L12 - Diversification: Adaptive Radiation

Question 1

What is adaptive radiation?

Answer 1

• Adaptation: Evolutionary changes shape traits allowing organisms to adapt to their environment, and convey fitness advantages
• Radiation: A sudden abundance of open niches

Question 2

Define ecological niche!

Answer 2

• The specific place a species holds in its biotic and a biotic environment
  
  • E.g., the physical space where it feeds/reproduces etc, role it plays in the food chain of its community, and its interactions with other organisms
  • No two species can occupy the same niche: one will outcompete the other

Question 3

Example: What are some components of the niche of a squirrel in Montreal?

Answer 3

• Trees & ground
• Pizza, contents of my bird feeder (raccoons will go into the compost whereas squirrels will not )
• Active during the day (VS raccoons are active at night)
• Cat predates squirrels, but probably not raccoons

Question 4

What kind of scenario would suddenly cause an abundance of unfilled niches?

Answer 4

• Extinctions & mass extinctions
• Large-scale disturbances that modify the habitat (fire, hurricanes, earthquakes)
• Dispersal to a new environment (like an island)
• Evolution of traits that open up new niche opportunities (e.g., birds learning to fly)

Question 5

Physical opening of new niches?

Answer 5

• Galapagos Islands: Darwin’s finches
• East African Rift (plate divergence) Lake: one of largest adaptive radiations we see today
  
  • 250 M Cichlids

Question 6

Emptying niches: End-Cretaceous Extinction

Answer 6

• Mass extinction (meteor impact and possibly other factors)
• <90% of mammal species may have been
  lost during the End-Cretaceous extinction
  too
• Loss of dinosaurs = abundance of empty niches
  
  • Led to incredible adaptive radiation in mammal species, that continues to this day

Question 7

Why did mammals recover and dinosaurs
did not?

Answer 7

• Small size: Fast reproduction rate (could fill niches quite rapidly)
• Generalist habit: could survive on a diverse
  range of foods (not picky eaters)

Question 8

Another example of adaptive radiation: Cambrian Explosion

Answer 8

• Rapid emergence of new traits: hard bodies, predation, eyes - creating new niches
• Increased levels of O2 created new environmental niches

Question 9

Adaptive radiation of Angiosperms (flowering plants)

Answer 9

• Experienced an extreme adaptive radiation after initial emergence of the innovation
• Angiosperm species increased at the expense of all other plant groups
  
  • New pollination niches
  • Major diversification of angiosperm pollinators and angiosperms
• First angiosperms arose in forests dominated by conifers
  
  • Angiosperms grow faster than conifers
  • Lots of annual angiosperms
  • Decomposing angiosperm leaves tend to make the soils more neutral, where conifers tend to prefer more acidic soils

Question 10

What is convergent evolution?

Answer 10

• INDEPENDENT evolution of a particular trait in two or more (unrelated) lineages where the ancestors of these lineages didn’t already have these traits
• Tusks in mammoths and elephants is not an example, they have a common ancestor
• Wings in birds of different species = example of convergent evolution
• Duck bill and platypus = convergent
• Legless lizards and snakes = convergent

Question 11

Flight and convergent evolution?

Answer 11

• Shown up in evolution at least 4 times in very diverse lineages
• Common ancestors of mammals and birds were flightless, the “wing” limb that they shared has evolved in different ways

Question 12

Venom injection and convergent evolution?

Answer 12

• Different lineages of venom injecting snakes cobras and vipers (not related)
• Platypi (from their feet)
• Insects

Question 13

Plants and convergent evolution?

Answer 13

• The North American cactus and the African Euphorb (not a cactus, but looks like one) didn’t share a common ancestor since before the continents split apart
• Common ancestor looks nothing like either of them, and yet they both look very similar and occur in similar environments

Question 14

Crab body-plan and convergent evolution?

Answer 14

• Carcinization: urge to become a crab
  
  • Good body plan: protective, can move very fast, ecologically successful
  • Whole lineage that is “false crabs”, imitating crabs body structure
• Also: urge to become a worm

Question 15

Photosynthesis and convergent evolution??

Answer 15

Phylogeny of seed plants shows that alternative photosynthesis paths evolved independently many times in diverse lineages

Question 16

What is the mechanism of convergent evolution?

Answer 16

• Natural selection and its selective forces can manifest similarly on organisms that are in similar environments/occupying similar ecological niches
• Sometimes, relatively small number of mutations required to make a given trait in a given lineage

Question 17

What is mimicry?

Answer 17

• Selective pressures causing one species to evolve to resemble the another
  
  • Ex: non-toxic species evolving to resemble an unrelated toxic/dangerous one to deter predation
• More energy conserving than actually being toxic alone

Question 18

What is parallel evolution?

Answer 18

• Similar development of a trait in distinct species that are not closely related, but share a similar original trait, in response to similar evolutionary pressure

Question 19

Example of parallel evolution: Mammals

Answer 19

• Placental mammals (us) and marsupials (with pouches) diverged 100M years ago (had a common ancestor)
• The two lineages were geographically isolated but still have evolved really similarly
  
  • Australia: evolved a marsupial that looks a lot like a wolf, and north America evolved actual wolves
  • Also an Australian equivalent of an ocelot

Question 20

What is the founder effect?

Answer 20

• New population colonizing an “island” = Founder Population
  
  • Smaller than source population
  • Reduced genetic diversity
  • May also have harmful recessive founder mutations
  • Example for deleterious mutation among humans: carrier gene for sickle cell anemia
• Founder mutation: rare mutation in one of the founding members of a population
  
  • Ex: Tristan da Cunha

Question 21

Does the founder effect lead to faster evolution?

Answer 21

• Small gene pools allow for increased chances that a new mutation spreads quickly

Question 22

What is the genetic bottleneck effect?

Answer 22

• Island colonization or intense population reduction (near extinction)
• Genetic diversity is reduced: weakening the population’s ability to adapt to selective pressures

Question 23

How do we expect degree of genetic diversity to differ between say African and American populations?

Answer 23

• All Homo sapiens’ genes moved out of Africa on pretty recent timescales
• Today we see:
  
  • Europe has lower genetic diversity
  • Africa has higher genetic diversity
  • South America has lowest genetic diversity
  • Greater proportion of deleterious mutations in European populations