Download 3.1 Patterns of Natural Selection

CHAPTER 3
Natural
Selection,
Speciation and
Extinctions
Prepared by
Peter Stiling
University of South Florida
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CHAPTER
3
Natural Selection, Speciation &
Extinctions
Outline
3.1
3.2
3.3
3.4
Patterns of natural selection
Patterns of speciation
The history of life
Continental drift, range collapse and
geographic distribution of species
3.5 Current patterns of extinction
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Natural Selection, Speciation and
Extinctions
Peter Ryan and colleagues (2007)
• Two islands Inaccessible and Nightingale in the Tristan de Cunha
archipelago in the South Atlantic ocean contained separate species
of Neospiza inches, whose ancestors had blown there from S.
America, 3000km distance.
• One small-billed seed generalist and one large-billed seed
specialist had evolved independently on each island.
 Generalists eat a variety of seeds of different plant species.
 Specialists feed upon seeds of just one species of plant.
• This work showed how the formation of species does not always
occur via the simplest route.
– One large billed species evolved on one island, one small billed
species on another island and both species dispersed onto the other
island, forming different species.
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We will be covering:
• How natural selection can follow four different patterns:
directional, stabilizing, balancing and disruptive.
• We will examine the species concept and the two main
mechanisms of speciation, allopatric and sympatric
speciation.
• We will briefly examine the history of life on Earth and
patterns of origination and extinction of species.
• We will discuss the current extinction crisis and current
factors endangering life on Earth.
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Natural Selection, Speciation and
Extinctions
3.1 Patterns of Natural Selection
Natural selection can follow different patterns
• Natural selection tends to lead to an increase
in the frequency of the allele that confers the
highest fitness in a given environment.
• Selection will act to maintain a number of
alleles in a population if the relative fitness of
different alleles changes on a spatial or
temporal scale.
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Patterns of Natural Selection
Natural selection can follow four different
patterns:
1. Directional
2. Stabilizing
3. Balancing
4. Disruptive (given time disruptive
selection can lead to speciation)
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Patterns of Natural Selection
3.1.1 Directional Selection
Favors phenotypes at one extreme
• Directional selection favors individuals at one extreme
of a phenotypic distribution that have greater
reproductive success in a particular environment.
• Directional selection may arise when a new allele is
introduced into a population by mutation, and the new
allele may confer a higher fitness in individuals that
carry it (e.g., the melanic forms of the peppered moth).
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Patterns of Natural Selection
Directional Selection
Figure 3.1 Graphical representation of
directional selection.
This pattern of selection selects for a darker phenotype that confers
higher fitness, for example in a polluted environment as with the
peppered moth, Biston betularia.
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Patterns of Natural Selection
Directional Selection
Figure 3.2 The study organism and study site of
the Grants’s work on natural selection.
(a) The Grants focused much of their work on one of the Galapagos
Islands known as Daphne Major.
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Patterns of Natural Selection
Directional Selection
Figure 3.2 The study organism and study
site of the Grants’s work on natural
selection.
(b) The small island of Daphne Major (0.3km2) has a resident
population of the medium ground finch, Geospiza fortis.
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Patterns of Natural Selection
Directional Selection
• The Grants quantified beak size of Galapagos
finches by measuring beak depth (a measure
of the beak from top to bottom)
• They compared beak sizes of parents and
offspring by examining broods over many
years.
• They discovered the depth of the beak was
inherited by offspring from parents regardless
of environmental conditions.
• Beak depth is a heritable trait.
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Patterns of Natural Selection
Directional Selection
In 1978, a drought
reduced seed
abundance on
Daphne Major,
leaving only large
seeds. Birds with
larger beaks were
more likely to survive
because they were
better at breaking
open the large seeds.
In the year after the
drought the average
beak depth of birds in
the population
increased almost 10%.
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Figure 3.3 Variation in the beak size of the
medium ground finch, G. fortis on Daphne
Major in 1976 and 1978.
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Patterns of Natural Selection
3.1.3 Stabilizing Selection
favors intermediate phenotypes
• Stabilizing selection favors the survival of individuals
with intermediate phenotypes.
