Notes From Reading
CHAPTER 26: S PECIATION (PGS.551-570)
- Speciation occurs when populations of the same species become genetically isolated by lack of
gene flow and then diverge from each other due to natural selection, genetic drift, or mutation
- Populations can be recognized as distinct species if they are reproductively isolated from each
other, if they have distinct morphological characteristics, or if they form independent branches
on a phylogenetic tree
- Populations can become genetically isolated from each other if they occupy different geographic
areas, if they use different habitats within the same area, or if one population is polyploidy and
cannot breed with the other
- When populations that have diverged come back into contact, several outcomes are possible
- When gene flow is reduced between populations, they may then diverge genetically as a result
of mutation, natural selection, and genetic drift.
- This genetic divergence may eventually lead to speciation, the creation of new species.
- Usually, speciation creates two or more distinct species from a single ancestral group.
26.1 How Are Species Defined and Identified?
- A species is defined as an evolutionarily independent population or group of populations.
- Gene flow eliminates genetic differences among populations, so evolutionary independence
starts with lack of gene flow.
- If gene flow between populations stops, then mutation, natural selection, and genetic drift
begin to act on populations independently.
- Allele frequencies and other characteristics of these independent populations diverge, and over
time the populations can become distinct species.
- Biologists commonly use the following three approaches to identify species:
o the biological species concept
o the morphospecies concept
o the phylogenetic species concept
The Biological Species Concept
- The biological species concept considers populations to be evolutionarily independent if they
are reproductively isolated from each other, i.e., they do not interbreed.
- Therefore, no gene flow occurs between these populations.
- Biologists categorize the mechanisms that stop gene flow between populations as being either
prezygotic or postzygotic.
- Prezygotic isolation occurs when individuals of different species are prevented from mating.
- Postzygotic isolation occurs when individuals from different populations do mate, but the
hybrid offspring produced have low fitness and do not survive or produce offspring.
- The criterion of reproductive isolation cannot be evaluated in fossils or in species that reproduce
asexually. In addition, it can only be applied to populations that overlap geographically. Notes From Reading
CHAPTER 26: S PECIATION PGS .551-570)
The Morphospecies Concept
- Under the morphospecies concept, biologists identify evolutionarily independent lineages by
differences in size, shape, or other morphological features.
- This concept is based on the idea that distinguishing features are most likely to arise if
populations are independent and isolated from gene flow.
- The morphospecies concept can be widely applied, but the features used to distinguish species
under this concept are rather subjective.
The Phylogenetic Species Concept
- The phylogenetic species concept is based on reconstructing the evolutionary history of
- On phylogenetic trees, an ancestral population plus all of its descendants is called a
monophyletic group or clade.
- Under this concept, a species is defined as the smallest monophyletic group on a tree that
compares populations. On such a phylogenetic tree, each tip is a phylogenetic species.
- The phylogenetic species concept can be applied to any population.
- However, phylogenies are currently available for only a tiny (though growing) subset of
populations on the tree of life.
- In practice, biologists use all three species concepts, the biological, morphospecies, and
Species Definition in Action: Dusky Seaside Sparrow
- Subspecies are populations that live in discrete geographic areas and have their own identifying
traits but are not distinct enough to be considered a separate species.
- Several subspecies of seaside sparrow live along the Atlantic and Gulf Coasts and are physically
isolated from one another; scientists believed that there was little or no gene flow between
- Based on the biological species and the morphospecies concepts, these subspecies were
considered to be separate species (Figure 26.4a).
- Scientists launched a conservation program for one subspecies thought to be nearing extinction,
the dusky seaside sparrow.
- However, phylogenetic analysis of gene sequences from different seaside sparrow populations
showed that only two distinct monophyletic groups of seaside sparrows exist.
- The dusky sparrow was shown to be genetically indistinguishable from the other Atlantic Coast
sparrows and thus did not need to be individually preserved to preserve the genetic diversity of
26.2 Isolation and Divergence in Allopatry
- Genetic isolation happens when populations become physically separated.
- Physical isolation occurs by dispersal or vicariance. Notes From Reading
CHAPTER 26: S PECIATION (PGS.551-570)
- Dispersal occurs when a population moves to a new habitat, colonizes it, and forms a new
- Vicariance occurs when a physical barrier splits a widespread population into subgroups that are
physically isolated from each other.
- Speciation that begins with physical isolation via either dispersal or vicariance is known as
- Populations that live in different areas are said to be in allopatry.
- Biogeography—the study of how species and populations are distributed geographically—can
tell us how colonization and range-splitting events occur.
Dispersal and Colonization Isolate Populations
- Colonization events often cause speciation because the physical separation reduces gene flow,
and genetic drift via the founder effect causes the old and new populations to diverge rapidly.
- In addition, natural selection may cause divergence if the newly colonized environment is
different from the origin