The Rise and Fall of Cichlids 

advertisement
Introduction: The Rise and Fall of Cichlids
 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?
Copyright © 2009 Pearson Education, Inc.
Introduction: The Rise and Fall of Cichlids
 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
Copyright © 2009 Pearson Education, Inc.
Introduction: The Rise and Fall of Cichlids
 Two closely related species of cichlid, Pundamilia
nyererei and Pundamilia pundamilia, feed at
different depths
 The males of the two species differ in color
Pundamilia pundamilia
Copyright © 2009 Pearson Education, Inc.
Pundamilia nyererei
Introduction: The Rise and Fall of Cichlids
 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
 Groups isolated by diet or female mate choice may
have lost the ability to interbreed
Copyright © 2009 Pearson Education, Inc.
Introduction: The Rise and Fall of Cichlids
 Over the last 30 years, 200 species of cichlids have
disappeared from Lake Victoria
– Some were eaten by the Nile perch, an introduced
predator
 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
Copyright © 2009 Pearson Education, Inc.
14.1 The origin of species is the source of
biological diversity
 Speciation is the emergence of new species
 Every time speciation occurs, the diversity of life
increases
 The many millions of species on Earth have all
arisen from an ancestral life form that lived around
3.6 billion years ago
Copyright © 2009 Pearson Education, Inc.
14.2 There are several ways to define a species
 Taxonomy is the branch of biology that names and
classifies species and groups them into broader
categories
 Carolus Linnaeus developed the binomial system of
naming organisms using physical characteristics to
distinguish over 11,000 species
 Similarities between some species and variation
within species can make defining species difficult
Copyright © 2009 Pearson Education, Inc.
14.2 There are several ways to define a species
 The morphological species concept classifies
organisms based on observable phenotypic traits
 It can be applied to asexual organisms, fossils, and
in cases when we don’t know about possible
interbreeding
 There is some subjectivity in deciding which traits to
use
Copyright © 2009 Pearson Education, Inc.
14.2 There are several ways to define a species
 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
Copyright © 2009 Pearson Education, Inc.
14.2 There are several ways to define a species
 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
Copyright © 2009 Pearson Education, Inc.
14.2 There are several ways to define a species
 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
 Reproductive isolation prevents gene flow and
maintains separate species
Copyright © 2009 Pearson Education, Inc.
14.2 There are several ways to define a species
 Can the biological species concept always
distinguish species from each other?
 Not if they reproduce asexually
 Not if they are extinct
 But since reproductive isolation is an essential factor
in the evolution of species most scientists find the
biological species concept most useful
Copyright © 2009 Pearson Education, Inc.
MECHANISMS
OF SPECIATION
How does reproductive isolation
occur
Copyright © 2009 Pearson Education, Inc.
14.4 In allopatric speciation, geographic isolation
leads to speciation
 In allopatric speciation, populations of the same
species are geographically separated, separating
their gene pools
 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
Copyright © 2009 Pearson Education, Inc.
14.4 In allopatric speciation, geographic isolation
leads to speciation
 Gene flow between populations is initially prevented
by a geographic barrier
– The Grand Canyon and Colorado River separate two
species of antelope squirrels
Copyright © 2009 Pearson Education, Inc.
14.5 In sympatric speciation, speciation takes
place without geographic isolation
 In sympatric speciation, new species may arise
within the same geographic area as a parent
species
 Gene flow between populations may be reduced
by factors such as polyploidy, habitat
differentiation, or sexual selection
Copyright © 2009 Pearson Education, Inc.
14.5 In sympatric speciation, speciation takes
place without geographic isolation
 Many plant species have evolved by polyploidy,
the multiplication of the chromosome number due
to errors in cell division
 A tetraploid (4n) plant can arise from a diploid
parent
Copyright © 2009 Pearson Education, Inc.
1
Parent species
2n = 6
Tetraploid
cells
4n = 12
2
1
Parent species
2n = 6
Tetraploid
cells
4n = 12
Diploid
gametes
2n = 6
3
2
1
Parent species
2n = 6
Selffertilization
Tetraploid
cells
4n = 12
Diploid
gametes
2n = 6
Viable, fertile
tetraploid
species
4n = 12
Species A
2n = 4
Species B
2n = 6
Gamete
n=2
Gamete
n=3
Chromosomes not
homologous
(cannot pair)
1
Species A
2n = 4
2
Gamete
n=2
Sterile hybrid
n=5
Species B
2n = 6
Gamete
n=3
Chromosomes not
homologous
(cannot pair)
1
Species A
2n = 4
2
3
Gamete
n=2
Sterile hybrid
n=5
Species B
2n = 6
Gamete
n=3
Viable, fertile
hybrid species
2n = 10
14.6 EVOLUTION CONNECTION: Most plant
species trace their origin to polyploid
speciation
 80% of all living plant species are the descendants
of ancestors that formed by polyploid speciation
 Hybridization between two species accounts for
most of these species
– What advantage might there be to hybridization?
