The Origin of Species

Chapter 24 – The Origin
of Species
Macroevolution: evolutionary change above the
species level, the formation of new taxonomic
Speciation: the origin of
new species
Anagenesis (phyletic
evolution): accumulation of
heritable changes
Cladogenesis (branching
evolution): budding of new
species from a parent species
that continues to exist (basis of
biological diversity)
What is a Species?
Biological Species Concept (Ernst Mayr) - a
population or group of populations whose
members have the potential to interbreed and
produce viable, fertile offspring (genetic
exchange is possible and that is genetically
isolated from other populations)
Reproductive barriers
Biological factors that impede production of viable, fertile
Both prezygotic and postzygotic barriers.
Prezygotic Barriers
Habitat isolation: 2
species that occupy
different habitats within
the same area will
rarely, if ever, come
into contact with each
Example: two species
of snakes in the same
geographic area but
one lives in water and
one on land
Prezygotic Barriers
Temporal Isolation: species that
breed during different times of
the day, seasons, or years;
prevents gamete mixing
Example: different species of
leopard frogs can live in the
same pond, but mate in different
Prezygotic Barriers
Isolation: 2
species have
different courtship
rituals thereby
attracting different
Example: the blue
footed boobies –
specific dance,
bird calls, insect
mating dances
Prezygotic Barriers: mating attempt
Mechanical Isolation: when
mating is attempted, but
morphological differences
prevent successful mating
Example: closely related
species of monkey flowers
Gametic Isolation: one
species cannot fertilize the
eggs of another species
Example: common in sea
creatures who discharge
their gametes into the water
column, like sea urchins and
sea anemones
Postzygotic Barriers
Reduced hybrid viability:
genes of different parent
species may interact and
impair the hybrid’s
development; offspring
usually die in the embryonic
stage of development
Reduced hybrid fertility:
hybrid are sterile
Example: donkey and
horse = mule (sterile)
Hybrid breakdown: first
generation hybrids are
viable and fertile, but the
second generation
offspring are feeble or
Hybrid Organism Assignment
Identify 10 different hybrid organisms.
 For each organism identify
The mother and father species
The post zygotic barrier that causes them to be a
hybrid and not all one species
One interesting fact about the hybrid or mating pair
You may identify/classify an additional 2 hybrids
for extra credit
 Assignment due on
Biological Species Concept
There are some limitations with the biological species
No method to measure reproductive success of fossils or
asexual organisms
Difficult to determine ability of one organism to mate with another
Other definitions of a species:
Morphological species concept – uses structural features
Palentological species concept – morphology of fossils
Ecological species concept – ecological niche
Phylogenetic species concept – genetic history
None of these are perfect, but a combination of each (especially
biological and morphological) is most accepted.
Modes of Speciation
Must have some
type of geographic
separation of
populations for
speciation to
occur; 2 different
 1 – Allopatric
 2 – Sympatric
Allopatric Speciation
“other country” or “other homeland”
Populations segregated by a geographical barrier
Can result in adaptive radiation
Example: white-tailed antelope squirrel and Harris’s antelope squirrel –
separated by Grand Canyon
Sympatric Speciation
“same country”
 Speciation takes place in
overlapping populations
 Can result from various
mechanisms including
chromosomal changes
(polyploidy in plants) and
non-random mating that
reduces gene flow
Pundamilia pundamilia
Pundamilia nyererei
Adaptive Radiation
The evolution of many diversely adapted
species from a common ancestor
Tempo of Speciation
Gradualism – gradual
divergence from a
common ancestor
Punctuated equilibrium
– changes in short
bursts, mostly when it
buds from a parent
species, with long
stretches of little
Helped explain the nongradual appearance of
species in the fossil