The Origin of Species

The Origin of Species
Chapter 16
Species
• Species is defined as a group of actually
or potentially interbreeding natural
populations, which are reproductively
isolated from other such groups
(biological species concept)
• Speciation is the process by which new
species arise
Mechanisms of Speciation
• Reproductive isolation occurs when members
of one population are unable to interbreed with
members of another
• Isolating mechanisms prevent interbreeding
and maintain reproductive isolation
– Premating (prezygotic) isolating mechanisms
prevent mating between species
– Postmating (postzygotic) isolating mechanisms
prevent the formation of viable, fertile offspring after
mating has occurred
Premating Isolating Mechanisms
• Premating isolating mechanisms include:
– 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 Africa
• Tigers live in India
• Geographically separated populations
aren’t necessarily distinct species
Premating Isolating Mechanisms
• Ecological isolation occurs when species
can’t mate because they occupy different
habitats
• White-crowned sparrows inhabit fields and
meadows, while white-throated sparrows
inhabit dense thickets
• 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
• Mechanical incompatibility occurs when
species cannot mate because their
reproductive structures are incompatible
• In animals with internal fertilization, male
and female sexual organs may not fit
together
– e.g., snails of species whose shells have lefthanded spirals may be unable to successfully
copulate with snails whose shells have righthanded spirals
Postmating Isolating Mechanisms
• Postmating isolating mechanisms include:
– 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
– Mule hybrids (a cross between a horse and a
donkey) are sterile
• Different chromosome numbers
– Liger hybrids (a zoo-based cross between a
male lion and a female tiger) are sterile
• Infertility is caused by the failure of chromosomes
to pair properly during meiosis, so eggs and
sperm never develop
How Do New Species Form?
• Speciation depends on two factors
– Isolation
– 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
Mechanisms of Speciation
• Allopatric speciation
– Isolating mechanism is a physical barrier
• Sympatric speciation
– Isolation occurs without geographic separation
• Parapatric speciation
– Ranges overlap,variations in mating habits
• Adaptive Radiation
– Pop. from 1 species spread out & inhabit new habitats
Allopatric 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
• Allopatric 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
Sympatric 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
• 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
Sympatric Speciation
• 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)
Parapatric Speciation
• Extremes of a species over a broad range
(cline) differ from each other.
• Ranges overlap a little or adjacent.
• Variations in mating habits allows
reproductive isolation
Adaptive Radiation
• Adaptive radiation is the rise of many
new species over a relatively short period
of time
• Occurs when populations of one species
invade a variety of new habitats
– Finch colonization of the Galápagos Islands
– Cichlid fish colonization of Lake Malawi
– Tarweed plant colonization of the Hawaiian
Islands
What Causes 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 may make a species
vulnerable to extinction include:
– Localized distribution
– Overspecialization
– Competition among species
– Habitat destruction
Localized Distribution
• Wide-ranging species normally do not
succumb to local environmental
catastrophes
• 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
– The habitat of the lupine has been
significantly reduced by development
– 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
• Exotic species – introduction of non-native
species to an area
• Usually can out compete native species
(no predators)
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
Patterns of Evolution
• Gradualism – proposes that evolutionary
change is slow, gradual, & continuous.
• Punctuated equilibrium – proposes that
species have long periods of stability
interrupted by geologically brief periods of
significant change during which many new
species evolved
• Patterns of evolution
• Macroevolution – large scale evolutionary
change
• Microevolution – gene frequency changes in
local populations
• Divergent evolution – one population becoming
isolated from the rest of the species & following
a different course of evolution
• Black bear (omnivore) Polar bear (carnivore)
Patterns of evolution
• Convergent evolution – populations of different
species exposed to similar selective pressures &
exhibit similar adaptations
• Whale/shark/seal/penguin body shape
• Parallel evolution – two ancestral species
resemble each other & have evolved along
similar lines
• Australian marsupials & placental mammals