Speciation

🌿 Speciation and Macroevolution

Part 1 of 7 — Species Concepts

🔑 Key Concept: Under the biological species concept, a species is a group of populations whose members can interbreed in nature and produce viable, fertile offspring. A new species arises when a population becomes reproductively isolated, so that gene flow with its parent population stops.

Microevolution vs. macroevolution

Earlier topics showed microevolution: changes in allele frequencies within a population from one generation to the next, caused by natural selection, genetic drift, gene flow and mutation. This topic asks the bigger question: how does one species become two, and how do those splits add up over millions of years to the diversity of life?

Speciation is the bridge between the two scales. Every branch point (node) on a phylogenetic tree represents a speciation event.

The biological species concept (BSC)

The AP Biology course uses the biological species concept, proposed by Ernst Mayr in 1942. Each phrase of the definition does a specific job:

Worked example: horse, donkey and mule

1. Horses have 64 chromosomes (2n = 64); donkeys have 62 (2n = 62).
2. A horse gamete carries 32 chromosomes and a donkey gamete carries 31, so a mule has 63 chromosomes.
3. The mule is viable: it grows into a strong, healthy adult.
4. In meiosis, the mule's horse-derived and donkey-derived chromosomes cannot pair up properly, so it makes few or no functional gametes. The mule is not fertile.
5. Conclusion: alleles cannot flow from the horse gene pool into the donkey gene pool through mules, so horses and donkeys are separate species.

The key test is never "do they look alike?" or "can they mate?" It is "can genes flow between them through fertile offspring in nature?"

Concept Check — Applying the Biological Species Concept 🎯

Gene flow: the glue that holds a species together

Gene flow is the movement of alleles between populations when individuals (or their gametes, such as pollen) move and reproduce. Gene flow has a homogenizing effect: it keeps the allele frequencies of connected populations similar, even if they live in somewhat different environments.

Why stopping gene flow matters

Think of two populations as two buckets of paint connected by a tube. As long as paint flows through the tube, the colors stay mixed. Cut the tube, and anything added to one bucket (a new mutation, a shift from drift, a different selection pressure) stays in that bucket. Over many generations the buckets drift apart in color.

Speciation follows the same logic:

1. Gene flow is reduced or stopped (by a mountain range, a new river, a chromosome change, a shift in breeding season).
2. The populations diverge: natural selection favors different traits in different environments, genetic drift changes allele frequencies at random, and new mutations arise separately in each.
3. Reproductive isolation evolves as a by-product of that divergence: mating signals, breeding times, or chromosome structure no longer match.
4. Speciation is complete when, even if the populations meet again, they do not exchange genes.

🔑 AP Connection: The CED states that speciation may occur when two populations become reproductively isolated from each other. On the exam, a strong answer about speciation names the event that stopped gene flow, the evolutionary mechanism that caused divergence, and the barrier that now keeps the populations apart.

Looks can deceive

• Similar appearance, different species. Eastern and western meadowlarks look almost identical, but where their ranges overlap they rarely interbreed, largely because their songs differ. They are separate species.
• Different appearance, same species. A Chihuahua and a Great Dane look very different, but all dog breeds can interbreed and produce fertile puppies. Human populations also vary in height, skin color and facial features, yet all humans are one species with extensive gene flow.

Appearance is a clue, not the test. The test is gene exchange.

Fill in the Blanks — Isolation and Gene Flow 🔍

Where the biological species concept breaks down

The BSC is the most widely used definition for sexually reproducing organisms, but it has clear limits. Biologists use other species concepts when the BSC cannot be applied.

Other species concepts (for context)

Worked example: choosing a concept

A microbiologist isolates two strains of a soil bacterium. They reproduce only by binary fission, so the BSC cannot be applied. Their ribosomal RNA gene sequences differ at many positions, and on a phylogenetic tree built from those sequences each strain sits on its own branch with its closest relatives. The microbiologist uses the phylogenetic (or a morphological or ecological) concept and treats them as separate species.

