Biology  by Bradford
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IB BIOLOGY · THEME A · UNITY AND DIVERSITY

How one species becomes two

Every species alive today began as part of another. Here is the evidence that life evolves, from DNA to the bones in your arm, and how a river, a season or a doubled set of chromosomes can split one species into two.

IB Biology A4.1SL and HL4 hours SL, 5 HL9 chaptersdry lab

Start learningRun the dry labTeach it

One populationThe sea splits itDifferent seeds,different beaksTwo species
Speciation in one loop: a population is split, each side is selected differently, and one species becomes two. Run it yourself in the dry lab.

01Learn

Nine chapters, the last three for higher level. Read one, play its figure through or tap its parts, and make each prediction before you open the answer.

  1. 1Evolution is a change in what a population inherits
  2. 2Sequences remember common ancestors
  3. 3Six vegetables, one wild weed
  4. 4Same bones, different jobs, and the reverse
  5. 5New species arise only by splitting
  6. 6One river, two apes
  7. 7Apart or together: two routes to a new species HL
  8. 8One ancestor, many niches HL
  9. 9The mule's dead end, the plant's way out HL

A4.1.1 What evolution is

Evolution is a change in what a population inherits

Evolution is not something one animal does in its lifetime. It is a change, over generations, in the heritable characteristics of a population. Genes carry those characteristics, so evolution shows up as some alleles becoming more common and others rarer.

The figure follows a real case. In 1977 a drought on Daphne Major, a tiny Galápagos island, left mostly big, hard seeds. Medium ground finches with deep beaks could crack them; many with shallow beaks starved. The survivors bred, their chicks inherited their alleles, and the next generation's beaks were about 4% deeper on average. No finch changed its own beak. The population changed, because some birds left more offspring than others.

Watch out A change gained during a life is not passed on: a finch's chipped beak, a weightlifter's muscles, a giraffe stretching for leaves. None of these changes the alleles in eggs or sperm. Lamarck thought such changes were inherited; they are not, and they are not evolution.
Why "theory"? Evolution by natural selection explains and predicts a vast range of observations, and nothing found so far contradicts it. But science cannot prove an idea true by checking it against the whole world; it can only fail to disprove it. So it is called a theory: a pragmatic truth, true for every practical purpose and very unlikely ever to be overturned.

A gardener clips a hedge into a perfect cube every year for 40 years. Seeds from the hedge are sown. What shape do the seedlings grow into?

789101112Beak depth / mmone finchGeneration 1beaks vary, and chicksresemble their parentsmean 9.25mean 9.65Droughtonly big, hard seeds left:shallow beaks cannot crack them12 survivors breedtheir chicks inherit theiralleles for beak depthGeneration 2the population has changed:beaks about 4% deeper on averagegeneration 1beak depthParentchipped beakbreedsChickwhole beak
  1. Each dot is one finch. Beak depth varies, and the variation is heritable: chicks resemble their parents.
  2. A drought leaves only big, hard seeds. Most finches with shallow beaks starve.
  3. The survivors breed and pass on their alleles, deep-beak alleles included.
  4. The next generation's beaks are deeper on average: the population has evolved. (Numbers simplified from the real study.)
  5. A beak chipped on a hard seed is not passed on: the chick hatches with a whole beak. An acquired change is not evolution.
Change the ground, then switch off heritable: with no inheritance the population stops changing, however many beetles are eaten.

A4.1.2 Evidence from sequences

Sequences remember common ancestors

Every living thing keeps its genes in DNA, and almost all of them read it with the same genetic code. That shared machinery already points to one common ancestor. The sequences themselves hold the detail.

When a species splits in two, each new line copies its own DNA, and mutations build up separately in each. So the longer ago two species shared an ancestor, the more differences there are between the same gene in each. Compare one gene across species, count the differences, and the fewest differences mark the closest relatives.

Base sequences, in DNA or RNA, change faster than the amino acid sequences of the proteins they code for. Most amino acids have several codons, so many base changes leave the protein the same. Switch the figure to amino acids and some differences vanish: base sequences sort out close relatives, amino acid sequences reach further back.

