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 concept.
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
•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
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
•The populations separated by such barriers are often, but not always, initially
•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
•Allopolyploids may also be produced when individuals of two different species
interbreed or hybridized.