---
title: "How Species Change"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 68
exercises: 12
source: https://one-course.com/books/biology/1/en/chapter/68-how-species-change
---

# Chapter 68 — How Species Change

The evidence hall established the fact; this chapter builds the engine. Its parts are all in your hands already: [mutation](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#prop-g9-genes-and-dna-explains) writing new spellings ([Proposition 65.6](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#prop-g9-genes-and-dna-explains)), the shuffle dealing unique decks ([Proposition 66.1](https://one-course.com/books/biology/1/en/chapter/66-unique-individuals#prop-g9-unique-individuals-theorem)), the [environment](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components)’s checks collecting most of every generation’s surplus ([Proposition 55.6](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#prop-g8-reproduction-and-environment-limits)). Assembled, they run themselves — that self-running is [natural selection](#prop-g9-how-species-change-selection), and it is the most consequential simple idea in [biology](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living).

## 68.1 The engine, assembled

**Proposition 68.1 (Natural selection).**

Three facts, none deniable, and their consequence:

1. *variety* : a [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population) ’s individuals differ in inherited [traits](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits) ( [Proposition 66.6](https://one-course.com/books/biology/1/en/chapter/66-unique-individuals#prop-g9-unique-individuals-reserve) banked it);
2. *over-production and checks* : far more young are produced than the [environment](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components) lets survive — the checks collect the surplus ( [Example 55.7](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#ex-g8-reproduction-and-environment-twosprings) );
3. *unequal collection* : the checks do not collect at random — individuals whose inherited [traits](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits) suit the current [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) escape them a little more often, and so leave more offspring, *who inherit those [traits](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits)* .

Consequence: generation after generation, the suited [alleles](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene)’ share of the [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population) grows. That drift of a [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population)’s inherited makeup, driven by nothing but variety meeting checks, is *natural selection* — and enough time makes it *evolution*.

**Proof.** *Admitted at this level.* ∎

**Example 68.2 (The moths, mechanized).**

[Example 67.6](https://one-course.com/books/biology/1/en/chapter/67-evidence-of-evolution#ex-g9-evidence-of-evolution-live)’s peppered moths, part by part. Variety: pale and dark inherited forms. Checks: [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) hunt moths by sight against bark. Unequal collection: on soot-blackened trunks the pale form is the visible one — eaten more, so the dark [allele](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene)’s share climbs toward the observed dark woods; the air clears, the bark pales, and the same engine runs in reverse. No moth changed; the *[population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population)’s* makeup did — selection edits the deck’s frequencies, not the individuals.

![The check at work: on soot-stained bark, the pale form is the visible one — and the visible one is the eaten one.](https://one-course.com/images/onecourse/chapters/biology-1/g9-how-species-change/img-3dd42c28f55d.jpg)

*The check at work: on soot-stained bark, the pale form is the visible one — and the visible one is the eaten one.*

**Remark 68.3 (What selection is not).**

Three standing confusions, retired:

- *no intention* : [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) select the way dampness “selected” woodlice ( [Example 37.7](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#ex-g6-exploring-our-environment-chooser) ) — by consequences, not choices; nothing plans;
- *no perfection* : selection only compares the variants *present* — it finds the better available, not the best imaginable (the shared [limb](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) plan’s awkward re-jobbing, [Exercise 67.7](https://one-course.com/books/biology/1/en/chapter/67-evidence-of-evolution#exo-g9-evidence-of-evolution-7) , is its signature);
- *no inheritance of effort* : the blacksmith’s [arm](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) dies with the blacksmith ( [Proposition 63.4](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#prop-g9-heredity-and-traits-sorting) ); selection works only on what [gametes](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#def-g8-sexual-reproduction-gametes) carry. Giraffes did not stretch their necks long — longer-necked variants out-fed and out-bred the shorter.

## 68.2 The engine observed and exploited

**Example 68.4 (Selection in the hospital).**

The medical case, mechanized for [Chapter 69](https://one-course.com/books/biology/1/en/chapter/69-microbes-and-infection#ch-g9-microbes-and-infection)’s use: a microbe [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population) holds rare resistant mutants ([mutation](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#prop-g9-genes-and-dna-explains)’s trickle); an antibiotic is the check; it collects the susceptible and spares the resistant, who inherit the ward. Days, not millennia — generations turn in hours, and selection’s speed scales with them. Every rule about finishing antibiotic courses and not scattering antibiotics into farming is engine management: weaken the check raggedly, and you breed what you meant to kill.

