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ORIGIN OF SPECIES
Chapter 24
AP Biology
Campbell 9e
WHAT YOU NEED TO KNOW
 Biological concept of species
 Prezygotic and postzygotic barriers that maintain reproductive
isolation in natural populations.
 How allopatric and sympatric speciation are similar and
dif ferent.
 How an autopolyploid or an allopolyploid chromosomal change
can lead to sympatric speciation.
 How punctuated equilibrium and gradualism describe two
dif ferent tempos of speciation.
(24.1) THE BIOLOGICAL SPECIES CONCEPT
EMPHASIZES REPRODUCTIVE ISOLATION
 Speciation – the process by which new species arise.
 Microevolution – chance in the genetic makeup of a
population from generation to generation. It refers to
adaptations that are confined to a single gene pool .
 Macroevolution – refers to evolutionary change above the
species level, such as the appearance of feathers and other
such novelties, used to define higher taxa.
(24.1) THE BIOLOGICAL SPECIES CONCEPT
EMPHASIZES REPRODUCTIVE ISOLATION
 The Biological Species Concept –
defines a species as a population or
group of populations whose members
have the potential to interbreed in
nature and produce viable, fertile
of fspring but are unable to produce
viable, fertile of fspring with members
of other populations.
(24.1) THE BIOLOGICAL SPECIES CONCEPT
EMPHASIZES REPRODUCTIVE ISOLATION
 Reproductive Isolation – the existence of biological barriers
that impede members of two species from producing viable,
fertile hybrids
 Prezygotic Examples:
 Habitat Isolation – two species can live in the same geographic area, but
not the same habitat (i.e. body of water or mountain range separation).
 Behavioral Isolation – different mating rituals (i.e. birds of paradise).
 Temporal Isolation – different mating seasons (i.e. early vs. late summer).
 Mechanical Isolation – anatomically incompatible (i.e. snails with opposite
spiraling shells).
 Gamete Isolation – sperm cannot survive the reproductive tract or
penetrate through the protein barrier around the egg.
 Postzygotic Examples:
 Reduced Hybrid Viability – genetic incompatibility
 Reduced Hybrid Fertility – successful mating, but the offspring is sterile.
 Hybrid Breakdown – successful hybrid mating, but the offspring is weak or
sterile.
(24.1) OTHER DEFINITION OF SPECIES
 morphological species concept - characterizes a species by
body shape and other structural features. The morphological
species concept can be applied to asexual and sexual
organisms. ( d e f i n i ti on i s s u b j e c t i ve , s o n o t a l l s c i e n t i s ts a gre e o n t h i s o n e ) .
 ecological species concept - views a species in terms of its
ecological niche. Can also be applied to asexual and sexual
organisms.
 phylogenetic species concept - defines a species as the
smallest group of individuals that share a common ancestor,
forming one branch on the tree of life. ( The dif ficulty with this
species concept is determinin g the degree of dif ferenc e required to
indicate separ ate species).
 This is not the full list of species definition.
(24.2) SPECIATION CAN TAKE PLACE WITH
OR WITHOUT GEOGRAPHIC SEPARATION
 Allopatric speciation - gene flow is interrupted when a
population is divided into geographically isolated
subpopulations. For example, the water level in a lake may
subside, resulting in two or more smaller lakes that are now
home to separated populations
(24.2) SPECIATION CAN TAKE PLACE WITH
OR WITHOUT GEOGRAPHIC SEPARATION
 Sympatric speciation - speciation occurs in
populations that live in the same geographic
area.
 Not as frequent as allopatric, but can occur by:
 Polyploidy
 Habitat differentiation
 Sexual selection
(24.2) SPECIATION CAN TAKE PLACE WITH
OR WITHOUT GEOGRAPHIC SEPARATION
 Polyploidy - a species may originate
from an accident during cell division
that results in extra sets of
chromosomes
 Habitat Dif ferentiation - occur when
genetic factors enable a subpopulation
to exploit a habitat or resource not
used by the parent population.
 Sexual Selection - (typically) females
select males based on their
appearance
(24.4) SPECIATION CAN OCCUR RAPIDLY OR SLOWLY AND
CAN RESULT FROM CHANGES IN FEW OR MANY GENES.
 Gradualism – proposes that
species descended from a
common ancestor and
gradually diverge more and
more in morphology as they
acquire unique adaptations.
 Punctuated equilibrium –
periods of apparent stasis
punctuated by sudden
change observed in the fossil
record.