Chapter 23

BiologySixth Edition

Raven/Johnson

(c) The McGraw-Hill Companies, Inc.

Speciation – the process that explains how one species can lead to several new species.

Species are the Basic Units of Evolution

Species that occur together (sympatric) are distinctive entities that are phenotypically different, utilize different parts of the habitat, and behave separately.

Species Population Community Ecosystem

Subspecies – Populations of species that live in separate geographical areas and have a different phenotype – assigned a name after the species.

Individuals in overlapping areas often exhibit a combination of features characteristic of both populations.

Biological Species Concept – ‘groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.’

Individuals that can not produce viable offspring are reproductively isolated.

What about populations that do not occur together in nature – allopatric?

However, the ability to exchange genes appears to be the hallmark of a species.

Maintaining Genetic Distinctiveness

• Prezygotic Isolating Mechanisms– Ecological (use of different habitats)

– Behavioral (color, vocalization, pheromones)

– Other• Temporal isolation, mechanical isolation,

prevention of gamete fusion

• Postzygotic Isolating Mechanism– Zygote doesn’t successfully develop– Hybrid sterility

Although two species able to hybridize and live in the same geographic area – they utilize separate habitats.

Reproductive barriers: Prezygotic

Behavior patterns can keep closely related species from mating.

Temporal Isolation – Time of year when reproduction occurs may be different for two different species able to hybridize.

Mechanical Isolation: Organisms are just physically not able to successfully mate.

Prevention of Gamete Fusion: Sperm prevented from fusing with an egg.

Reproductive barriers: Postzygotic

Postzygotic mechanisms usually are expressed by failure of an embryo to develop, adults not able to survive, or production of sterile adults.

Problem’s With the Biological Species Concept:

The extent of hybridization is much greater than previously believed.

Although not common, hybridization should be enough to cast doubt on whether reproductive isolation is enough to maintain separate species.

Stabilizing selection may also be a factor.

How does a species divide into two descended species?

Does the process of speciation equate to reproductive isolation?

Speciation is a two-part process:1) Identical populations must diverge2) Reproductive isolation must evolve to

maintain these differences

Allopatric divergence is the primary means of speciation.

Geographic Isolation

Selection May Reinforce Isolating Mechanisms.

Can reproductive isolation be perfected before gene flow destroys the differences between the populations?

Towards reproductive isolation - Selection

Gene Flow ?

The process of selection continually improving prezygotic isolating mechanisms is termed reinforcement.

Can speciation occur in sympatry?

Disruptive selection can lead to two phenotypes. Before they become two species, a mechanism of reproductive isolation must occur.

With genetic drift……this process of speciation is rare.

Random changes in small populations (genetic drift) may lead to changes that cause reproductive isolation.

Adaptation to different environments can lead to speciation.

More than 300 species of Drosophila exist on the Hawaiian islands – all probably evolved from a single ancestor by adaptive radiation.

Adaptive radiation – when a group of closely related species evolved from a common ancestor by occupying different habitats.

Darwin’s finches evolved from a single ancestor by utilizing different food types.

Lake Victoria Cichlid Fishes11 species of Cichlids in crater lake in Cameroon. All evolved from a common ancestor about 200,000 years ago. Changes in water level is probably what lead to speciation.

Hi water = more habitat; Low water = isolation

Nile perch, introduced in the 1950’s, has reduced the number of known cichlid species by 70% - does not include those not known.

Periodic isolation (glaciation) has also led to adaptive radiation of Alpine Buttercup’s.

Pace of Evolution:Punctuated equilibria – evolutionary process is a series of ‘spurts’ separated by periods of ‘stasis’.

Gradualism – gradual evolutionary change.

An ‘explosion’ of new species occurred during the Cambrian period. The number of species since then has increased.

The number of species change has not been consistent through time. Five major extinctions – we are currently in the sixth.

First Homonids = 2myaFirst Homo sapians about 100,000 ya

The End.

Genetic change does not have to be that large to create a new species.

Only a few genes – important ones – separate these two species of monkeyflower.