• The extreme values of a trait are selected against
• An example of stabilizing selection involves clutch size,
the number of eggs laid, in animals.
• Too many eggs dooms the chicks to starvation.
• Too few eggs results in fewer genes passed on.
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Patterns of Natural Selection
Stabilizing Selection
Figure 3.4 Graphical representation of
stabilizing selection.
Here the extremes of a phenotypic distribution are selected against
while individuals with intermediate traits are favored. These graphs
illustrate stabilizing selection in clutch size of birds.
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Patterns of Natural Selection
3.1.3 Balancing Selection
promotes genetic diversity
• Balancing selection is a type of natural selection that
maintains genetic diversity in a population.
• In balancing selection two or more alleles are kept in
balance and are maintained in a population over the
course of may generations.
• Balancing selection does not favor one particular allele
in the population.
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Patterns of Natural Selection
Balancing Selection
• Population geneticists have identified two
common pathways along which
balancing selection can occur.
1. Heterozygote advantage
2. Frequency-dependent selection
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Patterns of Natural Selection
Balancing Selection
Heterozygote advantage
•
•
•
•
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Classic example is the sickle cell allele of the
human β-globin gene.
A homozygous individual with two copies of this
allele has sickle-cell disease, a hereditary disease
which damages blood cells.
The sickle cell homozygote has a lower fitness than
a homozygote with two copies of the normal and
more common β-globin allele.
However, in areas where malaria is prevalent, the
heterozygote has the highest level of fitness and
the sickle cell allele is maintained.
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Patterns of Natural Selection
Balancing Selection
Frequency-dependent selection
•
Positive frequency-dependent selection:
common phenotypes have an advantage.
Example: Prey are warningly colored, to advertise bad taste or
toxicity, the prey gain most benefit where many individuals
have warning coloration and the prey is deterred.
•
Negative frequency-dependent selection:
rare phenotypes are favored over common
ones.
Example: visually searching predators form a search image
for the more common color forms of prey.
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Feature Investigation
John Losey and colleagues (1997) showed how the
existence of green and red color morphs of the pea
aphid, Acyrthosiphon pisum, are maintained by natural
enemies.
• Red morphs are more likely to be eaten by ladybird
predators than green morphs.
• Green morphs are more likely to be parasitized than
red morphs.
• The frequency of color morphs in a population
therefore changes according to the frequency of
parasites or ladybird predators in the area.
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Feature Investigation
Figure 3.6
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Patterns of Natural Selection
3.1.4 Disruptive Selection
favors the survival of two phenotypes
• Disruptive selection favors the survival of
individuals at both extremes of a range rather than
the intermediate
• Disruptive selection is similar but not identical to
balancing selection where individuals of average
trait values are favored against those of extreme
trait values.
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Patterns of Natural Selection
Disruptive Selection
Figure 3.5 Graphical representation of
disruptive selection.
In this example, the normal wild-type colonial bentgrass, Agrostis
tenuis, is intolerant to metals in the soil. A mutation creates a metal
tolerant variety, which can grow in soils contaminated with metals
from mining operations.
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Natural Selection, Speciation and
Extinctions
3.2 Speciation
Occurs where genetically distinct groups separate into species
• Over a long time span, disruptive
selection can result in speciation.
• Two alternative mechanisms by which
speciation occurs:
– Allopatric speciation
– Sympatric speciation
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Speciation
Table 3.1 Four different species concepts
There are over 20 species concepts, each with its own advantages and disadvantages. Here are
four of the more widely accepted species concepts, biological, phylogenetic, evolutionary and
ecological.
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Speciation
3.2.1 There are many definitions
of what constitutes a species
• The biological species concept defines
species in terms of interbreeding.
• The biological species concept has been
used to distinguish morphologically
similar yet reproductively isolated
species.
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Speciation
There are many definitions of species
“
Groups of populations that can
actually or potentially exchange
genes with one another and that are
reproductively isolated from other
such groups.
”
~Ernst Mayr (1942)
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Speciation
There are many definitions of species
Figure 3.7 The biological species concept
The northern leopard frog, Rana pipiens (left), and the
southern leopard frog, R. utricularia (right), appear very
similar but are reproductively isolated from each other.
