Plant of the Day Isoetes andicola Endemic to central and southern Peru Found in scattered populations above 4000 m Restricted to the edges of bogs and lakes Leaves lack stomata and so CO 2 is obtained, not from the atmosphere, but from sediment via the roots Carbon fixation occurs via the C 3 pathway by day, but via a CAM-like process at night Members of the quillwort family (Isoetaceae) are the nearest living relatives of the ancient tree lycophytes
Definition: Macroevolution Macroevolution Evolution at means or above evolution on the grand scale, and it is mainly studied in the fossil record. It is contrasted with microevolution, the study the of level evolution of the over short time periods, such as that of a human lifetime species or less. Microevolution therefore refers to changes in gene frequency within a population... Also, long-term trends, biases, or patterns (i.e. phylogeny!) in the evolution of higher taxonomic levels. Macroevolutionary events are much more likely to take millions of years. Macroevolution refers to things like the trends in horse evolution... or the origin of major groups, or mass extinctions, or the Cambrian explosion... Speciation is the traditional dividing line between micro- and macroevolution. - Mark Ridley, Evolution
Why are some clades more diverse?
Why are some clades more diverse? Net diversity = number of speciation events number of extinction events What affects the rates of speciation and extinction?
There are at least 3 different reasons for variation in species number between sister groups. 1. Stochasticity: if speciation is random, then clades with more species are more likely to be subdivided again than clades with fewer species 2. Extrinsic Factors: external factors, such as environment and geology, can affect speciation and extinction rates 3. Intrinsic Factors: a single character, or combinations of characters, moves a group into a new adaptive zone, either geographically or ecologically
There are at least 3 different reasons for variation in species number between sister groups. 1. Stochasticity: if speciation is random, then clades with more species are more likely to be subdivided again than clades with fewer species 2. Extrinsic Factors: external factors, such as environment and geology, can affect speciation and extinction rates 3. Intrinsic Factors: a single character, or combinations of characters, moves a group into a new adaptive zone, either geographically or ecologically KEY INNOVATION
What are the main extrinsic drivers of evolutionary change? Physical environment? Interaction with other species? Darwin puts a lot of emphasis on competition and predation Van Valen s Red Queen hypothesis
Red Queen hypothesis
Red Queen hypothesis The continual evolutionary change by a species that is necessary to retain its place in an ecosystem because of ongoing co-evolution by other species (and it s environment) Physical and biotic aspects of the environment of any species are forever in flux, with subtle changes in annual temperature and weather cycles, topography, food supply, predators and group dynamics the targets of selection keep changing, and so the organism is never perfectly adapted.
Red Queen vs. Court Jester Van Walen saw evidence for this through his observation of what he called the law of constant extinction : species or genera were as likely to become extinct at one time as at any other, irrespective of their geological age Paleontologists did not like that Other studies showed that extinction risk depended on taxon age Other models, collectively called Court Jester hypotheses, posit that random changes to the physical environment (e.g. climate change, tectonic events) are the key drivers of major changes in organisms and diversity.
Red Queen vs. Court Jester Evidence for either hypothesis from the fossil record? Difficult since it is hard to exclude the possibility of changes in physical environmental conditions Barnosky (2001): suggests that the different hypotheses may operate at different spatial & temporal scales Red Queen OK for local, ecosystem scale views, but in the grand schemes of things random geological events matter more Biologists tend to favor Red Queen, whereas Paleontologists tend to favor the Court Jester
Red Queen vs. Court Jester Venditti et al (2010): Test the Red Queen s prediction of constant speciation rate Used empirical data (trees from different organism groups) and tested five different statistical models for the frequency distribution of branch lengths Result: Best fit is to the simplest model of evolution, the exponential model, in which new species emerge from single events each event being infrequent but individually sufficient to cause speciation. Their interpretation: Rare stochastic events that cause reproductive isolation explain constant rate of speciation. Species do not so much run in place as simply wait for the next sufficient cause of speciation to occur. So, the saga continues
Nonetheless there are many examples of particularly diverse, and species rich clades!! Pictured here are the flowering plants.
