Transcript Secondary Growth

Biodiversity
• How did biological diversity come about?
• What are the principles of natural selection?
• What affects biodiversity?
What is biological diversity?
1. genetic diversity
2. species diversity
3. higher taxonomic diversity (taxonomy)
4. habitat diversity
How many species exist in the world?
• No one knows!
• Taxonomists have named and
described 1.4-1.7 million
species
– 56% insects
– 14% plants
– 3% vertebrates
– 15% are in oceans
• Highly biased sample
– Vertebrates much more
widely studied
– What about microbes?
• 4000 different bacteria
species per gram of
Norwegian soil!
– Also, mostly done in Europe
and N. America while most
of the biodiversity is in
tropical countries and in
oceans
1,000,000
750,000
500,000
250,000
protists
viruses
insects other higher
plants
fungi
bacteria
animals
So how many species are there?
14 mil
15,000,000
number
of
species
however, this
number
could be as
high as
100,000,000
10,000,000
5,000,000
1.7 mil
0
total
identified
total estimated
to exist
Global biodiversity seems to be at its peak
Where are these species?
• Oceans
– 1 to 10 million in oceans
– diverse in phyla
• 32 in oceans but only 12 phyla on land
• Tropics
– 7% of land mass
– 50% of species
How do species evolve?
Evolution is the change in the genetic
characteristics of a population over time.
This change may happen by:
1. genetic mutations
2. natural selection
3. geographic isolation and migration
4. genetic drift (most likely in small, isolated
populations)
Views of Species Change: Evolution
Organisms today descended by gradual changes from
ancient ancestors.
Age of the Earth: 238Uranium half-life of 4.5 billion
years, current amount present suggests earth is ~ 4.6
billion years old (…so what?)
• Lamarck (1809)
– Use and disuse
– Inheritance of acquired
characteristics
• Charles Darwin (1859)
• Alfred Wallace
Principles of Natural Selection
•
•
•
Genetic variation exists among organisms in
a population, these variations are
inheritable.
Populations produce more offspring than
environment can support and therefore only
a fraction survive (struggle for existence)
Individuals best adapted to environment
(more “fit”) will survive and leave more
offspring
• …..“Survival of the fittest”
Examples of natural selection
• Moths: “industrial melanism”
• DDT and mosquitos
What is “fit” changes with a changing environment
Galapagos finches
• Variety of finches filling many ecological niches
• Ground feeders, flower and fruit feeders, insectivores,
woodpecker finch, warbler finch
• Evolutionary divergence in < 3 million years
Island speciation in Galapagos finches
• Some islands have only one
species
– No competition for seeds
– beak sizes have a larger range
of variation
– “Generalists”
• Other islands have > 1 species
– Competition for seeds
– Leads to character displacement to
reduce competition
– “Specialists”
Character displacement and
biodiversity
• Helps explain how so many
species are able to coexist
• Competitive exclusion principle:
Two species that have exactly the
same requirements (niches)
cannot coexist in the same habitat.
• However, species that require the
same resources can coexist by
utilizing those resources under
different environmental conditions
(or niches)
• Also called “resource partitioning”
or “niche partitioning”
Speciation
• Speciation = origin of new species
– Central phenomenon of evolution
– Evolution ≠ speciation
• When is a subpopulation defined as a new
species?
• How do genes usually flow through a
population?
• Reproductive isolation prevents gene flow
and allows 2 populations to become
distinct.
Geographic isolation and
migration
If two populations
are geographically
isolated from each
other for a long
time, they may
change so much
that they cannot
reproduce
East ern Redbud
Cercis canadensis L.
California redbud
Cercis occident alis
Torr. ex Gray
Genetic drift
• Changes in the frequency of a gene in a population
due to chance (not mutation, natural selection, or
migration).
• Mostly an issue in small populations (endangered
species)
• Genetic variability is low in small populations, so
their ability to adapt to future changes in the
environment is low.
Where can expect to find high
biodiversity or low biodiversity?
Higher diversity in complex environments
• Larger number of niches in heterogeneous environments
• Also, high diversity at a supporting trophic level leads to high
diversity.
“Paradox of the Plankton”
• seemingly simple environment, many
species, no competitive exclusion
• environmental complexity can still
account for significant portion of diversity
• need just two limiting resources
Environments can be complex when
spatial component added
Highest diversity at intermediate
disturbance levels
• Intermediate Disturbance Hypothesis
– low disturbance, competitors dominate
– high disturbance, only a few stress-tolerators
Highest Diversity in Low Nutrient
Environments
What leads to low diversity?
• Environmental stress, extreme
environments, extreme disturbance, or
limitation of an essential resource
• Geographic isolation (real or ecological
islands)
• Recent introductions of exotic species