Macroevolution: Speciation and Phylogenesis

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Macroevolutionary Processes

Evolution of Humans

Evolution of Horses

Horses are thought to have evolved in North America from dog-sized herbivores. Slow adaptation to open, grassland habitats brought changes in the structure of the limbs by the gradual reduction of toes touching the ground. This has enabled the animals to run faster and succesfully escape predators. At the same time, the changing diet and feeding pattern (browsing to grazing) transformed the size and structure of dentition and individual teeth as well. Horses later spread into Asia where they have survived until today. The last wild horses (called Przewalskii horse [Equus przewalskii] after the Russian explorer who rediscovered them in the nineteenth century) live in very small numbers somewhere on the steppes of western Mongolia.

Evolution of Flight

Gradual Evolution and Punctuated Equilibrium

Species may arise by gradual transformation, or by separation from the ancestral lineage.

Speciation may take the form of slow, gradual accumulation of phenotypic differences over time (gradualism), or abrupt changes in the phenotype that will remain relatively constant over long periods of time (punctuated equilibrium). The latter was suggested by Niels Eldredge (Museum of Natural History, New York) and Stephen J. Gould (Harvard University) to explain the frequently observed pattern in the fossil record.

Phylogenetic Reconstructions

Using molecular data and phylogenetic methods, scientists were eventually able to answer the question whether the giant panda is a bear or not. The lesser panda, however, is a closer relative of the racoon.

On phylogenetic trees, species that are connected with the smallest number of nodes (locations of branches on the tree) are the most closely related.

Molecular Evolution

Based on pairwise comparisons between different taxa, different proteins, such as cytochrome-c (an electron transport molecule in cells) evolve at a constant rate. This rate then can be used to determine the degree of relatedness, or time of divergence of different taxa.

Long-term Trends in Biodiversity

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