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Historical Biogeography of The Tropics
Tropical Ecology has changed over time as a result of dynamic biogeography. Biogeography is the geographical distribution of plants and animals, and has a large impact on the course of evolution. Understanding the historical biogeography of the tropics is important for learning about the origins of biodiversity. Evolution around the world is impacted by the changing species distribution across tropical biomes because many species can be traced back to tropical ancestors. Species evolve rapidly in the tropics, changing the habitat ecology. The theory of a latitudinal species gradient suggests that species richness increases towards the equator for most taxa. This holds true for marine and terrestrial species, northern and southern located species, ectotherms and endotherms and active and passive dispersing species. By turning to the biogeographical history of the tropics, we gain a deeper understanding of how the current tropical ecology developed.

Biogeography is related to the physical conditions and geography of the habitat. The temperature and geographical area of the tropics enforce the latitudinal species gradients by facilitating higher rates of speciation. Tropical biomes are defined as warm, moist habitats, and cover the largest amount of the terrestrial area on earth. Tropical species do not face the evolutionary pressure of harsh climates experienced in northern and southern regions. Therefore, tropical adaptations are largely influenced by biotic interactions as opposed to climate pressures. Consequently, tropical evolution has been described as a “creative process” among biota rather than a survival process against the climate. Some hypotheses also suggest that the higher tropical temperatures affect speciation at the molecular level, leading to higher mutation rates, faster generation times and therefore faster rates of genetic divergence. The West Pacific Coral Triangle, a tropical ocean region, is the global peak of marine diversity because of its combination of warm temperatures, large area, minimal seasonal climate variation, diverse habitat types, important biotic relationships and influx of taxi from other tropical regions because of a complex mainland-island structure and ocean currents.

Tropical Glacial and Interglacial Periods
The Paleocene period experienced warm air and water temperatures, with tropical terrestrial biomes reaching as far north as current day London and tropical seas extending into the arctic. The Paleocene period was the first epoch of the Cenozoic era, taking place from 66 - 56 million years ago. The temperature gradient from the equator to the poles was extremely reduced. Cooling trends began about 45 million years ago, with interspersed warm periods. During this period, species diversification increased because glacial fragmentation of tropical biomes facilitated speciation. This cooling also gradually restricted taxa into shrinking tropical regions. The geographic and temperate history of the Cenozoic period is reflected in tropical ecology and more specifically the latitudinal species gradient. Tropical species have experienced a longer and less disturbed evolutionary history than species found in other biomes around the world. Therefore, temperate species that initially radiated out of tropical biomes are often nested in the phylogenies of tropical clades. The longevity of tropical environments constitutes species composition with more age variance and diversity.

The Quaternary glacial cycles that occurred during the Pleistocene era were the most recent major climate disturbance in the tropics. Glaciers caused the decline of life forms at high altitudes because they remained solid for the longest consecutive time at high altitudes. Tropical environments became interspersed by glaciers and dry climates. This fragmentation resulted in speciation due to geographic isolation, steepening the latitudinal species gradient. Today, Peru contains 70% of all tropical glacial area, hosting the two largest tropical glaciers: the Quelccaya ice cap and the Nevado Coropuna ice cap. Monitoring the shrinkage of these glaciers is important for understanding the rate of glacial melt. From 1998 to 2020, 26% of the Quelccaya ice cap melted and the average altitude of the ice cap rose from 5389 meters to 5449 meters. The degree of change in tropical glacial areas is rapidly accelerating. Future research aims to understand current glaciation trends in the tropics in order to give context to the relationship between tropical ecology and climate change.

The geography and biology of the tropics has the longest history of any biomes found on Earth today. Tropical glacial and interglacial cycles caused by a fluctuating climate have induced speciation in the tropics, explaining its outstanding ecological diversity.