7.10 Speciation

1. AWithout gene flow, the paired populations became separate species once any difference appeared.
   A student who thinks any genetic difference makes a new species picks this. The graph measures genetic difference, not the ability to interbreed; populations become separate species only when they are reproductively isolated.
2. BWith gene flow, neither population evolved, because the two populations stayed nearly identical.
   A student who equates evolution with populations becoming different picks this. The graph shows only the difference between the two populations; both could change in allele frequency while gene flow kept them similar to each other.
3. CWithout gene flow, each population mutated because it needed to adapt to the conditions in its cage.
   A student who thinks mutations arise when organisms need them picks this. Mutations arise at random with respect to need; without gene flow, new alleles and changes in allele frequency were not shared between the two populations.
4. DWith gene flow, the populations stayed similar, because moving flies let alleles pass between them. Correct
   With a few flies moved each generation, the genetic difference stays below 1% for 100 generations, while without movement it rises to about 7.4%. Moving flies is gene flow: alleles pass between the cages and keep the two populations' allele frequencies similar; without it, differences accumulate, the first step toward reproductive isolation.

CED 7.10.A.1 · Read this in Fix

1. ACross crickets from the two populations and count whether any hybrid eggs or nymphs are produced.
   A student who thinks producing any offspring shows that two groups are one species picks this. Hybrids may hatch yet die young or be sterile, as mules from horses and donkeys usually are, so survival and fertility must be tested.
2. BMeasure the body size, wing shape and color of crickets from each population and compare them.
   A student who defines species by appearance picks this. Members of one species can look very different and separate species can look alike; the concept is based on interbreeding, not appearance.
3. CCross crickets from the two populations and test whether the hybrid offspring survive and are fertile. Correct
   The biological species concept defines a species as a group whose members can interbreed and produce viable, fertile offspring. Testing whether hybrids survive and can reproduce addresses exactly that; producing some offspring is not enough.
4. DSequence a gene from each population and check whether all the crickets have identical DNA.
   A student who thinks members of a species share identical DNA picks this. Individuals of one species differ genetically, so finding differences would not show that the populations are separate species.

CED 7.10.A.2 · Read this in Fix

1. AThe change resulted from a single large mutation that produced the new form in one generation.
   A student who reads 'rapid' as 'in one generation' picks this. Punctuated equilibrium describes change that is rapid compared with geologic time, which still spans many generations.
2. BThe change was rapid on a geologic time scale but still spanned thousands of generations. Correct
   50,000 years at one to two years per generation is roughly 25,000 to 50,000 generations, enough for natural selection to change a population substantially. Against 4 million years of stasis, this is the rapid change after a long period of stasis that defines punctuated equilibrium.
3. CThe change was gradual, since the whole record spans more than 4 million years.
   A student who calls any change spread over millions of years gradualism picks this. The record spans millions of years, but almost all of the change happened within about 50,000 years after a long period with almost no change; that pattern is punctuated equilibrium, not slow, steady change.
4. DThe change happened quickly because the clams were evolving toward a more advanced form.
   A student who sees evolution as progress picks this. Selection favors traits suited to the current environment; there is no goal of a more advanced form.

CED 7.10.B.1 · Read this in Fix

1. ADivergent evolution: each bird reshaped its beak during its life to handle the local seeds.
   A student who thinks individuals evolve during their lives picks this. Beak shape is largely inherited; populations changed because birds with suitable beaks left more offspring.
2. BDivergent evolution: the seeds on each island caused the mutations needed for a suitable beak.
   A student who thinks the environment causes needed mutations picks this. The seeds did not cause mutations; they selected among heritable beak variants that already existed.
3. CDivergent evolution: adaptation to different seeds led to different beak shapes. Correct
   Populations on different islands faced different selective pressures from the available seeds, so birds with beaks suited to the local seeds survived and reproduced more. Over generations the populations diversified in phenotype, which is divergent evolution.
4. DSpeciation: every island population with a different beak shape is now a separate species.
   A student who defines species by appearance picks this. A difference in beak shape does not show reproductive isolation; the populations are separate species only if they could not interbreed to produce fertile offspring.

CED 7.10.B.2 · Read this in Fix

1. ASharks and dolphins inherited the body form from a common ancestor shared only with each other.
   A student who takes similar traits as evidence of close relationship picks this. The cladogram shows that dolphins share a more recent common ancestor with cows, lizards and frogs than with sharks, and the trait is marked twice.
2. BSimilar selective pressures in open water produced similar body forms in two distant lineages. Correct
   The cladogram places dolphins with cows, inside the groups with four limbs and with hair and milk, and sharks outside those groups. The streamlined body is marked as arising separately on the shark and dolphin branches: convergent evolution, in which similar selective pressures produce similar adaptations in different lineages.
3. CEach dolphin developed a streamlined body during its own life because it needed to swim fast.
   A student who thinks traits acquired during life are inherited picks this. A dolphin's body form is inherited; it evolved over many generations by natural selection in dolphin ancestors.
4. DBoth lineages were evolving toward the most advanced body form that an animal can have.
   A student who sees evolution as progress toward perfection picks this. Selection favors traits that work in a particular environment; there is no single most advanced body form.

