Chapter 23
Biology
Sixth Edition
Raven/Johnson
(c) The McGraw-Hill Companies, Inc.
Speciation – the
process that
explains how
one species can
lead to several
new species.
Species are the Basic Units of Evolution
Species  Population  Community  Ecosystem
Species that occur together (sympatric) are
distinctive entities that are phenotypically
different, utilize different parts of the habitat, and
behave separately.
Subspecies – Populations of species that live in
separate geographical areas and have a different
phenotype – assigned a name after the species.
Individuals in
overlapping areas
often exhibit a
combination of
features
characteristic of both
populations.
Biological Species Concept – ‘groups of actually or
potentially interbreeding natural populations which
are reproductively isolated from other such groups.’
Individuals that can not produce viable offspring are
reproductively isolated.
What about populations that do not occur together in
nature – allopatric?
However, the ability to exchange genes appears to be
the hallmark of a species.
Maintaining Genetic Distinctiveness
• Prezygotic Isolating Mechanisms
– Ecological (use of different habitats)
– Behavioral (color, vocalization, pheromones)
– Other
• Temporal isolation, mechanical isolation,
prevention of gamete fusion
• Postzygotic Isolating Mechanism
– Zygote doesn’t successfully develop
– Hybrid sterility
Reproductive barriers: Prezygotic
Although two species able to hybridize and live in the
same geographic area – they utilize separate
habitats.
Behavior patterns can keep closely related species
from mating.
Temporal Isolation – Time of year when reproduction
occurs may be different for two different species able
to hybridize.
Mechanical Isolation: Organisms are just physically
not able to successfully mate.
Prevention of Gamete Fusion: Sperm prevented from
fusing with an egg.
Reproductive barriers: Postzygotic
Postzygotic mechanisms usually are expressed by
failure of an embryo to develop, adults not able to
survive, or production of sterile adults.
Problem’s With the Biological Species Concept:
The extent of hybridization is much greater than
previously believed.
Although not common, hybridization should be enough
to cast doubt on whether reproductive isolation is
enough to maintain separate species.
Stabilizing selection may also be a factor.
How does a species divide into two descended
species?
Does the process of speciation equate to
reproductive isolation?
Speciation is a two-part process:
1) Identical populations must diverge
2) Reproductive isolation must evolve to
maintain these differences
Allopatric divergence is the primary means of
speciation.
Geographic Isolation
Selection May Reinforce Isolating Mechanisms.
Can reproductive isolation be perfected before gene
flow destroys the differences between the
populations?
The process of selection continually improving
prezygotic isolating mechanisms is termed
reinforcement.
Gene Flow ?
Towards reproductive
isolation - Selection
Can speciation occur in sympatry?
Disruptive selection can lead to two phenotypes.
Before they become two species, a mechanism
of reproductive isolation must occur.
With genetic drift……this process of speciation
is rare.
Random changes in small populations (genetic
drift) may lead to changes that cause
reproductive isolation.
Adaptation to different environments can lead to
speciation.
More than 300 species of Drosophila exist on the
Hawaiian islands – all probably evolved from a
single ancestor by adaptive radiation.
Adaptive radiation – when a
group of closely related
species evolved from a
common ancestor by
occupying different habitats.
Darwin’s finches evolved from a single ancestor
by utilizing different food types.
Lake Victoria Cichlid Fishes
11 species of Cichlids in crater lake in
Cameroon. All evolved from a common ancestor
about 200,000 years ago. Changes in water level
is probably what lead to speciation.
Hi water = more habitat; Low water = isolation
Nile perch, introduced
in the 1950’s, has
reduced the number of
known cichlid species
by 70% - does not
include those not
known.
Periodic isolation
(glaciation) has also led to
adaptive radiation of Alpine
Buttercup’s.
Pace of Evolution:
Punctuated equilibria – evolutionary process is a
series of ‘spurts’ separated by periods of ‘stasis’.
Gradualism – gradual evolutionary change.
An ‘explosion’ of new species
occurred during the Cambrian
period. The number of
species since then has
increased.
The number of species
change has not been
consistent through
time. Five major
extinctions – we are
currently in the sixth.
First Homonids = 2mya
First Homo sapians
about 100,000 ya
The End.
Genetic change does not have to be that large to create
a new species.
Only a few genes – important ones – separate these two
species of monkeyflower.