IB Biology HLEvolution & SpeciationPaper 1 & 2~12 min read
Evidence for Evolution
Nobody watched a fish become a mammal. So how do we know evolution happened? Three lines of evidence do most of the work: the sequences inside cells, the animals and plants we have bred ourselves, and the bones sitting in your own arm.
📚 What you need to know
Sequence data can be taken from DNA, RNA or the amino acid sequence of proteins.
Similarities in sequence data between species suggest that all species share a common ancestor.
Comparisons must use the same part of the DNA, and usually a highly conserved region.
The more similar two base sequences are, the more closely related the two species are.
Selective breeding changes heritable characteristics over generations, so it is evolution in action — just carried out by humans.
Homologous structures look and work differently but share the same underlying structure, e.g. the pentadactyl limb.
Homologous structures are explained by adaptive radiation from a shared ancestor, but they do not prove it.
Sequence data
Every cell carries a record of its own history, written in sequences. There are three places to read it.
Source
What is read
Why it is useful
DNA
The base sequence of DNA in the nucleus, mitochondria and chloroplasts
The complete genetic record, including genes that are switched off
RNA
The RNA base sequence, which is the product of transcription
Tells you the DNA sequence of the genes actually being expressed in that cell
Proteins
The amino acid sequence of expressed proteins
Any change in DNA shows up as a change in the protein, so proteins can be compared instead
When the same sequences turn up again and again in species that look nothing like each other, the simplest explanation is that they were all inherited from a shared ancestor.
Why biologists pick conserved regions
You cannot just grab any bit of DNA from two organisms and compare it. The sequences must come from the same part of the DNA, and they are usually taken from regions that are highly conserved — regions that have changed very little over time.
Like has to be compared with like. Comparing two completely different regions tells you nothing useful.
Conserved regions have relatively few differences, so the differences that do exist are easy to spot and count.
Conserved sequences are more likely to exist in a wide range of species, so you can compare distant relatives as well as close ones.
The most conserved sequences are the ones that code for essential proteins, such as haemoglobin or the enzymes involved in respiration. That makes sense: if a protein is vital, almost any change to it is harmful, so those changes get removed by selection instead of building up.
There is a nice logic here worth stating in an answer. A region is conserved because mutations in it are usually fatal. So the few differences you do find between two species are the ones evolution tolerated — which makes them a fair record of time passing.
Comparing DNA sequences
DNA is extracted from cells — blood or skin from living organisms, or from fossilised remains.
It is processed and analysed, and the base sequence is obtained.
That sequence is compared with the same sequence in other organisms to work out how closely they are related.
Data from several different genes is used, rather than one, to increase the level of certainty.
The results are used to build an evolutionary tree.
Counting differences is the whole method. Everything else — trees, timelines, family groups — is built on top of this simple count.
The headline example. In 2005 the chimpanzee genome was sequenced. Compared with the human genome, the two share almost 99% of their DNA sequences, making chimpanzees our closest living relatives.
Selective breeding
Selective breeding is the process in which humans choose organisms with desirable characteristics and breed them together repeatedly, so that those characteristics become stronger over many generations. It is also called artificial selection.
It works because naturally occurring variation already exists. Some plants happen to give a higher yield or resist disease better; some sheep happen to have thicker wool; some cows happen to produce more milk. Humans simply pick those individuals.
Step
What happens
1. Variation
The population contains individuals with different characteristics
2. Selection
Breeders pick the individuals with the desired characteristics
3. Breeding
The two selected individuals are bred together
4. Testing
The offspring grow to maturity and are tested; the best of them are kept for further breeding
5. Repeat
The process is repeated over many generations until all offspring show the characteristic
Compare that with the natural version and the only real difference is who does the choosing. In natural selection, advantageous alleles are passed on because they improve an organism’s chances of survival. In artificial selection, desirable alleles are passed on because humans decide which individuals get to breed.
Selective breeding involves changes to heritable characteristics over many generations, which is the definition of evolution — so it is evolution in action. It also happens faster than natural selection, because only the selected individuals are allowed to breed, while in nature some individuals with less favourable alleles still manage to reproduce.
