Origin of species
Cladogenesis
 One ancestral species becomes divided into two descendant species
 If species are defined by the existence of reproductive isolation, then
 the process of speciation is identical to the evolution of reproductive
isolating mechanisms
Reproductive isolation
 Populations can become isolated
 Geographically in this example
Evolution of reproductive isolation
 The formation of species is a continuous process
 Two populations may only be partially reproductively isolated
 If isolating mechanisms have not evolved, then two populations will interbreed
freely
 If populations are reproductively isolated, no genetic exchange will occur, two
populations will be different species
Reproductive Isolation
 During the time when reproductive isolation is incomplete (the intermediate
state):
 Hybrids are partly sterile
 Hybrids are not as well adapted to the habitat
 Selection would favor any alleles in the parental populations that prevent
hybridization
 Reinforcement - incomplete isolating mechanisms are reinforced by natural
selection until they are completely effective
Allopatric species
 Natural selection led to reproductive isolation where ranges overlapped
Gene flow may counter speciation
 Reinforcement is not inevitable
 Incompletely isolated populations have gene flow
 Hybrids may be inferior but serve as a conduit of genetic exchange
 Two populations will lose their genetic distinctiveness
 A race between complete reproductive isolation evolution and gene flow
Genetic drift
 Random changes may cause reproductive isolation
 Genetic drift in small populations
 Founder effects
 Population bottlenecks
Natural selection
 Adaptation can lead to speciation
 Natural selection produces a variety of differences in physiological and
sensory traits
 Promotes ecological and behavioral isolation
Anolis dewlap
Is geographic isolation required for speciation to occur?
 i.e., allopatric speciation
Sympatric speciation
 Occurs without geographic speciation
Polyploidy
 Individuals that have more than two sets of chromosomes
 e.g., 3n, 4n, etc.
 Offspring with altered chromosome number cannot breed with parent
population
 Common mechanism of speciation in flowering plants
 Occurs in insects, fish, and salamanders but is rare
Polyploidy
 Can occur when all chromosomes arise from a single species (autopolyploidy)
or when chromosomes arise from hybridization between two species
(allopolyploidy)
Autopolyploidy
Allopolyploidy
Disruptive selection
 Sympatric speciation may occur over the course of multiple generations
through disruptive selection
 Two phenotypes would have to evolve reproductive isolating mechanisms
Sympatric speciation in
African Cichlid fish
 Studied in two lakes in Cameroon
 Species in each lake are most likely descended from single ancestor
 No barriers within either lake, but 11 species found in one, 9 species in the
other
 Some ecological isolation by feeding preference
Adaptive radiations
 Closely related species that have recently evolved from a common ancestor by
adapting to different parts of the environment
 Occurs in an environment with few other species and many resources
 Newly formed islands such as Hawaiian and Galápagos Islands
 Catastrophic event leading to extinction of other species
Rapid speciation often follows the evolution of a key innovation
 A key innovation allows the species possessing it to use resources or other
aspects of the environment that were previously inaccessible
 Evolution of lungs in fish
 Wings in birds and insects
 Seeds in plants
Adaptive radiation with allopatric speciation
Adaptive radiation with sympatric speciation
Character displacement
 Natural selection in each species favors those individuals that use resources
not used by the other species
Hawaiian Drosophila
 > 1000 species of Drosophila on Hawaiian Islands
 Diversity of morphological and behavioral traits
 Empty habitats resulted in fruit flies that are:
 Predators
 Herbivores
 Detritivores
 Nectar eaters
 Parasites
Darwin’s finches
Lake Victoria cichlids
 Was home to over 300 species of cichlid until recently
 Recent radiation: sequencing of cytochrome b gene -- 2000,000 years ago
 Colonized from the Nile
Cichlid extinction
 Abrupt extinction in the last several decades
 1950’s Nile perch introduced into lake
 1990’s 70% cichlids extinct
New Zealand alpine buttercups
 Speciation in glacial habitats
 Periodic isolation
 14 species occupy 5 distinct habitats
The pace of evolution
 Gradualism: the accumulation of small changes
 Punctuated equilibrium: long periods of stasis followed by rapid change
 Proposed by Niles Eldredge and Stephen Gould in 1972
 Stabilizing and oscillating selection is responsible for stasis
Gradualism or punctuated equilbrium – which is correct?
 Evolution may include both types of change
 Rapid change and speciation are not linked
 Speciation can occur without substantial phenotypic change
 Phenotypic change can occur within species in the absence of speciation
Speciation and extinction
 Speciation, through time, has surpassed extinction
 Five mass extinctions have occurred
 Most severe at the end of the Permian period—96% of all species may
have perished
 End of the Cretaceous – K-T extinction
 Dinosaurs went extinct
Consequence of extinction
 previously dominant groups may perish, changing the course of evolution
 Dinosaurs went extinct, mammals began their radiation
 Rates of speciation after an extinction may take about 10 my
 Takes time for:
 Ecosystems to recover
 Processes of speciation and adaptive diversification to begin
 Not all groups of organisms are affected equally during extinctions
A sixth extinction is underway
 Estimates:
 1/4th of all species will become extinct in the near future
 Rebound in species diversity may be slower than following previous mass
extinction events
 A large proportion of the world’s resources will be taken up by human
activities
The future of evolution
 Human influences on the environment affect the evolutionary processes
 Changing patterns of natural selection
 Global climate change is major challenge for many species
 Decreased population sizes will increase the likelihood of genetic drift
 Geographic isolation will remove homogenizing effect of gene flow
 Chemicals and radiation could increase mutation rate
Tigers now exist in geographically isolated populations