Download Speciation

Sexual Selection in the harlequin
beetle riding pseudoscorpion
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Multiple Paternity in the HB-riding
Pseudoscorpion
• Males A & B
captured with
mother (M) on
same harlequin
beetle
• Numbered lanes
are
DNAsamples
from offspring
of the mother
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Female pseudoscorpions discriminate
against previous mates
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Why Polyandry in Cordylochernes?
(34,770 nymphs counted!)
• The average
lifetime
reproductive
success of females
mated to two
males is 32%
higher than that of
females mated
twice to a single
male
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Percent of Females Achieving Zero
Reproductive Success
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Jeanne Zeh’s Genetic Incompatibility
Avoidance Hypothesis
Polyandry enables females to exploit
postcopulatory mechanisms for reducing the risk
and/or cost of fertilization by genetically
incompatible sperm
• Sperm competition
• Female choice of sperm
• Re-allocation of maternal resources in
live-bearing species
In 33% of “infertile” couples, the
female’s cervical mucus
agglutinated partner’s sperm but
not donor sperm
Dondero et al. 1978
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Human Leukocyte Antigen Matching
and Fetal Loss
(Ober et al. 1998)
• 10-year study of HLA matching and pregnancy
in 111 Hutterite couples
• Matching at entire 16-locus haplotype and/or
allele sharing at HLA-B-linked locus confers
significant risk for fetal loss
New Topic: Speciation
• Speciation is the process by which one
genetically-cohesive population splits into two
or more reproductively-isolated populations.
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Ernst Mayr (1942) Biological
Species Concept
• Species are groups of actually or potentially
interbreeding natural populations which are
reproductively isolated from other such groups;
i.e., they have distinct gene pools
Allopatric Speciation
• Speciation in a different place. Argued to be the most common (least
controversial) form of speciation.
• Evolution of reproductive isolating mechanisms between populations that are
geographically separate
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Allopatric Distributions
• Vicariance: Widespread
population becomes divided:
– New river or mountain range
(barriers); emergence of the
Isthmus of Panama
– Extinction of intermediate
populations due to habitat
fragmentation
• Rare dispersal and
colonization
– Founder Effect (or Peripatric)
speciation (e.g., Galapagos
Finches)
Vicariant Speciation in Antelope
Ground Squirrels
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Sexual Selection and Allopatric Speciation
• Sexual selection can cause
rapid evolution of epigamic
traits and mating
preferences
• It can cause rapid evolution
of premating isolating
mechanisms in allopatry
• Evidence?
– Of relatively sexually
monomorphic and dimorphic
sister clades, the latter is
usually more speciose
Allopatric Speciation - Summary I
• No gene flow between gene pools due to
geographical separation
• Independent evolution --> divergence
• Divergence includes traits associated with
reproduction, which causes evolution of
reproductive isolating mechanisms
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Allopatric Speciation - Summary II
• Mutation + drift --> slow allopatric speciation
• Natural selection and sexual selection can lead
to faster allopatric speciation
• Extrinsic barriers to hybridization permit
independent evolution until speciation is
complete.
Mechanisms of Non-Allopatric Speciation:
I. Parapatric Speciation
• Speciation along side; development of
reproductive isolation among members of a
population in the absence of an obvious
geographic barrier
• Looks much like secondary contact
• May occur when a sharp environmental
discontinuity develops within the range of a
species.
• Plants growing on mine tailings flower at different
times than plants growing in surrounding pastures
in Wales
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Mechanisms of Non-Allopatric Speciation:
II. Sympatric Speciation
• Speciation in the same
place; development of
reproductive isolation
without geographic
barriers
• FAIRLY COMMON IN
PLANTS VIA
POLYPLOIDY
• 47% of flowering plant
species are polyploid
Polyploidy
• Multiplication of the number of chromosomes
– autopolyploidy - duplication of the chromosomes of a
single species
– allopolyploidy - duplication of a combination of
chromosomes from different species
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Polyploidy & Speciation
• Allopolyploidy can result in nearly instantaneous
speciation.
• Polyploid individuals cannot interbreed
successfully with members of either parental
population because polyploid individuals have
twice as many chromosomes as their parents.
Therefore the chromosomes cannot pair properly
during the first metaphase of meiosis.
Sympatric Speciation: Plants
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Sympatric Speciation: Animals
• Sympatric speciation is a controversial mechanism
of speciation in animals.
• How can reproductive isolation be achieved in the
absence of barriers to gene flow?
• Again, there must be selection for assortative
mating - i.e., like genotypes mate with like
• Many entomologists argue that sympatric speciation
is common in phytophagous insects via host plant
shifts
Sympatric Speciation:
Rhagoletis pomonella
• Tephritid fly which is a pest species on apples and
hawthorn
• Females infrequently oviposit in new host plants and
then both males and females show an affinity for the
new host plant
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