Macroevolution and Speciation Evolution creates (and destroys) new species, but … What is a species? Its not as straightforward a question as most believe. These are members of different species - eastern (left) and western (right) meadowlark. What is a Species? There is only one extant (existing) human species. What is a Species? And these are all members of a single species. Determining What Is and What Isn’t a Distinct Species Can Have Economic Consequences Northern spotted owl (left) and barred owl (right). What is a Species? The definition we’ll use is this: A species is a group of individuals capable of interbreeding to produce fertile offspring. This is the biological species concept. Like all attempts to define a species, it has many problems. One Problem in the Biological Species Concept For asexually-reproducing organisms, like these bacteria, what constitutes a species? …and another problem with the Biological Species Concept… Where do you draw the line between different ‘species’? How Many Species Are There? We don’t know. About 2 million species have been described. Estimates of existing species number range from 4 million to 100 million (with 10-15 million being a more commonly considered upper estimate). How did this diversity of life come to be? Species = basic unit Continuous lineage - information passed through genes Speciation - rise of new species Two Patterns of Speciation How Do Species Arise? The key to speciation is reproductive isolation of populations. There are extrinsic and intrinsic reproductive isolating mechanisms. Geographic isolation is the primary extrinsic reproductive isolating mechanism. Process of speciation: Parent 2 separate species species I. Barrier II. Diverge III. Don’t interbreed Reproductive Isolation May Occur With or Without Geographic Isolation Allopatric speciation occurs when geographic isolation creates a reproductive barrier (an extrinsic mechanism). Sympatric speciation occurs when a reproductive barrier is created by something other than geographic isolation (intrinsic mechanisms). Allopatric Speciation Harris’ antelope squirrel White-tailed antelope squirrel Two species of ground squirrel are postulated to have descended from a common ancestral population that was separated by formation of the Grand Canyon. Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation (different habits within an overlapping range) Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation Courtship rituals, like these, are critical for mating within a species, but ineffective for attracting members of other species. Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation Many Intrinsic Reproductive Isolating Mechanisms Drive Speciation Hybrid Infertility Was the Impetus for Cloning a Mule Speciation Dynamics - Gradualism or Punctuated Equilibrium? Gradualism-species is constantly making small changes over a long period of time until they are a new species Punctuated equilibrium: new species evolve fairly rapidly and then stay the same for a long time Punctuated equilibrium appears to be a more accurate view of speciation dynamics. Does Evolution Create the Perfect Organism? No, only better organisms as evolution is constrained by history and buffeted by random events. Essentially, every organism on earth is in significant part a sum of accidents. Species Come and Go Best estimates from the fossil record indicate that greater than 99% of species that have existed are now extinct. A typical “lifetime” for a species is about 1 million years. Mass Extinctions Are a Fact of Life 90% of all species that have ever existed are now extinct … The Cretaceous/Tertiary Mass Extinction Gary Larson Are We Now Causing a Mass Extinction? .