(1) Geographic barriers, for example, rivers, mountains, and roads.

(2) Differences in courtship behavior or fertile periods may result in organisms that breed only with individuals that aremost similar to themselves.

Contributors to Reproductive Isolation

The process in which one species gives rise to many species, is called adaptive radiation.

The process of adaptive radiation is also known as divergent evolution.

In adaptive radiation, a number of different species diverge, or move away, from a common ancestral form.

Speciation and Adaptive Radiation

Speciation and Adaptive Radiation

Adaptive radiations among different organisms often produce species that are similar in appearance and behavior.

This phenomenon is known as convergent evolution.

Convergent evolution has produced many of the analogous structures in organisms today.

Speciation and Adaptive Radiation

Analogous structures are similar in appearance and function, but they have different origins.

Due to having different origins, analogous structures usually have different internal structures.

For example, the wings of butterflies, birds, and bats are used for flight but are different internally.

Speciation and Adaptive Radiation

Geneticists have shown that an allele can become common in a population by chance.

This kind of random change in the frequency of a gene is called genetic drift.

Possible reasons for genetic drift are:

(1) An individual with a particular allele may produce more offspring just by chance.

Genetic Drift

(2) Environmental events wipe out many individuals who do not carry a particular allele.

Genetic drift implies that all characteristics of an organism do not have to contribute to its fitness.

For example, the Indian rhinoceros has one horn and the African rhinoceros has 2.

Genetic Drift

Although natural selection and genetic drift

are both powerful forces of change, they do not cause genetic alterations in all species all the time under all conditions.

One example of such a species is the horseshoe crab, Limulus.

Its living members are nearly identical to ancestors that lived hundreds of millions of years ago.

Such organisms are often called living fossils.

Unchanging Gene Pools

Darwin felt that biological change was slow and steady.

The theory that evolutionary change occurs slowly and gradually is known as gradualism.

In many cases the fossil record shows groups of organisms that have changed gradually over time.

But there is also evidence that evolution occurs in spurts of rapid change followed by long periods with little or no evolutionary change.

Gradual & Rapid Evolutionary Change

This theory is called punctuated equilibrium.

Punctuated equilibrium was proposed in 1972 by biologists Niles Eldredge and Stephen J. Gould.

The history of life on earth has now been proven to contain several events that add credence to this theory.

Gradual & Rapid Evolutionary Change

Gradual & Rapid Evolutionary Change

For example, six hundred million years ago all invertebrates seemed to have come into existence within a relatively short period of time.

Currently, neither gradualism nor punctuated equilibrium is universally accepted.

Gradual & Rapid Evolutionary Change