PSY270H5 Lecture Notes - Sympatric Speciation, Allopatric Speciation, Reproductive Isolation
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
Postzygotic barriers operate after fertilization
•If individuals of two different populations lack complete prezygotic
reproductive barriers, postzygotic reproductive barriers may still prevent
oLow hybrid zygote viability
oLow hybrid adult viability
•Individuals that mate with individuals of the related species should evolve
prezygotic reproductive barriers more rapidly that allopatric pairs of species.
Hybrid zones may form if reproductive isolation is incomplete
•If contact is re-established between formerly isolated populations before
complete reproductive isolation has developed, members of two populations
may interbreed. Three outcomes of such interbreeding are possible:
oIn hybrid offspring are as fit as those resulting from mating within each
population; hybrids may spread through both populations and reproduce
with other individuals. The gene pools are then combined, and no new
species result from the period of isolation.
oIf hybrid offspring are less fit, complete reproductive isolation may
evolve as reinforcement strengthens prezygotic reproductive barriers.
oEven if hybrid offspring are at some disadvantage, a narrow hybrid
zone may exist if reinforcement does not happen, or the zone may
persist for a long time while reinforcement may be developing.
•When a hybrid zone firs forms, most hybrids are offspring of crosses between
purebred individuals of the two species.
•Subsequent generations include a variety of individuals with different
proportions of their genes derived from the original two populations.
•Hybrid zones contain recombinant individuals resulting from many generations
23.4 Why Do Rates of Speciation Vary?
•Rates of speciation vary b/c many factors influence the likelihood that a lineage
will split to form two or more species.
•The larger the number of species in a group, the large the number of
opportunities for new species to form.
•For speciation by polyploidy, the more species in a group, the more species are
available to hybridize with one another.
•For allopatric speciation, the large the number of different species living in an
area, the larger the number of species whose ranges will be bisected by a
given physical barrier.
•Speciation rates are likely to be higher in species with poor dispersal abilities
than those with good dispersal abilities.
•Populations of species that have specialized diets are more likely to diverge
than are populations with generalized diets.
•Speciation rates in plants are faster in animal-pollinated than wind-pollinated
•The mechanisms of sexual selection also appear to result in increased rates of
speciation (most striking example are found in birds with promiscuous mating
•Animals with complex sexually selected behaviours are likely to form new
species at a high rate b/c they make sophisticated discriminations among
potential mating partners.
23.5 Why Do Adaptive Radiations Occur?
•Proliferation of a large number of daughter species from a single ancestor is
called an evolutionary radiation.
•If rapid proliferation of species results in an array of species that live in a
variety of environments and differ in the characteristics they use to exploit
those environments, the radiation is said to be adaptive.
•If radiation is not accompanied by any observed ecological differentiation
among the species, it is said to be non-adaptive.
•Adaptive radiation begins when genetic differentiation between populations
evolves in response to differences in the environments they inhabit and the
resources they use.
oThis is likely to occur in environments with abundant resources.
oA population is likely to encounter underutilized resources when it
colonizes a new environment that contains relatively few species.
•Adaptive radiations have frequently followed mass extinctions.