Exercises: The major groups of living organisms

List the major groups (kingdoms) of living organisms and write a distinctive characteristic for each that allows it to be recognized.

Differentiate bryophytes, pteridophytes, gymnosperms, and angiosperms based on the presence of vascular tissue, seeds, and flowers, and give an example of each group.

Explain the criterion separating vertebrates from invertebrates, name the five groups of vertebrates, and give a key evolutionary novelty for each.

Distinguish between a unicellular and a multicellular organism with two examples of each, and explain what advantage the division of labor between the specialized cells of a multicellular organism brings.

Classify each of these organisms into its major group (monerans, protoctists, fungi, plants, or animals) and justify your choice: an amoeba, a mushroom, an oak, a cyanobacterium, and a sponge.

Explain two traits that distinguish fungi from plants (composition of the cell wall, form of nutrition, reserve substance) and discuss why fungi are more closely related to animals than to plants.

Name three major groups of invertebrates (for example cnidarians, annelids, arthropods, mollusks or echinoderms) and one evolutionary innovation that characterizes each (radial symmetry, segmentation, jointed exoskeleton...).

What is a protoctist (protist)? Explain why it is said to be a "catch-all" group, what its members have in common, and give two examples that show its enormous diversity (a unicellular alga and a protozoan).

Identify which major group each organism belongs to based on these descriptions, justifying your answer: a) a unicellular organism without a distinct nucleus, with ether-linked membrane lipids, living in hot springs; b) a multicellular eukaryotic organism with heterotrophic nutrition by absorption; c) a photosynthetic unicellular eukaryotic organism from plankton.

Explain what anatomical and physiological adaptations allowed the first vertebrates to transition from aquatic to terrestrial environments, and what problems they had to solve (support, respiration, desiccation, and reproduction).

Based on the simplified phylogenetic tree in the diagram: a) state which present-day group shares the most recent common ancestor with animals; b) explain what the node marked with "?" represents and what type of cellular organization is shared by all groups descending from it; c) explain whether it is correct to claim that bacteria are "less evolved" than animals.

Order the appearance of these innovations in the history of plants and explain what independence from the aquatic environment each one provided: conducting vessels, the seed and the flower.

Order the appearance of these evolutionary novelties of the vertebrates and assign each one to the group in which it arose: jaws, lungs and limbs with digits, the amniotic egg, and the feather.

Explain what a synapomorphy (a shared derived character) is using the hair and mammary glands of mammals, and discuss why shared derived characters, and not general similarities, are used to build evolutionary trees.

Distinguish between divergent and convergent evolution using these examples: the human arm and the wing of a bat (homologous organs) versus the wing of a butterfly and the wing of a bird (analogous organs). Explain what each case indicates about relatedness.

Sponges are the living animals with the simplest organization. Describe their body plan (no true tissues, choanocytes that filter the water) and discuss what they suggest about how animals could have arisen from colonial protists.

A 1st-year Bachillerato student states in class: "Crocodiles and lizards belong to the same class (Reptiles) because they share characters such as keratinized scales and ectothermy, whereas birds are a completely different group because they have feathers and homeothermy. Grouping crocodiles and birds in the same clade contradicts morphology." Based on the principles of phylogenetic systematics:

1. a) Refute the student's statement by explaining why basing classification on keratinized scales or ectothermy creates a paraphyletic group. Distinguish in your answer between plesiomorphies (ancestral characters) and synapomorphies (shared derived characters).
2. b) Draw or sketch a cladogram showing the common ancestor of amniotes, lepidosaurs (lizards), crocodiles, and birds. Place on the corresponding branches at least two synapomorphies exclusive to the clade Archosauria (which encompasses birds and crocodiles).
3. c) Predict what would happen if in a fossil stratum you discovered an organism with an antorbital fenestra in the skull and a muscular gizzard, but covered in scales and without a trace of feathers: where would you place it on your cladogram, and what implication would this have for the traditional definition of a "reptile"?

In a high-mountain pond on the Iberian Peninsula, an 80% population collapse has been recorded over the last decade in the common midwife toad (Alytes obstetricans), whereas populations of the viperine water snake (Natrix maura) and dragonflies show no significant declines. Environmental analyses reveal an increase in UV-B radiation, a moderate concentration of nitrates from livestock farming (18.5 mg/L), and the presence of the pathogenic fungus Batrachochytrium dendrobatidis.

1. a) Interpret, relating it to amphibian anatomy and physiology (gas exchange across the integument, biphasic reproductive cycle, and metamorphosis), why these factors devastatingly affect amphibians and not the reptiles in the same body of water.
2. b) Design an action plan with two specific, viable field conservation measures to stop the disappearance of the midwife toad in this enclave. Justify the biological mechanism by which each of your proposals will counteract the specific causes of the decline.

In an experimental plot of a deciduous forest, a broad-spectrum systemic fungicide was applied to the soil for three years, keeping solar radiation, rainfall, and atmospheric CO₂ concentration unchanged. After that period, three empirical observations were recorded:

1. The thickness of the undecomposed leaf litter layer increased by 180%.

2. Oak seedlings (Quercus robur) experienced a 45% reduction in their growth and a marked foliar deficiency of phosphorus and nitrogen.

