control line is bred from rats that never train. Pups from both lines are then raised with no training at all. Which result would have supported Lamarck's hypothesis?
0 of 4 answered
22.2 Darwin's journey and how natural selection works
pp. 455–460
Topics 7.1–7.3 test this directly: explain natural selection as heritable variation plus too many offspring leading to unequal reproduction, and remember that individuals are selected but populations evolve. Darwin's life story and the Wallace episode are background.
In the course: Topic 7.1 Introduction to Natural Selection, Topic 7.2 Natural Selection, Topic 7.3 Artificial Selection (notes, videos and more questions)
Key points
- On a five-year voyage around the world (1831–1836), Darwin collected plants, animals and fossils. He noticed that species fit their local surroundings, and that the species of a region, even fossil ones, resembled each other more than they resembled species from similar climates on other continents.
- The Galápagos Islands, off South America, were full of species found nowhere else, yet clearly related to mainland species. Darwin suggested that a few colonists had arrived from the mainland and changed over many generations into new species, as they adapted to each island.
- Darwin called evolution 'descent with modification.' All life shares ancestors, which explains why living things have so much in common. As lineages split and adapt to different ways of life, they build up differences, which explains life's diversity. A branching tree is the best picture, and most branches end in extinction.
- An adaptation is a heritable feature that makes an organism better at staying alive and having young where it lives. Natural selection is how adaptations build up: in a given setting, some inherited traits help their owners leave more surviving young than others do, so those traits grow more common.
- Darwin's argument runs in four steps. No two members of a population are quite alike, and many of their differences are passed on. Every species makes more young than its surroundings can feed, so most die or never breed. The ones with helpful inherited traits tend to have more surviving young. Over many generations, those helpful traits pile up in the population. An essay by Malthus on human population growth helped him see this.
- Artificial selection is the same process with people choosing which plants or animals breed. Because it reshapes crops, livestock and pets in relatively few generations, Darwin used it to show how much change selection can make.
- Selection acts on individuals, but it's the population that evolves. Selection can only sort heritable variation that's already present, and the same trait can help in one setting and hurt in another, so what selection favors shifts when the environment does. Wallace reached the same idea on his own, and both men's work was presented in 1858, a year before Darwin's book was published.
Key terms (11)
- evolution
- Change in the inherited traits of a population over generations. Broadly, the idea that today's species came from different ancestral species.
- descent with modification
- Darwin's phrase for evolution: species descend from ancestors and pick up changes along the way.
- common ancestor
- A single ancestral population from which two or more later lineages are descended.
- adaptation
- A heritable feature, shaped by natural selection, that helps an organism survive and have offspring in the place where it lives.
- natural selection
- A process in which, because of their inherited traits, some individuals leave more surviving offspring than others in their environment, so those traits spread through the population over generations.
- artificial selection
- Selective breeding: people choose which individuals reproduce, based on traits they want, and so change the population over generations.
- heritable variation
- Differences among individuals that are passed from parents to offspring through genes. Selection can only change a population if this exists.
- overproduction
- The fact that organisms make far more young than their surroundings can feed and shelter, so most don't survive to reproduce.
- fitness
- How well an individual passes its genes on, measured by how many offspring it leaves that survive to reproduce, compared with others.
- selective pressure
- Any feature of the environment, such as a predator, drought or disease, that makes some traits better for survival or reproduction than others.
- population
- A group of the same species living in the same area that can interbreed. It's the unit that evolves.
Check yourself: 22.2 Darwin's journey and how natural selection works
4 questions on 22.2 Darwin's journey and how natural selection works. Pick an answer to see if you got it, and why.
Breeders plant corn and, each generation, save seed only from the 10% of plants with the highest oil content in their kernels (the selected line). A second line, started from the same seed stock and grown in the same fields, is replanted from randomly chosen plants (the unselected line). Mean kernel oil content (%), invented data: Generation | Selected line | Unselected line 0 | 4.7 | 4.7 10 | 7.2 | 4.8 20 | 9.9 | 4.6 30 | 12.3 | 4.7 Which conclusion is best supported?
A meadow plant reproduces only asexually, and every plant in one population is genetically identical, including all genes that affect leaf hairiness. During a long drought, plants growing in sunnier, drier spots respond by developing hairier leaves, lose less water and make more offspring. What will most likely happen to the population's genes for hairiness over the next few generations?
After two hurricanes strike an island, researchers find that the surviving lizards have, on average, larger toe pads than the lizards they measured before the storms. Toe-pad size is heritable. Which statement best describes what happened?
In a species of pond fish, every female spawns once in her life, lays about 2,000 eggs and then dies, and males also breed only once. The population has held steady in size for decades, with equal numbers of males and females. On average, what percentage of eggs survive to breed?
0 of 4 answered
22.3 The evidence for evolution
pp. 460–467
Topic 7.6 tests fossils, homologous and vestigial structures and DNA or protein comparisons, Topic 7.8 tests resistance evolving today, Topic 7.9 tests reading trees, and convergent evolution is in Topic 7.10. Biogeography counts as evidence but rarely gets its own question.
