The Evolution of Species

Explain what a species is according to the biological species concept (interbreeding to produce fertile offspring) and give an example of two closely related animals belonging to different species.

A horse and a donkey can interbreed and produce a mule, but they belong to different species. Explain, applying the biological species concept, why the mule does not "unite" the two species.

Define biodiversity and name its three levels, providing an example of each.

Distinguish between population and community with an example from the same place: all the sparrows in a park versus all the living things in that park. Why does the concept of species apply to populations and not to communities?

In a beetle population, there are individuals with lighter and darker shells, and predators spot the lighter ones more easily against dark tree bark. Explain step by step how natural selection will act on the frequency of each color in subsequent generations.

A patient diagnosed with a bacterial infection stops an antibiotic treatment prescribed for 10 days after only 4 days because they no longer have symptoms. Two weeks later, the infection recurs more aggressively, and the same antibiotic proves ineffective. Their relative claims: "The bacteria got used to the medication in order to survive and have now become immune." Refute their relative's claim based on the theory of natural selection and reasonably predict how the frequencies of sensitive and resistant bacteria in the bacterial population will have changed between the start of the treatment, day 4, and the relapse.

A classic illustration from an old book shows horse evolution as a single file: the small Hyracotherium (four-toed, forest browser) walks behind intermediate forms that steadily increase in size and lose toes until reaching the modern horse (Equus, tall, grassland runner with a single toe). The caption reads: "Evolution follows a fixed direction toward larger, more complex, and more perfect forms." Refute this statement by pointing out at least two conceptual errors, and justify why paleontologists today represent horse phylogeny as a branching bush adapted to climate changes rather than as a progressive ladder.

The subterranean fish Astyanax mexicanus lacks functional eyes and pigmentation, unlike individuals of the same species that inhabit illuminated surface rivers. A student proposes two hypotheses: 1) "By living in the dark, they stopped using their eyes, which atrophied, and their descendants inherited this lack of vision"; 2) "Losing organs cannot be evolution by natural selection, because evolving always consists of gaining more complex structures." Refute both hypotheses: demonstrate the Lamarckian error of the first one and justify, in terms of metabolic cost and energy balance in an environment with scarce food, why ocular and pigment atrophy may have represented an adaptive advantage.

The polar bear (Ursus maritimus) and the brown bear (Ursus arctos) diverged approximately 500 000 years ago and exhibit very different morphological and metabolic adaptations. Due to the retreat of Arctic ice, their ranges increasingly overlap and fertile hybrids have been documented in the wild ("grolar bears"). With these data: a) Question the limitations of the biological species concept applied to this case. b) Predict and justify what long-term genetic consequences this gene flow could have for the conservation of the specific adaptations of the polar bear if the ice continues to disappear.

Topic challenge. Give three examples of geographic barriers that can divide a population of living things (for example, a mountain range, a river that changes course, or an arm of the sea) and explain exactly what each barrier prevents the individuals on both sides from doing.

You will be graded on whether you name three different geographic barriers and explain that they prevent the individuals on both sides from meeting and breeding with each other.

Look at the diagram, which shows three stages in the evolution of a population divided by a geographical barrier. What is speciation? Explain stage by stage how geographical isolation can lead to the formation of a new species, and state what you would test in the two groups labelled ? to know if they are already distinct species.

Distinguish between geographical isolation and reproductive isolation, and explain why reproductive isolation marks the point at which two populations have become different species.

On opposite sides of the Grand Canyon, there are squirrel species that are very similar yet distinct. Explain what type of speciation this example illustrates and reconstruct the process that caused it.

Some flies lay their eggs on apple trees and others, closely related to them, on hawthorns, and each group almost always breeds within its own plant. Explain what sympatric speciation is and how it differs from the classic case of the geographical barrier.

Some fruit flies have been bred in a laboratory for hundreds of generations, isolated from the wild ones, and now hardly interbreed with them when they are brought together. Reason, applying the biological species concept, whether we can say that the laboratory flies are on their way to becoming a different species.

Explain what an adaptive radiation is and describe it with the example of the Galápagos finches: from an ancestral species that arrived from the mainland, several species arose with beaks adapted to different foods.

Explain why islands are considered "natural laboratories of evolution": what role isolation, new environments with few competitors, and adaptive radiations play. Give an example (the Galápagos, Madagascar, or Hispaniola).

Lake Victoria is home to hundreds of cichlid fish species that arose in an evolutionarily very short time. Explain which factors can speed up speciation in this way (a great variety of ecological niches and sexual selection based on colors) and what happened to many of those species after the Nile perch was introduced.

The shark (a fish), the penguin (a bird) and the dolphin (a mammal) have very similar streamlined bodies without being close relatives. Explain what convergent evolution is, how similar environments select similar solutions and why the resemblance does not indicate kinship.

On an oceanic island newly formed after an eruption, a storm washes up a small raft of vegetation carrying 8 beetles of a mainland species (6 green and 2 brown). On the mainland, millions of these beetles live, where 95 % are camouflaged brown and only 5 % are green, as birds easily prey on the green ones. After several generations on the island (where there are no predatory birds), 75 % of the beetles are green. Justify which evolutionary mechanism accounts for the initial difference and predict how genetic drift will influence the population compared to natural selection if the island population remains small.

An agricultural research center produces a cloned wheat variety (100 % genetically identical) that yields 40 % more than any traditional variety under moderate drought. The agronomist claims: "We have achieved the optimal phenotype; eliminating genetic variability from our fields is the safest long-term evolutionary strategy." Reasonably predict what will happen to this crop if an unforeseen new fungal pest emerges compared to a diverse traditional crop, and justify why, in changing environments, retaining genetic variability is advantageous for natural populations instead of homogenizing into a single supposedly optimal type.

Explain what an extinction is and when we speak of a mass extinction, naming two examples from Earth's history (for example, the one at the end of the Permian or the one that wiped out the dinosaurs at the end of the Cretaceous).

Name the main causes why human activities are accelerating the loss of biodiversity today (habitat destruction, invasive species, overexploitation, pollution and climate change) and briefly explain how each one acts.

The natural fossil extinction rate ("background extinction") is estimated at 1 species per million species per year, whereas the current rate due to human activity is 100 to 1 000 times higher. Knowing that after the great mass extinctions of the past (such as the Permian or Cretaceous extinctions) the biosphere took between 5 and 10 million years to recover its biodiversity through adaptive radiation, analyze and justify why the current pace of habitat destruction cannot be buffered by the emergence of new species and what impact this will have on a geological scale on the tree of life.