3. The soil respiration rate dropped drastically.

1. a) Justify observations 1 and 3 based on the metabolic strategy and the mode of external digestion characteristic of saprophytic fungi compared to plant autotrophic nutrition.
2. b) Explain the biological mechanism underlying observation 2: which specific symbiotic interaction has been destroyed in the soil, and what net nutrient flow between fungus and plant has been interrupted?
3. c) Predict, with reasoning, how the continued absence of fungi will affect the forest's net primary productivity and the biogeochemical carbon cycle in the medium term.

In a Cretaceous site, a fossil skeleton assigned to a non-avian theropod (Dromaeosauridae) is excavated. The fossil exhibits:

• Asymmetrical contour feathers on the forelimbs.

• An ossified V-shaped furcula and an incipient keeled sternum.

• Appendicular skeleton with hollow pneumatic bones.

• Serrated teeth in the jaws and a long, articulated bony tail.

1. a) Interpret these characters in light of the concept of exaptation (functional co-optation): demonstrate why structures associated with modern flight (feathers and pneumatic skeletons) did not initially evolve 'to' fly, but under different selective pressures in terrestrial dinosaurs.
2. b) A science outreach outlet publishes the headline: "A new fossil helps understand how dinosaurs went extinct and gave rise to birds." Identify the cladistic conceptual error in the headline and write a taxonomically rigorous phrasing based on the concept of a monophyletic group.
3. c) Explain what taxonomic consequences it would have for the class "Reptilia" to keep birds in a separate class: what type of group would Dinosauria turn out to be if we expressly excluded the lineage of living birds?

When analyzing the fossil record of continental deposits before and after the biological crisis at the Permian-Triassic boundary (~252 Ma), it is observed that therapsids (synapsids known as "mammal-like reptiles"), which dominated more than 70 % of the herbivore and carnivore faunal communities, suffered a mass extinction exceeding 85 % of their families. During the Triassic, basal archosaurs—previously scarce and marginal—underwent rapid diversification, occupying the vast majority of terrestrial ecological niches.

1. a) Interpret this faunal turnover by applying the concepts of ecological bottleneck, niche vacancy, and adaptive radiation. Was the diversification of archosaurs a consequence of a prior "competitive biological superiority" or of historical contingency?
2. b) Apply the same reasoning to the Cretaceous-Paleogene (K-Pg) boundary extinction event: justify whether the ecological rise of placental mammals following the meteorite impact supports the idea of cumulative evolutionary progress or that of contingency.
3. c) In the face of the current biodiversity loss crisis caused by human action, a report states: "The species that are dominant today (humans, poultry, rodents) guarantee the stability of future ecosystems because their abundance demonstrates their adaptive success." Critically assess this statement based on the survival patterns observed in the major extinctions of the fossil record.

Your city's environmental department wants to assess the ecological health of a peri-urban park subjected to two different regimes: Zone A (continuous lawn mowing, leaf litter removal, and herbicide use) and Zone B (rewilded, without systematic clearing or plant protection products).

1. a) Design a standardized sampling protocol to quantify and compare the diversity of two key taxonomic groups: soil arthropods and soil fungi. For each group, indicate: the field capture or collection method, sampling effort (number of replicates or area), and a specific quantitative biological variable that you would measure in the lab.
2. b) When analyzing the water of a park pond using visual sampling and conventional plankton nets, no presence of amphibians or threatened benthic macroinvertebrates is detected. Design an analytical strategy based on environmental DNA (eDNA) and metabarcoding to verify whether these species frequent or inhabit the water body, pointing out which common genetic target is used for taxonomic identification and two critical advantages of this technique compared to traditional survey methods.
3. c) Predict what quantitative and structural differences you would expect to find between the food web of Zone A and that of Zone B after two years of differentiated management, justifying how the management regime will impact the decomposition rate of organic matter and the ecosystem's resilience capacity.

Unit Challenge. Imagine you are working as a scientific advisor for a natural science museum. Your mission is to analyze how the major milestones in the history of life explain the biodiversity that inhabits your region today.

Follow these phases:

1. Gathering real data: Access the official GBIF platform (gbif.org) and consult the biodiversity data for your county or state. Extract the number of recorded species for four lineages: bacteria, plants, fungi, and animals.

2. Evolutionary reconstruction: Design a chronological table placing the appearance of each group on the geologic time scale (from the earliest evidence of cellular life $\approx 3.8$ Ga ago to the Cambrian explosion $\approx 0.54$ Ga ago). Include a key adaptive innovation for each (such as oxygenic photosynthesis, symbiogenesis, or multicellularity).

3. Synthesis report: Create a comparative graph of current species richness versus their antiquity, and write a reasoned proposal ($1$ page) arguing how extinctions and adaptive radiations have shaped the local biosphere you observe.

Assessment criteria:

• Rigor in extracting and tabulating real biological data from the official database.

• Scientific accuracy in dating evolutionary milestones and linking them to their biological innovations.

• Quality of the critical analysis connecting geologic history with current biodiversity patterns.