Campbell Biology Chapter 24 Review: The Origin of Species

24.1 What makes a species, and what keeps species apart

pp. 488–492

On the AP exam? Yes

Topic 7.10 covers the biological species concept and prezygotic and postzygotic barriers. The other species concepts (morphological, ecological, phylogenetic) are helpful background, but the current course doesn't name them.

In the course: Topic 7.10 Speciation, Topic 7.4 Population Genetics (notes, videos and more questions)

Key points

Speciation is one species splitting into two or more. It's the link between microevolution (allele frequencies shifting inside a population) and macroevolution (big patterns above the species level), and it explains why related species are both different and alike.
Under the biological species concept, a species is all the populations whose members could mate in nature and have offspring that survive and can breed themselves. Members of two different species can't. Looks don't decide it: two species can look almost identical, and one species can look very varied.
Gene flow between a species' populations keeps their gene pools similar. When gene flow stops, populations are free to drift apart genetically, and that's where new species start.
Reproductive isolation means biological barriers that keep two groups from producing fertile offspring together. Prezygotic barriers work before a zygote exists: living in different habitats (habitat), breeding at different times (temporal), using different courtship signals (behavioral), having parts that don't fit (mechanical), or having sperm and egg that can't fuse (gametic).
Postzygotic barriers work after a hybrid zygote forms: hybrids may die young or be frail (reduced hybrid viability), grow up healthy but be sterile (reduced hybrid fertility), or be fine themselves while their own offspring are weak or sterile (hybrid breakdown).
Barriers add up: any one of them may let a little gene flow through, but stacked together they can stop it almost entirely.
The biological definition can't be used on fossils or on organisms that only reproduce asexually, like most bacteria, and some clearly distinct species still swap a few genes. So biologists also define species by body form (morphological), by way of life or niche (ecological), or as the smallest group sharing one common ancestor on the tree of life (phylogenetic).

Key terms (15)

speciation: The process by which an ancestral species gives rise to new, separate species.
biological species concept: The idea that a species is a group whose members can interbreed in nature and have healthy, fertile offspring, but can't do so with other groups.
reproductive isolation: The state in which biological differences block two groups from interbreeding successfully, so their gene pools stay separate.
hybrid: An offspring whose parents belong to two different species.
prezygotic barrier: A barrier that stops a zygote from ever forming, either by preventing mating or by keeping sperm from fertilizing the egg.
habitat isolation: Two species live in the same region but use different places within it, so they seldom meet.
temporal isolation: Two species are ready to mate at different hours, months or years, so they never mate with each other.
behavioral isolation: Differences in courtship songs, dances, scents or other signals mean individuals don't recognize the other species as a mate.
mechanical isolation: Differences in the shape of reproductive structures (or flower parts) physically stop mating or pollen transfer from working.
gametic isolation: Sperm or pollen reaches the other species' egg but can't fuse with it, often because surface proteins don't match.
postzygotic barrier: A barrier that acts after a hybrid zygote has formed, by making hybrids die, fail to develop or be unable to reproduce.
reduced hybrid viability: Hybrids fail to develop properly, die young or are too weak to survive well.
reduced hybrid fertility: Hybrids are healthy but sterile, often because the two parents' chromosomes can't pair up in meiosis.
hybrid breakdown: The first hybrid generation is healthy and fertile, but the generation after it is weak or sterile.
morphological species concept: Grouping organisms into species by their shape and structure. It's handy for fossils and organisms that don't reproduce sexually, but where to draw the line is a judgment call.

Check yourself: 24.1 What makes a species, and what keeps species apart

4 questions on 24.1 What makes a species, and what keeps species apart. Pick an answer to see if you got it, and why.

Question 1 of 4

Two closely related wildflower species grow mixed together in the same meadows. One opens its flowers and releases pollen only in early April, and the other only in late June. The same insects visit both. Which barrier most directly keeps the species from interbreeding?