Non-adaptive radiation: rapid speciation in the absence of ecological diversification e.g. Aegean Nigella arvensis complex (12 taxa) -similar habitats on different islands -changes in sea level allow disperal, selfing -drift Comes et al 2008
Adaptive radiation: the evolution of ecological diversity within a rapidly multiplying lineage George G. Simpson (famous paleontologist and founder of the modern synthesis) first described this term. He believed that Adaptive radiations resulted from diversification accelerated by ecological opportunity, such as dispersal into a new territory, the extinction of competitors or the adoption of a new way of life.
This view is echoed by Verne Grant When a species succeeds in establishing itself in a new territory or new habitat, it gains an ecological opportunity for expansion and diversification. The original species may respond to this opportunity by giving rise to an array of daughter species adapted to different niches within the territory or habitat. These daughter species become the ancestors of a series of branch lines when they, in turn, produce new daughter species. The group enters its second phase of development, the phase of proliferation... Adaptive radiation is the pattern of evolution in this phase of proliferation. And speciation is the dominant mode of evolution in adaptive radiation. (1977: 309)
This view is echoed by Verne Grant Caveats: 1) Few examples rigorously document a causal relationship between rapid levels of adaptive diversification and speciation genes underlying any adaptive differences must be linked to/the same as those responsible for isolation ecological speciation has been documented in plants (e.g. sago palms, pollinator isolation in Aquilegia ) 2) The environment is not divided into predetermined niches in the absence of the organisms that inhabit them
...the greatest living example of adaptive radiation in plants Dolph Schluter The 28 Hawaiian-endemic species in silversword alliance include trees, shrubs, mat-plants, rosette plants, cushion plants, and vines; all have arisen in past 5.2 million years
Andean lupines: eighty species have arisen in last 1.2-1.8 Myr Andean uplift - between 2 and 4 Myr ago - created ecological opportunities for diversification
Andean lupines: eighty species have arisen in last 1.2-1.8 Myr Andean uplift - between 2 and 4 Myr ago - created ecological opportunities for diversification
Intrinsic Factors: Key innovations Key innovations are novel morphological or behavioral traits thought either to open new adaptive zones or to offer the ability to rapidly speciate after environmental change.
What about the angiosperm radiation?
Why are the angiosperms so species rich? Darwin's abominable mystery: the origin and diversification of flowering plants 90% land plants are angiosperms Ecologically widespread
Discovering adaptive radiations Step 1: construct a phylogenetic tree for the family to find a sister group. Sister groups are the descendants of a common and unique speciation event and therefore must be of equal age!! (ensure all else is equal) Step 2: test for a significant difference in species richness between sister-groups > 90% of the total diversity in the two sister groups in species-rich genus Step 3: which sister-group is unusual in the context of the whole tree?
Discovering key innovations As described in Hodges & Arnold, traits of interest must be optimized on the phylogenetic tree. Following the origin of the putative innovation on the tree, does the diversification rate increase? (Maximum-Likelihood based tests)
Difficulties - multiple innovations may work together to affect diversification, a key complex - the innovation only has an effect after an environmental change (therefore, change in rates delayed compared to origin of innovation) - the effects of innovations may be obscured over time by the evolution of other characters - require homoplasy (a character shared by a number of organisms but not present in their common ancestor) to do statistical tests
Back to the Angiosperms Gnetales 69 Ceratophyllaceae 6 Rest of the angiosperms > 250000 Angiosperm synapomorphies, possible key innovations
Distinguishing Angiosperm characteristics (1) Closed (angio-) carpels (sperm)
Distinguishing Angiosperm (2) Double fertilization characteristics
Distinguishing Angiosperm characteristics (3) Reduced male and female gametes
Distinguishing Angiosperm characteristics (4) Stamens with two pollen sacs
(5) Flowers Distinguishing Angiosperm characteristics
Back to the Angiosperms Gnetales 69 Ceratophyllaceae 6 Rest of the angiosperms > 250000 Angiosperm synapomorphies, possible key innovations
Three other characteristics have been proposed: biotic pollination (decreases extinction risk with more precise pollen transfer) biotic dispersal of seeds (enhances probability of speciation as seeds are dispersed over a larger area/range) NO YES herbaceous growth form (shorter generation times and less stable habitats enhance chances of speciation) YES Dodd et al. 1999
Phylogenetic relationships among the seed plants remain controversial
Conclusions There is no single key innovation involved in the angiosperm radiation, but many. The picture is more dynamic than a single origin of one key innovation & subsequent diversification. Innovations are gained and lost within the Angiosperma.