🔑 AP Connection: Unless a question says otherwise, use the biological species concept. When a question describes asexual organisms or fossils, expect to explain why the BSC cannot be applied: there is no interbreeding to observe.

AP-Style Check — Limits of the Definition 🎯

Part 1 Summary

Up next → Part 2: Allopatric Speciation, where a physical barrier splits a population and the two halves go their separate evolutionary ways.

🌿 Speciation and Macroevolution

Part 2 of 7 — Allopatric Speciation

🔑 Key Concept: In allopatric speciation ("other homeland"), a geographic barrier physically separates a population into isolated groups. With gene flow cut off, natural selection, genetic drift and mutation change each group independently, and reproductive isolation evolves as a by-product of that divergence.

The four steps

If the barrier later disappears and the groups meet but cannot exchange genes, speciation is complete.

Two ways a population gets split

Dispersal often involves a founder effect: the small founding group carries only a sample of the parent population's alleles, so its allele frequencies differ from the start. Genetic drift then acts strongly in the small new population.

A barrier is only a barrier for some organisms

Whether a feature isolates a population depends on how far and how easily the organisms move:

So the same geographic event can start speciation in one species and have no effect on another.

Concept Check — Barriers and Steps 🎯

Case study 1: snapping shrimp and the Isthmus of Panama

About 3 million years ago, the Isthmus of Panama rose and closed the seaway between the Pacific Ocean and the Caribbean Sea. Marine populations that had been continuous were suddenly split.

• Biologists studied snapping shrimp (genus Alpheus) and found 15 pairs of species. In each pair, one species lives on the Pacific side and one on the Caribbean side.
• DNA comparisons show that the closest relative of each Pacific species is a Caribbean species just across the isthmus, not another Pacific species.
• When males and females from a pair are put together in the lab, they rarely mate.

Reasoning: The pattern of sister species on opposite sides of one barrier is exactly what allopatric speciation predicts. The barrier split each ancestral species, and the two halves became reproductively isolated during roughly 3 million years apart.

Case study 2: fruit flies on different diets (a lab model)

Can reproductive isolation really arise as a side effect of adapting to different environments? Diane Dodd tested this with the fruit fly Drosophila pseudoobscura.

Design

1. She divided one laboratory population into several groups.
2. Some groups were raised on a starch medium and others on a maltose medium. No gene flow occurred between groups.
3. After about a year (roughly 40 generations), natural selection had changed the groups' ability to digest their own food.
4. She then offered flies a choice of mates from the starch and maltose groups and counted the matings.

Results (adapted from Dodd, 1989)

Worked example: reading the table

1. Same-medium matings: 22 + 20 = 42.
2. Different-medium matings: 9 + 8 = 17.
3. Total: 42 + 17 = 59.
4. Percent same-medium: 42 ÷ 59 x 100 ≈ 71%.
5. Expected with random mating: about 50%, because equal numbers of each type were offered.
6. Conclusion: Flies preferred mates from their own diet group. A prezygotic (behavioral) barrier had begun to form, even though nobody selected for mate choice. It arose as a by-product of adapting to different food.

Control: When groups raised separately on the same medium were tested against each other, flies showed no strong preference. This rules out "simply being kept apart" as the cause and points to the different diets.

Calculate — A Mating-Preference Test 🧮

A student repeats the diet experiment with two groups of flies kept apart for 50 generations, one on a corn medium and one on a rice medium. Equal numbers of each type are offered as mates. She records 40 same-medium matings and 10 different-medium matings.

What makes allopatric speciation more likely?

This is why islands and isolated habitats (lakes, mountaintops, caves) are hot spots of speciation. The Galápagos and Hawaiian archipelagos have many species found nowhere else.

Separation is not the same as speciation. If two groups are reunited before they have diverged much, they simply interbreed and merge back into one gene pool. Geographic isolation sets the stage; it does not guarantee a new species.

Secondary contact: what happens when the barrier disappears?