Question 2 of 4

When two species of tree frog are crossed in the lab, the eggs hatch normally and the hybrid tadpoles grow into healthy, vigorous adults. However, the hybrid adults never produce working eggs or sperm. Which barrier does this describe?

Question 3 of 4

Two populations of cicadas look identical, but their males' calls differ. Where both live together, genetic tests find almost no individuals of mixed ancestry. A student says they must be one species because they look the same. Which is the best response?

Question 4 of 4

A microbiologist isolates two strains of a soil bacterium that reproduce only by binary fission. Why can't the biological species concept be used to decide whether the strains are separate species?

0 of 4 answered

24.2 Splitting apart: allopatric and sympatric speciation

pp. 493–498

On the AP exam? Yes

Topic 7.10 names allopatric and sympatric speciation, and polyploidy connects to nondisjunction in Topic 5.2. You don't need the terms autopolyploid and allopolyploid, but knowing how polyploids form makes chromosome-count questions easy.

In the course: Topic 7.10 Speciation, Topic 7.4 Population Genetics, Topic 7.2 Natural Selection, Topic 5.2 Meiosis and Genetic Diversity (notes, videos and more questions)

Key points

A new species can only form once gene flow between two groups is cut down. The two main routes differ in how that happens.
In allopatric speciation, geography splits a population: a glacier advances, rising seas cut off a peninsula, a mountain range lifts, or a few individuals reach an island. Whether a barrier works depends on how far the organism can travel. A strip of salt water can stop a land snail but not a gull.
Once apart, each population collects its own mutations, and selection and drift change its allele frequencies in its own way. Reproductive barriers can then appear as a side effect of all that change. Selection isn't aiming at isolation.
Distance alone isn't a reproductive barrier. If separated populations meet again, only the biological barriers they've evolved will keep them apart. Evidence for allopatric speciation includes sister species on opposite sides of a barrier, more species in regions broken up by barriers, and isolation growing in lab populations kept apart under different conditions.
Sympatric speciation happens with no geographic split: the new species arises right alongside its parent species. It's less common, because contact allows gene flow, so it needs something that cuts gene flow quickly: polyploidy, a switch to a new habitat or food, or mate choice (sexual selection).
Polyploidy means having extra full sets of chromosomes, usually after an error in cell division. If one species doubles its own set (say 2n to 4n), crosses with the parent give triploid offspring that are mostly sterile, so the new form is isolated in one generation. If a sterile hybrid of two species doubles its chromosome number, every chromosome gets a partner, meiosis works, and the hybrid becomes a fertile new species.
Polyploidy is much more common in plants than animals, and many crops, such as bananas, strawberries, sugarcane and peanuts, are polyploid.

Key terms (13)

allopatric speciation: Speciation that starts when a population is split by a geographic barrier, so the separated groups can't exchange genes.
sympatric speciation: Speciation that happens while the new and parent species still live in the same area.
geographic barrier: A physical feature, like a river, mountain range or stretch of ocean, that keeps populations apart. How well it works depends on how far the organism can travel.
sister species: Two species that are each other's closest living relatives.
polyploidy: Having more than two complete sets of chromosomes. It's common in plants and can create a new species in a single generation.
autopolyploid: A polyploid whose extra chromosome sets all come from one species, for example after a diploid's chromosome number doubles.
allopolyploid: A polyploid that formed from a hybrid of two species, with chromosome sets from both parents. It can breed with its own kind but not with either parent.
tetraploid: Having four sets of chromosomes (4n).
triploid: Having three sets of chromosomes (3n). Triploids are usually sterile because three sets can't split evenly in meiosis.
nondisjunction: When chromosomes fail to separate properly during cell division. If every chromosome fails, a cell can end up with a doubled set.
unreduced gamete: A gamete that still has the full diploid chromosome number because meiosis didn't halve it.
habitat differentiation: Part of a population starts using a new habitat or food source, which cuts down mating with the rest of the population.
sexual selection: Selection driven by mate choice or competition for mates. Strong preferences for certain traits can split a population into groups that rarely interbreed.

