Questions on speciation and reproductive isolation

Question 1

A few individuals of a beetle species are carried by a storm to an island 400 km from the mainland and establish a population there. Describe how this could lead to the formation of a new species.

Accepted Answer

B1 the island population is geographically isolated from the mainland one, so gene flow between them stops and alleles no longer pass from one gene pool to the other B1 different random mutations occur in the two populations, and because they are no longer interbreeding those mutations are not shared B1 conditions on the island are not identical to those on the mainland, so different selection pressures act and different alleles are advantageous in each population B1 the island population is small and was founded by a sample of the mainland alleles, so genetic drift changes its allele frequencies by chance as well B1 over many generations the two gene pools diverge until the differences include features affecting reproduction, so the two forms could no longer interbreed to produce fertile offspring even if they met: they are reproductively isolated and are two species

Question 2

An allopolyploid forms when two related plant species hybridise and the chromosome number of the hybrid later doubles. The hybrid carries one set of chromosomes from each parent species, and the chromosomes of the two species are not homologous. Explain why the hybrid is sterile, why the plant becomes fertile once its chromosome number has doubled, and why it is then a new species.

Accepted Answer

B1 in the hybrid no chromosome has a homologous partner to pair with at meiosis, so the chromosomes are shared out unevenly and the gametes do not carry complete sets, which makes the hybrid sterile B1 after doubling, every chromosome has an identical partner to pair with, so meiosis produces balanced gametes and the plant is fertile with others like itself B1 crossed with either parent species, it gives offspring whose chromosomes cannot pair evenly at meiosis, so those offspring are sterile B1 it cannot produce fertile offspring with either parent species, so it is reproductively isolated from both from the moment it forms, and is a new species by the biological species concept. Spartina anglica arose this way on British salt marsh around 1890

Question 3

Isolating mechanisms are prezygotic if they stop a zygote forming, and postzygotic if a zygote forms but the hybrid dies or is sterile. Two species of frog breed in the same ponds. One spawns in early spring and the other about a month later. In the rare matings between them the eggs are fertilised, but the embryos die before hatching. Compare the two isolating mechanisms in this example, referring to the stage at which each acts and to what each costs the parents, and explain why both keep the two gene pools separate.

Accepted Answer

B1 the difference in breeding time is prezygotic, since the two species are rarely ready to mate at the same time, whereas the death of the embryos is postzygotic, since fertilisation happens but the hybrid does not survive B1 the prezygotic mechanism acts before any gametes are used, whereas the postzygotic one wastes eggs, sperm and the development of an embryo B1 the difference in timing prevents most matings, and the death of the embryos stops the few that do occur from producing offspring, so together they are more complete than either alone B1 neither produces a fertile hybrid, so no offspring carries alleles from one species' gene pool into the other's: there is no gene flow

Question 4

Two forms of a fish live at different depths in one lake. They breed at the same time of year, but each spawns only over the substrate at its own depth, and the two differ in colour. Kept together in a tank, fish of the two forms produce fertile offspring. Suggest whether these are two species, and suggest what is keeping the gene pools apart in the lake.

Accepted Answer

B1 either conclusion, justified: not yet two species if the test is whether they can produce fertile offspring, since the tank crosses show no postzygotic barrier; or effectively two species if the test is reproductive isolation in nature, since they do not interbreed in the lake B1 gene flow in the lake is nevertheless much reduced, because the two forms spawn in different places, which is ecological isolation acting before any gametes meet B1 the difference in colour may act as a behavioural barrier as well, if each form recognises and prefers a mate of its own colour, so matings between the forms are rare even where the fish do meet B1 this is sympatric divergence in progress: there is no geographical barrier, and if the separation persists the two gene pools may diverge far enough for reproductive isolation to become complete

Question 5

Describe two situations in which the biological species concept cannot be applied, and describe how biologists classify organisms in each case instead.