Worked example: reinforcement

Two bird populations were separated by a glacier and met again when it melted. Hybrids form but few survive to breed. A female that mates with a male of her own type leaves more surviving offspring than a female that mates with the other type. Over generations, alleles for choosing same-type mates increase in frequency, and the plumage of males in the overlap zone becomes more different between the two groups than it is far from the overlap. Selection against hybrids has completed the job that geography started.

🔑 AP Connection: In a free-response question about allopatric speciation, name (1) the barrier that stopped gene flow, (2) the mechanism of divergence (selection in different environments, drift in a small population, or mutation), and (3) the reproductive barrier that now keeps the groups apart.

AP-Style Check — Evidence and Secondary Contact 🎯

Part 2 Summary

Up next → Part 3: Sympatric Speciation, where new species arise with no geographic barrier at all.

🌿 Speciation and Macroevolution

Part 3 of 7 — Sympatric Speciation

🔑 Key Concept: In sympatric speciation ("same homeland"), a new species arises within the range of its parent population, with no physical barrier. Gene flow must be cut by something else: a chromosome change (polyploidy), a shift to a new habitat or food source, or sexual selection on mating signals.

Why sympatric speciation is harder

In allopatric speciation, geography does the work of stopping gene flow. In sympatric speciation, the diverging groups live side by side, so they could interbreed at any time. Any small difference that starts to build up is erased by gene flow unless something reduces mating between the groups from the very start. Three things can do that:

Polyploidy: a new species in one generation

A polyploid has more than two complete sets of chromosomes. Polyploidy usually arises from an error in cell division, such as nondisjunction of every chromosome during meiosis, which produces a diploid (2n) gamete instead of a haploid (n) one.

Autopolyploidy: extra sets from one species

1. A diploid plant (2n) has a meiotic error and makes some 2n gametes.
2. Two 2n gametes fuse (often by self-fertilization), giving a tetraploid (4n) offspring.
3. The tetraploid is fertile: in meiosis, its four copies of each chromosome pair up evenly and its gametes are 2n.
4. If the tetraploid mates with a diploid, a 2n gamete fuses with an n gamete to make a triploid (3n).
5. The triploid is sterile: three copies of each chromosome cannot be divided evenly between two daughter cells in meiosis, so its gametes have unbalanced chromosome sets.

Worked example: a diploid with 2n = 14

The tetraploids can breed with each other but not successfully with the diploids that produced them. By the biological species concept, they are a new species that arose in a single generation, in the same field as its parents.

Allopolyploidy: sets from two different species

1. Species A (2n = 10) and species B (2n = 14) occasionally cross. Their hybrid gets 5 + 7 = 12 chromosomes.
2. The hybrid is usually sterile: its 5 A-chromosomes and 7 B-chromosomes are not homologous, so they cannot pair in meiosis. It may still reproduce asexually (for example, by runners or bulbs).
3. A later error doubles the hybrid's chromosome number to 24. Now every chromosome has a homologous partner, meiosis works, and gametes carry 12 chromosomes.
4. The fertile allopolyploid (2n = 24) cannot produce fertile offspring with either parent species. It is a new species.

Real examples

• Goatsbeards (Tragopogon): Three European species were introduced to the Pacific Northwest of the United States in the early 1900s. By the mid-1900s, two new allopolyploid species had formed from hybrids between them, and botanists have documented them arising there.
• Bread wheat is a hexaploid (6n = 42): it carries six sets of 7 chromosomes, inherited from three ancestral species through two rounds of hybridization and chromosome doubling.

Concept Check — Why Polyploids Are Isolated 🎯

Habitat differentiation: the apple maggot fly

The apple maggot fly (Rhagoletis pomonella) originally laid its eggs only on the fruit of hawthorn trees, which are native to North America. Apple trees were brought to North America by European settlers, and in the 1800s some flies began laying eggs on apples instead.

Why does a change of fruit reduce gene flow?

