Speciation
Introduction
• Species continue to evolve
• Changes that occur within species
(microevolution) are easy to understand
– Faster-running cheetahs
– Colourful male songbirds
– Larger brains in humans
• But what factors lead to speciation- the
evolution of an entirely new species?
What is a Species?
• A species includes all members of a
population that can interbreed under natural
conditions
• Some species are quite distinct based on their
morphology but some species are only
distinguished by genetic comparisons
Modes of Speciation
• New species can evolve many ways
• Always includes the evolution of distinct
features that isolate the new species
reproductively, therefore genetically, from
other species
Mechanisms of Reproductive Isolation
• any biological factor that prevents the two
populations from interbreeding when living in
the same region.
• Prezygotic mechanisms: prevents interspecies
mating and fertilization
– Different breeding seasons, physical or
behavioural traits, habitat preferences or the
incompatibility of gametes
Mechanisms of Reproductive Isolation
• any biological factor that prevents the two
populations from interbreeding when living in
the same region.
• Postzygotic mechanisms: prevents maturation
and reproduction in offspring from
interspecies reproduction
Mechanisms of Reproductive Isolation
Prezygotic
Temporal Isolation: Species have different breeding
seasons.
Example: Western spotted skunks breed in the fall but
the Eastern species breeds in late winter
Western
Eastern
Mechanisms of Reproductive Isolation
Prezygotic
Behavioural Isolation: Species may have different
courtship or mating behaviours.
Example: Eastern meadowlark and western
meadowlark are almost identical in everything,
except their courtship rituals and songs
Eastern
Western
Mechanisms of Reproductive Isolation
Prezygotic
Mechanical Isolation: Species may have reproductive
structures that are physically incompatible.
Example: Snails of the same species have reproductive
structures that align, but snails from two different
species part the other species
Mechanisms of Reproductive Isolation
Prezygotic
Ecological Isolation: very similar species may occupy
different habitats within a region
Example: The mountain bluebird lives at high elevations, while
the eastern bluebird prefers lower elevations. The
mountain bluebird and eastern blue bird do not encounter
each other
Mechanisms of Reproductive Isolation
Prezygotic
Gametic isolation: male gametes may not be able to
recognize and fertilize an egg of a different species
Example: corals, clams and sea cucumbers release their
sperm and eggs into open water. The sperm
recognize the egg of their own species through
chemical markers on the egg surface.
Mechanisms of Reproductive Isolation
Postzygotic
Zygotic Mortality: mating and fertilization are possible,
but genetic differences result in a zygote that is
unable to develop properly
Example: If you have sheep and goats on a farm, some
species of sheep and goat are able to mate, but the
zygote is not viable.
Mechanisms of Reproductive Isolation
Postzygotic
Hybrid Inviability: a hybrid dies early in development,
or lives to maturity but is not healthy and suffers an
early death.
Example: Some salamanders species may mate with
each other, but most of the offspring do not live to
adulthood, or if they do they are weak.
Mechanisms of Reproductive Isolation
Postzygotic
Hybrid Infertility: The hybrid offspring mature into
adults but are infertile.
• Example: A mule is bred from a female horse (left)
and a male donkey (centre). Mules are physically
healthy but they are infertile.
Allopatric Speciation
• Most new species form when a
single species is separated into two
geographically isolated populations
• Once physically separated,
populations cannot share genetic
information
• Over many generations the
populations will gradually become
less alike
• Mutations and genetic drift may
have caused populations to change
• Reproductive isolating mechanisms
may have developed
Allopatric Speciation
Harris’s antelope squirrel
White-tailed antelope
squirrel
Sympatric Speciation
• The evolution of populations
within the same geographic area
into separate species
• Individuals within a population
become genetically isolated from
the larger population