Download Lecture2-k biodiv web

Lecture 2: Biodiversity
• What is biological diversity?
• Natural selection
• Adaptive radiations and convergent
evolution
• Biogeography
Biodiversity and Distributions
• Types of biological diversity:
– Genetic diversity
– Species diversity
– Ecosystem diversity
• Fundamental question:
– How is biodiversity distributed around the
globe, and why?
Biodiversity and Distributions
•
What determines the distributions of individual
species?
The process of natural selection
• Charles Darwin, Origin of Species (1859)
• What is natural selection?
1. Historical factors
-
Evolutionary history/geography
Speciation events
2. Ecological factors
-
– process by which organisms become uniquely
adapted to their environment
– Acts on individuals, but affects make-up of
populations
Environment varies dramatically across globe
Each species is specialized to deal with specific
ecological conditions
Adaptations that result from natural selection
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Evolution by Natural Selection
Variation in traits
50% green/brown
Speciation &
Adaptations
The process of natural selection
Requires the following 3 conditions in a
population:
1. variation among individuals in some trait
•
e.g. height, speed, coloration
2. This variation is heritable
•
Differential Reproduction
= “Fitness”
Natural Selection
Evolution!
Heredity
trait is coded by specific genes
3. There is a relationship between the trait and the
fitness of individuals
•
their survival and/or reproduction
End result: 100% brown
Natural selection: A case study
• Grants’ studies on the Galapagos finches
• Example: medium ground finch (G. fortis)
– Entire population found on Daphne island
– Finches use their beaks to open seed cases
and eat seeds inside
– They measured the beak sizes of individuals
and monitored reproduction and survival over
many years
Natural selection: A case study
Results:
1. Tons of individual variation in beak size
Number of
individuals
(frequency)
7 mm
12 mm
Beak depth/size
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Natural selection: A case study
Results:
Results:
2. Beak size is heritable
Each point is one
offspring plotted
against their own parent
Steepness of line
indicates heritability of
trait (i.e. no slope = not
heritable)
Beak depth
of offspring
(when full size)
Beak depth
of parent
3. Beak variation has ecological consequences
Birds with larger beaks
tended to eat larger and
harder seeds
Beak depth
Can only be measured if
offspring do not
disperse
Natural selection: A case study
Results:
4. Beak variation had survival consequences
-
Natural selection: A case study
Birds with smaller beaks
ate smaller and softer
seeds
Seed size
consumed
Natural selection: A case study
Results:
4. Beak variation had survival consequences
Big drought in 1977
Many plants died, much less food available
Shift to seeds that were harder and larger
⇒ Massive die off of finches from 1500 to 200
- Smaller beaked birds suffered most mortality
⇒ Next generation of birds had larger beaks
- Evolutionary shift in beak size in one generation!
Mean of survivors
(post-drought)
Mean of all birds
(pre-drought)
Beak depth
of offspring
Beak depth
of parent
Evolutionary shift
towards birds with
larger beaks in one
generation
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Case studies of natural selection
Summary:
• The study of natural selection in progress
requires these critical factors:
–
–
–
–
Variation in a trait that is related to fitness
Heritability of that trait
Populations with limited dispersal
A selection event to occur
Natural selection: Important points
3. NS is an evolutionary process (genetic
change) but is also profoundly ecological
-
Due to ecological interactions (biotic and
abiotic)
4. NS can be very strong and rapid
Natural selection: Important points
1. NS operates on individuals!
-
Does not work to ensure the survival of the
species
2. There is a lot of potential for NS because
most traits are heritable
-
Think of artificial selection in domesticated
animals
Mainly acts on existing genetic variation (not
necessarily on new mutations)
Adaptive radiations
= Speciation + ecological specialization
Definition of adaptive radiation:
rapid diversification of a lineage into
several species that exploit a variety of
different resources
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Adaptive radiations
• Classic example: Darwin’s finches (again!)
• Speciation: 14 species total
– 13 in archipelago of Galapagos islands
– 1 on Cocos island in Costa Rica
– Molecular work confirms common ancestor and that
speciation occurred within past 1 million years
• Ecological specialization
– Massive variation among species in beak size and
shape
– This morphological variation is associated with
different diets among species
Test
•
Recall: 13 species in
archipelago , but only 1 on
extremely remote Cocos island
– One island so no speciation
could occur
– Still is ecological selection for
specialization
⇒ Behavioral feeding specialization
•
Insect-eaters, ground foragers,
nectar specialists
How do adaptive radiations occur?
1. Empty region is colonized by one species
2. Isolation of populations allowed for
subsequent differentiation (no gene flow)
3. Competition leads to further divergence
Prediction: adaptive radiations require many
isolated populations w/ NS
Adaptive Radiations
• Two ways to become specialists:
morphology (across species)
or behavior (within a species)
• Adaptive radiation
(speciation + ecological specialization)
only occurred in archipelago!!
• Cool examples: Hawaiian honeycreepers,
African cichlid fishes
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• Despite divergence, share a common ancestor,
so there are still many similarities
• Do such similarities among species always
indicate that adaptive radiation has occurred?
Hawaiian honeycreepers
African Rfit Valley cichlids
Flying Squirrel
Convergent evolution
• Different taxonomic lineages converge in
ecological roles
• Evolutionary convergence is presumed to
be due to adaptation to similar
environments
• i.e. placental mammals and marsupials
Flying Marsupial
Convergent evolution
1. Similarities between forms can be very strong
-
Suggests that certain conditions or niches favor
very specific adaptations, regardless of the
taxonomic group that fills the role
2. Differences between forms still exist
-
-
Due to the fact that organisms come from different
taxonomic lineages that have very different
evolutionary histories
e.g. carnivores in Australia will always be
marsupials, whereas those in America will always
be placentals
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Biogeography
• The study of the geographic distributions
of organisms
– History of the earth is a major factor of interest
– Different continents have different histories
and different taxonomic groups
Wallace’s Line
• Alfred Russel Wallace, co-discoverer of Nat. Sel.
• Visited 2 islands separated by 26 miles
– On Bali found birds of Asian descent
– On Lombok found birds of Australian descent
• One side of line has Asian taxa: tigers, monkeys,
bears, woodpeckers
• Other side of line has Australian taxa:
kangaroos, marsupials, birds of paradise
Why?
Continental drift and Isolation (= Vicariance)
Biogeographic example: S. continents
Curious pattern– many similar looking taxa
between S. America and Africa
• Hummingbirds (S. America) and
Sunbirds (Africa) – both nectar feeders
• S. American and African Cattle egrets
• Ratites (flightless birds)
– Emu in Australia
– Ostrich in Africa
– Rhea in S. America
Biogeographic example: S. continents
What are the possible mechanisms?
1.Convergent evolution
- These species are not close relatives but NS
has caused them to resemble each other
2.Vicariance biogeographic hypothesis
- Originally stemmed from common ancestor in
Gondwanaland
3.Dispersal
- These birds are related and dispersed to
continents over water
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Biodiversity Hotspots
Biogeographic example: S. continents
•
Hummingbirds (S. America) and
Sunbirds (Africa)
Threats to Biodiversity
– not related species: convergent evolution
•
S. American and African Cattle egrets
– Dispersal between continents was
documented
•
Ratites (flightless birds)
– Are distantly related species
– Cannot disperse across continents
– Most likely due to continental drift
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Habitat destruction/fragmentation
Invasive Species
Overexploitation
Pollution
Climate Change
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