Check yourself: 24.2 Splitting apart: allopatric and sympatric speciation

4 questions on 24.2 Splitting apart: allopatric and sympatric speciation. Pick an answer to see if you got it, and why.

Question 1 of 4

Which scenario is the best example of sympatric speciation?

Question 2 of 4

Rising seas turn a coastal hill into an island, cut off from the mainland by 3 km of salt water. Which species living on the hill is most likely to form a separate island species over time?

Question 3 of 4

A fertile new plant species has 2n = 38, made of complete chromosome sets from two parent species. One parent species has 2n = 24. What is the other parent's diploid number, and how many chromosomes did the sterile hybrid that came first have?

Question 4 of 4

In one forest, male wolf spiders court by drumming on dry leaves, some with a fast rhythm and some with a slow one, a difference controlled by genes. Then a heritable female preference spreads: some females accept only fast drummers, and the rest accept only slow drummers. How could this lead to speciation without a geographic barrier?

0 of 4 answered

24.3 Hybrid zones: where young species meet

pp. 498–501

On the AP exam? Background

The current course doesn't name hybrid zones, reinforcement or fusion, but these cases are good practice for Topic 7.10's ideas about gene flow and prezygotic and postzygotic barriers.

In the course: Topic 7.10 Speciation, Topic 7.4 Population Genetics, Topic 7.2 Natural Selection (notes, videos and more questions)

Key points

A hybrid zone is a place where two species whose barriers aren't complete meet, mate and produce some hybrids of mixed descent.
Some hybrid zones are narrow bands along an environmental boundary, such as the edge between two soil types or climate zones. Others are scattered patches wherever the right conditions happen to occur.
Across a narrow zone, the share of one species' alleles drops sharply over a short distance. If alleles could pass freely, they'd spread far beyond the zone, so a steep change means something is blocking gene flow, often weak or infertile hybrids.
Over time a hybrid zone can go three ways: the barriers get stronger (reinforcement), they break down (fusion), or hybrids keep forming at a steady rate (stability).
Reinforcement: when hybrids do poorly, individuals that mate only with their own species leave more surviving, fertile offspring, so selection strengthens prezygotic barriers. That predicts stronger barriers where the species overlap than where they live apart.
Fusion: if barriers are weak and hybrids do well, gene flow can merge the two gene pools back into one. People can set this off, for example by bringing in a related species from elsewhere, and a rare species can disappear into a common one this way.
Stability: hybrids keep being made. Sometimes they do better than either parent in the zone's in-between habitat. Other times, steady arrivals of parent-species individuals keep replacing the losses, even though hybrids are less fit.

Key terms (8)

hybrid zone: An area where two species meet and interbreed, producing at least some hybrids.
reinforcement: Selection that strengthens prezygotic barriers because hybrids are less fit than offspring of matings within a species.
fusion: Two species merging back into one when weak barriers let so much gene flow happen that their gene pools blend.
stable hybrid zone: A hybrid zone that stays about the same over many generations, with hybrids continuing to form.
interspecific mating: Mating between members of two different species.
intraspecific mating: Mating between members of the same species.
gene pool: All the alleles present in a population or species at a given time.
hybrid fitness: How well hybrids survive and reproduce compared with purebred parents. It decides which way a hybrid zone goes.

Check yourself: 24.3 Hybrid zones: where young species meet

4 questions on 24.3 Hybrid zones: where young species meet. Pick an answer to see if you got it, and why.

Question 1 of 4

Two chickadee species meet in a band about 30 km wide. Inside the band, many birds have mixed ancestry, but outside it nearly every bird clearly belongs to just one of the two species. What does this pattern show most directly?

Question 2 of 4

Question 3 of 4

Two oak species meet on a mountainside: one grows on dry ridgetops and the other in moist valleys. Hybrid oaks do poorly in either parent's habitat but grow better than both parents on the mid-slopes between them. What is the most likely outcome over many generations?

