Evolution and speciation: exam questions

Answer: B ; [1]

• (a) chimpanzee; [1]
• (b) 12 ÷ 104 × 100 = 11.5 % / 11.5; [1] Accept 11.54 / 12 %
• (c) differences arise by mutation and accumulate over time after lineages split; [1]
• (c) fewer differences show more recent common ancestry, e.g. humans and chimpanzees / more differences show more distant ancestry, e.g. yeast; [1]
• (c) the same protein in all these organisms suggests a common origin / common ancestor; [1]
• (d) genetic code is degenerate / more than one codon codes for the same amino acid, so some base changes do not change the amino acid; [1] OWTTE
• max 5

• humans selected and bred plants with desired (heritable) traits over many generations; [1]
• large differences between varieties and from the wild species show heritable characteristics of a population can change; [1]
• change happened rapidly / in a short time, so natural selection over much longer periods could produce larger changes; [1]
• max 2

• homologous structures have the same basic structure / same arrangement of parts; [1]
• homologous structures are inherited from a common ancestor / have the same evolutionary origin, but may have different functions; [1]
• named homologous example, e.g. pentadactyl limbs of human arm, bat wing and whale flipper; [1]
• analogous structures have the same function but different evolutionary origins; [1]
• analogous structures result from convergent evolution / similar selection pressures on unrelated groups; [1]
• named analogous example, e.g. wings of birds and insects / eyes of octopus and vertebrates / fins of shark and dolphin; [1]
• max 4

• speciation is the splitting of a pre-existing species into two (or more); [1]
• a population becomes reproductively isolated / stops interbreeding; [1]
• so there is no gene flow between the populations; [1]
• geographical isolation is one cause / a barrier separates the populations; [1]
• ancestral Pan population separated by the Congo River, which the apes cannot cross; [1] Accept either the river forming or ancestral bonobos crossing it at low water.
• bonobos south of the river and common chimpanzees north of it; [1]
• differential selection / different selection pressures in each area favour different alleles; [1]
• e.g. gorillas compete for food north of the river but not south of it, OWTTE; [1]
• gene pools / allele frequencies diverge over many generations; [1]
• until the populations can no longer interbreed to produce fertile offspring / are separate species; [1]
• speciation increases the number of species; gradual change within one species is not speciation; [1]
• max 6

Answer: C ; [1]

• both require reproductive isolation / the end of gene flow between populations; [1]
• both involve divergence of gene pools / differential selection; [1]
• both produce new species that cannot interbreed (with the original); [1]
• allopatric speciation involves geographic isolation / populations in different areas; [1]
• sympatric speciation occurs in the same area / without a geographical barrier; [1]
• sympatric isolation is behavioural or temporal (or by polyploidy); [1]
• max 4. At least one similarity and one difference are needed for full marks.

• courtship behaviour (often) prevents hybridization in animals; [1]
• courtship signals are species-specific so individuals only mate with their own species; [1]
• interspecific hybrids are often sterile, e.g. mule (horse × donkey); [1]
• mule has 63 chromosomes / an odd number, which cannot form homologous pairs in meiosis, so no viable gametes; [1]
• so alleles are not passed from one species' gene pool into the other; [1]
• in plants, a hybrid of two species is sterile as its chromosome sets are not homologous; [1]
• doubling of the chromosome number (polyploidy) gives every chromosome a partner; [1]
• the polyploid can carry out meiosis and is fertile; [1]
• crosses with either parent give sterile offspring (e.g. triploids), so the polyploid is reproductively isolated / a new species; [1]
• this happens in one generation / abruptly; [1]
• Persicaria contains many species formed by hybridization and polyploidy; [1]
• max 6