In the course: Topic 7.6 Evidence of Evolution, Topic 7.8 Continuing Evolution, Topic 7.9 Phylogeny, Topic 7.10 Speciation, Topic 7.2 Natural Selection (notes, videos and more questions)
Key points
- You can watch evolution happen. When a population meets a new food source, a new predator or a new drug, its average traits can shift measurably within a few decades, and in bacteria within months.
- Antibiotics, pesticides, herbicides and chemotherapy drugs don't create resistance. They kill the susceptible individuals, so rare resistant ones that already existed survive, reproduce and take over. Bacteria can also swap resistance genes with each other, which spreads resistance even faster. Selection can only work with variants that already exist; it can't make the ones a population needs.
- Homology is similarity that comes from shared ancestry. Homologous structures are built on the same plan even when they do different jobs, and early embryos of related animals can share structures that later become very different adult parts.
- Vestigial structures are shrunken leftovers of parts that worked in an ancestor, like the tiny, useless wings of a kiwi. At the molecular level, all life uses DNA, RNA and almost the same genetic code, related species have similar gene and protein sequences, and broken gene copies (pseudogenes) can be passed down just like working ones.
- Homologies come in layers. A few very old ones, like the genetic code, are found in every living thing; others arose later and are found only in the smaller group descended from the ancestor that first had them. An evolutionary tree shows this, with each branch point standing for a common ancestor. To judge how closely two groups are related, find their most recent shared branch point, not how close their tips sit on the page.
- Convergent evolution is when unrelated lineages in similar environments evolve similar features on their own, like the streamlined bodies of sharks and dolphins. Those features are analogous: similar in function but not inherited from a shared ancestor.
- Fossils show that past life was different, that most species that ever lived are extinct, and that transitional forms link older groups to newer ones. Biogeography, where species live, fits evolution too: islands are full of endemic species related to those on the nearest mainland, and the drift of continents since the supercontinent Pangaea split up explains where many groups and their fossils are found. In science, a theory like evolution is a broad, heavily tested explanation, not a guess, and testing keeps refining it: new species can sometimes form quickly, and natural selection isn't the only thing that drives evolution.
Key terms (15)
- antibiotic resistance
- An inherited ability of a bacterium to survive a drug that kills others of its kind. It spreads when the drug kills off the susceptible cells.
- homology
- Similarity between species that comes from inheriting a trait from a common ancestor.
- homologous structures
- Body parts in different species built on the same basic plan because they come from a shared ancestor, even if they now do different jobs.
- vestigial structure
- A shrunken body part that does little or nothing now but worked in an ancestor, such as the tiny muscles that let some people wiggle their ears.
- pseudogene
- A copy of a gene that no longer works because of mutations, but is still carried in the genome, often in the same place in related species.
- molecular homology
- Similar DNA, RNA or protein sequences in different species, inherited from a common ancestor.
- evolutionary tree
- A branching diagram that shows a hypothesis about how groups are related, with each branch point marking a common ancestor.
- convergent evolution
- What happens when lineages that aren't closely related each evolve similar features on their own, usually because they face similar selective pressures.
- analogous structures
- Features that do a similar job in different species but evolved separately rather than from a common ancestor, such as bird wings and insect wings.
- fossil record
- All the fossils found so far, arranged by age. It shows which organisms lived when, and how groups changed.
- transitional fossil
- A fossil with a mix of features from an older group and a newer one that descended from it.
- biogeography
- The study of where species live around the world and why.
- continental drift
- The gradual shifting of the continents across Earth's surface over millions of years, which has split up and joined populations of organisms.
- Pangaea
- The single giant landmass that held nearly all of Earth's land about 250 million years ago, before it broke apart.
- endemic
- Found naturally in one place and nowhere else in the world. Islands often have many endemic species.
Check yourself: 22.3 The evidence for evolution
4 questions on 22.3 The evidence for evolution. Pick an answer to see if you got it, and why.
A researcher spreads bacteria that have never met an antibiotic on a plate with no drug, where they grow into about 200 colonies. She presses a velvet pad onto this plate and then onto a plate containing the antibiotic, copying the position of every colony. Three colonies grow on the antibiotic plate. When she tests cells from the matching three spots on the original plate, which never touched the drug, they are also resistant. What does this result best show?
Which pair of structures is homologous?
Researchers compare the amino acid sequence of the same 120-amino-acid enzyme in four mammal species (invented data): Species compared with W | Number of amino acid differences X | 2 Y | 11 Z | 25 Fossils show that the lineages leading to W and X split about 2 million years ago. If amino acid changes in this enzyme build up at a roughly steady rate, about when did the lineages leading to W and Z split?
Kiwis are flightless birds of New Zealand. Each has a wing only a few centimeters long, hidden under its feathers, with a reduced set of the usual bird wing bones. Which explanation best accounts for these wings?
0 of 4 answered