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1.6.1Inquiry Process

Biological Species Concept — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 15 of 160

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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Defining a Species

The concept judges whether two groups are separate species by whether their genes can mix successfully.

Species
  • Species A group of organisms with similar characteristics that can interbreed and produce fertile offspring.
Variation
  • Variation Individuals within a species differ in appearance, behaviour and genes, but remain able to interbreed.
Fertile offspring
  • Fertile offspring Hybrids that cannot make functional gametes, such as the mule, show that the parents are separate species.
Gene pool
  • Gene pool All the alleles of a population; a species shares one gene pool through interbreeding.

Part 2 of 5Keeping Species Separate

Keeping Species Separate
Geographical barriers
  • Geographical barriers Mountains, rivers or oceans prevent groups from meeting, so their genes cannot be exchanged.
Behavioural isolation
  • Behavioural isolation Different courtship songs, displays or pheromones mean individuals do not recognise each other as mates.
Temporal isolation
  • Temporal isolation Species that breed or flower at different seasons or times of day do not meet to mate.
Hybrid sterility
  • Hybrid sterility If mating does produce offspring but they are sterile, genes still cannot flow between the parent species.

Part 3 of 5Applying the Concept

1
Observe whether individuals from two groups can mate under natural conditions.
2
Check that any offspring are produced and survive to adulthood.
3
Test whether the offspring are themselves fertile and can produce a second generation.
4
If they are fertile, the groups belong to the same species; if sterile, they are separate species.
5
Where mating cannot be observed, other evidence such as DNA similarity is used.
CrossOffspringFertile?
Horse × horseFoalYes
Horse × donkeyMuleNo
Lion × tigerLigerAlmost always no

Part 4 of 5Worked Example

Question

  • A male lion and a female tiger can produce a hybrid offspring called a liger, which is almost always sterile.
  • Use the biological species concept to decide whether lions and tigers are the same species.

  1. The biological species concept defines a species as a group of organisms that can interbreed and produce fertile offspring.
  2. Lions and tigers can mate in captivity and a liger is born, so interbreeding is possible.
  3. However, the offspring are sterile, so the second requirement of fertile offspring is not met.
  4. In the wild the two species also live in different regions, so they do not meet.

Answer Lions and tigers are separate species because they cannot produce fertile offspring together.

Part 5 of 5Exam Checklist & Tips

1.6.1 – Key Points for the Exam

  • Species A group of organisms that interbreed to produce fertile offspring.
  • Hybrid The offspring of parents of different species.
  • Reproductive isolation Barriers preventing gene flow between populations.
  • Gene pool All alleles in an interbreeding population.

Common Mistakes

  • Defining a species only as organisms that can interbreed, forgetting that offspring must be fertile.
  • Thinking that all variation within a group means separate species.
  • Using appearance alone; very different-looking dogs are one species.
  • Saying the mule is a species; it is a sterile hybrid.

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