Today the apple and hawthorn groups (called host races) differ in allele frequencies and in the timing of their life cycles, and gene flow between them is low, all within the same orchards and hedgerows. They are widely studied as a case of speciation in progress.

Sexual selection: Lake Victoria cichlids

Lake Victoria in East Africa holds hundreds of species of cichlid fish, many of which have arisen recently within the same lake. Two closely related species illustrate how mate choice can split a population:

• Males of one species have blue backs; males of the other have red backs.
• Females of each species strongly prefer males with their own species' color, so the two species live together without interbreeding.

The key experiment

1. Researchers placed females with males of both species in tanks under normal light: females chose males of their own species.
2. They repeated the test under monochromatic orange light, which hides the blue/red difference.
3. Under orange light, females mated with males of both species, and the hybrid offspring were viable and fertile.

Reasoning: Nothing about the fish's genes or anatomy prevents interbreeding. The only barrier is behavioral: females' color preference. Sexual selection on male color has been enough to stop gene flow in a shared lake. (In parts of the lake where pollution has made the water murky, females can no longer see male color well, and hybridization has increased.)

🔑 AP Connection: For a sympatric scenario, identify what reduces gene flow without a geographic barrier: a change in chromosome number, a change in habitat or timing, or a mate-choice preference. Then name the reproductive barrier it creates.

Calculate — Chromosome Counts in Polyploid Speciation 🧮

Allopatric vs. sympatric: telling them apart

Worked example: classify the scenario

Scenario: A population of a wildflower grows in one meadow. A new form with twice the parent's chromosome number appears. Crosses between the new form and the parent produce seeds that grow into sterile plants.

1. Where? Same meadow, so no geographic barrier. This rules out allopatric speciation.
2. What stopped gene flow? Hybrids with the parent are sterile because of a chromosome-number difference.
3. Barrier type: Reduced hybrid fertility (a postzygotic barrier).
4. Claim: This is sympatric speciation by autopolyploidy.

Common misconceptions

• "Sympatric means no isolation." No: it means no geographic isolation. A reproductive barrier is still needed.
• "Polyploids are always weaker." Many polyploid plants are larger and more vigorous than their diploid parents, and many crops (wheat, cotton, potatoes, strawberries) are polyploid.
• "Only plants speciate sympatrically." Polyploidy is far more common in plants, but host shifts (insects) and sexual selection (fish, birds) work in animals.

AP-Style Check — Apple Flies and Cichlids 🎯

Part 3 Summary

Up next → Part 4: Reproductive Isolation, a full catalog of the prezygotic and postzygotic barriers that keep species apart.

🌿 Speciation and Macroevolution

Part 4 of 7 — Reproductive Isolation

🔑 Key Concept: Reproductive isolating barriers keep species from exchanging genes. Prezygotic barriers act before a zygote forms: they stop mating or fertilization. Postzygotic barriers act after a hybrid zygote forms: the hybrid dies, is sterile, or its offspring fail.

The barriers in the order they act

Picture the path from "two individuals in the same region" to "fertile grandchildren." Each barrier blocks one step on that path.

Memory aid

The prezygotic barriers follow the order of meeting, mating and fertilizing:

• Where? Habitat
• When? Temporal
• How do they court? Behavioral
• Do the parts fit? Mechanical
• Do the gametes fuse? Gametic

The postzygotic barriers follow the life of the hybrid: does it live (viability), can it reproduce (fertility), can its offspring reproduce (breakdown)?

Concept Check — Naming the Barrier 🎯

Why the timing of a barrier matters

Both kinds of barriers stop gene flow, but they differ in cost to the organisms:

This difference explains reinforcement (introduced in Part 2). Where two species meet and their hybrids are unfit, an individual that avoids mating with the other species leaves more offspring than one that wastes matings on hybrids. Natural selection therefore strengthens prezygotic barriers in areas of overlap.

