1. To understand how the term "species" is defined in the Modern Synthesis

2. To understand how new species can arise

3. To become familiar with some current debates about the patterns of speciation


What is a species?

The morphological species concept vs. the biological species concept

In the Modern Synthesis, the criteria for deciding whether 2 individuals are in the same or different species must be stated in terms of population genetics. The main question is: is there a potential for gene flow?

What is speciation?

  • cladogenesis
  • anagenesis

How does speciation occur?

Allopatric speciation
One mechanism for the phenomenon of cladogenesis is allopatric speciation:
1. A geographical barrier splits the range of a species into 2, preventing gene flow.
2. The 2 sub-populations diverge genetically. Mechanisms for this include:
  • genetic drift
  • natural selection
  • mutation
3. If contact is then reestablished...
  • if the two groups have not diverged genetically, then they will still be the same species, and merge
  • if there has been adequate time to diverge, then one or more isolating mechanisms, which prevent successful reproduction between the two groups, will have arisen.

Reproductive isolating mechanisms:

  • Habitat isolation
  • Temporal isolation
  • Behavioral isolation
  • Mechanical isolation


  • Gametic isolation


  • Hybrid inviablity
  • Hybrid sterility
  • Hybrid breakdown (or F2 breakdown)

If only post-mating barriers have arisen, pre-mating barriers will be selected for, since those that exhibit them will have higher fitness.

Allopatric speciation is especially common on island chains.

Sympatric speciation

Occurs without geographical barriers. It has not generally been considered to be an important mechanism of cladogenesis, but there are some situations where it must acount for rapid speciation without geographical barriers;
  • Cichlids in Lake Victoria
  • Apple and Hawthorn maggots in N America

Pre-mating barriers must arise first, and be very effective at preventing gene flow if cladogenesis is to take place.

It can also occur in plants via polyploidy. This is the mechanism that has produced many of our grain crops, e.g., modern corn and wheat.

Gradualism vs punctuated equilibrium

There is debate about the tempo and mode of evolutionary change

The fossil record suggests that new species may arise relativly quickly, at least in some cases.

Gradualists say that long-term change occurs gradually, by means of natural selection (adaptive change)

Punctualted equilibrists say that natural selection tends to maintain equilibrium, but that this "stasis" is punctuated with rapid change by means on non-adaptive mechanisms (genetic drift) during speciation.



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