  • Fewer differences: a more recent common ancestor.
  • Different genes and different proteins give largely the same branching pattern, and so does the fossil record. Independent lines of evidence agreeing is what makes this powerful.
Example Cytochrome c, a protein every aerobic cell uses, is 104 amino acids long in humans. A chimpanzee's is identical, a rhesus monkey's differs at one position, and a yeast's at more than 40.
Watch out Similar sequences do not mean one living species came from the other. Humans did not evolve from chimpanzees: both evolved from an ancestor that lived about 6 to 7 million years ago, and both lines have changed since.

Before you compare: which species' sequence will differ from the human one at the most positions?

one gene, positions 1 to 18HumanATGGCCTACGTGAACTTCMetAlaTyrValAsnPheChimpanzeeATGGCCTACGTGAATTTCMetAlaTyrValAsnPheMouseATGGCTTACATGAATTTTMetAlaTyrMetAsnPheChickenATGGCATTTGTAGATTTTMetAlaPheValAspPheFrogATGACGCATATAGATCTTMetThrHisIleAspLeueach bracket: one codon, one amino acidthe same stretch, read as its six amino acidsThe treeHumanChimpanzeeMouseChickenFrogCompare a species:its branch joins herebase differences from human
Compare:

Tap a species to show its sequence under the human one. Count the pink differences, then watch where its branch joins the tree.

A made-up 18-base stretch of one gene, built the way real comparisons turn out. Real ones use thousands of bases and give the same pattern.

A4.1.3 Evidence from selective breeding

Six vegetables, one wild weed

Wild cabbage grows on sea cliffs in western Europe: a straggly, leafy plant with yellow flowers. Over the last few thousand years, farmers kept seed only from the plants they liked best. This is selective breeding.

Choose the plants with the biggest leaves, generation after generation, and you get kale. Choose a fat terminal bud and you get cabbage; fat side buds, Brussels sprouts; a swollen stem, kohlrabi; thick flower stalks, broccoli; a dense mass of flower buds, cauliflower. Every one is still Brassica oleracea.

Why is this evidence for evolution? The cultivars differ from each other, and from wild cabbage, far more than the wild plant varies, and the changes are heritable: their seeds grow true. People choosing which plants breed changed a species this much in a few thousand years. Natural selection, choosing by survival and reproduction over millions of years, can change species far more.

Example Maize was bred from teosinte, a wild grass of Mexico. A teosinte ear holds fewer than a dozen kernels, each in a stone-hard case; a cob of maize holds hundreds of soft ones. Dogs were domesticated from wolves more than 15,000 years ago, and most of today's breeds, from chihuahua to Great Dane, were bred in the last few hundred years.
Watch out Dog breeds and cabbage cultivars look like different species but are not: a chihuahua and a Great Dane share one gene pool, which their alleles can still cross through dogs of in-between sizes. Selective breeding shows how fast heritable characteristics can change. It is not an example of new species forming.

Remember it The three lines of evidence for evolution in A4.1

  • Darwin’sDNA and protein sequencesfewer differences, closer relatives
  • BigBreedingbig heritable changes, fast
  • HunchHomologous structuresone bone plan, many jobs

Kohlrabi and Brussels sprouts look nothing alike. A grower lets them cross-pollinate. What happens?

terminal budleavesflowersside budsstemWild cabbageBrassica oleraceaon a sea cliffKaleleavesCabbageterminal budBrussels sproutsside budsKohlrabistemBroccoliflower stalksCauliflowerflower budsall one species
  1. Wild cabbage: leaves, buds at the tip and along the stem, and yellow flowers.
  2. Selected for leaves: kale. Selected for a big terminal bud: cabbage.
  3. Selected for side buds: Brussels sprouts. Selected for a swollen stem: kohlrabi.
  4. Selected for flower stalks and buds: broccoli and cauliflower. All six can still interbreed: one species.