Worked example: barriers stack

Most pairs of species are kept apart by several barriers at once. Suppose two closely related fly species share a forest:

1. Habitat: They feed on different trees, so 80% of potential encounters never happen. 20% of encounters remain.
2. Behavioral: Of the encounters that do happen, half end without mating because the courtship songs differ. Now 20% x 0.5 = 10% of potential matings occur.
3. Reduced hybrid fertility: Every hybrid that forms is sterile. 0% of potential matings pass genes into the next generation of either species.

Even the leaky early barriers matter: they mean fewer resources wasted on hybrids. But it takes only one complete barrier to stop gene flow entirely.

Barriers in plants

Plants cannot move to find mates, so their barriers often involve pollen and pollinators:

🔑 AP Connection: The CED names these eight barriers, split into five prezygotic mechanisms (habitat, temporal, behavioral, mechanical, gametic) and three postzygotic mechanisms (reduced hybrid viability, reduced hybrid fertility, hybrid breakdown). Expect to name the barrier in a scenario and state whether it acts before or after the zygote forms.

Classify — Barriers in Plants and Animals 🔍

A step-by-step method for classifying any scenario

Work through these questions in order. The first "no" names the barrier.

Worked example 1

Two frog species live in the same pond. One breeds in early April, the other in late May.

• Step 1: same pond, yes. Step 2: different breeding times, no.
• Answer: temporal isolation (prezygotic).

Worked example 2

Two moth species mate readily in the lab, and the eggs hatch into healthy hybrid caterpillars that become adults. The hybrid adults produce no eggs or sperm.

• Steps 1–6: yes (they mate, fertilize and survive). Step 7: no gametes, no.
• Answer: reduced hybrid fertility (postzygotic).

Worked example 3

Two fish species share a reef and spawn on the same night. Females of each species ignore the courtship dances of males of the other species.

• Steps 1–2: yes. Step 3: no mate recognition, no.
• Answer: behavioral isolation (prezygotic).

Tricky distinctions

AP-Style Check — Barriers and Selection 🎯

Part 4 Summary

Prezygotic barriers cost less, so where hybrids are unfit, selection can strengthen them (reinforcement).

Up next → Part 5: Macroevolution Patterns, where we zoom out to the tempo of speciation, adaptive radiations and extinctions across Earth's history.

🌿 Speciation and Macroevolution

Part 5 of 7 — Macroevolution Patterns

🔑 Key Concept: Over long time scales, the diversity of life is set by two rates: how fast new species form (speciation) and how fast species disappear (extinction). Speciation can be slow and steady or rapid after long stasis, it is especially fast during adaptive radiations when new habitats open up, and extinctions, including mass extinctions, open those habitats.

Two tempos of evolution

"Rapid" means rapid on a geological scale. A change that takes 50,000 years looks instantaneous in rock layers that each represent hundreds of thousands of years.

Worked example: reading two fossil graphs

Lineage A (a marine snail). A graph plots average shell length (mm) against time:

• 9 to 6 million years ago (mya): shell length stays near 20 mm.
• Around 6 mya: within about 50,000 years, shell length shifts to 30 mm, and the new form appears alongside the old one.
• 6 mya to 1 mya: shell length stays near 30 mm.

Interpretation: long stasis, a geologically sudden shift associated with the appearance of a new form, then stasis again. This is punctuated equilibrium.

Lineage B (a grazing mammal). A graph plots average tooth height (mm) against time:

• From 10 mya to 2 mya, tooth height rises from 12 mm to 28 mm, about 2 mm per million years, with no long flat stretches.

Interpretation: a steady slope across 8 million years. This is gradualism.

Why might punctuated patterns be common?

1. New species often form in small, isolated populations (Parts 2 and 3), where selection and drift can change traits quickly.
2. Small populations leave few fossils, so the transition is rarely captured.
3. Once a species is well adapted to a stable environment, stabilizing selection can hold its traits nearly constant for millions of years.

Both tempos occur in nature. The AP exam expects you to recognize each from a description or graph, not to declare one "correct."

