[MUSIC] Okay, your key lesson from what we have seen so far. Continued diversification happens as the radiating groups move into new environments that unfold as a consequence of the Earth's history. But note that not all lineages respond, so innovations may also be required. Very few of the South American bird groups managed to respond to the establishment of the Panama Isthmus. But the song birds on the northern hemisphere did so very efficiently, maybe because they were migratory and adapted to tolerate a wide climatic range already. There was an enormous increase as with evolution of more than thousand species of sparrows, tanagers, warblers, orioles, etc. Very soon they reached a similar diversity pattern in South America as the suboscine birds as we can see on the map here. And there was a very high rate of speciation now along the Andes and down in the southern savanna regions. There is little sign of competition, because the new groups had different ecological adaptations as the old South American groups. For instance, they could digest grain, grass seeds which the old South American groups could not do. So the new groups filled up the continent in about the same way as the old groups had done. On this graph we see a divergence of sister groups up through the phylogeny. Each dot is a comparison of two sister groups where we on the y-axis can see the correlation coefficients and the x-axis shows the time since divergence. And we can see that all the groups that diverged long ago, more than 40 million years ago, they have almost the same age and diversity pattern/ correlation coefficient of about 90%. This is based on all pairwise comparisons of clades with more than 10 species in each. The dot at 69 million years ago shows the convergent similarity between the New World groups and the songbirds which colonized from the North during the upper Tertiary. Also, the deep clades among the neotropical passerines show very similar spatial distributions and the plus mark shows the comparison between the diversity of lineages that existed in the late Oligocene, 25 million years ago, and the present day diversity pattern. We can see it is about the same. But the recently divergent groups differ greatly in this spatial distribution. Though, although the theory of niche conservatism should predict closely related species to inhabit similar environments, we see a predominating tendency to expand into new environments, move into new geographical areas. One of the ideas to explain the high diversity of species in the tropics is called the Tropical Niche Conservatism Theory. This assumes that the whole world was warm in the past, much warmer than today, and that species were all adapted to warm conditions. And actually experiments with heat tolerance of species, or heat preference, show that most species actually like it warm. When the globe then cools down, during from about 15 million years ago, all these warm adaptive species are forced towards the equator, so we get the very large concentration there. Species differ only in their ability to tolerate cold. So, some species adapt to the colder conditions, and then are able to move out of the tropics, towards the poles to utilize the other resources that exist there. We have a high dynamism, out of the tropics, into the tropics there are different possibilities, but a high turnover, that high turnover in the harsh environment, since towards the poles conditions are more fluctuating, species may adapt to these conditions for a while, but then they simply are wiped out again. But a high persistence in the tropical rainforest leads to gradually a buildup of an extraordinary tropical diversity; therefore, a general pattern develops convergently over tens of millions of years among different groups. They all reach an equilibrium with most diversity in the tropics and fluctuating lower diversity up on the higher latitudes. [MUSIC]
