BIOL 180 - Reading 5-3:
24.1 - How are Species Deﬁned and Identiﬁed?:
Speciation - A splitting event that forms two or more distinct species from a single
ancestral species- can occur abruptly or gradually
There are things you need to deﬁne to understand when two species are actually
Although there is no single, universal answer for this, scientists do agree there is a
distinction between the general deﬁnition of a species and the criteria used in the
practical identiﬁcation of species in particular cases
Species - An evolutionary independent population or group of populations.
Of the many criteria that can be used to identify species, the three most
common ones are
1. The biological species concept
2. The morphospecies concept
3. Phylogenetic species concept
The Biological Species Concept:
Biological species concept - According to this concept, the main criterion for
identifying species is reproductive isolation
If two diﬀerent populations do not interbreed in nature, or if they fail to produce
viable and fertile oﬀspring when matings take place, then they are considered
The criterion of reproductive isolation oﬀers clear evidence of evolutionary
Reproductive isolation can result from a wide variety of events and processes
To organize the various mechanisms that stop gene ﬂow between populations,
biologists distinguish between:
Prezygotic isolation - Prevents individuals of diﬀerent species from mating
Postzygotic isolation - The hybrid of matings between members o diﬀerent
species either do not survive or do not reproduce
The criterion of reproductive isolation cannot be evaluated in fossils or in species that
reproduce asexually, and is diﬃcult to apply when closely related populations don’t
overlap with each other geographically.
In the case of the lack of geographical overlap, biologists are left to guess
whether interbreeding and gene ﬂow would occur if the populations happened
to come info contact
Reproductive isolation can also sometimes be a complex gradient rather than an all-or-
For example, if a male lion and female tiger produce a rare “liger” that survives
and reproduces in captivity (the ranges of these cants do not overlap in nature),
are lions and tiger necessarily the same species?
Mechanisms of Reproductive Isolation: Prezygotic Isolation:
Type Process Example
Temp Populations are isolated because Bishop pines and Monterey pines release
oral they breed at diﬀerent times. their pollen at diﬀerent times of the year.
Habit Populations are isolated because Mainland mice and beach mice mate in
at they breed in diﬀerent habitats. diﬀerent habitats.
Populations do not interbreed
Beha because they courtship display To attract females, male songbirds sing
vioral diﬀer. species-speciﬁc songs.
Diﬀerences in bindin protein determine
etic Matings fail because eggs and which sea urchin sperm will penetrate
barri sperm are incompatible. eggs.i
Mech Matings fail because male and Very large dogs and very small dogs
anica female reproductive structures are cannot mate due to the incompatible
l incompatible. sizes.
Type Process Example
Hybri When ring-necked doves mate with rock
d Hybrid oﬀspring do not develop doves, less than 6 percent of the eggs
viabili normally and die as embryos. hatch.
Hybri Horses and donkeys can mate and
d Hybrid oﬀspring mature but are
sterili sterile as adults. produce viable oﬀspring, but their hybrid
ty oﬀspring (mules) are usually sterile.
The Morphospecies Concept:
Morphospecies concept - According to this concept, researchers identify evolutionarily
independent lineages by diﬀerences in size, shape, or other morphological features
The logic behind the morphospecies concept is that distinguishing features are
most likely to arise if populations are independent and isolated from gene ﬂow
The morphospecies concept is compelling simply because it is so widely applicable
It is a useful criterion when biologists have no data on the extend of gene ﬂow,
and it is equally applicable to sexual, asexual, or fossil species Its disadvantages are that
1. It may lead to the n