BIOL 4100 Lecture Notes - Sympatric Speciation, Allopatric Speciation, Hybrid Zone

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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 hybridized.
What Happens when Newly Formed Species Come Together?
2Reproductive 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
oHabitat Isolation
oTemporal Isolation
oMechanical Isolation
oGametic Isolation
oBehavioural 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.
oLow hybrid zygote viability
oLow hybrid adult viability
oHybrid infertility
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 of hybridization.
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 systems).
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.
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.