Adaptive radiation and the rise of mammals

Question 1

Adaptive radiation definition

Answer 1

*Rapid speciation in an area of new ecological opportunity

*“Evolution of ecological and phenotypic diversity within a rapidly multiplying lineage” (Schluter 2000)

Question 2

The process of adaptive radiation

Answer 2

1.Divergent natural selection between environments
*Different environments = different resources available = different selection pressures
*Resource availability is not uniformly distributed—intermediate phenotypes have lower fitness
*The availability of ecologically accessible resources that may be evolutionarily exploited e.g. Hawaiian islands ‘hotspots’

2.Divergence driven by resource competition
*Character displacement: “trait evolution that arises as an adaptive response to resource competition or deleterious reproductive interactions between species” - Pfennig & Pfennig 2015 Evolution’s Wedge
^e.g. an island with just a green lizard species
Then a blue lizard species arrives and the two species have exploitative competition for resources.
*selection favours divergence e.g. green lizards evolve becoming smaller and green lizards evolve becoming larger resulting in them occupying different niches

3.Ecological speciation
*When the processes described (divergent natural selection, character displacement) proceed over longer time periods, this can lead to speciation.
*“Ecological speciation” is a ‘process by which barriers to gene flow evolve between populations as a result of ecologically-based divergent selection’-Rundle & Nosil 2005, Ecology Letters

Question 3

Adaptive radiation example: rose finches
(Losos and Riclefs 2009)

Answer 3

~5.5-6million MYA an ancestral rose finch from mainland Asia found its way to the emerging Hawaiian islands over 5 million years Hawaiian Honey creepers radiated into 56 distinct species with different dietary specialisations.

The majority of species in this group are now extinct due to humans.

Question 4

In contrast to Non- Adaptive radiation (diversity in phylogeny without improving adaptation)

Answer 4

e.g. in Damselflys organs on males used to grasp females during reproduction are diverse

Question 5

Characteristics of an adaptive radiation (according to Schluter 2000)

Answer 5

1.Common ancestry

2.Phenotype-environment correlation

3.Trait utility

4.High rates of speciation

In combination, these four characteristics provide evidence that a clade has adaptively radiated

Example: Galapagos finches aka Darwin’s finches each exploit different food sources available on different islands and have beaks adapted to these foods some ground finches have even adapted to feed on blood in the manner of a vampire bat.
^ They all share a common ancestor

Question 6

Rapid speciation (example: finches)

Answer 6

*Phylogeny, time-calibrated using coalescent models on genomic data.
may have all diversified in less than 1 million years – very short period for this

Grant and Grant Galapagos Finch Studies on Daphne Major island:
– beaks of medium ground finches measured every year of the study allowing realtime tracking of evolution of beak size

How did drought impact them?
- Drought of 1978 led to a population crash of medium ground finches
- Due to the fact that one of the only abundant seeds at the time were large and hard.
- Requiring larger bills to survive the population phenotype changes dramatically.

Another crash occurred in 2004 drought but this time small billed birds did better – why?

Question 7

Class: Mammalia

Answer 7

*~5,500 species described

*Distinguished from other groups by:
-Jaw traits: differentiated dentition (back,front,top and bottom), middle ear bones
-Large brain relative to body size
-Mammary glands & nursing of young

*Three main clades:

1. Monotremes (~5 species)
2. Marsupials (~330 species)
3. Placentals (~5165 species)

Question 8

Mammalia: Monotremes

Answer 8

*Oviparous (egg laying)
*Hindlimb spurs
*Examples: Platypus and echidnas

Question 9

Mammalia: Marsupials

Answer 9

*Viviparous, but with short gestation – giving birth to offspring that crawl in to the pouch and develop/grow there
*Development continues in pouch
*Examples: Kangaroos, opossums

Question 10

Mammalia: Placentals

Answer 10

*Viviparous, with relatively long gestation periods
*Largest clade, with enormous morphological diversity.

Hypothesis: radiation of mammals was made possible by the extinction of dinosaurs, and subsequent availability of niches (~66 MYA K/PG Mass Extinction)

Question 11

Cretaceous-Paleogene (K-Pg) Extinction (66 Mya)

Answer 11

Caused by an asteroid between 6.2 and 9.5 miles wide
Wiping out ~75% of all species

We believe it was an asteroid as the same strata is observable worldwide high in an element rare on earth and common in asteroids.