Concept Check — Reading Tempo from Fossil Data 🎯

Divergent evolution and adaptive radiation

Divergent evolution occurs when related populations adapt to different habitats or ways of life and their phenotypes diversify. When divergence happens many times in a short period from one ancestor, the result is an adaptive radiation: the rapid evolution of many species, each adapted to a different niche.

The CED notes that speciation rates can be especially rapid during adaptive radiations, as new habitats become available. Habitats become available in three main ways:

Case: Hawaiian silverswords

• The Hawaiian Islands formed by volcanic activity far from any continent.
• Molecular evidence indicates that the roughly 30 silversword species descend from a single ancestral tarweed that reached Hawaii from North America, probably as a seed carried by a bird.
• The descendants now range from low cushion-shaped plants to shrubs, vines and tree-like forms, in habitats from wet forests to dry volcanic slopes.

Case: Galápagos finches

• A small founding population of finches reached the Galápagos from South America.
• Its descendants diverged into more than a dozen species whose beaks are adapted to different foods: large crushing beaks for hard seeds, slender probing beaks for insects, and others.

Divergent vs. convergent evolution

Reading radiations on a phylogenetic tree

On a tree drawn with a time axis, an adaptive radiation appears as many branch points clustered within a short time span soon after a trigger (an island forming, a mass extinction). Branch points spread evenly through time suggest a steadier rate of speciation.

🔑 AP Connection: When explaining an adaptive radiation, connect three links: (1) a new habitat or empty niche became available, (2) populations faced different selection pressures in different niches, (3) divergence plus reproductive isolation produced many new species in a short time.

Calculate — Diversity as Speciation Minus Extinction 🧮

The number of species in a group changes by: new species formed − species lost to extinction.

Extinction throughout Earth's history

Extinction is a normal part of the history of life: the vast majority of species that have ever lived are now extinct. Biologists distinguish two scales:

The CED's key point: extinction rates can be rapid during times of ecological stress.

The five largest mass extinctions (approximate)

Human activity and extinction

The CED states that human activity can drive changes in ecosystems that cause extinctions. Current extinction rates are estimated to be far above the background rate.

Small populations are at greatest risk. A small population has less genetic variation, suffers more from genetic drift and inbreeding, and can be wiped out by a single disease or disaster. Habitat fragmentation turns large populations into many small ones.

Diversity = speciation − extinction

The CED states that the amount of diversity in an ecosystem can be determined by the rate of speciation and the rate of extinction:

• Speciation rate > extinction rate → diversity increases.
• Speciation rate = extinction rate → diversity stays steady.
• Extinction rate > speciation rate → diversity decreases (as in a mass extinction).

Extinction opens the door to adaptive radiation

The CED also states that extinction can make new environments available for adaptive radiation. After the end-Cretaceous extinction removed the non-avian dinosaurs and many other groups, mammals, which had existed for well over 100 million years mostly as small animals, radiated into the empty niches. Within roughly 15 million years, early members of the lineages leading to whales, bats, primates and hoofed mammals had appeared.

Extinction and speciation are linked: a mass extinction lowers diversity sharply, then the survivors' radiations rebuild it, usually with different dominant groups.

AP-Style Check — Extinction and Radiation 🎯

Part 5 Summary

Common misconceptions in this topic

• "Evolution always proceeds slowly and steadily." Fossils show both gradual change and long stasis broken by rapid change.
• "Organisms evolve because they need to." Populations change only when heritable variation already exists and selection acts on it; individuals do not develop new traits on demand.
• "Extinction means a species was poorly designed." Most species that went extinct in mass extinctions were well adapted to conditions that then changed suddenly.
• "Mammals replaced dinosaurs by outcompeting them." Mammals coexisted with dinosaurs for over 100 million years; the extinction, not competition, opened the niches.
• "Speciation stops in a mass extinction." Speciation continues, but extinction outpaces it, so diversity falls.

Up next → Part 6: Problem-Solving Workshop, where you will use the whole topic on AP-style scenarios, data and experiments.