Question 4 of 4

Hybrids between two katydid species have low fitness wherever the species meet. In one region they meet in a hybrid zone only 3 km wide; in another, the zone is 80 km wide. Katydids move about 1 km per generation. Where is selection most likely to succeed in making females choosier about mates?

0 of 4 answered

24.4 The pace and genetics of speciation

pp. 501–504

On the AP exam? Yes

Topic 7.10 asks you to compare punctuated equilibrium and gradualism. How many genes control reproductive isolation is background; the exam won't ask about it directly.

In the course: Topic 7.10 Speciation, Topic 7.6 Evidence of Evolution (notes, videos and more questions)

Key points

The fossil record shows two tempos. In punctuated equilibrium, a species shows up fairly suddenly, then barely changes (stasis) for a long time. In gradualism, change builds slowly and steadily over hundreds of thousands to millions of years.
Sudden in the rocks doesn't mean overnight. Sediment piles up slowly, so change that took tens of thousands of years can fit between two layers and look instant in a species that lasted millions of years.
Once populations start to diverge, speciation can finish quickly. Polyploids can be isolated in one generation, and some hybrid and lab populations have started to become reproductively isolated within just dozens of generations.
The time from one speciation event to the next in a lineage is wildly irregular, anywhere between a few thousand and more than ten million years. That's because a split depends on chance events that cut populations off, and the groups then have to become isolated before they reconnect.
Two species can sometimes give rise to a third without any change in chromosome number. Selection in a hybrid population keeps the gene combinations that work well together, and the hybrids end up isolated from both parents.
Sometimes a change in one gene, for example one that alters a mating signal, body shape or flower trait, is enough to start isolation. Other times many genes and their interactions are involved.
Speciation repeated many times, along with extinction, adds up to the big changes in life's history that we call macroevolution.

Key terms (9)

punctuated equilibrium: A pattern in which species change fairly quickly when they form and then stay nearly the same for long periods.
gradualism: A pattern in which species change slowly and steadily over very long periods of time.
stasis: A long stretch of time in which a species shows little or no change.
stratum: A single layer of sedimentary rock. Lower strata are usually older, so fossils in them are older too.
hybrid speciation: A new species forming from hybrids of two existing species, with or without a change in chromosome number.
divergent evolution: Related populations becoming more and more different as they adapt to different conditions.
microevolution: Changes in allele frequencies within a population from one generation to the next.
macroevolution: Large-scale evolutionary change above the species level, such as the rise of whole new groups of organisms.
locus: The specific spot on a chromosome where a particular gene is found.

Check yourself: 24.4 The pace and genetics of speciation

4 questions on 24.4 The pace and genetics of speciation. Pick an answer to see if you got it, and why.

Question 1 of 4

Two related clam lineages are traced through the same 2 million years of rock. Lineage 1's shell shape is the same in every layer until a branch with a new shape appears, and that new shape then stays the same too. Lineage 2's shell shape shifts a little from each layer to the next. Which description fits?

Question 2 of 4

A paleontologist measured the average shell diameter of a fossil plankton lineage in a series of rock layers (invented data). Age of layer (millions of years ago) | Mean shell diameter (mm) 6.0 | 0.50 5.0 | 0.56 4.0 | 0.61 3.0 | 0.67 2.0 | 0.72 Which description best fits this record?

Question 3 of 4

In a quarry, fossil samples come from rock layers about 30,000 years apart. A snail's shell shape stays the same through 2 million years of layers, then a new shape appears in the next layer and stays unchanged after that. A student concludes that the new shape arose in a single generation. The snails breed once a year. What is the best response?

Question 4 of 4

Two isolated populations of a beetle both start out as aabb. In population 1 a new allele A spreads until all are AAbb; in population 2 a new allele B spreads until all are aaBB. Both populations stay healthy and fertile. When the populations are crossed, the AaBb hybrids are sterile. Which explanation fits best?

0 of 4 answered