CED 7.10.B.3 · Read this in Fix

1. AThe groups became two species as soon as the river separated them, since they no longer meet.
   A student who equates separation with speciation picks this. The groups are geographically isolated, but they remain one species until they could no longer produce viable, fertile offspring together, which takes many generations of divergence.
2. BGenetic differences will build up in each group, and the groups may become reproductively isolated. Correct
   With gene flow stopped, mutation, genetic drift and different selective pressures change allele frequencies independently in each group. Over many generations the differences may become large enough that snails from the two groups could not produce viable, fertile offspring together: allopatric speciation.
3. CEach snail will change its body during its life to suit its side, and these changes may form new species.
   A student who thinks individuals evolve during their lives picks this. Changes an individual acquires are not inherited; populations change as allele frequencies change across generations.
4. DThe groups will remain one species forever, because species do not change into new species.
   A student who thinks species are fixed picks this. Isolated populations evolve independently and can become reproductively isolated over time.

CED 7.10.C.1 · Read this in Fix

1. AFlies will mate with flies from either group equally often, as they still form one species.
   A student who confuses the prediction with the null hypothesis picks this. Equal mating between groups is what the null hypothesis of no isolation predicts.
2. BFlies from the two groups will mate as often as before, but their hybrid offspring will be sterile.
   A student who confuses prezygotic with postzygotic mechanisms picks this. Hybrid sterility acts after fertilization and is a postzygotic barrier; the question asks about isolation that acts before.
3. CFlies will not mate at all with flies from the other group, since they now form separate species.
   A student who thinks reproductive isolation is all or nothing picks this. Isolation that has begun to develop is expected to reduce, not eliminate, mating between the groups.
4. DFlies will mate more often with flies from their own group than with flies from the other group. Correct
   A prezygotic mechanism acts before fertilization, for example through mate choice. If the groups have begun to diverge in mating preferences, flies will mate more often within their own group; isolation that has only begun to develop is expected to be partial.

CED 7.10.C.2 · Read this in Fix

1. AA and B are one species and C is another, since all hybrids with C were sterile. Correct
   A × B crosses give eggs that hatch (91%) and adults that are fertile (93%), much as A × A crosses do, so A and B can exchange genetic information. A × C and B × C eggs hatch, but none of the resulting adults are fertile, so C is reproductively isolated from both A and B and is a separate species.
2. BA, B and C all belong to one species, since every cross produced offspring that hatched.
   A student who thinks producing living offspring is enough picks this. The biological species concept requires fertile offspring; the hybrids from crosses with C are all sterile, so genes cannot pass between C and the other populations.
3. CA and C belong to one species, since frogs from A and C have the same color pattern.
   A student who defines species by appearance picks this. A × C hybrids are all sterile, so A and C cannot exchange genes despite looking alike.
4. DA, B and C are three different species, since each population lives in a separate forest.
   A student who equates living apart with being separate species picks this. A and B live in separate forests yet produce viable, fertile offspring together, so they meet the definition of one species.

CED 7.10.A.2 · Read this in Fix

1. APunctuated equilibrium: each jump in shell length was produced by one large mutation in one generation.
   A student who reads 'rapid' as 'in one generation' picks this. The fossil record shows each rise within an interval of up to half a million years, which spans many generations; the graph does not show that one mutation produced it.
2. BPunctuated equilibrium: no mutations occurred in the flat periods because the environment was stable.
   A student who thinks mutations occur only when the environment changes picks this. Mutations arise at random all the time; during stasis, selection and other processes can keep the mean shell length steady.
3. CGradualism: shell length increased slowly and steadily across the whole 10 million years.
   A student who calls any change spread over millions of years gradualism picks this. The graph shows long flat stretches and short steep rises, not the steady slope that gradualism would produce.
4. DPunctuated equilibrium: long periods of stasis were interrupted by short periods of rapid change. Correct
   Mean shell length stays near 20 mm from 10 to 7 million years ago and near 28 mm from 6.5 to 3 million years ago, then near 35 mm. Each rise occurs between two samples half a million years apart, short compared with the multimillion-year flat periods: rapid change after long stasis.

CED 7.10.B.1 · Read this in Fix

1. AAfter the island formed, a colonizing population adapted to many new habitats and rapidly split into species. Correct
   All four island speciation events occur between about 4.6 and 3.5 million years ago, soon after the island formed, and each species uses a different part of the habitat. Rapid speciation as populations adapt to newly available habitats is adaptive radiation, a case of divergent evolution.
2. BThe island lizards split rapidly because they were more advanced than their mainland relative.
   A student who sees evolution as progress picks this. The island lineages are not more advanced; speciation was rapid because new habitats imposed different selective pressures.
3. CEach island species formed at the moment its ancestors first moved into a different habitat.
   A student who thinks populations become species as soon as they are separated picks this. Using a new habitat reduces gene flow, but reproductive isolation develops over many generations.
4. DEach lizard adjusted its body to its habitat during its life, and these changes formed new species.
   A student who thinks traits acquired during life are inherited picks this. Heritable variation was selected differently in each habitat over generations.

