Chapter 26: Speciation
If mutation, natural selection and genetic drift cause isolated population to diverge
sufficiently, distinct types of species, form. This process is called speciation.
Speciation is the evolutionary process in which a new biological species arise.
o Once speciation is complete, a new branch is added to the tree of life.
o Begins when gene flow between populations is reduced or eliminated.
Speciation results from genetic isolation and genetic divergence.
Genetic isolation results from lack of gene flow.
Divergence occurs because selection, genetic drift and mutation process
independently in the isolated population.
26.1: How Are Species Defined and Identified?
Species are distinct types of organisms and represent evolutionarily independent
population or group of populations.
Species are distinct from one another in terms of:
o Habitat use
o Other traits
Species differ in genetic characteristics. Genetic distinction is due to mutation,
natural selection and genetic drift.
What makes one species “evolutionarily independent” of other species?
o Answer: Lack of gene flow
If gene flow between populations stops, then mutation, natural selection, and drift
begins to act on the populations independently.
If a new mutation creates an allele that changes the phenotype of individuals in
one population, there is no longer a way for that allele to appear in the other
When allele frequencies change sufficiently over time, populations become
3 criteria in identifying species are:
1. Biological species concept
2. The morphospecies concept
3. The phylogenetic species concept
The Biological Species Concept (pg. 552)
According to biological species concept, to identify a species, you use the
o No gene flow occurs in a population with reproductive isolation.
If two different populations do not interbreed or produce a viable offspring then
they are considered distinct species.
To organize specific populations, biologist distinguish: REFER TO FIGURE
1. Prezyogtic means before-zygote
2. Isolation which prevents individuals of different species from mating
3. Postzygotic means after-zygote
Chapter 6: Speciation Prezygotic Isolation
Type Description Example
Populations are isolated Bishop Pines and Monterey
Temporal because they breed at pines release their pollen at
different times. different times of the year.
Populations are isolated Parasites that begin to
Habitat because they breed in exploit new host species
different habitats. are isolated from their
Populations do not To attract male fireflies,
Behavioural interbreed because their female fireflies gives a
wooing ability differs from species-specific sequence
other species. of flashes
Mating fail because eggs Closely related population
and sperm are unsuited for cannot mate with each
Gametic barrier each other. other. Only population of
the same kind can mate.
Ex. A goldfish can’t mate
with a shark fish
Mating fails because male Species of the same kind; if
and female genitalia are a change in either
Mechanical unsuited for each other. reproductive organ can
isolation between them.
Type Description Example
Hybrid viability Hybrid offspring do not When ring-necked doves
develop normally and die as mate with rock doves, less
embryos. than 6% of eggs hatch.
Hybrid sterility Hybrid offspring mature but Eastern meadowlarks and
are sterile as adults. western meadowlarks are
morphologically, but hybrid
offspring are largely
Chapter 6: Speciation 2 The Morphospecies Concept (pg. 552)