Formation of Species

Section 3

The Concept of Species

• Speciation – species formation.

• Existing species are changed versions of older species.

Morphological Concept of Species• Morphology – internal and external structure

and appearance of an organism.CONCEPT:• Species is defined according to their

morphology.PROBLEM:-phenotype differences amongspecies.

Biological Species Concept

• Proposed by Ernst Mayer 1904• A species is a population of organisms that can

interbreed and produce healthy offspring.• They cannot breed with other species.

Isolating Mechanisms

• Speciation begins with isolation.

Geographic Isolation

• Physical separation of members of a population• Ex. Deep canyon, river, crying climate in a valley• When the subpopulations become isolated, gene flow

between them stops.• Natural selection and genetic drift cause the two

subpopulations to diverge, eventually making them incompatible for mating. (2 new species)

Example

• Pupfish• Death valley has many ponds.• Each pond has a different species of fish that lives only

in that pond.• Why did this happen? The valley was covered by a

lake during the last ice age. • The ponds formed when the ice age ended.• Fish species became isolated and the environments of

each pond differed enough that the separate populations of fish diverged.

Reproductive Isolation

• Results from barriers to successful breeding between population groups in the same area.

Two types of Reproductive Isolation:1. Prezygotic isolation- occurs before

fertilization2. Postzygotic isolation- occurs after

fertilization.

Prezygotic Isolation

• Mating calls that are not recognized by the mate.• Difference in mating seasons.• Mechanical Isolation – difference in anatomy not

allowing them to mate.

• Ex. Wood frog and leopard frog, mating calls and mating times differ so they do not mate in the wild.

• They do mate in captivity.

Postzygotic Isolation

• Offspring our not healthy (sterile).

• Zygote dies after fertilization.

• Offspring die quickly.

Rates of Speciation

• Speciation sometimes requires millions of years but some can form more rapidly.

• Punctuated equilibrium – sudden shift or change; species arise quickly or die quickly.

• Homework• Pg. 312• 1-6