**Example 68.5 (Selection with a human hand on the check).**

[Problem 54.1](https://one-course.com/books/biology/1/en/chapter/54-sexual-reproduction#pb-g8-sexual-reproduction-1)’s breeder ran the same engine with herself as the check — keeping the crisp and resistant, culling the rest. Every dog breed, every [crop](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#def-g6-producing-our-food-farming) variety, every dairy line is accumulated *artificial selection*: proof of the engine’s power on a human timescale, wolf to lapdog inside some thousands of generations. Darwin — the naturalist who assembled this chapter’s argument — opened his great book with exactly that comparison.

![Charles Darwin, photographed around 1854, in the years he was assembling this chapter’s argument.](https://one-course.com/images/onecourse/chapters/biology-1/g9-how-species-change/img-28a24b6cd2c1.jpg)

*Charles Darwin, photographed around 1854, in the years he was assembling this chapter’s argument.*

**Proposition 68.6 (From change to new species).**

Selection explains a [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population) changing; one more ingredient explains [populations](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population) *splitting*. Separate two [populations](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population) of one [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) — a sea rises, a range divides, an island receives castaways ([Exercise 39.11](https://one-course.com/books/biology/1/en/chapter/39-how-plants-spread#exo-g6-how-plants-spread-11)) — and each runs the engine under its own [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions), accumulating its own [alleles](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene). Given enough divergence, the two no longer interbreed where they meet: by [Definition 30.1](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species)’s own definition, two [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) stand where one stood. The tree’s forks ([Definition 44.6](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#def-g6-classification-kinship-tree)) are exactly such splits, deep time’s worth of them.

**Proof.** *Admitted at this level.* ∎

**Method 68.7 (Running the engine on any case).**

For any claimed example of [evolution](#prop-g9-how-species-change-selection):

1. name the *variety* and check it is inherited ( [Method 63.8](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#met-g9-heredity-and-traits-audit) );
2. name the *check* , and how it collects unequally across that variety;
3. follow the *bookkeeping* : whose [alleles](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene) gain share in the next generation?
4. for a split, add the *separation* and the diverging [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) ;
5. timescale check: generations, not years, are the engine’s ticks.

## 68.3 Exercises

**Exercise 68.1 ★.**

State [natural selection](#prop-g9-how-species-change-selection)’s three facts and their consequence.

**Solution of Exercise 68.1.**

Variety of inherited [traits](https://one-course.com/books/biology/1/en/chapter/63-heredity-and-traits#def-g9-heredity-and-traits-traits); over-production met by the [environment](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components)’s checks; unequal collection — the suited escape a little oftener and leave more heirs. Consequence: the suited [alleles](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene)’ share grows generation by generation — [natural selection](#prop-g9-how-species-change-selection), and with time, [evolution](#prop-g9-how-species-change-selection).

**Exercise 68.2 ★.**

In the moth case, name variety, check, and the unequal collection — in both directions.

**Solution of Exercise 68.2.**

Variety: inherited pale and dark forms. Check: sighted bird predation against bark. Unequal collection: on sooty trunks the pale are seen and eaten — dark’s share climbs; on clean pale bark the dark are seen — pale’s share recovers. One engine, either direction.

**Exercise 68.3 ★.**

What changes under selection — the individuals, or the [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population)? Explain.

**Solution of Exercise 68.3.**

The [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population): no individual moth darkens. Selection shifts the *frequencies* of inherited forms across generations — individuals are dealt and spent; the deck’s composition is what drifts.

**Exercise 68.4 ★.**

Correct, mechanism in hand: “giraffes stretched their necks and passed the stretch on.”

**Solution of Exercise 68.4.**

Effort is not inherited — a stretched neck is the blacksmith’s [arm](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts). The mechanism: necks varied heritably; longer-necked variants fed higher, survived the lean checks oftener and left more heirs carrying the longer-neck [alleles](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene) — the [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population)’s necks lengthened, no giraffe’s did.

**Exercise 68.5 ★.**

Why does antibiotic resistance evolve in days where moths took decades?

**Solution of Exercise 68.5.**

The engine ticks in generations, and microbe generations turn in hours: days hold thousands of ticks. Add a savage check (the antibiotic) and rare resistant mutants inherit the ward within the week.

**Exercise 68.6 ★.**

What did the apple breeder and the moth’s [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) have in common, and what differed?

**Solution of Exercise 68.6.**

Both were the unequal check on an inherited variety — deciding who breeds. The difference: the breeder chose on purpose toward a goal; the [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) chose nothing — their hunting’s consequences did the selecting.

**Exercise 68.7 ★★.**

Why can selection not produce “the best imaginable” organism? Which of its three facts limits it?

**Solution of Exercise 68.7.**

Fact 1: it can only compare the variants *present*. Selection is a sieve over the current deck — it cannot summon the unmutated, unshuffled better design, only promote the best on hand; hence re-jobbed [limb](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) plans, not fresh engineering.

