IB Biology A4.1: Evolution and speciation worked solutions

B. A rise in the proportion of bacteria carrying a resistance allele is a change in heritable characteristics of a population over generations. A, C and D are changes in single organisms during a lifetime and are not written into the gametes.

C. I together with II: sequence similarity and a shared structural plan are both evidence of common ancestry. III is the Lamarckian idea, which is not supported, because acquired features are not written into the gametes.

B. The human arm and the whale flipper share the humerus, radius, ulna and digits inherited from a common ancestor. A, C and D pair structures with similar functions or appearance but separate origins.

B. A similar function that arose independently makes the eyes analogous structures. Homologous structures are inherited from a common ancestor, vestigial structures are reduced and functionless, and divergent evolution describes homologous structures adapting differently.

D. The proportions of positions that differ are 5%, 4% and 3%, so R is most similar even though it has the most differences. Always normalise for the number of positions compared.

B. A barrier first stops gene flow, different pressures then act on each side, allele frequencies diverge, and the populations end up unable to interbreed. A starts with divergence before there is a barrier, C and D reverse the order.

B. Sympatric speciation occurs in the same area, with isolation arising from, for example, breeding time or behaviour. A describes allopatric speciation, C is gradual change within a species, and D describes extinction.

A. A difference in courtship signals stops mating, so it is behavioural isolation before the zygote forms. Temporal isolation concerns timing, hybrid sterility acts after the zygote forms, and geographical isolation is a physical barrier.

C. Several species that descend from one recent ancestor, each using a different resource, is the pattern of adaptive radiation. A and D describe population growth or an extinction, and B describes one species with no splitting.

The limbs have corresponding bones in the same arrangement, humerus, radius, ulna, wrist bones and digits, despite different functions (1). The shared plan is explained by inheritance from a common ancestor (1). Natural selection modified it in each lineage for different uses, which is divergent evolution (1). Similar function alone would not be evidence of shared descent.

Any four of: homologous structures share an origin in a common ancestor, analogous structures have separate origins (1); homologous structures may have different functions, analogous structures have similar functions (1); homologous structures arise by divergent evolution, analogous structures by convergent evolution (1); an example of homologous structures, such as the pentadactyl limbs of a human and a bat (1); an example of analogous structures, such as the wings of an insect and a bird (1). The comparison must pair each feature, not give two separate definitions.

Sharks and dolphins evolved separately from different ancestors (1). Both faced a similar selection pressure, the need to move fast through water (1). In each lineage, individuals with a more streamlined shape survived and reproduced more, and passed on the alleles (1). Over generations the frequency of those alleles rose in both, which is convergent evolution producing analogous structures (1).

(a) Chimpanzee, with 1 difference (1); 100-14=86 identical positions of 100, so 86% (1). (b) Differences arise from mutations that accumulate independently in each lineage after they separate (1); the similar sequences are inherited from a common ancestor (1); the fewer the differences, the less time has passed since the common ancestor, so the chimpanzee shares the most recent common ancestor with humans (1).

(a) (8000-5000)\div5000\times100=60\% (1). (b) The cattle with high yield were selected as parents, and the yield trait is inherited, so the allele frequencies in the herd changed over generations, which is evolution (2); large heritable change can happen in a relatively short period when selection is strong (1).

One ancestral population was divided by a geographical barrier (1). Gene flow stopped, so alleles were no longer exchanged (1). The two parts had different conditions, so different selection pressures acted (1). Beetles with alleles suited to each part survived and reproduced more, so the allele frequencies diverged (1). Mutations arose independently in each population (1). After many generations they could no longer produce fertile offspring together, so reproductive isolation persisted and two species existed (1).

(a) Pairs 2 and 3 (1), because sterile hybrids or no offspring stop alleles passing between the populations (1). (b) The claim is not supported (1): pair 1 produces fertile offspring, so alleles can still flow even though courtship differs (1). (c) Any one: test the fertility of the second generation from pair 1; observe whether the populations interbreed under natural conditions (1).

