Ch 3 · 17.3 Evolution · AS & A Level Biology 9700
Topic 3 of 3 in Selection and evolution — AS & A Level Biology 9700 — 58 lessons.
Introduction
Evolution explains how the huge variety of species alive today, and in the fossil record, arose from earlier species. This lesson outlines the theory, shows how DNA sequences reveal how closely species are related, and explains how one species can split into two. (Syllabus section 17.3.)
Key ideas — The theory of evolution
A gene pool is all the alleles of all the genes in a population. The theory of evolution states that new species form from pre-existing species over time, as a result of changes to gene pools from generation to generation:
🧑🏫 Teacher's explanation
Imagine a massive library containing every single book (gene) and every single word (allele) available to a specific community. This entire collection is the 'gene pool'. Evolution is essentially the story of how this library changes over hundreds of years. Sometimes, a typo occurs in a book (mutation), creating a new word. Sometimes, certain books become more popular and are copied more often (natural selection), while other books are lost by accident (genetic drift).
📖 Definitions
🔍 Worked example — Easy
A population of beetles has a gene pool where the allele for green color (A) has a frequency of 0.6 and the allele for brown color (a) has a frequency of 0.4. Calculate the percentage change in the frequency of allele a if it increases to 0.5 in the next generation.
🔍 Worked example — Standard
A scientist compares the DNA sequence of a specific protein in two species. Species X has 10 differences in a sequence of 100 base pairs compared to Species Y. Species Z has 40 differences in the same 100 base pair sequence compared to Species Y. Determine which species (X or Z) is more closely related to Species Y and explain why using the concept of common ancestry.
🔍 Worked example — Exam-style
Explain how a physical barrier, such as a rising sea level, leads to the formation of two new species from an original population of terrestrial lizards.
⚠️ Common errors
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🎯 Exam question
(a) Name the type of speciation that occurs in this scenario. [1] (b) Explain how the two populations may eventually become two different species. [3] [4 marks]
Key ideas — Evidence from DNA sequences
Mutations build up in DNA at a roughly steady rate over time. So when two species split from a common ancestor, their DNA sequences start to become different, and the longer ago they split, the more differences there are.
🧑🏫 Teacher's explanation
Imagine you are comparing two editions of a classic novel. If you compare the 1st edition to the 2nd edition, they might only have a few typos or changed words. If you compare the 1st edition to the 50th edition, there will be massive differences. In biology, DNA is the 'text' of life. Every time a cell divides, tiny errors (mutations) occur. Over millions of years, these errors accumulate.
📖 Definitions
🔍 Worked example — Easy
A scientist compares the amino acid sequence of the cytochrome c protein in three species. Species A and B have 2 differences in the sequence, while Species A and C have 15 differences. Determine which species are more closely related to Species A.
🔍 Worked example — Standard
A segment of mitochondrial DNA is 1000 bases long. Species X and Y show 40 differences in this sequence. If the mutation rate is 1 mutation per 10,000 bases per million years, calculate the estimated time since divergence.
🔍 Worked example — Exam-style
Explain why mitochondrial DNA is more useful than nuclear DNA for studying recent evolutionary divergences.
⚠️ Common errors
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🎯 Exam question
\nSequence X: A-G-T-C-C-G-A-T-T-A Sequence Y: A-G-T-C-C-G-A-T-G-A Sequence Z: A-G-T-C-C-A-A-T-T-T
Key ideas — Speciation and isolation
Speciation is the formation of a new species. It requires genetic (reproductive) isolation: two populations stop exchanging alleles, so their gene pools change independently (different mutations, different selection pressures, drift) until they can no longer interbreed to produce fertile offspring.
🧑🏫 Teacher's explanation
Imagine a large group of people who all speak the same language and live in the same city. If a massive mountain range suddenly rises between two halves of this group, they can no longer visit each other. Over hundreds of years, the group on the left might start using different slang, and the group on the right might adopt different customs. Eventually, they speak such different languages that they can no longer communicate or understand one another. In biology, this is how one species becomes two.
📖 Definitions
🔍 Worked example — Easy
A scientist compares the DNA sequence of a specific protein in two species. Species A has 10 differences in a sequence of 100 nucleotides compared to Species B. Species C has 40 differences in the same 100-nucleotide sequence compared to Species B. Which species is more closely related to Species B? Justify your answer.