🌿 Speciation and Macroevolution

Part 6 of 7 — Problem-Solving Workshop

🔑 Key Concept: AP questions on this topic rarely ask for definitions alone. They give you a scenario or data set and ask you to identify a barrier or mode of speciation, design or critique an experiment, analyze data (often with percentages or chi-square), and predict and justify an outcome.

The task verbs and what they demand

A reliable pattern is claim → evidence → reasoning: state the answer, point to the specific data or scenario detail, then connect them with a biological principle.

Scenario 1: a mate-choice experiment

A researcher divides a laboratory population of seed beetles into two groups. Group 1 is raised on mung beans and Group 2 on adzuki beans for 50 generations, with no contact between groups. She then places equal numbers of males and females from both groups together in mating chambers and records 60 matings.

Step 1: identify the parts of the design

The control group matters: it separates the effect of different diets from the effect of simply being kept apart.

Step 2: analyze the data with chi-square

Results: 45 same-group matings and 15 mixed-group matings.

If mating were random, the expected counts would be 30 and 30 (half of 60 each).

$\chi^2 = \sum \frac{(o-e)^2}{e} = \frac{(45-30)^2}{30} + \frac{(15-30)^2}{30} = \frac{225}{30} + \frac{225}{30} = 15$

• Degrees of freedom = number of categories − 1 = 2 − 1 = 1.
• Critical value at p = 0.05 with 1 df = 3.84.
• Because 15 > 3.84, reject the null hypothesis.

Step 3: claim, evidence, reasoning

• Claim: Beetles raised on different beans prefer mates from their own group; a prezygotic (behavioral) barrier is forming.
• Evidence: 75% of matings (45 of 60) were same-group, and χ² = 15 exceeds the critical value of 3.84.
• Reasoning: Without gene flow, selection for using different beans changed the two groups, and mate preference changed as a by-product. If this continues, gene flow between the groups could stop entirely, the first step toward speciation.

Calculate — Chi-Square for Mating Data 🧮

A second researcher repeats the experiment with a different pair of beetle groups and records 60 matings: 36 same-group and 24 mixed-group. Use a null hypothesis of random mating.

Scenario 2: hybrid fertility data

Two wildflower species, A (2n = 12) and B (2n = 18), grow in the same region. A botanist measures the percentage of pollen grains that are viable (able to fertilize an ovule).

Model analysis

1. Identify the barrier: The F1 hybrid grows but makes almost no viable pollen: reduced hybrid fertility, a postzygotic barrier.
2. Explain the low fertility: The hybrid's 6 chromosomes from A and 9 from B are not homologous, so they cannot pair in meiosis and most gametes are unbalanced.
3. Explain the recovery: After doubling to 30, each chromosome has a homolog, meiosis works, and pollen viability (84%) approaches that of the parents.
4. Predict: The 30-chromosome plant crossed with either parent would give offspring with unmatched chromosome sets (21 with A, 24 with B) and low fertility. It is reproductively isolated from both parents: a new species by allopolyploidy.

Scenario 3: justifying allopatric vs. sympatric claims

Data: A small volcanic crater lake, about 2 km across and with no internal barriers, was colonized once by a cichlid fish from a nearby large lake. Today the crater lake holds two cichlid species. DNA shows they are each other's closest relatives. One species feeds on the open-water surface; the other feeds along the bottom, and each mates in its own feeding zone.

Model answer

• Claim: The two species arose by sympatric speciation.
• Evidence: They are sister species descended from a single colonization, and the small lake has no geographic barrier that could have split the population.
• Reasoning: Gene flow was reduced by habitat differentiation: fish adapted to different feeding zones mated within those zones, so divergent selection and assortative mating split one population into two without physical separation.

Contrast: If the two species were found in two separate lakes, each other's closest relatives, with the lakes once joined by a river that dried up, the claim would be allopatric speciation.

AP-Style Check — Data and Claims 🎯

Use Scenarios 2 and 3 above.

Scenario 4: diversity over time

A paleontologist counts genera of marine invertebrates in successive rock layers. (Hypothetical data modeled on the end-Permian extinction.)

