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. 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
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
o Low hybrid zygote viability
o Low hybrid adult viability
o Hybrid infertility
• Individuals that mate with individuals of the related species should evolve