Text of Major themes in 100A Evolution - descent with modification adaptations - co-evolution among...
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 dont 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
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 Adaptive Radiations
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
Mackenzie 2003 Seedless plants Gymnosperms dominate Angiosperms dominate First gymnosperms # of families First 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 Plant Evolution following Mass Extinctions
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 cant 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 O 2 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 leg fly mutant head, fly normal head antenna
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 doesnt 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 - 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 cant open healthy mussels Malgas Marcus
Two nearby African islands have very different communities - 70% of animals are lobsters - snails are immediately preyed on (eaten) by lobsters Malgas Marcus
Two nearby African islands have very different communities Malgas Marcus - 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
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 termite mounds beaver dam 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 dont 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 1/3 rd 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 80s 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) Brown tree snake was accidentally brought to Guam; ate all the birds killer algae Caulerpa taxifolia took over the Mediterranean cost $6 million to eliminate from San Diego
Threats to Biodiversity Habitat loss Over half of a