Chapter 16
The Origin of Species
“Species” Definitions:
• Pre-Darwin:
– Different kinds of organisms (based on
appearance)
• Biological species concept:
– Species are groups of actually or potentially
interbreeding natural populations, which are
reproductively isolated from other such
groups.
– Appearance Can Be Misleading
Appearance Can Be Misleading
• Same species?
– Cordilleran flycatcher and Pacific slope
flycatcher
– They do not interbreed
Appearance Can Be Misleading
• Same species?
– The myrtle warbler and Audubon’s warbler
used to be classified as different species
– Interbreed where their ranges overlap
Species
• Limitations of species definition
– Asexually reproducing organisms or fossils
– Often difficult to observe whether members
of two different groups interbreed
Speciation
• Process by which new species form. It
depends on 2 factors:
– A. Isolation
– B. Genetic Divergence
• When populations are isolated, gene flow
between them is blocked
• Isolated populations subsequently evolve
genetic differences large enough to
prevent interbreeding
– Differences arise by chance (genetic drift) or
through natural selection
Hypothetical mechanisms of speciation:
• Allopatric speciation
occurs when two populations
of a species become
separated by a geographical
barrier
– Colonization of remote islands
by mainland organisms
– Geological changes such as
volcanism, earthquakes,
continental drift, and rivers
changing course
– speciation occurs when
isolated populations diverge
genetically
Allopatric Speciation
• Is believed to be the most common type of
speciation, especially among animals
• Two allopatric populations, the Kaibab
squirrel and Abert’s squirrel, may be
evolving into two separate species
Hypothetical mechanisms of speciation:
• Sympatric
speciation occurs
when
– Two populations of a
species living in the
same geographical
area become restricted
to different habitats
– Isolated populations
diverge genetically
Sympatric Speciation
• Two sympatric populations of
fruit flies (Rhagoletis pomonella)
may be evolving into two
separate species.
– One population lays its eggs in
hawthorn fruit, while the other
prefers apples
– The two populations experience
very little interbreeding
– Males and females prefer the
same type of fruit in which they
developed
– Apples mature two or three weeks
later than hawthorn fruit (flies
mature and mate at different
times)
Sympatric isolation, eating either
hawthorn or
Apple trees: Rhagoletis pomonella
New Species
• Mechanisms of speciation and reproductive isolation lead
to forking branches in the evolutionary tree of life, as one
species splits into two
Adaptive Radiation
• Adaptive radiation is the rise of many
new species over a relatively short period
of time
Adaptive Radiation
• Occurs when
populations of one
species invade a
variety of new habitats
– Cichlid fish
colonization of Lake
Malawi
– Finches in the
Galapagos Islands
Adaptive Radiation
– Tarweed plant colonization of the Hawaiian
Islands
Maintenance of Reproductive
Isolation Between Species
• Premating isolating mechanisms
(prevents the mating between 2
organisms)
– Geographical isolation
– Ecological isolation
– Temporal isolation.
– Behavioral isolation
– Mechanical incompatibility
Premating Isolating Mechanisms
• Geographical isolation occurs when
populations cannot mate because of physical
barriers
– In nature, lions do not mate with tigers
• Lions live in African Savanna
• Tigers live in Siberian lands.
Premating Isolating Mechanisms
• …Geographical isolation Kaibab and Abert
squirrels; in different areas of the Grand
Canyon. Very similar, but have they diverged?
Premating Isolating Mechanisms
• Ecological isolation occurs when species
might live in the same area, but use
different portions of the habitat:
– White-crowned sparrows inhabit fields and
meadows, while white-throated sparrows
inhabit dense thickets
Premating Isolating Mechanisms
• …Ecological isolation
– Each species of fig wasp breeds in the fruits
of a particular species of fig
Premating Isolating Mechanisms
• Temporal isolation occurs when species
can’t mate because they breed at different
times
• In nature, Bishop pines and Monterey
pines do not interbreed
– Bishop pine pollination occurs in
summer
– Monterey pine pollination occurs in early
spring
Premating Isolating Mechanisms
• Behavioral isolation occurs when species
can’t mate because they have different
courtship and mating rituals
• Songs and plumage
of male songbirds
are species specific
– Attract females of
the same species
– Females of other
species are
unresponsive
Premating Isolating Mechanisms
• …Behavioral isolation
• Male frogs embrace any female regardless
of species
– Female frogs encountering males of a
different species utter the “release call”
Premating Isolating Mechanisms
• Mechanical
incompatibility. In plants
or animals: Size or
structure of reproductive
organs don’t fit.
