[MUSIC] So, what are the essential processes involved in maintaining large scale diversity? The Latitudinal Diversity Gradient is not just a consequence of the warm Earth history and "out of the tropics" dispersal. It goes the opposite way. Groups that originated in Antarctica or Australia dispersed and diversified into the tropics. The group that diversified in Asia colonized the Americas and underwent a terrific radiation as they reached the tropics in South America. So groups go out of the tropics and into the tropics. Diversity patterns converge and large radiating groups will, given time and opportunity to disperse end up with roughly the same similar species richness gradient that reflect the gradient of resources. Many groups succeed very well in taking advantage of new opportunities, colonizing new environments that emerge. However, not all these opportunities are long lasting. In the harsh environments in the high north, species come and go, and the number, the high turnover leads to "speciation in reverse" as we have already seen and extinction. On the other hand, the largest tropical rainforests offer predictable conditions, leading to accumulation of species over endless periods of time. The landscape dynamics in these areas may create the kind of patchwork where each of these species may permanently find a niche on their own. So we have a long term persistence of those groups that no longer diversify. They simply hang on, their extinction is very slow and the diversity is therefore maintained within these areas. The evolutionary process happens mainly through gradual adaptational change within local species assemblies something that is referred to as the Red Queen Hypothesis, based on a term introduced by Leigh van Valen in 1973. It is based on Lewis Carroll's book "Through the Looking Glass" with a Red Queen. It tells Alice now, here you see it takes all the running you can do to keep in the same place. This was taken as a metaphor for the arms race of evolutionary developments and counter-developments in complex biological communities that cause co-evolving species to mutually drive each other to adapt. Every time a species is about to be well adapted to a certain condition, the competitive interaction changes and it has to start moving into another direction. So Jim Brown suggests that the Red Queen runs particularly fast when it is hot, in other words, in the warm tropics. And it seemed that this is part of the kinetics involved in maintaining the extraordinary biodiversity in the tropics. However, the shifting interactions caused by landscape dynamics may also cause a zig zag course of evolution. You don't evolve in one particular direction. It goes in different directions because of changing selective pressure. Speciation studies often aim to identify barriers to gene flow which allow different geographical populations to diverge in independent directions, in a presumed gradual, (Brownian motion) process. However, the most remarkable diversification takes place where groups can invade new environments, even archipelagos in the ocean. New opportunities and maybe enhanced by innovation and new selection pressures. Maybe we should pay more attention to population expansion, dispersal, feeding innovation, abilities to adapt, etc. The alternative to the Red Queen hypothesis is called the Court Jester hypothesis. It is a term introduced by Anthony Barnosky at Berkeley University. This is an antithesis to the Red Queen, that environmental change, rather than biotic interaction in communities is the major force driving bio- diversity at macro scale. The Court Jester is a metaphor like a wild card, the Joker or the Tarot card. Just in biology it is the group that rapidly responds to new environmental challenges that initiate expansions. And the hypothesis implies that events random in respect to the biota occasionally change the rules on the biotic playing field, leading to accelerated responses, accelerated evolution, as in the case of rapid proliferation of modern birds and mammals after the Cretaceous/Paleogene boundary extinction crisis, or as illustrated by the global expansion of songbirds. Now to the hot spots in the tropical mountain areas. If conditions are perfectly stable, the predicted zig zag course that we may expect under the reign of the Red Queen, are no longer Valid. Species accumulation is slow and gradual, and the species may actually have a chance to stay under constant conditions, and specialize in a very specific direction until they are perfectly adapted to the environment as suggested for instance for the hummingbird example that we saw some time ago. Every hummingbird adapted to one particular flower. [MUSIC]