*aka the K-T = Cretaceous/Tertiary border (nowadays K-Pg border, Pg = Paleogene)
*~ 66MYA
*Associated with the extinction of all (non-avian) dinosaurs

Question 12

A number of hypotheses that might explain mammalian radiation around this time

Answer 12

A- Explosive model
all crown placental mammals came after K-Pg
The extinction of non-avian dinosaurs led to a rapid adaptive radiation of placental mammalian lineages

B - Soft explosive
crown placental mammals began to diversify before K-Pg

C - Trans KPG
crown placental mammals began to diversify a while before K-Pg

D -Long fuse
stem but not crown diversify pre K-Pg
Interordinal diversification happened in the Cretaceous, with intraordinal diversification in the early Paleogene

E - Short fuse
stem and crown diversify pre K-Pg
The extant orders of placentals originated and diversified in the Cretaceous

A is assumed true as we have no fossils of crown placental mammals before K-Pg event

Question 13

New whole genome phylogenies

Answer 13

new methods show: Stem orders were diversifying before K-Pg but crown after – supporting long fuse model

also see figure in notes

Interordinal diversification after K-Pg event

Some intraordinal diversification before K-Pg event

Question 14

New models of the fossil record

Answer 14

^account for fossil preservation through time

^develops models based on fossil data sets not connected to genetics or current phylogeny

More accurately assigning origin dates to fossils

Question 15

Mammalian radiation across the K-Pg boundary

Answer 15

Fossil data and molecular estimates still don’t converge on the same answer:

-Fossil record incomplete and/or

-Molecular clock techniques biased

Within the last year we are starting to see fossil records and molecular clock techiniques converging

Question 16

Body size evolution at the K-Pg boundary

Hypothesis: mammals were small prior to the K-Pg mass extinction, and small mammals preferentially survived evolving larger body size

Answer 16

Does the data support this claim?

Phylogeny and trait measurements (e.g. body mass) -> trait evolution models -> Inferences about tempo (i.e., rate) and mode (e.g., drift-like? constrained?) of evolution

Body size evolution at the K-Pg boundary

Slater 2013 Methods in Ecology & Evolution: Fitted several models to body mass data for mammals and a time-calibrated phylogeny.

including:
*Brownian Motion
*Ornstein-Uhlenbeck

*K-Pg “release and radiate”, where mass evolves via OU prior to the K-Pg boundary, and via BM afterwards

the standard way to assess support for a model is finding a p value but a new method is using AIC value instead by calculation of model support as an AIC weight.

strong support for size limitation before K-Pg event which was no longer a limiting factor after the event

Body size evolution at the K-Pg boundary
*Phylogenetic models of trait evolution are powerful tools for testing macroevolutionary hypotheses.
*Evidence from such models supports the hypothesis that body mass evolution was constrained prior to the K-Pg mass extinction, but proceeded in an unbounded fashion afterwards

Question 17

More trait evolution: tooth evolution

Answer 17

*The spread of grasslands in the Miocene corresponds to an increase in hypsodont tooth morphology of equids. Did selection drive tooth evolution to a new adaptive optimum?

Grazing vs browsing in horses:
*Imagine fitting several models to dataset of crown heights in fossil horses:
*Brownian motion
*Ornstein-Uhlenbeck (with a single optimum)
*Ornstein-Uhlenbeck (with different optima for browsers and grazers)

*Minute paper: How could you measure support for these different models? What would you expect to find if selection drove shifts in dental morphology?

Question 18

Lecture summary

Answer 18

1.Adaptive radiation is the ‘evolution of ecological and phenotypic diversity within a rapidly multiplying lineage’.

2.Adaptive radiations arise from a combination of ecological adaptation, competition, and speciation.

3.We can detect adaptive radiations by testing for patterns that are predicted to emerge from the process.

4.Galápagos finches meet the predictions of adaptive radiation theory, using comparative approaches and long-term field studies.

5.Placental mammal diversity expanded rapidly after the K-Pg boundary, but whether that expansion occurred solely in the aftermath of the extinction remains debated.

6.Fossil estimates and molecular estimates are still not in agreement, but they are getting closer.

7.Fitting several phylogenetic models of trait evolution to a comparative dataset, we can ask: which of the processes represented by candidate models is the most likely to have generated a datset like the one we observe?

8.Such models support the hypothesis that mammalian body size was constrained prior to the K-Pg boundary, but not afterward, leading to the evolution of enormous mammals after the K-Pg extinction event.