Chapter 14
The Origin of Species
Introduction: The Rise and Fall of Cichlids
A.)
Until recently, over 500 species of cichlid fishes lived in East
Africa’s Lake Victoria
– Where did these species come from?
– Why are they disappearing?
B.)
Lake Victoria’s cichlids diversified 100,000 years ago
– Different species of cichlid have specialized mouthparts that allow them
to specialize on different food sources
– The bright colors of the males vary with species, as females chose males
with specific colors
C.)
Groups isolated by diet or female mate choice may have lost
the ability to interbreed
D.)
A species can be defined as a group of organisms whose
members can breed and produce fertile offspring, but who do
not produce fertile offspring with members of other groups
E.)
Two closely related species of cichlid, Pundamilia nyererei and
Pundamilia pundamilia, feed at different depths
F.)
The males of the two species differ in color
G.)
Females of each species prefer brightly colored males with the
“right” color
H.)
How do females benefit from this choice?
I.)
Over the last 30 years, 200 species of cichlids have disappeared
from Lake Victoria
– Some were eaten by the Nile perch, an introduced predator
J.)
In the polluted waters of Lake Victoria, it is more difficult for
females to choose brightly colored males of the right species
– As a result, the gene pools of separate species are mixing, as two
species fuse back into one
14.1 The origin of species is the source of
biological diversity
A.)
Speciation is the emergence of new species
B.)
Every time speciation occurs, the diversity of life increases
C.)
The many millions of species on Earth have all arisen from an
ancestral life form that lived around 3.6 billion years ago
CONCEPTS OF SPECIES
14.2 There are several ways to define a species
A.) Taxonomy is the branch of biology that names and classifies
species and groups them into broader categories
B.)
Carolus Linnaeus developed the binomial system of naming
organisms using physical characteristics to distinguish over
11,000 species
C.)
Similarities between some species and variation within species
can make defining species difficult
D.)
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 fertile offspring
E.)
Reproductive isolation prevents gene flow and maintains
separate species
F.)
Can the biological species concept always distinguish species
from each other?
G.)
The morphological species concept classifies organisms
based on observable phenotypic traits
H.)
It can be applied to asexual organisms, fossils, and in cases
when we donít know about possible interbreeding
I.)
There is some subjectivity in deciding which traits to use
J.)
The ecological species concept defines a species by its
ecological role or niche
– Consider the cichlids, which are similar in appearance but feed at
different depths in the lake
K.)
The phylogenetic species concept defines a species as a set
of organisms representing a specific evolutionary lineage
– Morphological or DNA similarities or differences can be used to define a
species
– Defining the amount of difference required to distinguish separate
species is a problem
14.3 Reproductive barriers keep species separate
A.) Reproductive barriers serve to isolate a species gene pool
and prevent interbreeding
B.)
Reproductive barriers are categorized as prezygotic or
postzygotic, depending on whether they function before or
after zygotes form
C.)
Prezygotic Barriers
D.)
– Prezygotic barriers prevent mating or fertilization between species
– In temporal isolation, two species breed at different times (seasons,
times of day, years)
– In habitat isolation, two species live in the same general area but not in
the same kind of place
Prezygotic Barriers
– In behavioral isolation, there is little or no sexual attraction between
species, due to specific behaviors
– In mechanical isolation, female and male sex organs are not compatible
– In gametic isolation, female and male gametes are not compatible
E.)
Postzygotic Barriers
– Postzygotic barriers operate after hybrid zygotes are formed
– In reduced hybrid viability, most hybrid offspring do not survive
– In reduced hybrid fertility, hybrid offspring are vigorous but sterile
– In hybrid breakdown, the first-generation hybrids are viable and fertile, but the
offspring of the hybrids are feeble or sterile
– The process of speciation depends on whether reproductive barriers
prevent gene flow between populations
F.)
If two related species live in the same area, would natural
selection favor the evolution of prezygotic or postzygotic
reproductive isolating mechanisms?
MECHANISMS
OF SPECIATION
14.4 In allopatric speciation, geographic isolation leads to
speciation
A.)
In allopatric speciation, populations of the same species are
geographically separated, separating their gene pools
B.)
Changes in the allele frequencies of each population may be
caused by natural selection, genetic drift, and mutation,
unaffected by gene flow from other populations
C.)
Gene flow between populations is initially prevented by a
geographic barrier
– The Grand Canyon and Colorado River separate two species of antelope
squirrels
D.)
Likelihood of allopatric speciation increases when a population
is small and isolated
– A small population may have a different gene pool due to the founder
effect
– Genetic drift and natural selection may have a greater effect in a small
population in a new habitat
14.5 In sympatric speciation, speciation takes place without
geographic isolation
A.)