**Exercise 68.8 ★★.**

Run [Method 68.7](#met-g9-how-species-change-run) on the finches’ drought ([Example 67.6](https://one-course.com/books/biology/1/en/chapter/67-evidence-of-evolution#ex-g9-evidence-of-evolution-live)): [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) harden, big beaks crack what small ones cannot.

**Solution of Exercise 68.8.**

Variety: beak depth, heritable (ringers’ pedigrees show it). Check: the drought’s hard [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) — small beaks cannot crack the remaining food. Bookkeeping: deep-beaked [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) feed, survive, breed; next season’s hatchlings average deeper — the deep [alleles](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene) gained share. No split: one [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population), one tick, measured.

**Exercise 68.9 ★★.**

Assemble speciation for the island castaways of [Exercise 39.11](https://one-course.com/books/biology/1/en/chapter/39-how-plants-spread#exo-g6-how-plants-spread-11): separation, divergence, and the test that would declare two [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species).

**Solution of Exercise 68.9.**

Separation: the sea — castaways breed alone. Divergence: each island’s [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) run the engine its own way, accumulating its own [alleles](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene), generation on generation. The test: reunited [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) failing to interbreed — by the [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) definition, the fork is complete.

**Exercise 68.10 ★★.**

Explain, engine in hand, why unfinished antibiotic courses and farm-scattered antibiotics both breed resistance.

**Solution of Exercise 68.10.**

Both apply the check raggedly. The unfinished course kills the susceptible and releases the survivors — enriched for resistance — mid-selection; farm-scattered doses run the same partial sieve across vast microbe [populations](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population) daily. A check that culls without finishing is a breeding program for what it spares.

**Exercise 68.11 ★★.**

“Selection is the checks of [Chapter 55](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#ch-g8-reproduction-and-environment), falling on the variety of [Chapter 66](https://one-course.com/books/biology/1/en/chapter/66-unique-individuals#ch-g9-unique-individuals).” Justify the sentence as the book’s own summary.

**Solution of Exercise 68.11.**

The checks chapter established that most of every generation’s surplus is collected — predation, hunger, [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions); the uniqueness chapter established that the collected differ heritably, deck by deck. Selection is nothing added: it is the observation that those checks, falling on that variety, cannot fall equally — the two chapters multiplied together.

**Exercise 68.12 ★★★.**

Why does the engine *need* [sexual reproduction](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual)’s shuffle and [mutation](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#prop-g9-genes-and-dna-explains)’s trickle — what happens to selection in a [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population) of perfect clones ([Exercise 66.8](https://one-course.com/books/biology/1/en/chapter/66-unique-individuals#exo-g9-unique-individuals-8)) once [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) turn?

**Solution of Exercise 68.12.**

Selection spends variety; the shuffle and [mutation](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#prop-g9-genes-and-dna-explains) restock it. A clone [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population) offers the sieve one deck: when [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) turn, selection has nothing to promote — the vineyard stands or falls as one [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant). Without the trickle of new spellings and the deal of new combinations, the engine seizes: variation is not a by-product of the mechanism, it is the fuel.

## 68.4 Problem: The Island Notebooks

**Problem 68.1.**

Weekend problem — two centuries on a volcanic archipelago

A (composite, historically inspired) archipelago: volcanic islands, never joined to the mainland, hosting finch-like [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) whose beak forms differ island to island — and differ from their one evident mainland kin. Naturalists’ notebooks span two centuries.

**Part I — The pattern.**

1. The islands hold only [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) that could cross the sea — [birds](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) , [drift-seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) , one bat, no native land [mammals](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) or frogs. Which chapter predicted the passenger list ( [Exercise 39.11](https://one-course.com/books/biology/1/en/chapter/39-how-plants-spread#exo-g6-how-plants-spread-11) )?
2. All the archipelago’s finches resemble the one mainland [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) more than any other bird on Earth. What does [kinship](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#prop-g6-classification-kinship-kinship) reasoning ( [Proposition 44.4](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#prop-g6-classification-kinship-kinship) ) conclude about their origin?
3. Beaks differ by island’s diet: seed-crackers on [seed](https://one-course.com/books/biology/1/en/chapter/9-from-seed-to-plant#def-g2-from-seed-to-plant-seed) islands, probers on cactus islands. Read the match with [Proposition 28.4](https://one-course.com/books/biology/1/en/chapter/28-adapted-to-their-world#prop-g4-adapted-to-their-world-answers) — then say what this chapter adds that the [adaptation](https://one-course.com/books/biology/1/en/chapter/28-adapted-to-their-world#def-g4-adapted-to-their-world-adaptation) chapter could not.
4. Why are volcanic, never-joined islands the engine’s showcase? Name the speciation ingredient they supply wholesale.