CED 7.10.B.2 · Read this in Fix

1. ADarker beetles will become more common in both species, making them look more alike. Correct
   On black rock, birds find pale beetles more easily, so darker beetles survive and reproduce more in both species. The same selective pressure acting on two unrelated species produces similar adaptations: convergent evolution.
2. BThe two species will merge into a single species, because they will come to look alike.
   A student who defines species by appearance picks this. Looking alike does not allow two species to interbreed and produce fertile offspring, and these species live on different islands.
3. CEach beetle will gradually darken during its own life to match the black rock it lives on.
   A student who thinks individuals change their heritable traits to suit the environment picks this. The color of each beetle is inherited; the population changes as darker beetles leave more offspring.
4. DNeither species will darken, because similar traits evolve only in closely related species.
   A student who thinks similar traits arise only from close common ancestry picks this. Similar selective pressures can produce similar traits in distantly related species.

CED 7.10.B.3 · Read this in Fix

1. AIt cannot lead to speciation, because no physical barrier separates the two groups of flies.
   A student who thinks speciation needs a geographic barrier picks this. Sympatric speciation occurs in populations with geographic overlap; differences in host fruit and timing can reduce gene flow without a barrier.
2. BIt is probably allopatric speciation, because apple flies are isolated from hawthorn flies.
   A student who treats any reproductive isolation as allopatric picks this. Allopatric speciation requires geographic isolation; these flies share the same areas.
3. CIt may be an early stage of sympatric speciation, as isolation is arising between flies in one area. Correct
   The two host groups live in the same geographic area, but because flies mate on the fruit they grew up in and the fruits ripen at different times, flies on different hosts mate with each other less often. Gene flow is reduced without geographic isolation, as in sympatric speciation.
4. DIt will not lead to speciation, because apple and hawthorn flies can still mate when they meet.
   A student who thinks isolation must be complete before speciation can begin picks this. Speciation can proceed as gene flow is gradually reduced, and mating between the host groups is already less frequent.

CED 7.10.C.1 · Read this in Fix

1. AReject the null hypothesis, as 4.16 is more than the critical value for 1 degree of freedom.
   A student who takes the degrees of freedom as the number of groups of flies minus one (2 − 1 = 1) picks this and compares 4.16 with 3.84. The counts fall into four categories of mating, so there are 3 degrees of freedom and the critical value is 7.81.
2. BReject the null hypothesis, as a χ² of 4.16, below 7.81, shows that the difference is significant.
   A student who treats a χ² smaller than the critical value as significant picks this. A small χ² means the observed counts are close to the expected ones; the null hypothesis is rejected only when χ² exceeds 7.81.
3. CFail to reject the null hypothesis, as 4.16 is less than the critical value for 3 degrees of freedom. Correct
   Each kind of mating is expected 100/4 = 25 times. χ² = 1.44 + 0.64 + 1.44 + 0.64 = 4.16. With 4 − 1 = 3 degrees of freedom, the critical value at p = 0.05 is 7.81; 4.16 is smaller, so the departure from equal numbers could be due to chance, and these data do not show prezygotic isolation.
4. DFail to reject the null hypothesis; χ² = 4.16 proves that the flies mate at random.
   A student who thinks failing to reject proves the null hypothesis picks this. The data are consistent with random mating but do not prove it; a larger sample might show a preference.

Working Expected count for each kind of mating = 100/4 = 25. χ² = (31 − 25)²/25 + (21 − 25)²/25 + (19 − 25)²/25 + (29 − 25)²/25 = 1.44 + 0.64 + 1.44 + 0.64 = 4.16. Degrees of freedom = 4 categories − 1 = 3; critical value at p = 0.05 = 7.81. Because 4.16 < 7.81, the null hypothesis of random mating is not rejected; the data do not show a mating preference, but they do not prove that mating is random.

CED 7.10.C.2 · Read this in Fix

1. AGene flow will occur freely, because the two species can mate and produce hybrids that survive to adulthood.
   A student who thinks producing living offspring means genes are exchanged picks this. Alleles enter the other species only if hybrids reproduce; sterile hybrids are a dead end for gene flow.
2. BLittle or no gene flow will occur, because the hybrids cannot pass alleles on to either species. Correct
   Hybrid sterility is a postzygotic mechanism: fertilization occurs and hybrids develop, but because they cannot reproduce, alleles do not pass through them into either parent species. Reproductive isolation is maintained even though the species meet and mate.
3. CGene flow will continue, because only a geographic barrier can block the exchange of alleles between species.
   A student who equates reproductive isolation with geographic separation picks this. Prezygotic and postzygotic mechanisms can block gene flow between species that live in the same place.
4. DThe sterile hybrids will form a third species that carries alleles from both of the species.
   A student who thinks any new kind of hybrid is a species picks this. Sterile hybrids cannot reproduce, so they cannot form a population that persists as a species.

CED 7.10.C.2 · Read this in Fix