Living Things · Living things and their classification
The Major Groups of Living Things
At a tianguis (open-air market) in Oaxaca you can see a fresh fish, a frog in a tank, a spider in a corner, and a fern in a pot. They are all living things, but very different from one another. Scientists sort them into major groups according to their shared characteristics and how closely related they are. Every classification is a useful proposal, with strengths and limits, and it can be revised when new data appear.
Vertebrates
If you eat a sardine, you find a central bone. If you touch your back, you feel a row of bones. A worm or a jellyfish, on the other hand, has a soft body inside. What internal structure separates animals into two large body plans?
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Name the five groups of vertebrates and give an example of an animal for each. Also state the characteristic shared by all vertebrates, which gives the group its name.
Name the five groups of vertebrates and give an example of an animal for each. Also state the characteristic shared by all vertebrates, which gives the group its name.
The diagram shows the five groups of vertebrates with a typical trait for each, but one group is missing, marked with "?". State which group it is, describe its defining…
The diagram shows the five groups of vertebrates with a typical trait for each, but one group is missing, marked with "?". State which group it is, describe its defining characteristics, and give two examples of animals from that group.
From this list of animals —dog, ant, spider, earthworm, snail, jellyfish, and trout— first separate the vertebrates from the invertebrates. Then, group the invertebrates…
From this list of animals —dog, ant, spider, earthworm, snail, jellyfish, and trout— first separate the vertebrates from the invertebrates. Then, group the invertebrates into insects, arachnids, worms, molluscs, and jellyfish, stating a feature that helped you decide in each case.
Mosses have no true roots or conducting vessels; they grow in moist, shady places, such as logs and damp walls. Ferns have roots, stems, and leaves, but no seeds; they usually live in moist places, such as forests and ravines. Both reproduce by spores and need water for fertilization.
The plants with seeds are the gymnosperms, such as pines, whose seeds are not inside a fruit, and the angiosperms, such as the geranium, the prickly pear cactus, or corn, with flowers and seeds inside fruits. They are the most abundant and live in almost every environment.
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State which group each of these living things belongs to and justify each answer with a feature of the corresponding group: a frog, a sardine, a cat, a spider, a fern,…
State which group each of these living things belongs to and justify each answer with a feature of the corresponding group: a frog, a sardine, a cat, a spider, a fern, and a geranium.
Explain the difference between mosses, ferns, and flowering plants, and give an example of each group that you could find near where you live. What type of habitat does…
Explain the difference between mosses, ferns, and flowering plants, and give an example of each group that you could find near where you live. What type of habitat does each typically grow in?
Create a table with three columns —group, main feature, and example— and fill it in for these six groups of living things: fish, amphibians, reptiles, birds, mammals,…
Create a table with three columns —group, main feature, and example— and fill it in for these six groups of living things: fish, amphibians, reptiles, birds, mammals, and insects.
Groups and kingdoms: from the cell to the organism
The groups of animals and plants are subdivisions within a kingdom. To place an organism in its kingdom, you first look at what its cells are like and how it gets food.
Feature
Plants
Animals
Fungi
Protists
Monera
Cells
eukaryotic
eukaryotic
eukaryotic
eukaryotic
prokaryotic
Body
multicellular
multicellular
mostly multicellular
mostly unicellular
unicellular
Nutrition
autotrophic (photosynthesis)
heterotrophic: ingests
heterotrophic: absorbs
autotrophic or heterotrophic
autotrophic or heterotrophic
Cell wall
yes
no
yes
depends on the group
yes
Animals ingest food and form tissues. Fungi, such as huitlacoche (corn smut) or mushrooms, absorb nutrients without ingesting them. Sea sponges live attached to rocks and have no true tissues, but they ingest their food: that is why they are animals and not fungi.
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In a school laboratory, three unknown organisms are analysed and their cellular and nutritional characteristics are recorded: Organism A: unicellular, eukaryotic, with…
In a school laboratory, three unknown organisms are analysed and their cellular and nutritional characteristics are recorded:
Organism A: unicellular, eukaryotic, with chloroplasts and autotrophic nutrition.
Organism B: multicellular, heterotrophic by absorption, with a cell wall and lacking true tissues.
Organism C: multicellular, heterotrophic that ingests food, lives attached to a marine rock, and lacks differentiated tissues.
Determine with reasoning which kingdom each organism belongs to based on its cellular and nutritional data. In addition, explain why organism C belongs to the animal kingdom despite living immobile like a plant.
A scientist does not only classify: they propose a hypothesis and put it to the test. For example: "The bat is more closely related to the dolphin than to the sparrow."
To test it, they look for data that support or contradict it: Do they have hair and mammary glands? How are their young born? What are the bones of their limbs like? Then you compare your data with reliable sources, such as textbooks, CONABIO, natural history museums, or UNAM articles. If the sources agree with your observations, the hypothesis is strengthened. If not, it is corrected.
Remember that every classification has strengths and limits: it is based on the available evidence and can change when new data on relatedness are discovered.
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An explorer finds an unknown marine animal: it has a streamlined body, lacks hind limbs, moves by means of flippers, and spends its entire life underwater. However, he…
An explorer finds an unknown marine animal: it has a streamlined body, lacks hind limbs, moves by means of flippers, and spends its entire life underwater. However, he observes that it periodically surfaces to take in air and that it gives birth to live young which it feeds with milk. The explorer notes in his journal: "Because of its shape and because it always lives in water, it is a new species of fish." Refute his conclusion by justifying why it should be classified as a mammal, and predict what type of reproduction, fertilisation, and thermal regulation (homeothermic or poikilothermic) you would expect to find in this animal.
Look at these three animals: the bat, the sparrow, and the dolphin. If we used a classification criterion based exclusively on the medium through which they move, which…
Look at these three animals: the bat, the sparrow, and the dolphin. If we used a classification criterion based exclusively on the medium through which they move, which two animals would you group together? Explain why this criterion is misleading for biologists and justify, by comparing the internal anatomy of their limbs and their mode of reproduction, why the bat is much more closely related to the dolphin than to the sparrow.
Back to the beginning: the fish and the frog are vertebrates because they have a backbone; the spider is an invertebrate (an arachnid) with eight legs, and the fern is a plant without seeds. Each one falls into its group because of its relatedness and its features, not because of where it lives.
Summary
Let's practice! 3 medium exercises in this section