Nobody watched a fish turn into a frog, so how do we know evolution happened? The honest answer is that we do not rely on one killer piece of proof. We rely on several completely separate kinds of evidence — DNA, bones, and breeding experiments we ran ourselves — that all point to the same story.
📚 What you need to know
Sequence data from DNA, RNA and proteins can be compared between species. More similarity means a more recent common ancestor.
Comparisons use highly conserved sequences — regions that have barely changed — so you are comparing like with like.
Selective breeding shows heritable characteristics changing over generations. It is evolution happening fast enough to watch.
Homologous structures look and work differently but share the same underlying layout, which points to a shared ancestor.
The pentadactyl limb is the standard example: five digits, same bone plan, completely different jobs.
Adaptive radiation is the idea that one ancestor’s descendants spread into different environments and were shaped differently by them.
Evidence 1: sequence data
Every living thing stores its instructions in the same four-letter DNA code. That is already a hint. But the real evidence comes from comparing the actual order of those letters between species.
You can compare three kinds of sequence, and they all tell you the same thing:
DNA base sequence — from the nucleus, and also from mitochondria and chloroplasts.
RNA base sequence — mRNA is a copy of a gene, so it tells you which genes a cell is actually using.
Amino acid sequence — proteins are built from the DNA, so differences in DNA show up as differences in proteins.
The rule everything rests on
More similar sequences → fewer changes have built up → a more recent common ancestor
Why we use “conserved” sequences
You cannot just grab any bit of DNA from two species and compare it. Scientists deliberately pick highly conserved regions: stretches of DNA that have changed very little over enormous spans of time, usually because they code for something essential like haemoglobin or a respiratory enzyme. Change those and the organism does not survive to reproduce.
🤔 Why conserved regions and not any old gene?
Three reasons, and examiners like all three. First, you must compare like with like — lining up two unrelated stretches of DNA tells you nothing. Second, because conserved regions change slowly, there are few differences, so each one stands out and can be counted. Third, conserved sequences are found in a wide range of species, so the same gene can be used to compare a bacterium, a mushroom and a whale.
Read the tree from left to right as time passing. The point where two lines meet is the last ancestor those species shared. Notice B and C differ the most, which fits: they have both been changing since the earliest split.
A real number worth quoting: when the chimpanzee genome was sequenced in 2005 and lined up against ours, roughly 99% of the sequence matched. That is the strongest single reason chimpanzees are called our closest living relatives.
Biologists never rely on a single gene. They compare several different genes from the same pair of species, because one odd gene could mislead you. Agreement across many genes is what makes the conclusion trustworthy.
Evidence 2: selective breeding
This one is powerful because humans did it on purpose and wrote it down. In selective breeding (also called artificial selection) we pick the individuals with the characteristics we want and breed them together, over and over.
🧩 The process, in five steps
Variation exists. The starting population already contains individuals with different characteristics.
Select. The breeder picks the individuals showing the desired characteristic most strongly.
Breed. Those chosen individuals are bred together.
Test the offspring. When the offspring mature, check which ones show the characteristic best.
Repeat over many generations until the whole population shows it.
Look at that list again and compare it with natural selection. The only thing that changed is who does the choosing. In nature the environment decides who breeds; in selective breeding a farmer decides.
NATURAL SELECTION
The environment selects
Alleles that aid survival spread
Slower — less-suited individuals still breed sometimes
No goal or plan
SELECTIVE BREEDING
Humans select
Alleles humans find useful spread
Faster — only chosen individuals are allowed to breed
A clear goal, e.g. more milk
Why does this count as evidence? Because it proves the mechanism works. A single wild cabbage-like plant has been bred into cabbage, cauliflower, broccoli, kale, kohlrabi and Brussels sprouts — all still the same species, all looking wildly different, and all produced by selecting on natural variation. If small heritable changes can pile up that far in a few centuries, it is not a stretch to accept what millions of years could do.
Evidence 3: homologous structures
Homologous structures are body parts that may look completely different and do completely different jobs, but are built to the same underlying plan.
Definition
Homologous structures = same basic structure, different function, inherited from a common ancestor
The classic case is the pentadactyl limb — any limb with five digits. Look at a human arm, a whale flipper, a bat wing and a bird wing. They are used for completely different things, yet the bones inside are laid out in the same order every time: one long upper bone, then two bones side by side, then a cluster of small wrist bones, then digits.
If each of these limbs had been designed from scratch for its job, there is no reason a whale flipper would need finger bones inside it. The layout only makes sense as something inherited and then reshaped.
Adaptive radiation
The usual explanation for homologous structures is adaptive radiation: one ancestral species spreads out into different habitats, and natural selection reshapes the same body parts for different lives. Wrist bones become part of a paddle in the sea, and finger bones stretch into a wing frame in the air.
Be precise: adaptive radiation does not prove a common ancestor. It is the best available explanation for a pattern we can see. IB likes you to be careful with words like “prove”.
Worked examples
WORKED EXAMPLE
Three species were compared for the same gene. A and B differ at 2 bases, A and C at 9, and B and C at 10. Which two species are most closely related, and explain your reasoning. [3]
Step 1: find the smallest number of differencesA vs B = 2, which is the lowest of 2, 9 and 10Species A and B are the most closely relatedStep 2: explain what the number meansBase differences build up by mutation after two species separate. Fewer differences means less time has passed since they shared an ancestor.Step 3: place CC differs a lot from both, so it branched off earlier and is the more distant relative.
WORKED EXAMPLE
Explain why scientists compare highly conserved DNA sequences rather than randomly chosen ones. [3]
Point 1Conserved sequences are present in a wide range of species, so the same region can be found in all of the organisms being compared.Point 2You must compare the same region in each species. Comparing unrelated stretches of DNA gives meaningless results.Point 3Conserved regions change slowly, so there are few differences and each one can be identified and counted clearly.Same region, present everywhere, few enough differences to count
WORKED EXAMPLE
Explain how selective breeding provides evidence for evolution. [3]
Step 1: show it fits the definitionSelective breeding changes heritable characteristics of a population over many generations, which is exactly what evolution means.Step 2: name the mechanismIt works on naturally occurring variation, and desirable alleles become more frequent because only chosen individuals breed.Step 3: draw the conclusionIt shows small changes in DNA can accumulate into large differences, and we can watch it happenAdd the cabbage example if the question says “using an example”.
💡 Exam tip
For “most closely related” questions, always pick the pair with the fewest differences or the highest percentage similarity, then say why.
Say “more recent common ancestor”, not just “more related”. That phrase is usually the marking point.
Give three separate reasons for using conserved sequences: same region, few differences, wide range of species.
When asked to define homologous structures, include all three parts: same structure, different function, common ancestor.
The pentadactyl limb is the safest example to memorise. Name two animals and what their limbs are used for.
Avoid the word “prove”. Evidence supports or suggests a common ancestor.
⚠ Common mix-up
Homologous is not the same as “looks similar”. Homologous means the same underlying structure, even when the outside looks nothing alike.
Mixing up homologous and analogous. Homologous = shared ancestor. Analogous = similar job, no shared ancestor. That comes next page.
Saying pentadactyl means “five fingers on a hand”. It means five digits on any limb — the bones may be fused or reduced.
Forgetting that selective breeding is still evolution. Heritable characteristics change over generations, so it qualifies.
Reading a similarity table backwards. High percentage similarity means closely related; a high number of differences means distantly related.
Claiming DNA evidence proves evolution. It supports it very strongly. Keep the wording scientific.
Up next: Convergent Evolution — what happens when two unrelated species end up looking almost identical, and how DNA lets us catch the coincidence.
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