Adaptation and speciation
IB Biology SLΒ· Theme D: Continuity and Change, Unit 4Β· 15 min read
1. Adaptations and Fitnessβ β ββββ± 4 min
Adaptation
A heritable characteristic that increases an organism's ability to survive and reproduce in its environment, compared to individuals without the trait.
Example:
Thick insulating fur in Arctic foxes that reduces heat loss in cold tundra climates.
Adaptations fall into three main categories: structural (physical features), physiological (functional metabolic traits), and behavioral (actions that improve survival/reproduction). All adaptations act to increase an organism's fitness, a measure of relative reproductive success compared to other members of the population.
Classify the following adaptation as structural, physiological, or behavioral: migrating songbirds moving south for the winter to access more food.
- 1
- Recall the three categories of adaptation:
- 2
- Structural: physical morphological features; Physiological: functional metabolic traits; Behavioral: actions that improve survival
- 3
- Migrating is an action performed by the bird to improve survival and reproduction in changing seasonal climates.
- 4
- Therefore, this adaptation is classified as behavioral.
Exam tip:
IB exams often ask for one example of each type of adaptation, so memorize a clear example for each category.
2. Core Species Conceptsβ β β βββ± 4 min
Species
No single universal definition exists, but the most widely used definition in IB Biology describes species as groups of actually or potentially interbreeding populations that produce fertile offspring, reproductively isolated from other groups.
Biologists use multiple concepts to define species because no single approach works for all organisms (asexual organisms, fossils, and geographically isolated populations do not fit the biological species concept well). The table below summarizes the three key concepts for SL:
Concept | Core Definition | Limitations |
|---|---|---|
Biological | Groups that interbreed to produce fertile offspring, reproductively isolated | Does not work for asexual organisms or fossils |
Ecological | Species defined by their unique niche and adaptations to the environment | Niche boundaries are often ambiguous and subjective |
Phylogenetic | Smallest group sharing a common ancestor, defined by genetic similarity | Can lead to unnecessary splitting of established species |
Why is the biological species concept not useful for classifying bacteria?
- 1
- Recall the biological species concept relies on the ability to interbreed and produce fertile offspring.
- 2
- Bacteria reproduce asexually via binary fission, so they do not interbreed as defined by the concept.
- 3
- Therefore the concept cannot be applied to bacteria, and alternative concepts (like phylogenetic) are used instead.
Exam tip:
Comparison of species concepts is a common 5-mark extended response question, so prepare at least one pro and one con for each concept.
3. Allopatric Speciationβ β β βββ± 3 min
Allopatric Speciation
Speciation that occurs when populations of the same species become geographically isolated, stopping gene flow and leading to genetic divergence over time.
Geographic isolation forms when a physical barrier (mountain range, river, canyon, ocean) splits a single population into two separate groups that cannot cross to interbreed. Different selection pressures act on each isolated population, leading to genetic differences accumulating over generations. Eventually, the populations become so different that even if the barrier is removed, they can no longer interbreed to produce fertile offspring: speciation is complete.
Outline how allopatric speciation occurred in squirrels separated by the Grand Canyon.
- 1
- A single population of squirrels lived across continuous connected land before the canyon formed.
- 2
- The canyon formed as a physical barrier, splitting the population and stopping all gene flow between the two groups.
- 3
- Different food sources and predators on each side of the canyon created different selection pressures.
- 4
- Over thousands of generations, genetic differences accumulated via natural selection and genetic drift.
- 5
- Eventually the two groups could no longer interbreed, resulting in two distinct separate species.
4. Sympatric Speciationβ β β β ββ± 4 min
Sympatric Speciation
Speciation that occurs within the same geographic area, with no physical barrier to gene flow, due to the emergence of reproductive isolation from other factors.
Common causes of sympatric speciation include polyploidy, temporal isolation (different breeding seasons), and behavioral isolation (different mating rituals). Polyploidy (having more than two full sets of chromosomes) is the most common example in plants: an error in meiosis produces unreduced gametes with extra chromosomes, which can lead to a new reproductively isolated species in a single generation.
Explain how polyploidy leads to instant sympatric speciation in flowering plants.
- 1
- A diploid parent plant (2n) produces an unreduced gamete with 2n chromosomes (instead of n).
- 2
- Two unreduced gametes fuse to form a tetraploid offspring (4n) that lives in the same area as the parent population.
- 3
- The tetraploid cannot interbreed with the original diploid parent: any cross produces sterile triploid (3n) offspring that cannot produce viable gametes.
- 4
- The tetraploid can self-pollinate or breed with other tetraploids, so it forms a new reproductively isolated species instantly.
Exam tip:
Always mention that reproductive isolation is the final required step for any speciation event, whether allopatric or sympatric.
5. Common Pitfalls
Wrong move:
Claiming that individuals adapt during their lifetime to become better suited to the environment
Why:
This is incorrect Lamarckian evolution. Adaptation occurs at the population level over generations, not in individuals.
Correct move:
State that genetic variation exists in populations, and individuals with advantageous adaptations are more likely to survive and reproduce, passing the advantageous allele to the next generation.
Wrong move:
Confusing the root words for allopatric and sympatric speciation
Why:
Students often mix up the definitions because they assume 'sym-' means same species, not same location.
Correct move:
Remember: allo- = different (different geographic location), sym- = same (same geographic location).
Wrong move:
Stating that any polyploid organism is automatically a new species
Why:
Speciation requires permanent reproductive isolation from the parent population. Some polyploids can still interbreed with parent species.
Correct move:
State that polyploidy leads to speciation when the new polyploid is reproductively isolated from the original parent population.
Wrong move:
Claiming the biological species concept is the only correct definition of a species
Why:
IB exams expect recognition that multiple concepts are used, depending on the organism and context.
Correct move:
Acknowledge the limitations of the biological species concept and compare it to other concepts when asked.
6. Quick Reference Cheatsheet
Term | Key Feature |
|---|---|
Adaptation | Heritable trait increasing fitness |
Biological Species Concept | Interbreeding β fertile offspring, reproductively isolated |
Allopatric Speciation | Geographic barrier stops gene flow |
Sympatric Speciation | Speciation in same geographic area |
Polyploidy | Extra chromosome sets, instant plant speciation |
Reproductive Isolation | Required for all speciation |
7. Frequently Asked
What is the difference between adaptation and acclimation?
Adaptation is a heritable genetic change in a population over multiple generations that increases fitness. Acclimation is a reversible, non-heritable physiological change in an individual organism in response to environmental change, for example increased red blood cell production in humans at high altitude.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 Β· Paper 1
Adaptation vs acclimation distinction
- 2024 Β· Paper 2
Allopatric speciation example
- 2023 Β· Paper 1
Species concept comparison
- 2022 Β· Paper 2
Polyploidy in sympatric speciation
Going deeper
What's Next
Adaptation and speciation are core evolutionary concepts that connect to broader IB SL Biology topics including biodiversity, classification, and the impact of human activity on evolution. Understanding how new species form explains the origin of Earth's biodiversity, and informs modern conservation efforts to protect endangered species and manage invasive populations. The concepts you learned here build a foundation for understanding patterns of evolutionary change across geological time, and the genetic basis of variation that drives selection and speciation.