 Polyploid food plants include oats, potatoes,
bananas, peanuts, barley, plums, apples,
sugarcane, coffee, and bread wheat
 Cotton is also polyploid
Copyright © 2009 Pearson Education, Inc.
´
AA
Triticum monococcum
(14 chromosomes)
BB
1
Wild
Triticum
(14 chromosomes)
Hybridization
AB
Sterile hybrid
(14 chromosomes)
2
Cell division error
and self-fertilization
´
AA BB
DD
T. turgidum
Emmer wheat
(28 chromosomes)
T. tauschii
(wild)
(14 chromosomes)
3
Hybridization
ABD
Sterile hybrid
(21 chromosomes)
4
Cell division error
and self-fertilization
AA BB DD
T. aestivum
Bread wheat
(42 chromosomes)
14.5 In sympatric speciation, speciation takes
place without geographic isolation
 Sympatric speciation in animals more commonly
occurs through habitat differentiation and sexual
selection
– Remember the cichlids in Lake Victoria!
Copyright © 2009 Pearson Education, Inc.
a.
b.
14.7 Reproductive barriers may evolve as
populations diverge
 So in order for speciation to occur
reproductive barriers have to arise.
 How do reproductive barriers arise?
Copyright © 2009 Pearson Education, Inc.
14.3 Reproductive barriers keep species separate
 Reproductive barriers serve to isolate a species
gene pool and prevent interbreeding
 Reproductive barriers are categorized as prezygotic
or postzygotic, depending on whether they function
before or after zygotes form
Copyright © 2009 Pearson Education, Inc.
14.3 Reproductive barriers keep species separate
 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
Copyright © 2009 Pearson Education, Inc.
Zygote
Gametes Prezygotic barriers
• Temporal isolation
• Habitat isolation
• Behavioral isolation
• Mechanical isolation
• Gametic isolation
Postzygotic barriers
• Reduced hybrid
viability
• Reduced hybrid
fertility
• Hybrid breakdown
Viable,
fertile
offspring
Initial sample
of fruit flies
Starch medium
Maltose medium
Female
Starch Maltose
22
9
8
20
Results
Mating frequencies
in experimental groups
Female
Population Population
#1
#2
Male
Pop#2 Pop#1
Maltose Starch
Male
Mating experiments
18
15
12
15
Mating frequencies
in starch control groups
14.8 Hybrid zones provide opportunities to study
reproductive isolation
 What happens when isolated populations renew
contact?
 In hybrid zones, members of different species
meet and mate to produce hybrid offspring
Copyright © 2009 Pearson Education, Inc.
New
species
Ancestral
species
1
3
Hybrid
zone
2
4
Gene
flow
Gene flow
Population
(five individuals Barrier to
gene flow
are shown)
Hybrid
14.8 Hybrid zones provide opportunities to study
reproductive isolation
 What may happen in a hybrid zone?
 Reinforcement: If hybrids are less fit than parent
species, natural selection strengthens reproductive
barriers
 Fusion: Weak reproductive barriers between the
two species, with considerable gene flow, reverses
speciation and two species become one again
Copyright © 2009 Pearson Education, Inc.
14.8 Hybrid zones provide opportunities to study
reproductive isolation
 Stability: Many hybrid zones are stable,
continuing to produce 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?
Copyright © 2009 Pearson Education, Inc.
Reinforcement
Fusion
Stability
14.10 Adaptive radiation may occur when new
opportunities arise
 In adaptive radiation, many diverse species
evolve from a common ancestor
 Adaptive radiations occur
– When a few organisms colonize new unexploited
areas
– After a mass extinction
 Adaptive radiations are linked to new
opportunities: lack of competitors, varying habitats
and food sources, evolution of new structures
Copyright © 2009 Pearson Education, Inc.
14.11 Speciation may occur rapidly or slowly
 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
Animation: Macroevolution
Copyright © 2009 Pearson Education, Inc.
Punctuated Equilibrium-speciation is abrupt, speciation
to have occurred relatively rapidly based on Fossil evidence
Time
Gradualism- species diverged gradually
over long periods of time
Time
You should now be able to
1. Explain how the diverse assemblage of cichlid
species evolved in Lake Victoria; explain why
many of these species no longer exist
2. Compare the definitions, advantages, and
disadvantages of the different species concepts
3. Describe five types of prezygotic barriers and
three types of postzygotic barriers that prevent
populations belonging to closely related species
from interbreeding
Copyright © 2009 Pearson Education, Inc.
You should now be able to
4. Explain how geographical processes can
fragment populations and lead to speciation
5. Explain how sympatric speciation can occur,
noting examples in plants and animals
6. Explain why polyploidy is important to modern
agriculture; explain how modern wheat evolved
7. Explain how reproductive barriers might evolve in
isolated populations of organisms
Copyright © 2009 Pearson Education, Inc.
You should now be able to
8. Explain how hybrid zones are useful in the study of
reproductive isolation
9. Explain the conditions that can lead to adaptive
radiation
10. What is difference between gradualism model of
speciation and the punctuated model of speciation
Copyright © 2009 Pearson Education, Inc.
Download