Accepted Answer

B1 asexual organisms such as bacteria and many fungi never interbreed at all, so the interbreeding test cannot be applied to them B1 such organisms are instead grouped by the similarity of their DNA or rRNA sequence B1 fossils cannot be tested for interbreeding because the organisms are dead, so the concept cannot be applied to them directly B1 palaeontologists classify fossil species from their morphology and from stratigraphy, the order and position of the rock layers they are found in, treating the resulting species as working hypotheses

Question 6

Polar bears and brown bears can produce fertile offspring and have exchanged genes repeatedly where their ranges overlap, yet biologists usually treat them as two species. Suggest why this fertile-hybrid case is a difficulty for the biological species concept, and suggest what other evidence, besides interbreeding, could be used to decide whether they are one species or two.

Accepted Answer

B1 the biological species concept requires interbreeding to produce fertile offspring and reproductive isolation from other groups, but polar and brown bears meet the first condition while apparently failing the second, so the definition gives contradictory guidance in this case B1 this shows the concept defines a discrete boundary although real populations can show partial or graded reproductive isolation rather than being either completely interbreeding or completely isolated B1 additional evidence could include the frequency of hybridisation actually observed in the wild, since rare hybridisation alongside mostly within-form mating differs from freely interbreeding populations B1 additional evidence could also include differences in ecology, morphology or DNA sequence that have persisted despite some gene flow, indicating that each population maintains a largely separate gene pool

Question 7

A diploid plant occasionally produces an unreduced, diploid gamete. Fusion of two such gametes produces a tetraploid plant, with four copies of every chromosome. Explain why this tetraploid is fertile, and why it is a new species, separate from the diploid population it came from.

Accepted Answer

B1 at meiosis every chromosome in the tetraploid has homologous partners to pair with, so it produces balanced diploid gametes and is fertile with itself and with other tetraploids B1 a cross with a diploid plant joins a diploid gamete to a haploid one, giving a triploid B1 the triploid has three copies of each chromosome, which cannot pair evenly at meiosis, so its gametes are unbalanced and it is largely sterile B1 the tetraploid cannot produce fertile offspring with the diploids, so it is reproductively isolated from them from the moment it forms, which makes it a new species in a single generation

Question 8

The apple maggot fly, Rhagoletis pomonella, fed only on native hawthorn in North America until apple trees were introduced, and some flies began to use apples instead. Describe how this host shift could lead towards sympatric speciation, without a geographical barrier.

Accepted Answer

B1 flies mate on the fruit they emerged from as larvae, so hawthorn flies tend to mate with other hawthorn flies and apple flies with other apple flies, even where both host trees grow in the same orchard B1 apples ripen earlier than hawthorn fruit, so the two host races are further separated because they tend to breed at slightly different times of year, adding temporal isolation to the host preference B1 gene flow between the two host races is greatly reduced by these two factors even though no physical barrier separates them, so mutation, selection and drift can begin to make the allele frequencies of the two races diverge independently B1 over enough generations the two races could accumulate enough genetic difference, including differences affecting host and mate choice, to become reproductively isolated and count as two species, though after roughly 150 years the two races are only measurably diverging and still interbreed at a low rate rather than being fully separate species yet

Question 9

Grizzly bears and polar bears can produce fertile cubs when they interbreed, yet the two have distinct ecologies, appearances and genomes and have been separate lineages for hundreds of thousands of years. Discuss whether grizzly bears and polar bears should be classified as one species or as two.

Accepted Answer

B1 for one species: the biological species concept requires reproductive isolation, and fertile cubs show that gene flow between the two is still physically possible, which is the concept's central test B1 for two species: the two forms have been diverging independently for a very long time and differ substantially in ecology, morphology and genome, which an operational species concept based on overall similarity would treat as two species B1 the disagreement arises because the boundary between two recently diverged lineages is often genuinely gradual rather than sharp, since isolating mechanisms accumulate gradually rather than appearing all at once B1 a judgement: whichever the two are called depends on which operational definition is applied and on how much weight is given to interbreeding versus overall divergence, rather than on an unmeasured biological fact waiting to be discovered

Question 10

Explain why sympatric speciation is harder to achieve than allopatric speciation.

