Chapter 17
Speciation
17.1 Species are reproductively
isolated lineages on the tree of life
• Speciation – the divergence of biological
lineages and the emergence of
reproductive isolation between lineages
• It comes down to what a species is
Speciation
The splitting of one species into two or
more species.
OR
The transformation of one species into a
new species over time.
Figure 24.1 Two patterns of speciation
We can recognize many species by
their appearance
• Groups of organisms that mate with one
another are commonly called species
• Carolus Linnaeus developed a binomial
nomenclature system that we still use
today
• He classified certain species only by
appearances alone
• He used a morphological species concept
which assumes that species are made of
individuals that look alike (vise versa)
• Using morphology as a way of identifying
species has limitations:
– Do not always look alike
– Cryptic species (two or more species look almost
identical but do not interbreed)
– Cannot rely on appearance alone
• Scientists now use genetic and behavioral data
as well to group species
Cryptic Species
http://ntdtv.org/en/news/science-technology/2011-11-24/dna-barcoding-uncovers-new-butterfly-species-in-mexico.html
Reproductive isolation is key
• Reproductive isolation – a state in which two
groups of organisms can no longer exchange
genes
• Most important factor in long-term isolation
• Ernst Meyer proposed the biological species
concept – species are groups of actually or
potentially interbreeding natural populations
which are reproductively isolated from other
such groups
The lineage approach takes a longterm view
• Lineage species concept – when one species
splits into two or more daughter species, which
thereafter evolve as distinct lineages
• Lineage is an ancestor-descendant series of
population over time
• Either ends in extinction or another speciation
event
• Can happen over thousands of generations or
rather quickly
Lineage species concept
http://earthlingnature.wordpress.com/2012/06/05/whats-a-species-2-vertical-species-concepts/
17.2 Speciation is a Natural
Consequence of Population
Subdivision
• Not all evolutionary changes result in new
species
• Speciation requires the interruption of
gene flow within a species
• Is genetic change prevents reproduction
how can such a change spread thorough a
species in the first place?
Incompatibilities between genes
can produce reproductive isolation
• Lets look at figure 17.3 which illustrates
how this can happen
Dobzhansky-Muller model of
hybrid incompatibility
http://www.nature.com/scitable/content/dobzhansky-muller-model-of-hybrid-incompatibility-7883
Reproductive isolation develops
with increasing genetic divergence
• As species diverge genetically, they
become more reproductively isolated
• Can happen over millions of years or
develop in just a few generations
http://www.sci.sdsu.edu/class/bio100/Lectures/Lect12/lect12.html
17.3 Speciation may occur through
geographic isolation or in sympatry
http://www.tokresource.org/tok_classes/enviro/syllabus_content/4.1_biodiversity/index.htm
Physical barriers give rise to
allopatric speciation
• Allopatric speciation – (geographic speciation)
speciation that results when a population is
divided by a physical barrier
• Dominant mode of speciation
• Examples: water, mountains, dry land formed by
continental drift or climate changes
• Usually large populations
Allopatric speciation
http://evolution.berkeley.edu/evosite/evo101/VC1bAllopatric.shtml
• Can also happen when members of a pop.
Cross a barrier and establish a new
isolated pop.
