Chapter 23 Species and Their Formation
the mechanisms by which a population splits into new species and how such separations are maintained. We will look at
different factors that can make speciation a rapid or a very slow process. Finally, we will look at the conditions that give rise to
the great species diversifications known as evolutionary radiations.
23.1 What Are Species?
The word species o]ooÇuvU^l]vX_tv}Pv]Ìv]v](ÇuvÇ]ÇZ]
appearance and over large geographic areas because many species change little in appearance (ex.,
birds and lizards). More than 200 years ago, Swedish Carolus Linnaeus originated the binomial system
of Latin nomenclature classification based on appearance which is a morphological species concept.
Although the Linnean system seems practical, not all species look alike, take male red-winged blackbirds
and female brown ones for example. They are sexually dimorphic.
Species form over time. Each species has a history that starts at a speciation event and ends at either
extinction or another speciation event, at which it produces two daughter species. It is often gradual.
Speciation is the process by which one species splits into two or more daughter species, thereafter
evolving its own lineages.
An important component of speciation is the development of reproductive isolation. It is where
organisms mate with each other, but not with other populations, constituting a distinct group. They
become separate branches of the tree of life. Ernst Mayr in 1940 proposed the following definition of
species, known as the biological species conceptW^^]P}µ}(µooÇ}potentially
23.2 How Do New Species Arise?
Not all evolutionary changes result in new species. A single lineage can change without speciation. If tow
populations are isolated from each other, and sufficient differences in their genetic structure
accumulate, then the two populations may not exchange genes. Gene flow may be interrupted in two
Allopatric speciation (allo, different; patris, country) or geographic speciation requires almost complete
genetic isolation. The physical barrier that divides the range of a species may be a water body or a
mountain range, or dry land. Barriers can form when continents drift, sea levels rise, glaciers rise and
retreat, and climates change. This can happen when members of a species immigrate past a barrier and
found a new isolated population.
arid lands to forested mountains and are far enough that a population will not diverge any
farther to another island.
x Many of the 800 species of fruit fly genus Drosophila in the Hawaiian Islands are restricted to