Accepted Answer

B1 in allopatric speciation a physical barrier stops gene flow completely from the start, so the two gene pools change independently of each other B1 in sympatric speciation the two groups remain in the same place, so individuals can still meet and interbreed and gene flow can continue B1 gene flow mixes the alleles of the two groups and so erases the divergence as it accumulates, which means an ecological, behavioural or temporal separation, or else polyploidy, has to reduce interbreeding from the outset for divergence to build up at all

Question 11

Two recently separated bird populations meet again after their geographical barrier disappears. Where the two forms interbreed, hybrid offspring survive less well than offspring of matings within either form. Explain how this could lead to complete reproductive isolation through the process called reinforcement.

Accepted Answer

B1 a bird that mates with the other form produces hybrid offspring that survive or reproduce less well, so it leaves fewer surviving offspring than one that mates within its own form B1 any heritable feature that helps an individual distinguish the two forms, such as a difference in song or courtship display, is therefore favoured by selection, because it reduces the chance of a costly hybrid mating B1 as those distinguishing features become more pronounced over generations, matings between the two forms become rarer still, until the two are completely reproductively isolated

Question 12

In some crosses between related plant species, the first hybrid generation is healthy, but crossing those hybrids with each other gives a second generation containing many weak or infertile individuals. Explain why this pattern, called hybrid breakdown, is evidence of reproductive isolation between the parent species even though the first-generation hybrids themselves are fine.

Accepted Answer

B1 the two parent species have accumulated different gene combinations while evolving apart, and each parent's own combination works well together within itself B1 the first-generation hybrid inherits one complete set of alleles from each parent, so its two sets still work together reasonably well, which is why first-generation hybrids are often healthy B1 recombination during meiosis in the first-generation hybrid breaks up those parental combinations, so the second generation carries new mixtures of alleles from both species that have never been tested together by selection, and many of these combinations do not work well, producing weak or infertile offspring; gene flow between the species therefore cannot proceed past the first generation in practice

Question 13

The cichlid fish species flock of Lake Victoria evolved into hundreds of species within about 15 000 years, an allopolyploid plant becomes a new species in a single generation, and many pairs of related animal species took somewhere between 10⁴ and 10⁶ generations to separate. Suggest why the timescale of speciation varies so much between these examples.

Accepted Answer

B1 an allopolyploid is reproductively isolated the instant it forms, because its chromosome number alone prevents fertile crosses with the parent species, so no time is needed for divergence to accumulate B1 most animal speciation instead depends on many mutations, selection differences and drift slowly changing courtship, gamete recognition or other traits in two separated populations, and accumulating enough difference for full reproductive isolation takes many generations B1 the cichlid flock is fast for an animal group because a large lake offers many different ecological opportunities such as depth and diet for strong divergent selection to act on quickly, and because sexual selection on colour can proceed rapidly once mate choice starts to differ

Question 14

State the biological species concept in full, and state what reproductive isolation means.

Accepted Answer

B1 a group of organisms with similar characteristics that can interbreed to produce fertile offspring, and which are reproductively isolated from other such groups B1 reproductive isolation is the state in which two populations cannot exchange alleles because of features of the organisms themselves, such as courtship, breeding time or place, gamete compatibility or hybrid fertility, so they stay separate even where they meet

Question 15

A prezygotic isolating mechanism stops a zygote forming between two species. In two related species of flowering plant, the flowers differ in shape, so an insect visiting one species carries pollen on a part of its body that does not touch the stigma of the other. Two species of sea urchin release sperm and eggs into the same water at the same time, but the sperm of one species cannot bind to the surface of the other species' eggs. Explain why each of these is a prezygotic mechanism, and suggest why the second kind matters most in animals that release their gametes into water.

Accepted Answer

B1 in the plants, pollen is not delivered to the stigma of the other species, so fertilisation cannot occur; in the sea urchins the gametes meet but cannot fuse; in both, no zygote forms B1 animals that spawn into water cannot choose a mate, so gametes of different species meet freely, and recognition between sperm and egg surfaces is the only point at which fertilisation by the wrong species can be stopped