A4.1.4 A4.1.5 Homologous and analogous structures

Same bones, different jobs, and the reverse

Your arm, a bat's wing, a whale's flipper and a horse's front leg do very different jobs. Inside, they are built from the same bones in the same order: one upper arm bone, two in the forearm, a block of wrist bones, a row of palm bones, then the digits. This shared plan is the pentadactyl limb, and structures built on a plan inherited from a common ancestor are homologous structures.

No engineer would build a wing, a paddle and a leg from the same parts. Evolution does not design from scratch: it modifies what an ancestor already had. The shared plan makes sense only if all four limbs were inherited from one common ancestor, an early four-legged vertebrate, and each line then modified it for its own way of life.

Now switch the figure. A bird's wing and an insect's wing both fly, but one is a forelimb full of bones and the other a thin fold of the body wall with no bones at all. Features with the same function but different evolutionary origins are analogous structures, and they arise by convergent evolution: unrelated lines meeting similar demands with similar solutions. The camera eyes of octopuses and vertebrates are another pair.

Watch out Penguin, turtle and whale flippers are often given as analogous structures, but all three are forelimbs with the pentadactyl bones inside: homologous as limbs, with only the flipper shape converging. And "pentadactyl" names the plan, not a count: the horse has one digit left, on the same plan.

Remember it The pentadactyl limb, shoulder to fingertip

  • HungryHumerusupper arm, one bone
  • RatsRadius and ulnaforearm, two bones
  • ChewCarpalswrist
  • MyMetacarpalspalm
  • PencilsPhalangesfingers

A bat's wing and a bird's wing: homologous or analogous?

HumangraspingBatflyingWhaleswimmingHorserunningHumerusRadius and ulnaCarpalsMetacarpalsPhalangesBirdwing: flyingInsectwing: flyingbones inside, feathers outsideno bones: cuticle and veinswings evolvewings evolvevertebratesarthropodscommon ancestor: no wings

Tap a bone, or a name, to light that bone in all four limbs.

A4.1.6 Speciation

New species arise only by splitting

A species, in the definition biologists use most, is a group of organisms that can interbreed and produce fertile offspring. Its populations share one pool of alleles, because individuals move and mate between them.

Speciation happens when that sharing stops. Split a species into two populations that no longer interbreed and each changes in its own way: different mutations, different chance events, different selection. Given long enough, they differ so much that they could not interbreed even if they met again. One species has become two.

Every new species has appeared like this, by splitting from one that already existed. Speciation adds to the number of species on Earth and extinction takes species away; the diversity alive today is the balance of the two.

Watch out A species that changes gradually without splitting is still one line. It has evolved, but no species has been added, so that is not speciation. Play the figure to step 4.
Where the definition struggles Organisms that reproduce asexually, like bacteria, do not interbreed, and fossils cannot be tested, so these are grouped by their characteristics and sequences instead. And because splitting takes time, some pairs of populations are part-way: they interbreed a little where they meet, and whether they count as one species or two is a judgement.

A line of snails changes its shell shape steadily for two million years and never splits. The oldest and newest shells look so different that fossil experts give them different names. How many species has the line added to the Earth?

speciesalive now1232a populationtimenowa barrier: gene flow stopscannot interbreedchanges gradually, stays one linea new split: +1extinction: a line ends, −1
  1. One species: every population can interbreed with every other.
  2. A barrier divides it into two populations. Gene flow stops, but for now they are still one species.
  3. Each population changes separately until they cannot interbreed: two species.
  4. The lower species changes gradually but never splits: still one species, so the count stays at two.
  5. The upper species splits again: three.
  6. The lower species dies out: extinction takes the count back to two.
Switch on gene flow and run it again: a few animals crossing each generation keeps them one species.

A4.1.7 Reproductive isolation and differential selection

One river, two apes

Speciation needs two things. First, reproductive isolation: the populations must stop interbreeding, so alleles no longer pass between them. A sea, a mountain range or a river can do it: geographical isolation. Second, differential selection: different conditions on each side favour different characteristics, so the populations diverge. New mutations and chance (genetic drift), different on each side, add to it.