– e.g., snails of species
whose shells have lefthanded spirals may be
unable to successfully
copulate with snails whose
shells have right-handed
spirals
Maintenance of Reproductive
Isolation Between Species
• Postmating isolating mechanisms
(prevents the production of vigorous
and fertile offspring)
– Gametic incompatibility
– Hybrid inviability
– Hybrid infertility
Postmating Isolating Mechanisms
• Gametic incompatibility occurs when
sperm from one species cannot fertilize
eggs of another
– In animals, fluids of the female reproductive
tract my weaken or kill sperm of another
species
– In plants, pollen from one species may fail to
germinate when it lands on the stigma of
another species
Postmating Isolating Mechanisms
• Hybrid inviability occurs when hybrid
offspring fail to survive to maturity
• Hybrid may abort early in development
• Hybrid may be unable to reproduce
because it display behaviors that are
mixtures of the two parental types
– Lovebird hybrids have great difficulty
learning to carry nest materials during
flight
Postmating Isolating Mechanisms
• Hybrid infertility occurs when hybrid
offspring are sterile or have reduced fertility
• Horse + donkey  sterile mule
• Lion + tiger  sterile liger
• Infertility is caused by the failure of
chromosomes to pair properly during
meiosis, so eggs and sperm never develop
R.M.L. Lulu born 06-19-04. Very active Bay with
black legs, mane and tail. Molly Mule Foal. Very
good conformation. Mother is dark brown/black
quarter horse mare, 15-1 hands. Sire is Knute's
Centinela Gomez Black Catalonian Mammoth Jack,
14.2 Hands. $1,650.00
Ligers
Ecological isolation: liger only in captivity
Extinction
• Extinction is the death of all members of
a species
• At least 99.9% of all species that ever
existed are now extinct
• Factors that can cause extinction:
– Localized Distribution
– Overspecialization
– Competition for limited resources
– Habitat Destruction (living and non-living)
Localized Distribution
• Species inhabiting extremely limited
ranges may become extinct if the area is
disturbed
– Devil’s Hole pupfish is found in only one
spring-fed water hole in the Nevada desert
Overspecialization
• Species that develop adaptations that
favor survival in a specific environment are
at risk of becoming extinct
– The Karner blue
butterfly feeds only on
the blue lupine plant
– Lupine’s habitat is
now reduced.
– Loss of the lupine will
lead to extinction of
the Karner blue
butterfly
Competition Among Species
• Species that are unable to exploit
resources more efficiently and effectively
than their competitors may become extinct
• After formation of land bridge between
North and South America 2.5 million years
ago, many species in South America were
displaced and became extinct.
Habitat Destruction
• Habitat destruction is the single greatest cause of
extinction
• Human activities are the primary cause of present-day
habitat destruction
– Clearing of tropical rainforests could lead to loss of up to half
of all current species over the next 50 years
The Ethiopian wolf - the most endangered
canid in the world - faces a variety of threats
due to conflict with Ethiopia's ever-expanding
human population. The EWCP works with local
herders and communities to reduce conflict
with livestock, protect wolves from hunting,
and to reduce disease transfer from, and
inbreeding with, local domestic dogs. With only
an estimated 400-500 Ethiopian wolves left in
a few isolated populations throughout the
country, research and community involvement
are crucial to the survival of this unique wolf.
Other notes on speciation
• 1. Polyploidy in plants can lead to
speciation
Case 1: Selffertilization
meiosis
fertilization
Case 2: Breeds with
normal diploid parent
Speciation by
polyploidy
meiosis
viable, diploid gametes
meiosis fails;
no viable gametes