In sympatric speciation, new species may arise within the
same geographic area as a parent species
B.)
Gene flow between populations may be reduced by factors
such as polyploidy, habitat differentiation, or sexual selection
C.)
Many plant species have evolved by polyploidy, the
multiplication of the chromosome number due to errors in cell
division
D.)
A tetraploid (4n) plant can arise from a diploid parent
E.)
Most polyploids arise from hybridization of two different species
F.)
Haploid gametes from two different species combine to
produce a sterile hybrid
– Why is the hybrid sterile? How can it reproduce?
G.)
Chromosome duplications may produce a fertile polyploid
species
– What is the chromosome number of the new species
H.)
Sympatric speciation in animals more commonly occurs through
habitat differentiation and sexual selection
– Remember the cichlids in Lake Victoria!
14.6 EVOLUTION CONNECTION: Most plant species trace
their origin to polyploid speciation
A.)
80% of all living plant species are the descendants of ancestors
that formed by polyploid speciation
B.)
Hybridization between two species accounts for most of these
species
– What advantage might there be to hybridization?
C.)
Polyploid food plants include oats, potatoes, bananas, peanuts,
barley, plums, apples, sugarcane, coffee, and bread wheat
D.)
Cotton is also polyploidy
E.)
Wheat has been domesticated for 11,000 years
– It is the most widely cultivated plant in the world
F.)
Bread wheat, Triticum aestivum, is a polyploid with 42
chromosomes
14.7 Reproductive barriers may evolve as populations diverge
A.) How do reproductive barriers arise?
14.8 Hybrid zones provide opportunities to study reproductive
isolation
A.)
What happens when isolated populations renew contact?
B.)
In hybrid zones, members of different species meet and mate
to produce hybrid offspring
C.)
What may happen in a hybrid zone?
D.)
Reinforcement: If hybrids are less fit than parent species,
natural selection strengthens reproductive barriers
E.)
Fusion: Weak reproductive barriers between the two species,
F.)
Stability: Many hybrid zones are stable, continuing to produce
with considerable gene flow, reverses speciation and two
species become one again
hybrids; this allows some gene flow between populations, but
each species maintains its own integrity
– Which of these three outcomes—reinforcement, fusion, or stability—is
happening to the Pundamilia species of cichlids in Lake Victoria?
14.9 TALKING ABOUT SCIENCE: Peter and Rosemary
Grant study the evolution of Darwin’s finches
A.)
Peter and Rosemary Grant have worked on medium ground
finches on tiny, isolated, uninhabited Daphne Major in the
Galapágos Islands for 35 years
B.)
Medium ground finches and cactus finches occasionally
interbreed
– Hybrid offspring have intermediate bill sizes and survive well during wet
years, when there are plenty of soft, small seeds around
– During dry years, hybrids are outcompeted by both parental types
C.)
D.)
The occasional hybridization between finch species introduces
new genes into both populations
During drought years, hybrids die out
– This keeps medium ground finches and cactus finches on separate
evolutionary paths
14.10 Adaptive radiation may occur when new opportunities
arise
A.)
In adaptive radiation, many diverse species evolve from a
common ancestor
B.)
Adaptive radiations occur
– When a few organisms colonize new unexploited areas
– After a mass extinction
C.)
Adaptive radiations are linked to new opportunities: lack of
competitors, varying habitats and food sources, evolution of
new structures
14.11 Speciation may occur rapidly or slowly
A.) What is the total length of time between speciation events
(between formation of a species and subsequent divergence of that
species)?
– In a survey of 84 groups of plants and animals, the time ranged from
4,000 to 40 million years
– Overall, the time between speciation events averaged 6.5 million years
and rarely took less than 50,000 years
You should now be able to
§ Explain how the diverse assemblage of cichlid species evolved in
Lake Victoria; explain why many of these species no longer exist
§ Compare the definitions, advantages, and disadvantages of the
different species concepts
§ Describe five types of prezygotic barriers and three types of
postzygotic barriers that prevent populations belonging to closely
related species from interbreeding
You should now be able to
§ Explain how geographical processes can fragment populations and
lead to speciation
§ Explain how sympatric speciation can occur, noting examples in
plants and animals
§ Explain why polyploidy is important to modern agriculture; explain
how modern wheat evolved
§ Explain how reproductive barriers might evolve in isolated
populations of organisms
You should now be able to
§ Explain how hybrid zones are useful in the study of reproductive
isolation
§ Explain the conditions that can lead to adaptive radiation