Transcript Document

Secondary Contact
• If populations come back into contact
after steps 1 and 2, have opportunity to
interbreed
• Hybridization is common in plants and
birds
– Will hybrids be viable and fertile?
– Will hybrids have characteristics of parent
species or new characteristics?
– Depends on outcome of speciation event
Secondary Contact
• Theodosius Dobzhansky formulated the
reinforcement hypothesis about the third
stage of speciation
– If populations sufficiently diverged in allopatry,
hybrids should have reduced fitness
– Should be strong selection for assortive mating
– Selection that reduces fitness of hybrids is
Reinforcement
• Completes reproductive isolation
Secondary Contact
• Reinforcement Hypothesis
– Predicts that pre-mating isolation will
evolve in species in secondary contact
– Prezygotic isolating mechanisms prevent
fertilization
• Mate choice, time of breeding, genetic
incompatibility
– Postzygotic isolation
• Offspring are sterile
• Reinforcement not necessary
Secondary Contact
• Study by Coyne and Orr showed that prezygotic
isolation occurs more often in sympatric than
allopatric species
Secondary Contact
• Hybridization
– Hybrids should have reduced fitness by
reinforcement hypothesis
– Some have normal or increased fitness
– Sorghum is an important crop
– Johnsongrass is closely related weed
– Agriculturalists worried that if they
genetically engineered herbicide
resistance into sorghum it might be
transferred to johnsongrass
Secondary Contact
• Hybridization
– Significant gene flow occurred and herbicide
resistance introgressed into johnsongrass
– If hybrids have increased fitness, will they
form their own species?
– Biologists attempted to recreate the
hybridization event that lead to the formation
of Helianthus anomalous
• Found that certain crosses had higher fitness
and certain had low
– High fitness hybrids became new species
Secondary Contact
• Hybridization
– Hybrid zones occur where recently
diverged populations overlap
– May occur by secondary contact or during
parapatric (or peripatric) speciation
– Hybrid zones are often present where
hybrids have equal fitness to parental
species
– Size of hybrid zone depends on fitness of
hybrids
Secondary Contact
• Hybridization
– Study on sagebrush in western US
• Basin sagebrush found at low elevations
• Mountain sagebrush found at high elevations
• Species make contact at middle elevations and
hybridize
– Graham compared several fitness measures of
hybrids and parentals
– Found that hybrids have superior fitness in
transitional habitats
Sympatric
Speciation
• Among the most
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controversial
subjects in
evolutionary
biology
Complete panmictic
mating and a
reproductive
isolating
mechanism evolves
within the
population
Sympatric Speciation –
Possible Mechanisms
1. Polyploidy
2. Disruptive selection, whereby certain
homozygous genotypes have high
fitness on one or the other of two
resources & intermediate
(heterozygotes) has low fitness
Sympatric Speciation &
Polyploidism
Hybridization in Helianthus
Sympatric Speciation –
Disruptive Selection
• Selection may favor alleles in homozygous condition
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and you get assortive mating
But, antagonism likely to arise from alternative
selection, which promotes association between
alleles for adaptation and alleles for assortative
mating, and recombination which destroy this
association
This is the greatest opponent to the process of
Sympatric Speciation
In such a model strong selection and tight linkage
are required... BUT recombination will break this
down!
Sympatric Speciation –
Disruptive Selection
• Another similar model was proposed by Bush based
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on his work on the apple maggot flies
Rhagoletis – economically important pest which are
parasites on the fruits of trees in the hawthorn clade
Rhagoletis ID host by sight, tough and smell
Courtship and mating occur on the fruit
Females lay eggs in fruit on the tree
eggs hatch in 2 days and develop after fruits fall to
ground
insects burrow into ground, overwinter, and emurge
next spring
Mechanisms of Divergence
– Apple and hawthorn maggot flies
• Are they different populations?
• Live in sympatry on adjacent trees
• Recently diverged because apples are not
native to US
• A mark-recapture allozyme study revealed
they do form distinct populations
• How have they diverged without initial
isolation?
– Did they skip Step 1?
Mechanisms of Divergence
– Apple and hawthorn maggot flies
• There are other mechanisms for speciation
besides allopatric
• Through assortive mating there is only 6%
gene flow among populations
• Separated in time by pupating at different
times of year
• They are able to maintain distinct
populations even with gene flow because
of strong natural selection
Genetics of Differentiation and
Isolation
• What changes in the genome are necessary
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for speciation?
Most F1 hybrids are sterile
– Postzygotic isolation is pronounced
• Which genes are responsible for sterility?
• If one sex is sterile, it is usually the
heterogametic sex
– Human males have Y
– Bird females have W
Genetics of Differentiation and
Isolation
• Haldane’s Rule
– Pattern of sterility is in heterogametic sex,
regardless of which is male or female
Genetics of Differentiation and
Isolation
• Why does Haldane’s Rule work?
– Consider an autosomal locus A and an X-linked
locus B
– Individuals from one species are fixed for A1 and
B1
– Sister species fixed for A2 and B2
– A2 and B1 interact to cause inviability
– If females from first species mate with second:
A1A2B1 males and A1A2B1B2 females
– Males are inviable, females are viable
Genetics of Differentiation and
Isolation
• Quantitative Trait Loci (QTL) Mapping
– Most traits involved in reproductive
isolation are quantitative
– QTL mapping attempts to locate genes
with small effects on quantitative traits
– Find genetic markers that are unique to
each parental species and the value of that
trait in hybrids
Genetics of Differentiation and
Isolation
• Quantitative Trait Loci (QTL) Mapping
– If statistically significant associations are
found between traits and markers that
have been mapped, implies that at QTL
near the marker contributes to that trait
– Try to identify “speciation genes” in a
diverse array of organisms to understand
the genetics behind speciation
CHAPTER 17
Character Evolution:
Form and Function
Character Evolution
• Why do species vary in temperature
tolerance?