🔍 Worked example — Standard
A population of birds is split by a new canyon (geographical isolation). Population 1 lives in a forest and Population 2 lives in an open grassland. Explain how this leads to speciation.
🔍 Worked example — Exam-style
Calculate the percentage difference in a DNA sequence between two species where Species X has 125 base pairs and Species Y has 125 base pairs, but they differ at 5 specific positions.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Name the type of speciation that occurs in this scenario. [1] (b) Explain how the accumulation of genetic differences leads to the formation of two distinct species. [3] [4 marks]
Key ideas — Allopatric speciation
In allopatric speciation, the populations are separated by a geographical barrier, such as a sea, a mountain range, a river, or a desert. Individuals on the two sides cannot meet, so there is no gene flow between them. The two environments differ, so different alleles are selected on each side, and mutations and drift add further differences. Eventually, even if the barrier is removed, the two populations can no longer interbreed successfully: they are separate species. Example: the different finch species on the Galápagos islands.
🧑🏫 Teacher's explanation
Imagine a large group of friends who all love playing the same game. One day, a massive canyon opens up right in the middle of their playground. Now, the group is split into two. The group on the left side starts playing a different game because the ground is sandy, while the group on the right starts playing a different game because it is rocky. Because they can't cross the canyon to play together, they stop sharing new rules or strategies. Over many years, the two groups become so different in how they play that even if the canyon filled in, they wouldn't recognize each other's games anymore.
📖 Definitions
🔍 Worked example — Easy
A population of beetles is split by a new river. Population A has a mean body length of 5.0 mm and Population B has a mean body length of 5.2 mm. Calculate the percentage increase in body length from Population A to Population B.
🔍 Worked example — Standard
Explain why a physical barrier leads to speciation.
🔍 Worked example — Exam-style
Two species of birds have DNA sequences for a specific protein. Species A has 95% identical base sequences to Species B, while Species C has only 80% identical base sequences to Species B. Interpret this data to determine the evolutionary relationship.
⚠️ Common errors
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🎯 Exam question
Key ideas — Sympatric speciation
In sympatric speciation, new species form in the same area, without a geographical barrier:
🧑🏫 Teacher's explanation
Imagine a large, crowded food court in a shopping mall. Even though everyone is in the same room (the same area), one group of people only eats at the pizza stall, while another group only eats at the sushi stall. Because they are always in different spots and eating at different times, they stop interacting. Over time, these two groups might develop different habits, different ways of talking, and eventually, they become two completely different social clubs that never mix.
📖 Definitions
🔍 Worked example — Easy
A population of insects lives in a single forest. Half the population feeds on oak leaves, while the other half feeds on maple leaves. Explain how this could lead to sympatric speciation.
🔍 Worked example — Standard
Two populations of birds live in the same woodland. Population A sings a high-pitched song during early spring, while Population B sings a low-pitched song during late spring. Describe the mechanism of speciation occurring here.
🔍 Worked example — Exam-style
A researcher compares the DNA sequences of two closely related species of fruit flies living in the same orchard. The DNA sequence of a specific protein differs by 2\% between the two species. How does this data support the theory of evolution?
⚠️ Common errors
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🎯 Exam question
Worked example
Question. The table shows the number of amino acid differences in the cytochrome c protein between humans and three other species: chimpanzee 0, rhesus monkey 1, horse 12. (a) State which species is most closely related to humans, and explain your answer. [2] (b) Suggest why comparing DNA base sequences may give more information than comparing amino acid sequences. [2]
🔍 Worked example — Easy
A scientist compares the cytochrome c protein sequence between two species. Species A and Species B have 5 amino acid differences. Species A and Species C have 25 amino acid differences. Which species (B or C) is more closely related to Species A? Explain your reasoning.
🔍 Worked example — Standard
A population of birds is divided by the formation of a new mountain range. Over time, the two groups develop different mating songs and different beak shapes. Explain why this leads to speciation.
🔍 Worked example — Exam-style
Calculate the percentage similarity between two DNA sequences where 12 out of 100 bases are different.
⚠️ Common errors
Common mistakes
Practice
Answers