Speciation and the Origin of Reproductive Isolation
How Species Form
Speciation basics
Speciation is the evolution of reproductive isolation between populations. Once gene flow is sufficiently reduced, populations can diverge genetically and ecologically, eventually becoming distinct species.
Types of reproductive isolation
Prezygotic barriers prevent fertilization, while postzygotic barriers reduce the viability or fertility of hybrids. Isolation can arise from geography, ecology, behavior, timing, or genetic incompatibilities.
Common isolating barriers
| Barrier | Category | Example |
|---|---|---|
| Habitat isolation | Prezygotic | Populations use different host plants |
| Temporal isolation | Prezygotic | Different breeding seasons |
| Behavioral isolation | Prezygotic | Different mating signals |
| Hybrid sterility | Postzygotic | Sterile hybrid offspring |
Important distinction
Reproductive isolation can evolve before full geographic separation ends, and geographic separation is not required for all speciation.
Which barrier prevents fertilization before a zygote forms?
Temporal isolation is a prezygotic barrier that prevents mating or gamete encounter.
Correct answer: Temporal isolation
Why does reduced gene flow promote speciation?
Isolation allows lineages to follow separate evolutionary trajectories.
Correct answer: Because divergent selection, drift, and mutation can cause populations to accumulate differences without being homogenized by interbreeding.
Modes of speciation
Allopatric speciation
- Occurs after geographic separation
- Commonly involves divergence in isolation
Sympatric speciation
- Occurs without physical separation
- Often requires strong disruptive selection or niche separation
A hybrid that survives but is sterile represents which kind of barrier?
Hybrid sterility is a postzygotic reproductive barrier.
Correct answer: Postzygotic barrier
Reasoning through a speciation case
-
1
Ask whether populations exchange genes freely.
-
2
Identify barriers to mating, fertilization, or hybrid success.
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3
Determine whether geographic separation is present.
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4
Consider how selection and drift could accumulate divergence.