CHAPTER 14The Origin of Species The origin of new species is called speciation
What is a species? • Similarities between some species and variation within a species can make defining species difficult • The biological species concept defines a species as • a population or group of populations whose members can interbreed and produce fertile offspring
The Evolution of Species • The evolution of new species, a process called speciation occurs when members of similar populations no longer interbreed to produce fertile offspring within their natural environment.
MECHANISMS OF SPECIATION 1. Reproductive isolation can result in speciation Prezygotic and postzygotic reproductive barriers prevent individuals of different species from interbreeding • 2. Geographic isolation can lead to speciation ~Geographic isolation occurs whenever a physical barrier divides a population
Geographic isolation occurs whenever a physical barrier divides a population • When a population is cut off from its parent stock, species evolution may occur • An isolated population may become genetically unique as its gene pool is changed by natural selection, genetic drift, or mutation • This is called allopatric speciation
Islands are living laboratories of speciation • On the Galápagos Islands, repeated isolation and adaptation have resulted in adaptive radiation of 14 species of Darwin’s finches
Adaptive radiation on an island chain • The evolution of many diversely-adapted species from a common ancestor is called an adaptive radiation.
. The Evolution of Species • When geographic isolation divides a population of tree frogs, the individuals no longer mate across populations • Tree frogs are a single population.
The Evolution of Species • The formation of a river may divide the frogs into two populations.
The Evolution of Species • Over time, the divided populations may become two species that may no longer interbreed, even if reunited
New species can also arise within the same geographic area as the parent species • In sympatric speciation, a new species may arise without geographic isolation • A failure in meiosis can produce diploid gametes • Self-fertilization can then produce a tetraploid zygote • Any individual or species with a multiple of the normal set of chromosomes is known as a polyploid.
The tempo of speciation can appear steady or jumpy Speciation rates: 1. Gradualism:new species evolve by the gradual accumulation of changes brought about by natural selection • However, few gradual transitions are found in the fossil record
Speciation rates 2. Punctuated Equilibrium(proposed by Niles Eldredge and Stephen J. Gouldin 1972) • The punctuated equilibrium model suggests that speciation occurs in spurts • Rapid change occurs when an isolated population diverges from the ancestral stock • Virtually no change occurs for the rest of the species’ existence