Outline
1. The What: Defining species
2. The How: Mechanisms for speciation
Speciation
Part One:
What is a Species, and
How Does a Species Form?
Sections 1.1 and 9.2
Three different definitions
1. Morphological species
2. Biological species
3. Phylogenetic species
Brainstorm:
What is a species?
(1) The Morphological Species definition
A species is a group of organisms that
shares certain structural features.
based on comparing structural
of organisms
Advantage:
Simple!
features
e.g., body
shape, size,
colouration
Limitations:
 Morphological
variation within
species
 Morphological
similarities between
species
(2) Biological species definition
A species is a group of interbreeding
organisms that can produce viable, fertile
offspring under natural conditions.
Requirements of a biological species:
• Viable offspring
• Offspring are able to develop to birth, and live.
• Fertile offspring
• Offspring are capable of producing children of their own.
Based on reproductive
compatibility between
organisms.
• Natural conditions
• In nature, the organisms are likely to encounter each
other and mate.
All of these organisms are hybrids.
Why are the parents still considered
separate species?
Advantages
widely-used
Lion + Tiger = Liger (sterile)
Limitations
spatially-isolated
populations of the same
species
• can’t interbreed in nature
can’t apply to fossil
species
asexually-reproducing
organisms?
Jackal + Dog = Hybrid (fertile)
Donkey + Horse = Mule (sterile)
(3) Phylogenetic species definition
A species is a group of organisms that shares
a common ancestor and evolutionary history.
When does a
species begin?
When reproductive
isolation occurs
between related
groups.
Examines the
evolutionary line of
descent of organisms,
through DNA analysis
When does a
species end?
When lineages
become extinct.
Learning Checkpoint
Advantage
can be applied to
extinct species
Limitation
evolutionary histories
are not known for all
species
• The definition of species has evolved over time.
• The phylogenetic definition of a species is the
ultimate goal of modern classification.
• However, the biological definition is still the most
commonly-used among biologists and students.
Speciation is...
Speciation happens through the
• the formation of new species from existing ones
▫ an ancestral species branches off into
two or more new lineages
reproductive isolation of populations.
 eventual result: two populations cannot interbreed
 requires a stop of gene flow between populations
• a type of macroevolution
gene flow
Population
A
gene flow between
populations maintains
reproductive compatibility
Population
B
Pop’ns A and B remain
part of the same species.
Reproductive isolation
in Lizards (2:38)
Speciation happens through the
reproductive isolation of populations.
 eventual result: two populations cannot interbreed
 requires a stop of gene flow between populations
Reproductive isolation requires
barriers to gene flow:
Reproductive Isolating
Mechanisms
gene flow
SPECIES
A
genetic differences
accumulate in each
population;
prevents reproduction
The two populations
diverge into two
separate species.
SPECIES
B
Pre-zygotic
Post-zygotic
aka “pre-fertilization”
aka “post-fertilization”
prevention of:
• mating
• fertilization
affect the survival
or viability
of hybrid offspring
Pre-zygotic mechanisms act
before fertilization.
Prevention of mating:
• behavioural isolation
• ecological/habitat isolation
• temporal isolation
• mechanical isolation
Prevention of fertilization:
• gametic isolation
Behavioural isolation
Pre-zygotic mechanisms
• behavioural isolation
•ecological/habitat isolation
•temporal isolation
•mechanical isolation
•gametic isolation
Sexual cues/mating rituals are not recognized by
individuals of different species.
Example:
Songs of different
species of birds or
insects; mating
“dances”
Temporal isolation
Pre-zygotic mechanisms
• behavioural isolation
•ecological/habitat isolation
•temporal isolation
•mechanical isolation
•gametic isolation
Different species are fertile
at different times, preventing mating.
 different seasons, different times of day
Example:
B. americanus and
B. fowleri can produce
fertile hybrids in a lab;
in nature they are fertile
during different seasons.
Habitat isolation
Pre-zygotic mechanisms
• behavioural isolation
•habitat isolation
•temporal isolation
•mechanical isolation
•gametic isolation
Geographical isolation prevents
individuals from
meeting and mating.
Can occur even within the same area,
if the populations occupy
different habitats.
Example:
Two species of garter snakes
(genus Thamnophis) occur in the
same geographic area. One lives
mainly in water, and the other is
terrestrial.
Mechanical isolation
Pre-zygotic mechanisms
• behavioural isolation
•ecological/habitat isolation
•temporal isolation
•mechanical isolation
•gametic isolation
Genitals are mechanically incompatible
(though copulation may be attempted).
Example:
Insects
Plants (mode of pollination)
Pre-zygotic mechanisms
• behavioural isolation
•ecological/habitat isolation
•temporal isolation
•mechanical isolation
•gametic isolation
Gametic isolation
Mating and sperm transfer may
take place, but gametes are incompatible.
sperm may not survive in female reproductive tract
fusion is prevented by some biochemical mechanism
Post-zygotic mechanisms act
after fertilization.
• zygotic mortality/hybrid inviability
• hybrid sterility
• hybrid breakdown
Example:
Common in marine
invertebrates, which
utilize external
fertilization.
Zygotic mortality or
Hybrid inviability
Post-zygotic mechanisms
• hybrid inviability
• hybrid sterility
• hybrid breakdown
Zygote does not survive to birth, or
surviving hybrid is frail.
Example:
Goat/sheep hybrid
zygote mortality
Hybrid sterility
Post-zygotic mechanisms
• hybrid inviability
• hybrid sterility
• hybrid breakdown
The hybrid cannot produce normal gametes,
so it cannot reproduce.
When parents differ in
chromosome number,
meiosis cannot occur
normally.
Example:
Mules
(donkey/horse hybrid).
Hybrid breakdown
Post-zygotic mechanisms
• hybrid inviability
• hybrid sterility
• hybrid breakdown
F1 hybrids are viable and fertile,
but subsequent generations are not.
Example:
Some rice cultivars
produce robust hybrids,
but the next generation
is small and sterile.
Case study: Salamanders of California (3:22)
Four species of leopard frog:
What keeps these
species isolated?
• mating calls differ
• defective hybrid
embryos
1) Rana pipiens
2) R. blairi
3) R. utricularia
4) R. berlandieri
• Reproductive isolating mechanisms maintain
species by preventing gene flow and
hybridization.
Which specific
mechanism?
▫ Can be categorized as either pre-zygotic or
post-zygotic.
• Post-zygotic mechanisms act after fertilization.
Hybrids are produced, but with reduced viability
or fertility.
•
•
•
•
Read 9.2 (pg. 360-363)
Pg. 363 #13, 15, 17
Pg. 373 #1-4
worksheet: Speciation
• Speciation involves reproductive isolation of
populations.
Pre-zygotic or
post-zygotic?
• Pre-zygotic mechanisms prevent the
production of hybrids, by preventing either
mating or fertilization.
Homework
Summary