• Why variance in reproductive rate or
reproductive mode?
• Why differences in size?
Character Evolution
• The next four chapters discuss
evolutionary theory and how it relates
to:
– 17: Form and Function
– 18: Species Interactions
– 19: Life History Evolution
– 20: Behavior
Character Evolution
• We need to understand Morphology &
Physiology to understand biological
diversity
• We can then use this understanding to
gain insight into the mechanisms by
which organisms function
• This approach will allow us to develop
hypotheses about How and Why traits
have evolved
Morphological &
Physiological Adaptations
Morphology
rm,p
Performance
rp,f
Fitness
rm,p = correlation between morphology
and performance
rp,f = correlation between performance and
fitness
rm,p is easy to find
rp,f is difficult!
Morphological &
Physiological Adaptations
• We need to understand the benefits
that an adaptation has, along with its
costs and constraints
•  Trade-offs!
Flight in Birds
• Costs  work required to keep a bird
aloft increases with “wing-loading”
(=weight of the body relative to wing
area)
• Large birds have low wing loading by
having long wings that are broad at the
tip
Flight in Birds
• Terrific innovation!!!
• Primary flight feathers separated so
each feather acts as an individual
pointed wing (reducing drag and
adding loft)
Flight in Birds
Desert Animals & Heat
• Counter current heat exchangers
• Panting cools blood in nasal area (in
capillaries)
• This cooled blood then runs into the
arteries just before the brain and cool
blood cools the brain
• This keeps the brain from overheating
(denaturing proteins necessary for fxn)
Desert Animals & Heat
Body Size
• Spans 1021 Magnitude as measured by
mass!!!
• Small organisms take less time to grow
to maturity (generation time is shorter)
• Therefore, fitness is greater, all else
being equal...
• Small organisms require less food than
large ones and can inhabit smaller
microhabitats
Body Size
• Large plants set more seeds
• Large animals carry more eggs
• Large males can often win more
contests with smaller males
• Thus, Large size also has reproductive
advantages that may outweigh the
reproductive advantages of rapid
development and fewer necessary
resources...
Body Size
• Although they require more food, Large
animals can often overpower, handle,
or swallow a greater range of potential
prey or food items
• Predators select for small or large size
in prey species depending on predators
size and mode of feeding
Allometry and Isometry
• y changes as a function of x
• Allometric equation  y = bxa
– (where y= one char, x=another, a=coeff. of
allometry, and b=constant proportion relating y
and x)
• if “a” = 1 then b = y/x which means
that y changes in direct proportion to x
• a<1  y increases less rapidly than x
• a>1  y increases more rapidly than x
Allometry and Isometry
• Many times it is easiest to express this
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equation like this:
log y = log b + a log x
This gives a straight line with slope = a and
an intercept = log y
Most morphological evolution can be
described in terms of Allometric
relationships.
Allometric relationships with body mass are
often the consequence of adaptation
• Structures that support an organism
must change disproportionately in
shape as weight increases.
• Tree trunk mass to cross sectional are
is 3/2 power of height
Evolution of Tolerance
• In animals, a series of responses occur
sequentially in response to stress
• Lets examine these steps...
Allometry and Isometry
• Example:
• In cold environments, large size is
advantageous in birds and mammals
because they lose heat more slowly,
Thus requiring less food to maintain
constant body temp.
• Bergmann’s Rule  “birds and
mammals larger in colder climates than
same/related species in warmer
climates
Evolution of Tolerance
1. Changes in behavior
2. Hormone-modulated biochemical and
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4.
5.
physiological functions
Slower, longer lasting changes in
physiology (“acclimation”)
In some instances, developmental changes
in morphology
At population level, genetic changes due to
differences among genotypes in survival
and reproduction rates caused by the
stress
Evolution of Tolerance
• If the responses of individual
organisms cannot fully compensate for
the stress, fitness is reduced
• This may lead to genetic changes
• Some changes entail developmental
responses and these are reversible
– e.g., Seasonal Responses
What Limits Geographical
Ranges of Species?
• Some ranges are set by biotic factors,
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interspecific competition & predation, or by
abiotic factors such as temperature and
water availability
This question is thus complex and difficult to
answer
The simplest hypothesis is the lack of
genetic variation for tolerance of
physiological stress
– However, in general this is not likely...
What Limits Geographical
Ranges of Species?
• Successful colonization of sites may
require numerous coincident adaptive
changes
• This suite of adaptations may be an
improbable concatenation of genetic
variants for many characteristics
– e.g. Seasonal timing of reproduction &
Growth...
What Limits Geographical
Ranges of Species?
• Trade-offs exist between adaptation to
conditions within and beyond the
margin of the range
• Trade-offs limit adaptation to a new
environment due to gene flow from
old  new
(center of range  periphery)
What Limits Geographical
Ranges of Species?
• The explanation put fourth by Mayr
• Gene flow from the main range of a species into the
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marginal populations prevents them from further
adapting by breaking down adaptive combinations of
interacting genes
So, a marginal population may be better if able to
adapt & expand range if it could not exchange genes
with interior populations
Perhaps species have evolved broader ranges then
we give them credit  because the adapted
extralimital population we call different species