**Part II — The engine’s notebook years.**

5. A drought year: soft [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) fail, hard [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) remain; the ringers’ calipers show next year’s hatchlings averaging deeper beaks. Run the engine’s bookkeeping through that sentence.
6. Wet years follow, soft [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) return, and the average drifts back. What does the reversal prove about selection’s “direction”?
7. One island’s finches, transplanted by storm to another, breed poorly with the residents. Which proposition is being watched mid-run?
8. A creationist-era notebook objects: “no one has seen one [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) become another.” Answer with timescale and the convergent-witness method ( [Method 67.7](https://one-course.com/books/biology/1/en/chapter/67-evidence-of-evolution#met-g9-evidence-of-evolution-weigh) ).

**Part III — The synthesis lecture.**

9. Deliver the archipelago’s story in the engine’s five steps ( [Method 68.7](#met-g9-how-species-change-run) ), castaway arrival to island [species](https://one-course.com/books/biology/1/en/chapter/30-biodiversity#def-g5-biodiversity-species) .
10. Explain why every island’s finch is “the better available, not the best imaginable” — what could each island’s deck offer selection?
11. Connect the archipelago to the hospital ward of [Example 68.4](#ex-g9-how-species-change-resistance) : same engine, different tick rate — state what sets the rate.
12. Close the lecture with Darwin’s comparison made explicit: what the pigeon-fancier’s hand and the drought’s hard [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) have in common.

**Solution of Problem 68.1.**

**1.** The [dispersal](https://one-course.com/books/biology/1/en/chapter/39-how-plants-spread#def-g6-how-plants-spread-dispersal) chapter: only the long-range services — wings, drift, the storm-blown — reach an oceanic island; the passenger list is the services’ manifest.

**2.** That the archipelago’s finches descend from mainland castaways — shared [attributes](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#def-g6-classification-kinship-attribute) in detail mean shared ancestry, and their nearest kin names the source [population](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population).

**3.** The [adaptation](https://one-course.com/books/biology/1/en/chapter/28-adapted-to-their-world#def-g4-adapted-to-their-world-adaptation) chapter reads the fit — each beak answers its island’s food problem. This chapter adds the *coming-about*: castaway variety, island checks, unequal collection, generations of bookkeeping — the fit’s manufacture, where before we could only see the fit ([Remark 28.9](https://one-course.com/books/biology/1/en/chapter/28-adapted-to-their-world#rem-g4-adapted-to-their-world-how)’s promised answer).

**4.** Separation: sea-bound [populations](https://one-course.com/books/biology/1/en/chapter/55-reproduction-and-the-environment#def-g8-reproduction-and-environment-population) breed alone from the start, each island a sealed engine room — speciation’s first ingredient supplied by geography wholesale.

**5.** Check arrives (hard [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) only); the inherited variety in beak depth is collected unequally (small beaks starve); the survivors’ breeding writes the shift into the next generation’s average — one tick of the engine, caliper-documented.

**6.** That its “direction” is only ever the current [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions): selection tracks the check, and a reversed check reverses the drift — no destination, just consequences, season by season.

**7.** [Proposition 68.6](#prop-g9-how-species-change-speciation) mid-run: divergence far enough advanced that interbreeding falters — a fork closing before the notebooks’ [eyes](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense).

**8.** Speciation’s ticks are generations by the thousands — watched fully only in fast-turning life (the ward’s [microbes](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#def-g6-producing-our-food-fermentation)). For finches, the method stands witness instead: pattern, [kinship](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#prop-g6-classification-kinship-kinship), live ticks and half-closed forks converge on the process no single lifetime spans.

**9.** (1) Variety: castaway founders, shuffled and mutating. (2) Checks: each island’s own foods and dangers. (3) Bookkeeping: each island’s suited [alleles](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#def-g9-genes-and-dna-gene) gain, island by island. (4) Separation: the sea between, holding each ledger apart. (5) Timescale: thousands of generations — and the forks stand complete.

**10.** Only what its founders carried plus its own [mutations](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#prop-g9-genes-and-dna-explains) since: each island’s deck is a castaway’s sample, and selection could promote only the best *available* beak from it — islands with richer decks or luckier [mutations](https://one-course.com/books/biology/1/en/chapter/65-genes-and-dna#prop-g9-genes-and-dna-explains) got nearer answers.

**11.** The generation time: hours in the ward, years on the islands — same three facts, same bookkeeping, ticks apart by a factor of thousands.

**12.** Both are checks deciding who breeds: the fancier’s deliberate hand and the drought’s mindless hard [seeds](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) do identical bookkeeping on inherited variety — which is why the changes wrought in pigeon lofts in centuries certify what nature’s checks can write in millions of years.