🦎 Evolution Explains Biodiversity
Big Idea: Biodiversity exists because species change over time. This long-term change is called evolution—like nature’s way of trying new things!
Evolution happens when some individuals have traits that help them survive and have more babies. Over time, these helpful traits become more common.
Natural Selection: How Evolution Works
Natural selection is like a filter—nature keeps what works best.
Nature doesn’t pick the strongest, just what works best for the environment.
Simple Example: Birds and Beaks
Imagine a group of birds that eat different foods.
- Small beaks are great for picking up tiny seeds
- Big beaks are better for cracking hard nuts
- Birds with the right beak for the food survive more easily
- Over time, the population changes to match the food available
Best fit for the environment = more babies!
The Four Steps of Natural Selection
- Genetic variation – no two are exactly alike (like different hair colors in people)
- Survival advantage – some traits help survival (e.g., faster, better camouflaged)
- Reproduction – survivors have more babies
- Inheritance – helpful traits get passed on
Exam tip: Always mention variation → survival → reproduction → inheritance.
Camouflage Example
If an insect blends in with tree bark, birds can’t see it as easily.
- Harder to spot = less likely to be eaten
- Lives longer
- Has more babies
- Camouflage trait becomes more common
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Changing Environments
Environments don’t stay the same. When things change, species must adapt or they might disappear.
- Food sources can change (e.g., drought kills plants)
- Climate can shift (e.g., gets hotter or colder)
- Adapting takes time
- If change is too fast, some species can’t keep up and may go extinct
If the environment changes faster than species can adapt, they may die out.
Speciation: How New Species Form
Speciation happens when groups get separated and change in different ways.
- A group gets isolated (e.g., on an island)
- Different environments = different adaptations
- Genetic differences build up over time
- Eventually, they can’t breed with the original group
Island Example
If a few birds end up on a new island, they might evolve differently from the original group. Over time, they can become a new species.
Real example: The dodo (Mauritius): Ancestors of the dodo were flying pigeons that reached the island of Mauritius. Once isolated, there were no large predators and food was available on the ground. Over many generations, these birds became larger, lost the ability to fly, and evolved into a new species — the dodo.
Isolation + different conditions + time = new species
Why Evolution Matters for Biodiversity
- Evolution creates new species
- Natural selection helps species adapt
- Speciation increases biodiversity
- More biodiversity = stronger, more resilient ecosystems
For top marks: Link evolution → biodiversity → resilience.
Practice with real exam questions
Answer exam-style questions and get AI feedback that shows you exactly what examiners want to see in a full-marks response.
IB-style question — Mechanism of natural selection [4]
Outline the mechanism by which natural selection could increase drought tolerance in a population of savanna grass over successive generations. [4]
How to answer it, step by step
- Variation + selection pressure
• Individuals vary in drought tolerance (e.g. deeper roots, waxy cuticles) due to genetic mutations.
• During drought, tolerant individuals survive and reproduce; intolerant ones die before reproducing. - Inheritance → allele frequency change
• Tolerant parents pass the advantageous alleles to offspring.
• Over generations, the frequency of the tolerance allele increases in the population.
Final answer
The chain the examiner wants: variation → selection pressure → differential survival → inheritance → allele frequency change. Hit all five links.
IB-style question — Tectonic speciation essay [7]
Explain how the movement of tectonic plates can ultimately result in the formation of entirely new species, covering both the initial geographic separation of populations and the subsequent biological mechanisms that lead to reproductive isolation. [7]
How to answer it, step by step
- Barrier created → isolated populations
• Tectonic movement forms mountains, oceans, or new islands, splitting one population into two isolated sub-populations (allopatric isolation).
• Each sub-population now faces different environments and accumulates different mutations independently. - Divergence → new species
• Natural selection favours different traits in each environment; allele frequencies shift in different directions over many generations.
• Eventually the two populations can no longer interbreed and produce fertile offspring — reproductive isolation is complete and two species now exist.
Final answer
Hit all 7 marks in sequence: (1) barrier, (2) population split, (3) different environments, (4) genetic variation, (5) different selection, (6) adaptations accumulate, (7) reproductive isolation. State the last point explicitly.