The Congo River is a real case. Common chimpanzees live north of it and beyond its upper reaches; bonobos live only in the forests south of its great bend. Neither ape can swim, and the river is wide and deep. Genetic evidence suggests their ancestors were divided roughly 1 to 2 million years ago, perhaps when a few reached the south bank during a dry spell and were then cut off.

South of the river there are no gorillas, and plants on the forest floor, which the apes eat when fruit is scarce, are plentiful. North of it, chimpanzees share the forest with gorillas and food is patchier. One widely held explanation is that these differences selected for different behaviour: bonobos live in large, calm groups in which females form strong alliances, while chimpanzee groups are led by males and compete more fiercely. Their bodies differ too: bonobos are more slender, with longer legs and smaller skulls.

Watch out The river did not make the apes change. It stopped gene flow. Different selection on each side, with mutation and chance, did the changing.
How clean was the split? Their genomes show that a few apes did cross the river long after the split, a few hundred thousand years ago, but too rarely to stop the two populations diverging.

Suppose the river had divided the apes but the forests on both banks were identical. What would you expect after a million years?

AtlanticOceanequatorCongo RiverKasai Riverno gene flowgorillanorth: gorillas compete, food patchiersouth: no gorillas, herbs all yearChimpanzeePan troglodytesnorth bankBonoboPan paniscussouth bank
  1. Long ago: one population of ancestral apes, north of the Congo River.
  2. Perhaps in a dry spell, the river runs low and a few apes reach the south bank.
  3. The water rises. Neither side can cross: no gene flow between the populations.
  4. Different conditions on each bank: differential selection favours different behaviour and bodies.
  5. Over about 1 to 2 million years they diverge into two species: chimpanzees north, bonobos south.

A4.1.8 Allopatric and sympatric speciation

Apart or together: two routes to a new species

Every speciation needs the same first step: gene flow between two groups has to stop, so they can change separately. The two routes differ in what stops it.

In allopatric speciation ("other homeland") a geographical barrier does it: a sea, a mountain range, or a river, as for the bonobos. In sympatric speciation ("same homeland") the groups live side by side, and something else keeps them from mating. They may breed at different times (temporal isolation) or choose mates differently (behavioural isolation).

Apple maggot flies in North America show it happening. They used to lay their eggs only in hawthorn fruit, and adults find mates on the kind of fruit they grew up in. After apple trees were planted in North America, some flies switched to apples; apple-infested fruit was first recorded in the 1860s. Apples ripen weeks before hawthorns, so apple flies now emerge, meet and mate earlier, on apples. The two groups share the same orchards, yet interbreed far less than they would by chance: they are on their way to becoming separate species.

Watch out Sympatric does not mean the groups never meet; it means they could meet and still do not mate. It is rarer than allopatric in animals, because any interbreeding mixes their alleles again. In plants it is common, by polyploidy (chapter 9).

Remember it Three kinds of reproductive isolation

  • GrumpyGeographica barrier between them
  • BirdsBehaviouraldifferent courtship
  • TweetTemporalbreed at different times

Two populations of a cricket share one meadow. One sings and mates in spring, the other in late summer. Which kind of isolation is this, and which route to speciation?

Allopatric"other homeland"Sympatric"same homeland"the sea rises: a barrierdifferent seeds each sidebig seedssmall seedshawthornappleapples arrive: some flies switcheach mates on its own fruitripe Sepripe Auggeographic isolationtemporal + behaviouralSame in bothgene flow stops, so each group changes on its own: differentselection, mutations and drift, until they cannot interbreed.
  1. Two populations, each of one species: finches on an island, flies on hawthorn trees.
  2. Left: the sea rises and splits the island. Right: apple trees arrive and some flies switch to them; no barrier.
  3. Left: different seeds each side select different beaks. Right: apple flies mate earlier, on apples. Gene flow stops in both.
  4. Both groups diverge. What stopped gene flow differs: geography, or timing and behaviour.

