Checkpoint Quiz 2, Speciation, Human Evolution and Evolutionary Change

• 1. C Directional selection: the environment favours one extreme of a trait distribution. Over generations, the population mean shifts toward that extreme. Example: antibiotic resistance (most resistant bacteria survive), industrial melanism in peppered moths (dark form selected on dark trees).
• 2. C Speciation is the evolutionary process through which one or more new species arise from an existing species. The key criterion: reproductive isolation, the new lineages can no longer interbreed successfully.
• 3. B Homo includes modern humans (H. sapiens) and extinct relatives: H. erectus, H. habilis, H. neanderthalensis, H. heidelbergensis, H. floresiensis, etc. Australopithecus is a closely related but distinct genus (ancestral to Homo).
• 4. C In convergent evolution, unrelated species facing similar environments/selection pressures independently evolve similar features (analogous structures), e.g. the streamlined bodies of sharks and dolphins.
• 5. C A transitional fossil shows intermediate features between an ancestral group and its descendants (e.g. Archaeopteryx between reptiles and birds), documenting gradual change. It is not simply the oldest fossil, nor a fossil of a present-day species.
• 6. D Radiometric dating uses the predictable decay of a radioactive isotope at a known half-life to calculate an absolute age. Counting layers or comparing size gives only relative information.

7. In allopatric speciation a physical barrier geographically separates populations, stopping gene flow between them; they diverge under different conditions until they are reproductively isolated (for example island populations of finches). In sympatric speciation new species form without geographic separation, for example by polyploidy in plants, where a sudden change in chromosome number prevents interbreeding with the parent population. In both cases reduced or absent gene flow is what allows the populations to diverge into separate species.

8. The fossil record shows that several hominin species existed at the same time rather than a single line replacing the one before it, so evolution did not proceed as a straight ladder. Humans did not descend from living apes; instead humans and apes share common ancestors from which lineages diverged. Traits such as bipedalism and larger brain size appeared in different lineages at different times. A branching tree represents this shared ancestry and divergence accurately, whereas a ladder wrongly implies linear, directed progress.