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Speciation
There are many definitions of species
The biological species concept suffers
from at least three disadvantages:
1. For many species with widely separate ranges, we have no
idea if the reproductive isolation is by distance only or whether
there is some species isolating mechanism.
2. In plants, hybrids often form when parents from two different
species are crossed with each other, and sometimes these
hybrids can be fertile. This greatly blurs species distinctions.
Example: Oaks from different species interbreed and their offspring are
themselves viable, capable of reproducing with other oaks.
3. The biological species concept cannot be applied to asexually
reproducing species such as bacteria and some plants and
fungi, or to extinct species.
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Speciation
3.2.2 Phylogenetic Species Concept
advocates species are identified by a unique combination of
characters.
• This definition incorporates the classic
taxonomic view of a species based on
their morphological characters and
molecular features such as DNA
sequences.
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Speciation
Phylogenetic Species Concept
The disadvantage of the phylogenetic species
concept is:
•
Determining how much difference between
populations is enough to call them species.
–
Using this definition, many currently recognized
subspecies or distinct populations would be elevated to
species status.
–
Example: The black racer, Coluber constrictor.
–
Example: Cotton mouse, Peromyscus gossypinus
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Speciation
Phylogenetic Species Concept
Figure 3.8 Difficulties with the phylogenetic
species concept
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The subspecies of the racer, Coluber
constrictor, appear different yet are members of
the same
species.
Modified
from
Conant,
1975.
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Speciation
Phylogenetic Species Concept
Figure 3.9 Difficulties with the phylogenetic species concept
The cotton mouse, Peromyscus gossypinus, exists as 6 subspecies. Each
subspecies possesses a slightly different coat color which resembles the
color of the local habitat and sometimes different lengths of tail and
hindfeet which
are
related
climbing
ability
infordifferent
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display.
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Speciation
3.2.3 Evolutionary Species Concept
Whereby a species is distinct from other lineages if it has its
own evolutionary tendencies and historical fate
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Speciation
3.2.4 Ecological Species Concept
Each species occupies a distinct ecological niche, a unique
set of habitat requirements (Leigh Van Valen, 1976).
• Competition between species is likely to
result in each individual species occupying a
unique niche.
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Hybridization and
Extinction
Introduced species can bring about a form of extinction via
hybridization.
Figure 3.10
(a) Mallard duck, Anas platyrhynchos, (b) New Zealand gray duck,
A. superciliosa, and (c) the hybrid between them.
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Speciation
3.2.5 The Main Mechanisms of Speciation
Two mechanisms: Allopatric and Sympatric speciation
• Allopatric Speciation involves spatial
separation of populations by a geographical
barrier.
• Sympatric Speciation is when members of a
species that initially occupied the same
habitat within the same range diverge into two
or more different species.
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Speciation
The Main Mechanisms of Speciation
Example of Allopatric Speciation:
•
Non-swimming populations separated by a
river may gradually diverge because there is
no gene flow between them.
•
Aquatic species separated by the emergence
of land may undergo allopatric speciation.
•
Upthrusting of mountains can also divide
populations among which speciation occurs.
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Speciation
The Main Mechanisms of Speciation
Figure 3.11 Allopatric speciation in porkfish.
About 3.5 million years ago the Isthmus of Panama arose, separating
porkfish into two separate populations with no opportunity for mixing.
Since then, genetic changes in each population have led to the
formationCopyright
of two
species,
one inInc.the
Caribbean
and one
in the Atlantic.
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Speciation
The Main Mechanisms of Speciation
Example of Sympatric Speciation:
•
Metal tolerant plants in Wales, Agrostis tenuis are
starting to show a change in their flowering season.
– Over time this population may evolve into a new
species that cannot interbreed with the original
metal-sensitive species.
•
Individual herbivorous insects are restricted to
individual host plant species. As plants speciate each
develops its own species of herbivores.
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Speciation
The Main Mechanisms of Speciation
•
Sympatric speciation is more common in
plants than animals because:
–
Plants commonly exhibit polyploidy (contain
three or more sets of chromosomes).
–
Such changes can result in sympatric
speciation.
•
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Some insects and about 30 species of
reptiles and amphibians are also polyploids.
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