Chapter 24 Macroevolution and Speciation

Macroevolution Macroevolution refers to any evolutionary change at or above the species level. Speciation is the splitting of one species into: two or more species or the transformation of one species into a new species over time

What is a species? Linnaeus separated species based on morphology, i.e., their traits differed Darwin saw that similar species are related by common descent. Ernst Mayr (1942) developed the biological species concept: a species is a group of interbreeding populations that are reproductively isolated from other such groups.

*The biological definition of a species - members of one species interbreed and have a shared gene pool, and each species is reproductively isolated from every other species.

Species are based on interfertility, not physical similarity. For example, the eastern and western meadowlarks may have similar shapes and coloration, but differences in song prevent interbreeding between the two species. In contrast, humans have considerable diversity, but we all belong to the same species because of our capacity to interbreed. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Speciation The key to speciation is reproductive isolation. Two types: Anagenesis – small, progressive evolutionary changes over long periods Cladogenesis – 2 or more populations of an ancestral species split and diverge, also called divergent speciation Anagenesis

Cladogenesis

Allopatric Speciation Allopatric speciation occurs when new species result from populations being separated by a geographical barrier that prevents their members from reproducing with each other. While geographically isolated, variations accumulate until the populations are reproductively isolated. Examples: rivers shifting their courses, glaciers melting, mountain ranges forming, large lakes diminishing into several smaller, geographically separated pools Ex. Kaibab squirrel

Sympatric Speciation Sympatric speciation would occur when members of a single population develop a genetic difference (e.g., chromosome number) that prevents them from reproducing with the parent type. The main example of sympatric speciation is in plants. Failure to reduce chromosome number produces polyploid plants that reproduce successfully only with polyploids.

Evolutionary Change Can Occur Rapidly or Gradually Punctuated Equilibrium – history of a species, long periods of stasis (little or no evolutionary change) are punctuated, or interrupted, by short periods of rapid speciation i.e. evolution proceeds in “spurts” Gradualism – evolution proceeds continuously over long periods Difficult to observe because fossil record is not always complete Maintains that populations evolve slowly by accumulating adaptations due to selective pressures

Adaptive Radiation The evolutionary diversification of many related species from one or a few ancestral species in a short time period. Ex. Hawaiian honeycreepers Ex. Finches of Galapagos Islands 13 species from 1 founder mainland species