IB Biology HLEvolution & SpeciationPaper 1 & 2~9 min read
Convergent Evolution
A shark and a dolphin have almost the same body shape, but one is a fish and one is a mammal. They did not inherit that shape from a shared ancestor. They arrived at it separately, because the sea sets the same problem for anything that wants to swim fast.
📚 What you need to know
Analogous structures have a similar form and function, but a different evolutionary origin.
Analogous structures are produced by convergent evolution.
Convergent evolution happens when distantly related species live in habitats with similar selection pressures.
The advantageous characteristics evolve separately in each lineage — they do not come from one shared mutation.
Analogous structures are evidence for the passing on of advantageous characteristics during natural selection.
Examples: dolphins and sharks (body shape), cacti and euphorbias (spines and succulent stems), butterflies and bats (wings).
Homologous structures point to shared ancestry; analogous structures point to shared conditions.
What convergent evolution is
Definition
Analogous structures have a similar form and function, but a different evolutionary origin
Here is how you get them. Two species that are only distantly related happen to live in habitats with similar selection pressures. In both habitats, the same sort of characteristic gives a survival advantage. So natural selection acts on each population separately, and pushes both towards the same solution.
That last word matters: separately. The advantageous characteristic is not the result of one mutation in one shared ancestor that both species inherited. Each lineage came up with it on its own, from its own mutations.
Notice the direction of the branches. Homologous structures spread apart from one point; analogous ones travel towards each other from two.
The examples you should know
Dolphins and sharks
Both are groups of aquatic animals that share a similar body shape, but they belong to different classes: dolphins are mammals and sharks are fish.
Their streamlined body shapes evolved separately rather than coming from one common ancestor.
The selection pressure is the same in both cases: water is dense, so anything that reduces drag saves energy and helps you catch food and escape predators.
Cacti and euphorbias
Two groups of desert plants, both recognisable by their spiny leaves and branching, succulent stems.
They belong to different orders of plants. Cacti are found in the deserts of the Americas; euphorbias are found in Africa.
They evolved separately but adapted to similar environments, so they ended up looking alike.
Both families solved the same three problems in the same three ways. That is what convergent evolution looks like from the outside.
Butterflies and bats
Both have wings, and both use them to fly. But a bat wing is a modified pentadactyl limb with bones inside it, while a butterfly wing is made of thin membranes and veins and contains no bones at all. Same job, completely different construction — the definition of an analogous structure.
🧠
Which one is which?
Homologous — think “homo” as in “the same family”. Same origin, different jobs. Analogous — an analogy is when two unrelated things happen to be alike. Same job, different origin.
Homologous or analogous?
Feature
Homologous structures
Analogous structures
Underlying structure
The same, e.g. the pentadactyl limb
Different, even when the outside looks alike
Function
Often very different
Similar
Evolutionary origin
A shared common ancestor
Separate ancestors
Produced by
Divergence from one ancestor into different environments
Convergent evolution under similar selection pressures
What it is evidence for
Shared ancestry between species
Advantageous characteristics being passed on by natural selection
Analogous structures are still evidence for evolution — just for a different part of it. They show natural selection repeatedly favouring the same advantageous characteristic. If evolution were random, two unrelated desert plants ending up with the same spines and the same fat stems would be an enormous coincidence.
Why this caused trouble for taxonomy
Before sequence data existed, biologists classified organisms mainly by how they looked. Analogous structures caused real confusion for scientists working in taxonomy, because two species could look like close relatives while being nothing of the sort.
Comparing DNA or amino acid sequences fixes this. A dolphin and a shark may share a shape, but their base sequences are very different, while a dolphin and a cow share far more sequence than either shares with a shark. Appearance can be misleading; sequence data usually is not.
Worked examples
WE 1
Explaining a shared body shape
Sharks and dolphins have similar streamlined body shapes. Explain how this similarity arose. (3 marks)
Point 1: they are not close relatives
Sharks are fish and dolphins are mammals, so they belong to different classes and did not inherit the shape from a common ancestor.
Point 2: the same pressure
Both live in habitats with similar selection pressures, where a streamlined shape gives a survival advantage.
Point 3: separate evolution
Natural selection acted on each group separately, so the shape evolved twice. This is convergent evolution and the shapes are analogous structures.
Same selection pressure, two separate lineages, one shared solutionthe word “separately” is usually worth a mark on its own
WE 2
Telling the two apart
State one difference between homologous and analogous structures, and give an example of each. (3 marks)
Point 1: the difference
Homologous structures share the same underlying structure and a common ancestor; analogous structures share a function but have different evolutionary origins.
Point 2: homologous example
The pentadactyl limb of a human, whale and bat.
Point 3: analogous example
The wings of a bat and a butterfly, or the body shape of a shark and a dolphin.
Same bones = homologous. Same job only = analogous.learn at least one analogous example properly; it is specifically asked for
💡 Exam tips
Learn the definition of analogous structures word for word, including different evolutionary origin.
Have at least one analogous example ready — this is specifically expected.
Always name the selection pressure when you explain convergence, e.g. drag in water, water shortage in a desert.
Say the characteristics evolved separately, not from one shared mutation.
If a question gives two look-alike species, check their group first — different class or order is your clue that it is convergent evolution.
Mention that sequence data can distinguish the two cases.
⚠ Common mistakes
Calling a dolphin a fish. It is a mammal — that is the whole point of the example.
Saying the two species were “trying” to become streamlined. Random mutation, then selection, in each line separately.
Using analogous structures as evidence of shared ancestry. They are evidence of shared conditions.
Mixing up the definitions under pressure. Analogous = analogy = alike by coincidence.
Giving only one example when the question asks for one of each. Read the command term.
Up next: Speciation — how one population becomes two species that can never breed together again.
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