Why this counts as evidence. Selective breeding shows that small changes to DNA can accumulate into large differences. Cabbage, cauliflower, broccoli, kale, kohlrabi and Brussels sprouts were all bred from one wild brassica plant — if humans can do that in centuries, nature can do far more in millions of years.
Homologous structures
Definition
Homologous structures are body parts that may look and function very differently, but share the same underlying structure
Limbs are the classic case. Animals move in wildly different ways, yet the basic arrangement of bones in their limbs is very similar. Birds, bats, crocodiles, whales, horses and monkeys use their limbs for completely different things, and those limbs look nothing alike — but structurally they match.
The pentadactyl limb
A pentadactyl limb is any limb with five digits, meaning five fingers or toes. Pentadactyl limbs are found in many species across many groups: mammals, birds, amphibians and reptiles.
The human foot evolved for upright walking and running.
Whale flippers let them propel themselves through a marine environment.
Bird wings are highly adapted for flight.
Frog limbs allow them to walk, jump and swim.
Alligator limbs are used for walking and swimming.
The individual bones are very different shapes and sizes because the animals move in different ways — but the layout is almost exactly the same.
If each limb had been built from scratch for its job, there would be no reason for a wing and a flipper to start with the same bone.
One explanation for these surprising similarities is adaptive radiation: the idea that organisms with homologous structures all evolved from a shared common ancestor but adapted to different environments along the way.
Be careful with the word “proof”. Adaptive radiation does not prove that these animals came from a common ancestor — it is the best available explanation for why homologous structures exist. Writing “suggests” or “provides evidence for” rather than “proves” keeps you on the right side of the mark scheme.
Worked examples
WE 1
Why conserved sequences?
Explain why highly conserved regions of DNA are chosen when comparing different species. (3 marks)
Point 1: like with like
The same region must be compared in both species, or the comparison means nothing.
Point 2: few differences
Conserved regions have changed very little over time, so there are few differences and they can be identified easily.
Point 3: wide coverage
Conserved sequences exist in a wide range of species, often because they code for essential proteins such as haemoglobin, so distant species can also be compared.
Same region, few differences, found in many speciesan example of an essential protein is a cheap extra detail that often secures the mark
WE 2
Selective breeding as evidence
Explain how selective breeding provides evidence for evolution. (3 marks)
Point 1: it fits the definition
Selective breeding changes heritable characteristics over many generations, which is exactly what evolution means.
Point 2: the mechanism matches
Desirable alleles become more frequent because only the chosen individuals breed, in the same way advantageous alleles spread in natural selection.
Point 3: the conclusion
It shows that small changes to DNA can accumulate into large differences, e.g. cabbage, broccoli and kale from one wild brassica.
Same process, human hand on the selection, much fastersay why it is faster: only the selected individuals are allowed to breed
WE 3
What limb bones tell you
The forelimbs of a bat and a whale contain the same bones arranged in the same way. Explain what this suggests. (3 marks)
Point 1: name it
These are homologous structures — the pentadactyl limb.
Point 2: the inference
A shared underlying structure suggests both species inherited that limb layout from a common ancestor.
Point 3: the difference explained
The bones differ in size and shape because the two species adapted to different environments and different ways of moving.
Same plan inherited, then reshaped by different selection pressuresdo not write “proves”; homologous structures are strong evidence, not proof
💡 Exam tips
Learn the three sources of sequence data: DNA, RNA and amino acid sequences.
Remember the direction of the rule: more similar sequence, more closely related.
Quote the chimpanzee figure — almost 99% shared DNA, sequenced in 2005.
Mention that several genes are compared to increase certainty.
Learn one homologous example (the pentadactyl limb) and be able to name the animals and their different uses.
Use “suggests” and “provides evidence”, not “proves”.
⚠ Common mistakes
Comparing different genes in the two species. The comparison must be like with like.
Thinking conserved means “important but changing fast”. Conserved means it has barely changed.
Saying selective breeding is not evolution. It changes heritable characteristics over generations, so it is.
Mixing up homologous and analogous. Homologous means same structure and shared ancestor.
Describing what a limb does instead of what it is made of. The evidence is in the bone layout, not the function.
Up next: Convergent Evolution — what happens when two unrelated species end up looking almost identical, and how to tell that apart from shared ancestry.
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