Biology: A Global Approach
Chapter 24 Species and Speciation 2
Concept 24.1 The biological species concept emphasizes reproductive isolation 2
Concept 24.2 Speciation can take place with or without geographic separation 3
Concept 24.3 Hybrid zones reveal factors that cause reproductive isolation 4
Concept 24.4 Speciation can occur rapidly or slowly and can result from changes in few
or many genes 5
1
, Chapter 24 Species and Speciation
Speciation is the process by which one species splits into two species. Speciation forms a
conceptual bridge between microevolution changes over time in allele frequencies in a
population, and macroevolution, the broad pattern of evolution above the species level.
Concept 24.1 The biological species concept emphasizes reproductive isolation
The Biological Species Concept
The primary definition of species used in this textbook is the biological species concept.
According to this concept, a species is a group of populations whose members have the
potential to interbreed in nature and produce viable, fertile offspring, but do not produce
viable, fertile offspring with members of other such groups. Thus, the members of a
biological species are united by being reproductively compatible, at least potentially.
Typically, gene flow occurs between the different populations of a species. This ongoing
allele tends to hold the populations together genetically.
Because biological species are defined in terms of reproductive compatibility, the formation
of a new species hinges on reproductive isolation, the existence of biological factors that
impede members of two species from interbreeding and producing viable, fertile offspring.
Such barriers block gene flow between the species and limit the formation of hybrids,
offspring that result from an interspecific mating. Although a single barrier may not prevent
all gene flow, a combination of several barriers can effectively isolate a species’ gene pool.
Prezygotic barriers block fertilization from occurring. Such barriers typically act in one of
three ways: by impeding members of different species from attempting to mate, by
preventing an attempted mating from being completed successfully, or by hindering
fertilization if mating is completed successfully. If a sperm cell from one species overcomes
prezygotic barriers and fertilizes an ovum from another species, a variety of portzygotic
barriers may contribute to reproductive isolation after the hybrid zygote is formed.
Developmental errors may reduce survival among hybrid embryos.
One strength of the biological species concept is that it directs our attention to a way by
which speciation can occur: by the evolution of reproductive isolation. However, the number
of species to which this concept can be usefully applied is limited.
Other Definitions of Species
While the biological species concept emphasizes the separateness of different species due
to reproductive barriers, several other definitions emphasize the unity within a species. The
morphological species concept distinguishes a species by body shape and other structural
features. The morphological species concept can be applied to asexual and sexual
organisms, and it can be useful ever without information on the extent of gene flow. A
disadvantage of this approach, however, is that it relies on subjective criteria.
The ecological species concept defines a species in terms of its ecological niche, the sum of
how members of the species interact with the nonliving and living parts of their environment.
Unlike the biological species concept, the ecological species concept can accommodate
asexual as well as sexual species. It also emphasizes the role of disruptive natural selection
as organisms adapt to different environments.
2
Chapter 24 Species and Speciation 2
Concept 24.1 The biological species concept emphasizes reproductive isolation 2
Concept 24.2 Speciation can take place with or without geographic separation 3
Concept 24.3 Hybrid zones reveal factors that cause reproductive isolation 4
Concept 24.4 Speciation can occur rapidly or slowly and can result from changes in few
or many genes 5
1
, Chapter 24 Species and Speciation
Speciation is the process by which one species splits into two species. Speciation forms a
conceptual bridge between microevolution changes over time in allele frequencies in a
population, and macroevolution, the broad pattern of evolution above the species level.
Concept 24.1 The biological species concept emphasizes reproductive isolation
The Biological Species Concept
The primary definition of species used in this textbook is the biological species concept.
According to this concept, a species is a group of populations whose members have the
potential to interbreed in nature and produce viable, fertile offspring, but do not produce
viable, fertile offspring with members of other such groups. Thus, the members of a
biological species are united by being reproductively compatible, at least potentially.
Typically, gene flow occurs between the different populations of a species. This ongoing
allele tends to hold the populations together genetically.
Because biological species are defined in terms of reproductive compatibility, the formation
of a new species hinges on reproductive isolation, the existence of biological factors that
impede members of two species from interbreeding and producing viable, fertile offspring.
Such barriers block gene flow between the species and limit the formation of hybrids,
offspring that result from an interspecific mating. Although a single barrier may not prevent
all gene flow, a combination of several barriers can effectively isolate a species’ gene pool.
Prezygotic barriers block fertilization from occurring. Such barriers typically act in one of
three ways: by impeding members of different species from attempting to mate, by
preventing an attempted mating from being completed successfully, or by hindering
fertilization if mating is completed successfully. If a sperm cell from one species overcomes
prezygotic barriers and fertilizes an ovum from another species, a variety of portzygotic
barriers may contribute to reproductive isolation after the hybrid zygote is formed.
Developmental errors may reduce survival among hybrid embryos.
One strength of the biological species concept is that it directs our attention to a way by
which speciation can occur: by the evolution of reproductive isolation. However, the number
of species to which this concept can be usefully applied is limited.
Other Definitions of Species
While the biological species concept emphasizes the separateness of different species due
to reproductive barriers, several other definitions emphasize the unity within a species. The
morphological species concept distinguishes a species by body shape and other structural
features. The morphological species concept can be applied to asexual and sexual
organisms, and it can be useful ever without information on the extent of gene flow. A
disadvantage of this approach, however, is that it relies on subjective criteria.
The ecological species concept defines a species in terms of its ecological niche, the sum of
how members of the species interact with the nonliving and living parts of their environment.
Unlike the biological species concept, the ecological species concept can accommodate
asexual as well as sexual species. It also emphasizes the role of disruptive natural selection
as organisms adapt to different environments.
2