• Example: Finches of the Galapagos
• The environments and food sources are
different on all the islands
Allopatric speciation
http://science.kennesaw.edu/~jdirnber/Bio2108/Lecture/LecEvolution/Evol4MacroEvol.html
Sympatric speciation occurs
without physical barriers
• Speciation without physical isolation is
called sympatric speciation
• Can happen with disruptive selection
where certain organisms have distinct
microhabitats where mating takes place
• Example: flies that lay their eggs in
different types of fruits (page 338 read)
Sympatric speciation
http://evolution.berkeley.edu/evosite/evo101/VC1eSympatric.shtml
• Most common is polyploidy or the
duplication of sets of chromosomes
• Can happen either from chromosome
duplication in a single species
autopolyploidy
• or from the combining of the
chromosomes of two different species
allopolyploidy
sympatric speciation due to polyploidy
(more common in plants than animals)
Allopolyploid– “allo” =
other, so two species-more
common, can reproduce
asexually, or occasionally,
sexually
Autopolyploid– “auto” =
self, so one species-- in this
case, the tetraploid can selffertilize or mate with other
tetraploids
• Happens much more in plants than
animals
• Reason being plants can self-fertilize
Figure 24.6 Two modes of speciation
17.4 Reproductive Isolation is
Reinforced When Diverging
Species Come into Contact
• Reproductive isolation may be incomplete when
two diverging species come back into contact
• This will result in hybridization
• If hybrids are less fit, selection will favor parents
that do not produce hybrids
• Selection results in strengthening, or
reinforcement of isolating mechanisms that
prevent hybridization
• Prezygotic isolating mechanismsmechanisms that prevent hybridization
from occurring are called
• Postzygotic isolating mechanismsmechanisms that reduce the fitness of
hybrid offspring
• Postzygotic mechanisms result in
selection against hybridization, which
leads to the reinforcement of the
prezygotic mechanisms
Figure 24.5 A summary of reproductive barriers between closely related species
Prezygotic mechanisms prevent
hybridization
• Prezygotic isolating mechanisms, which
come into play before fertilization, can
prevent hybridization in several ways
• 4 Examples
Mechanical isolation
• structural differences in genitalia or flowers
prevent copulation or pollen transfer
• Examples:
– male dragonflies must grasp females with
special appendages
– floral anatomy is often adapted to a specific
pollinator that transfers pollen only between
members of the same species.
Mechanical isolation
Can occur in even very closely-related species
(genital openings cannot be aligned)
Temporal Isolation
• Two species breed at different times of the
day or in different seasons
• Example:
• Brown trout breed in the spring and rainbow
trout in the fall
Temporal isolation
Breed at different times
Behavioral isolation
• Little or no sexual attraction exists
• Examples:
– Fireflies and blinking signals
– Other species specific courtship behavior
– Male gypsy moths and female pheromones.
Behavioral Isolation
Only other blue-footed boobies recognize and respond to
these mating behaviors
Gametic Isolation
• Sperm cannot reach egg to fertilize it (Important
in water species)
• Examples:
– sperm may not be able to survive in the environment
of the female’s reproductive tract
– gamete recognition may be based on specific
molecules coating the egg which only adhere to
complementary molecules on sperm cells of the same
species.
Gametic isolation
Sperm and eggs of a purple sea urchin and a red sea
urchin are releases, but do not fuse outside their species
Postzygotic isolating mechanisms
result in selection against
hybridization
• Genetic differences between two diverging
lineages may reduce the survival and
reproductive rates of hybrid offspring
• Three ways:
Low hybrid zygote viability
• Hybrid zygotes may fail to mature either
dying or developing phenotypic
abnormalities
• This prevents them from becoming
reproductively capable adults
Reduced hybrid viability
Salamander subspecies occasionally
hybridize, but embryos are often miscarried,
or live young are frail
Low hybrid adult viability
• Hybrid offspring may have lower
survivorship than non-hybrid offspring
• Also called hybrid breakdown
Low hybrid adult viability
Hybrids between these strains of rice (technically still
one species) are viable and fertile, but an accumulation
of too many recessive alleles makes them
small and sterile
Hybrid infertility
• Hybrids mature into infertile adults
• Example: the offspring of horses and
donkeys (mules) are sterile
• Healthy but produce no descendants
Reduced hybrid fertility
A mule (hybrid of horse and donkey) is viable,
but not fertile
• Hybrids that are less fit will leave less
species that can produce viable offspring
• Individuals that avoid breeding with
members of other species will have a
selective advantage
• Any trait that contributes to avoidance of
hybridization will be favored
Hybrid zones may form if
reproductive isolation is incomplete
• If reproductive isolation is not complete, it will
result in the formation of hybrid zones where
those two groups overlap
• When they first start they are crosses between
purebred individuals
• Over many generations this will result in many
recombinants of the original two species
Page 344 Lets read about hybrid
zones