Chapter 23: Species and Their Formation
23.1 What Are Species?
We can recognize and identify many species by their appearance
Linnaeus described hundreds of species on the basis of their appearancemorphological
Species form over time
Each species starts at a speciation event and ends at either extinction or another speciation
event, at which it produces two daughter species. This process is often gradual.
Speciation is the process by which one species splits into two or more daughter species, which
thereafter evolve as distinct lineages.
The gradual nature of most speciation guarantees that in many cases, two populations at
various stages in the process of becoming new species will exist.
An important component to speciation is reproductive isolation. If individuals of a population
mate with one another, but not with individuals of other populations, they constitute a distinct
group within which genes recombine.
23.2 How Do New Species Arise?
Allopatric speciation requires almost complete genetic isolation
Speciation that results when a populations is divided by a physical barrier.
Is thought to be the dominant mode of speciation among most groups of organisms.
The populations separated by such barriers are often, but not always, initially large.
They evolved difference for reasons including gene drift, but especially because the
environments in which they live are, or become, different.
Allopatric speciation may also result when some members of a population cross an existing
barrier and found a new, isolated population.
A physical barrier’s effectiveness at preventing gene flow depends on the size and mobility of
the species in question.
Sympatric speciation occurs without physical barriers
A partition of a gene pool without physical isolation.
What is required is some form of disruptive selection in which certain genotypes have high
fitness on one or the other of two resources.
Sympatric speciation via ecological isolation may be widespread among insects, many of which
feed on a single plant species.
BUT most common means of sympatric speciation is polyploidy—the production within an
individual of duplicate sets of chromosomes.
Polyploidy can arise from chromosomes duplication in a single species (autopolyploidy) or from
the combing of the chromosomes of two different species (allopolyploidy).
Allopolyploids may also be produced when individuals of two different species interbreed or
23.3 What Happens when Newly Formed Species Come Together?
Reproductive isolation can evolve as an incidental by-product of genetic changes in allopatric
populations. Geographic isolation does not necessarily lead to reproductive isolation, however, b/c genetic
divergence does not cause reproductive isolation to appear as a by-product.
Prezygotic barriers operate before fertilization
Mechanisms that operate before fertilization—prezygotic reproductive barriers—may prevent
individuals of different species or populations from interbreeding:
o Habitat Isolation
o Temporal Isolation
o Mechanical Isolation
o Gametic Isolation
o Behavioural Isolation
Postzygotic barriers operate after fertilization
If individuals of two different populations lack complete prezygotic reproductive barriers,
postzygotic reproductive barriers may still prevent gene exchange.
o Low hybrid zygote viability
o Low hybrid adult viability
o Hybrid infertility
Individuals that mate with individuals of the related species should evolve prezygotic