A4.1.9 Adaptive radiation

One ancestor, many niches

When a species reaches a place with food and ways of life that nothing else is using, its descendants can split again and again, each line adapting to a different way of life. This is adaptive radiation. The unused ways of life are vacant niches.

About 1 to 2 million years ago a few seed-eating finches reached the Galápagos from the American mainland. The islands had few other small birds, so niches stood empty: big seeds, small seeds, cactus flowers, buds and fruit, insects, grubs hidden in bark. Today about 18 species of Darwin's finches descend from that one ancestor, each with a beak suited to its food. The woodpecker finch even pries out grubs with a cactus spine held in its beak.

Because each species uses a different food, closely related species can live on the same island without competing much. That is why adaptive radiation raises biodiversity: one species has become many, living side by side. In Lake Malawi, cichlid fish have radiated into hundreds of species in one lake, scraping algae, crushing snails, eating scales or other fish.

Watch out The finches did not grow new beaks because they needed them. In each population, birds whose beaks happened to suit an unused food survived and bred better: natural selection acting on variation that was already there.

Suppose the first finches had landed on islands already full of small birds of many kinds. What would you expect?

Vacant niches: foods no other small bird is usingseed-eating ancestorfrom the mainlandLarge groundbig seedsSmall groundsmall seedsCactuscactus flowersVegetarianbuds, fruitWoodpeckergrubsWarblerinsectsdifferent foods,so little competition:18 species, one ancestor
  1. A seed-eating finch arrives on islands with vacant niches: foods no other small bird is using.
  2. The first split: one line takes to eating insects, with a thin, pointed beak.
  3. The rest split into ground finches and tree finches.
  4. Each line splits again, each species with a beak suited to its food.
  5. Different foods mean little competition: many related species live side by side.

A4.1.10 A4.1.11 Hybrids and polyploidy

The mule's dead end, the plant's way out

Even where two species meet, their alleles rarely mix. Barriers act before fertilisation and after it. Before, courtship usually prevents hybridisation: many animals choose mates by songs, dances, colours or scents that belong to their own species, and ignore the rest. Each firefly species flashes its own code, and a female answers only her own species' code.

After fertilisation, a hybrid may die young, or live but be sterile. A horse (2n = 64) and a donkey (2n = 62) can produce a mule, with 32 chromosomes from the horse and 31 from the donkey: 63. A mule is strong and healthy, but at meiosis its chromosomes cannot all pair up, because horse and donkey chromosomes differ in number and arrangement. Its gametes get unbalanced sets and almost never work, so horse and donkey alleles go no further.

Plants have a way out. A hybrid of two plant species is usually sterile for the same reason. But if its chromosome number doubles, every chromosome gains an identical partner, meiosis works, and the plant is fertile. It is a polyploid, and it cannot breed successfully with either parent: a new species in a generation or two. This is abrupt speciation.

The smartweeds and knotweeds, genus Persicaria, are full of examples: DNA studies show that several of their species, some of them common weeds, began as hybrids like this. Redshank, Persicaria maculosa (2n = 44), is one, with pale persicaria (2n = 22) as one of its parents.

Watch out The hybrid's number is the sum of the gametes, not of the parents: 32 + 31 = 63 for the mule, 11 + 11 = 22 for the smartweed hybrid. Doubling then gives 44. Japanese knotweed, the invasive garden weed, is a polyploid too, but it belongs to a different genus (Reynoutria, also called Fallopia); name a Persicaria for this example.

Redshank (2n = 44) is crossed with one of its parents, pale persicaria (2n = 22). How many chromosomes do the offspring have, and are they fertile?

