Transcript Lecture 17, adaptive radiation + ecology

Major themes in 100A
Evolution
- descent with modification
adaptations
- co-evolution among unrelated organisms
 mutualisms
- radiations: when lineages diversify (= become different)
from each other and from their ancestors, and
fill many different ecological niches
Ecology
- alternative stable states
- ecosystem engineers: corals, trees
- ecosystem services: earthworms, pollinating insects
- forces that maintain versus threaten biodiversity
- keystone species
Major themes in 100A
Evolution
- descent with modification
adaptations
Natural selection is good at taking a feature and tweaking it to
serve a new function
- sometimes, a key adaptation opens up a whole new range
of ecological niches (like dry land)
Why do bryophytes, hermit crabs, and frogs still depend on
water to complete their life cycle?
What adaptations allowed insects, snails and reptiles to
complete the transition to terrestrial life?
Adaptive Radiations
The fossil record shows that most species don’t change much,
over long periods of time
Groups of “living fossils” show no morphological change for
hundreds of millions of years
- horseshoe crabs, coelocanth fishes, nautilus
Then there are adaptive radiations, periods of intense change
when a group undergoes an explosive diversification into many
ecologically different species
- many new types of species appear in a short period of time,
all derived from the same common ancestor
Adaptive Radiations
Radiations often follow mass extinctions, when the dominant
organisms are wiped out  new ecological opportunities
open up for the lucky survivors, who fill all the empty niches
Mass extinction is when >60% of all living species are wiped
out within 1 million years
Results from extreme, sudden, temporary change in the global
environment
 rapid change in conditions can eliminate whole groups of
organisms – not just species, but entire families or classes
“Big 5” mass extinctions
Each of these events eliminated 20-60% of all families
of plants and animals on the planet
- not species, not genera -- whole families wiped out
end-Permian extinction: 90% of all marine
species gone
end-Cretaceous extinction,
65 million years ago:
bye-bye dinosaurs
Background extinction = natural selection eliminating poorly
adapted lineages; this is happening all the time, at a low rate
Adaptive radiations
Until 65 MYA, dinosaurs filled all available ecological niches
- air
- water
- land predators
- herbivores
- air
- water
- big herbivores
- predators
# of families
Plant Evolution following Mass Extinctions
Seedless
plants
Gymnosperms
dominate
Angiosperms dominate
First
First
gymnosperms angiosperms
Note that each major group of plants appeared long before
that group became dominant
could not take over until a mass extinction ( )
wiped out the group that was previously dominant
Mackenzie 2003
The Cambrian Explosion
565 million years ago, only sponges + cnidarians, maybe some
simple worms were here
Just 40 million years later, all animal phyla were present
- arthropods, annelids, molluscs...even primitive vertebrates
- some forms so wild, they can’t be classified
All major body plans appeared “overnight,” in geological terms
Since the Cambrian Explosion, no new animal phyla evolved
This diversification in such a short window of time was the
most important adaptive radiation in the history of life
Why did so many new body plans evolve in such a short time?
What led to the Cambrian explosion?
3 hypotheses for the explosive innovation in body plans:
(1) major groups diversified due to changes in new master
control genes that regulated development
(2) higher O2 levels made larger, more complex bodies possible
(3) ecological interactions
a) arms races (between predators and prey) fuelling diversity
b) mass extinction may have wiped out dominant jellyfish
allowing other life forms to thrive and diversify
(1) Changes in developmental genes
Network of developmental control genes was in place in
the earliest bilaterian animal; present in all modern animals
- govern how complex bodies develop from a fertilized egg
- mutations affecting when these master genes switch on
big changes in the body that develops
Flies with a single mutation grow legs where antennae go
antenna
leg
fly
head,
normal
mutant
fly
head
5 things to know about ecology
1) ecosystems have alternative stable states
2) ecosystem engineers create habitat from their bodies
or changes they make to the environment
(corals, trees, beavers, deep-sea vent worms)
3) ecosystem services: our profit from natural processes
(earthworms, pollinating insects, decomposers)
4) biodiversity = ecosystem stability
- protection from invasive species, fisheries collapse
5) keystone species: lose one species  lose 1000s
of other, dependant species
1. Alternative stable states
ecosystems are often in balance, or equilibrium, meaning they
go on without changing for long periods of time
A) however, this doesn’t mean the ecosystem will return to that
state following a disturbance
B) same community of species living in same environmental
conditions may end up in different “stable states” – meaning,
they can reach a very different balance
- may depend on random fluctuations in population sizes
- may reflect different starting conditions (which species
was initially more abundant, for example)
Example: Rainforest trees
Common trees produce few seeds
Rare trees produce many seeds
What happens if you cut down the common
trees?