Similarity, either one: both involve the splitting of a pre-existing species (1); both need reproductive isolation so that allele frequencies diverge (1). Difference: allopatric populations are separated by a geographical barrier, whereas sympatric populations live in the same area (1); sympatric isolation arises from temporal or behavioural differences or polyploidy, where allopatric begins with the barrier (1). Each point must be paired for both routes.

(a) 60\div200\times100=30\% (1). (b) The largest loss is at the pollen-tube stage, where 140 of 200 flowers failed (1); this is before fertilisation, so pre-zygotic (1). (c) Two of the hybrid stages: only 24 of 60 fertilised seeds germinated, so hybrid inviability (1); none of the 20 flowering hybrids produced viable pollen, so hybrid sterility (1).

(a) The proteins of M and N are identical in all 150 positions (1). (b) The genetic code is degenerate, so different codons can code for the same amino acid (1); a base substitution at the third position can therefore change the gene without changing the protein (1). (c) The conclusion is not supported: the gene sequences differ at 9 bases, so the species are not identical (1); one protein does not show whether they can interbreed (1). (d) The equal counts of 6 differences from O do not show whether M or N is closer to O (1); further evidence from other genes, or non-coding regions, from more than one sample would be needed (1).

(a) Gametes of P carry 14\div2=7 and of Q carry 18\div2=9, so the hybrid has 7+9=16 chromosomes (2). (b) The chromosomes come from two species and have no homologous partners, so they cannot pair at meiosis and balanced gametes are not formed (2). (c) Doubling gives 2\times16=32 chromosomes, which is two sets of P and two sets of Q, forming 16 pairs (2). (d) With partners for every chromosome meiosis works, so the plant is fertile (1); crossing it with P or Q gives offspring with an odd number of sets, which are sterile, so it is reproductively isolated from both and is a new species (1).

(a) Acceptance is 90% within an island and 12% between islands (1); 80% of the hybrids' offspring are fertile (1). (b) The sea stopped gene flow (1); different conditions on each island led to different selection, so allele frequencies diverged (1). (c) Evaluation: the low acceptance between islands shows partial behavioural isolation (1); but 80% fertility means that alleles can still pass between the islands, so isolation is incomplete and speciation is not yet complete (1); a further test of whether lizards interbreed when released together would be needed (1).

(a) Sympatric speciation (1), because the five species live in one lake and there is no geographical barrier (1). (b) Any three: the ancestor found vacant niches with unused food (1); jaw variation was heritable and selected for different foods (1); fish feeding on the same food bred together, so allele frequencies diverged (1); competition between species fell because they used different resources (1). (c) Adaptive radiation produces several species from one ancestor, which is an increase in species number, not only in the number of fish (1).

(a) The base A has been replaced by G, a substitution (point) mutation (1). (b) Both codons specify the same amino acid, so the protein is unchanged (1), because the genetic code is degenerate: several codons specify one amino acid (1). (c) The difference arose by mutation after the lineages separated (1); a single base difference shows the DNA is not identical, so the protein comparison alone understates the divergence (1). (d) Comparing more genes gives a more reliable estimate of relationship (1).

(a) Gametes carry 6 and 7 chromosomes, so the hybrid has 13 (1). (b) The chromosomes come from two species and have no homologous partners (1), so they cannot pair at meiosis I and balanced gametes are not produced (1). (c) The hybrid grows normally because mitosis does not need pairing of homologues (1). (d) Doubling the number to 26 gives 13 pairs, so each chromosome has a partner and meiosis can produce balanced gametes (1); the plant is fertile but isolated from both parents, so it is a new species (1).

(a) The wings of bats and birds are analogous structures that arose independently (1), as a result of convergent evolution, because flight was favoured by selection in both lineages (1). (b) Two of: homologous forelimb bones, hair, mammary glands (2). (c) Comparing corresponding gene sequences from more than one gene (1); the bat sequences would be more similar to the human sequences than to the bird sequences, giving a clade of mammals (1).