Animals: the mulehorse gametedonkey gameten = 32n = 31+horse 2n = 64donkey 2n = 62mule2n = 32 + 31 = 63meiosis: pairingsome cannot pairsterilePlants: the way outpale persicariasecond speciesn = 11n = 112n = 222n = 22+hybrid2n = 11 + 11 = 22no partners: steriledoubled2n = 44every chromosome has a partnerRedshank, Persicaria maculosa (2n = 44): fertile, butits offspring with either parent are sterile: a new species.
  1. A horse gamete (32 chromosomes) and a donkey gamete (31) make a mule with 63. A few of each set are drawn.
  2. At meiosis, matching chromosomes pair up, but horse and donkey sets differ in number and arrangement: some cannot pair.
  3. So the gametes get unbalanced sets and do not work: the mule is sterile.
  4. Two smartweed species make a hybrid with 11 + 11 = 22. No chromosome has a matching partner: sterile, like the mule.
  5. The chromosome number doubles to 44: every chromosome now has an identical partner, so meiosis works.
  6. A fertile polyploid whose offspring with either parent are sterile: a new species, made in a generation or two.

02Dry lab: Finches on islands

Split a population of finches, decide what acts on each side, run the generations, then bring them back together and test whether you have made a new species.

Your questionWhat does it take to turn one population into two species? Is cutting off gene flow enough, or do the two sides also need different selection? How much gene flow undoes it?
What you can changeThe scenario (split by the sea, a control with the same food both sides, and at higher level one island with two foods), the difference in food, the number of birds, how many cross each generation, how strongly females choose mates by song, mutation, and how many generations to run.
What to recordFor each run: the settings, the difference in mean beak depth when you stop, and the isolation index and verdict when you reunite them. Change one setting at a time and repeat each set-up at least three times: the runs scatter, as real populations do.

Find out

  • Is isolation alone enough? Run the control (same food both sides) three times with no birds crossing, then the split with very different food three times. Reunite them after each run. Compare the isolation indexes.
  • How much gene flow undoes it? Keep the food very different and raise the birds crossing each generation: 0, 1, 2, 4, 8. Three runs at each. At what rate does the verdict change from two species to one?
  • Small or large? Run the control with 20 birds per island and with 300. Which pair of islands drifts further apart by chance alone?
  • Higher level: one island, two foods. Set mate choice to none, then moderate, then very strong. When does the population split into two groups that do not interbreed?

Record

  • For every run: the scenario, the settings you changed, the generations, the difference in mean beak depth, and the isolation index and verdict when you reunite or test them.
  • Change one setting at a time and keep the rest fixed.
  • Repeat each set-up at least three times and take a mean: no two runs are the same, because chance is part of the model, as it is of real populations.
  • Use Graph your results under the lab to see the mean isolation index for each number of birds crossing each generation, or download the CSV and plot it yourself.

In the control, the islands have the same food. After 200 generations the mean beaks usually differ a little, but the test says one species. Why?

With very different food, a handful of birds crossing each generation is enough to keep the two islands one species. How?

Higher level. On one island with two foods, the population only splits into two groups when mate choice is strong. Why is mate choice needed here, but not when the sea splits the island?

03Check

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LessonCoversFocusChaptersLesson planSlides
1What evolves, and what DNA shows A4.1.1, A4.1.260 min Evolution as a change in a population's heritable characteristics, and what base and amino acid sequences show 1, 2 Lesson planPDF 663 KB SlidesPPTX 3.9 MB
2Breeding, bones and wings A4.1.3 to A4.1.560 min Selective breeding, the pentadactyl limb, and homologous against analogous structures 3, 4 Lesson planPDF 674 KB SlidesPPTX 678 KB
3How one species becomes two A4.1.6, A4.1.760 min Speciation by splitting: reproductive isolation and differential selection in bonobos and chimpanzees 5, 6 Lesson planPDF 673 KB SlidesPPTX 5.1 MB
4The island dry lab A4.1.6 to A4.1.860 min The finches dry lab: what isolation, gene flow and population size do to two populations 5, 6, dry lab Lesson planPDF 645 KB SlidesPPTX 5.6 MB
5Higher level HL A4.1.8 to A4.1.1160 min Allopatric and sympatric speciation, adaptive radiation, and new plant species by polyploidy 7, 8, 9 Lesson planPDF 700 KB SlidesPPTX 1.7 MB
PDFEvery lesson planAll five plans in one PDF, with this overview at the front. 1.3 MB KEYNote template, filled inThe class’s note template with every gap filled. 1.5 MB

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