The formerly rare species may now take over,
out-competing what used to be the common species
Ecosystem may never return to its previous balance after
a disturbance (for example, human-caused de-forestation)
Two nearby African islands have very different communities
Malgas
Marcus
- 70% of animals are lobsters
Why are there lobster on one
island, and snails on the other?
- beds of mussels, algae &
snails cover the rocks
- snails can’t open healthy
mussels
Two nearby African islands have very different communities
Malgas
- 70% of animals are lobsters
- snails are immediately preyed on
(eaten) by lobsters
Marcus
Two nearby African islands have very different communities
Malgas
- lobsters transplanted to Marcus Island
were swarmed by >300 snails, and
totally eaten within 15 minutes !
 thus, prey become the predators
when their starting numbers
are high enough
Marcus
2. Ecosystem engineers
ecosystem engineers modify their environment, building
physical structures that serve as habitat for other organisms
- can be their bodies: corals, trees
2. Ecosystem engineers
ecosystem engineers modify their environment, building
physical structures that serve as habitat for other organisms
- can be their bodies: corals, trees
- can be their homes, or products of their activities
beaver dam
termite mounds
 these organisms increase biodiversity (understand why)
3. Ecosystem services
We are used to paying for human services (economics), but
are used to getting free services from our ecosystems
- we typically don’t notice these services until they disappear,
due to disruption of an ecosystem by our activities
 people need to recognize the economic benefits of intact
ecosystems, to be motivated to conserve nature
- estimated that life on earth would end in ~6 months if
insects disappeared... why?
 example: coastal communities are protected from storm surge
by coral reefs and wetlands, both of which are
endangered by development, agriculture,
climate change and pollution
3. Ecosystem services
Colony collapse: huge loss of honeybees started in 2006
- lost 30% of bee hives in U.S. but problem is global
- bees are critical pollinators for 1/3rd of human food crops
- source of bee die-off is unclear: combination of pesticides
and poor nutrition may make bees vulnerable to viruses,
mites and fungal pathogens
In one region of China, excessive pesticide use in the 80’s
wiped out all bees... farmers must now hand-pollinate every
flower on every tree and plant to fertilize their crops
4. Biodiversity
Biodiversity includes
- the variety of species present
- the genetic diversity they encompass
- a major goal of biology is to catalogue biodiversity, and
understand how it maintains the health of an ecosystem
 organisms interact with each other in complex ways that
maintain a natural balance, and permit many species to
co-exist together
 disruption of that balance, by removal of even one species,
can have disastrous effects on an ecosystem
Threats to Biodiversity – Invasive species
- introduced (accidentally or intentionally) to a new area
- with no native predators, quickly over-run native species,
disrupt ecosystem at a cost of $137 BILLION per year
in the U.S. alone (Pimentel et al. 2000)
“killer algae”
Caulerpa taxifolia
took over the
Mediterranean
Brown tree snake was
accidentally brought to
Guam; ate all the birds
cost $6 million
to eliminate
from San Diego
Threats to Biodiversity – Habitat loss
Over half of all species predicted to be gone by end of this
century; each plays a role in its ecosystem that will be lost
habitat loss is implicated in 3/4th of extinctions
- 7% of earth is currently set aside as nature preserves
- a major goal now it to preserve hotspots of biodiversity
Together, 1/3rd of all plants and vertebrates inhabit only 1.5%
of the earth (mainly in rain forests and chaparral)
By recognizing its economic value (ecotourism, ecosystem
services), local people may be convinced to preserve the
biodiversity and health of their ecosystems
Environment & human health
The link between environmental health and human health
is increasing clear, but not appreciated by most voters
Lifetime cancer risk
(per million residents)
Why is cancer risk directly tied to race?
Asian
Latino
Black
white
American
Indian
Morello-Frosch & Jesdale, 2006 Environ Health Perspect. 114: 386–393
Why care about ecology?
The link between environmental health and human health
is increasing clear, but not appreciated by most voters
check out Natural Resources Defense Council --
http://www.nrdc.org
check out Environmental Justice link
- learn how minority communities are targeted by corporate
polluters and what is being done to combat this injustice