Model analysis

1. Identify the mass extinction: 252–250 mya. Losses (380) vastly exceed originations (5); 380 ÷ 470 ≈ 81% of genera lost in about 2 million years.
2. Describe the trend before it: From 260–252 mya, losses (70) already exceed originations (30), so diversity is declining, consistent with rising ecological stress before the crisis.
3. Explain the recovery: From 250 mya on, originations far exceed losses. Survivors radiated into niches emptied by the extinction (EVO-3.K).
4. Quantify the recovery: By 230 mya, the group has 250 genera, still only about half of the 510 present at 260 mya. Recovery from a mass extinction takes millions of years.
5. Connect to the rule: In every interval, change in diversity = new genera − genera lost (EVO-3.J).

Scenario 5: predicting the outcome of secondary contact

Two populations of a songbird were separated for 300,000 years by an ice sheet. After the ice melted, their ranges now overlap in a 50-km band.

AP-Style Check — Extinction Data and Experimental Controls 🎯

AP Connection: earning every point

Up next → Part 7: AP Review, which connects the whole topic in one concept map and a mixed set of questions.

🌿 Speciation and Macroevolution

Part 7 of 7 — AP Review

🔑 Key Concept: Speciation links the two scales of evolution. Microevolution (selection, drift, mutation, gene flow) changes allele frequencies; when gene flow stops, those changes accumulate until reproductive isolation makes two species. Repeated over millions of years, speciation and extinction together produce the patterns of macroevolution: tempo, adaptive radiation and the rise and fall of diversity.

The concept map

Reading the map as a cause-and-effect chain

1. Gene flow keeps a species together. As long as alleles move between populations, local differences are erased.
2. Cut gene flow and evolution acts separately. Selection, drift and mutation now change each group's allele frequencies independently.
3. Isolation is a by-product. Nobody "aims" for a new species. Differences in mating signals, timing or chromosomes build up as side effects of divergence (Dodd's fruit flies).
4. Hybrid fitness decides what happens on contact. Unfit hybrids lead to reinforcement of prezygotic barriers; fit hybrids lead to fusion.
5. Many speciation events make macroevolutionary patterns. Clusters of branch points on a tree mark adaptive radiations; long unbranched stretches with little change mark stasis.
6. Extinction resets the stage. A mass extinction lowers diversity, then the survivors radiate into the empty niches.

Mechanism vs. pattern

A frequent exam trap is mixing up mechanisms (processes that cause change) with patterns (what we observe in the record).

Integrated Check — From Populations to Species 🎯

One case, every idea: the island lizards

Follow one hypothetical lineage through the whole topic.

Worked FRQ-style answer: stage 7

Prompt: Explain how one colonizing species could give rise to five species on a single island.

• Claim: The island lineage underwent an adaptive radiation.
• Evidence: The five species are each other's closest relatives and each uses a different perch type.
• Reasoning: The island had open niches with no competing lizards. Populations using different perches experienced different selection pressures on limb length and body size. Because lizards tend to mate where they live, gene flow between perch types was reduced (habitat isolation), so the populations diverged and became reproductively isolated, producing several species in a short time.

Worked FRQ-style answer: stage 9

Prompt: Predict how the arrival of the snake will affect the island's lizard diversity over the next 100 years and justify your prediction.

• Prediction: Diversity will decrease.
• Justification: The lizards evolved without this predator and lack defenses against it, so extinction rates will rise far above the rate at which new lizard species can form; with extinction exceeding speciation, the number of species falls.

Fill in the Concept Map 🔍

Connections to other units

Self-check: can you do each of these?

Integrated Check — Patterns and Mechanisms 🎯

Final Summary

🔑 AP Connection: The strongest answers on this topic always name the process (what stopped gene flow and what drove divergence) and the evidence (the barrier, the data, or the tree pattern). Practice explaining the chain from allele frequencies to new species in your own words, and you will be ready for any speciation question on the exam.