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Species and Speciation LectureChapter 22 (Part 2) (pp. 450-459)
Speciation
• How do two populations become reproductively isolated from each other?
Over time :
- Random mutations
genetic divergence
- Eventually many genetic differences accumulate (genetic drift, natural
selection)
• What is speciation? The development of reproductive isolation between species
• It is just a byproduct of the genetic divergence
• Gradual process, development of reproductive isolation between species
• During the process of speciation can interbreeding occur?
• Yes, if populations have not diverged enough genetically
• Partially reproductively isolated populations
Modes of Speciation
Allopatric vs. Sympatric Speciation
Allopatric speciation characteristics:
o Speciation 2 groups of organisms becoming reproductively isolated from each
other
- Rewuires genetic isolation betweeb the diverging populations
- Easiest way for this to happen is via geography
- Allopatric speciation begins with formation of allopatric populations
Geographical separation
Long periods of time genetic divergence
Subspecies
o Populations can become allopatric in 2 ways:
1. Dispersal – individuals colonize a distant place such as an island
2. Vicariance – geographic barrier arises within population
o Regardless of the mode by which populations become allopatric, the end result is
2 separate populations that will diverge genetically until speciation occurs
Page | 1
, Example of speciation in action: Salamander (Ensatina spp.) Speciation in
California Video
o Notes on video:
Allopatric Speciation:
Two ways populations become allopatric:
Dispersal - individuals colonize a distant place such as an island
Vicariance - geographic barrier arises within population
• Dispersal – Peripatric Speciation (a type of allopatric speciation)
• Characterized by:
• Importanr in peripatric speciation
• Dispersal from mainland population to new remote location
- May be accidental or intentional (migration)
2 populations then evolve separately
• Result:
Distant, isolated island population
Island population often small and in an environment that is slightly different from
mainland population
• What is meant by “island population”?
Habitat patch, not necessarily an island
• Why do changes accumulate faster in “island population”?
- Change accumulates faster in island population because…
Genetic drift is more pronounced in smaller population
Island population faces new selective pressures in their new environment
Page | 2
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