🦎 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
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 exists
• Within the grass population, individuals vary in their ability to tolerate dry conditions — this genetic variation arose from random mutations in previous generations. - Selection pressure acts
• During a prolonged drought, individuals with deeper root systems or waxy cuticles (drought-tolerant traits) survive and reproduce at higher rates.
• Individuals lacking these traits die before reproducing — this is differential survival and reproduction (natural selection). - Inheritance
• Drought-tolerant individuals pass their advantageous alleles to offspring.
• Over many generations, the frequency of the drought-tolerance allele increases in the population.
Final answer
The chain examiner wants: variation → selection pressure → differential survival/reproduction → inheritance → allele frequency change. Award 4 from these points. Named examples (root depth, waxy cuticle) earn credit as illustration of the mechanism.
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
- Tectonic mechanism creates a barrier
• Tectonic plate movement causes continental separation, mountain building, or the opening of new ocean basins.
• These physical features form geographic barriers that divide a previously continuous population into two or more isolated sub-populations (allopatric isolation).
• Volcanic activity can also create new islands, colonised by a subset of an existing population. - Isolated populations face different environments
• The separated sub-populations experience different climates, food resources, and predators on each side of the barrier.
• Each group carries its own subset of the original genetic variation (and accumulates new mutations independently). - Natural selection drives divergence
• In each isolated environment, individuals with locally advantageous traits survive and reproduce more successfully.
• Allele frequencies shift in different directions in each population — different adaptations accumulate over many generations. - Reproductive isolation → new species
• After sufficient divergence, the two populations can no longer interbreed and produce fertile offspring — they are now reproductively isolated.
• By definition, they are now distinct species: speciation via allopatry has occurred.
• Over geological time, repeated tectonic events produce many such splits, increasing global species diversity.
Final answer
Examiners use a 7-point credit list: (1) barrier created by tectonics, (2) population split, (3) sub-populations in different environments, (4) genetic variation exists, (5) natural selection favours different traits, (6) adaptations accumulate over generations, (7) reproductive isolation → new species. Hit all 7 in sequence for full marks. Phrase the final sentence explicitly — 'they can no longer interbreed and produce fertile offspring' — to secure the speciation mark.
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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.