Then think about what you know about the groups of animals they are in - are they in fact closely related? Which of the two would be more closely related to a human?

The science of classifying organisms is called taxonomy and it uses a hierarchical system to group organisms based on shared characteristics Taxonomic Rank | Dogfish (a type of shark) | Dolphin (e.g., Bottlenose Dolphin)

Kingdom | Animalia | Animalia

Phylum | Chordata | Chordata

Class | Chondrichthyes (cartilaginous fishes) | Mammalia (mammals)

Order | Squaliformes | Cetacea

Family | Squalidae | Delphinidae

Genus | Squalus | Tursiops

Species | Squalus acanthias (spiny dogfish) | Tursiops truncatus (bottlenose dolphin)

Features that show a common evolutionary origin, like bone structure or genetics are more reliable for classifying organisms than traits like body shape.

In “On the Origin of Species” (1859), Darwin emphasised that shared ancestry is the key to understanding biological classification.

For example, dolphins and dogfish both have streamlined bodies, but they are not closely related.

In contrast, dolphins share key features with all mammals, showing they evolved from the same ancestor.

two reproductively isolated groups,

causing each group to develop different

traits due to their respective selective

pressures and natural selection

This occurs when unrelated species evolve similar traits independently due to similar environmental pressures or lifestyles.

Example: Wings in birds, bats, and insects – they all developed flight, but not from a common winged ancestor.

2. Divergent Evolution:

This happens when related species evolve different traits and become more dissimilar over time, often due to adapting to different environments.

Example: Darwin’s finches – all evolved from a common ancestor but developed different beak shapes based on their food sources.

These are body parts that are similar in structure and position but may have different functions.

Example: The forelimbs of humans and bats – same bones, different uses.

Analogous Structures:

These are body parts that have similar functions but different structures and evolutionary origins.

Example: Wings of birds and insects – both used for flying, but built very differently.

Aquatic habitat | Both live in the ocean (marine environments).

Streamlined body | Both have smooth, tapered bodies for fast swimming.

Fins | Both use fins to move and steer in water.

Tail (caudal fin) | Both have tails that help propel them forward.

Vertebrates | Both have a backbone (they’re in the phylum Chordata).

Carnivores | Both are meat-eaters – they hunt fish and other sea animals.

Well-developed senses | Both have strong senses (e.g., vision, hearing/smell).

Live in similar ecosystems | Both are part of the marine food web and are top predators.

Using a consistent, evolutionary-based method of classification (such as grouping organisms by common ancestors and genetic similarities) ensures that dolphins are correctly placed with mammals and dogfish with fish. This helps scientists communicate clearly, avoid confusion, and understand the true biology and evolutionary history of each organism. Without a standard system, scientists might interpret the similarity in appearance between dolphins and dogfish as a sign of close relation, which would be misleading.