Chapter 24

The Origin of Species

Macroevolution is the origin of new taxonomic groups, as opposed to microevolution, which is genetic variation between generations within a species. A.  What is a species?            1.  Biological species concept                        - A species is a population or group of populations whose members have the potential to interbreed with one another and produce viable offspring, but who cannot produce viable offspring with other species. 

The biological species concept is based on inter-fertility, rather than physical similarity.

Speciation - process by which a new species originates.

- Involves the creation of a population of organisms that are novel enough to be classified in their own group. 

- Two processes by which this can occur: - Anagenesis is the accumulation of heritable traits in a population, that transforms that population into a new species.

- Cladogenesis is branching evolution, in which a new species arises as a branch of from the evolutionary tree.  The original species still exists.  This process is the source of biological diversity.

- For a new species to form, there needs to be isolation of some members of a species as a separate population.

- Forms of isolation, that interfere with breeding include both…           2.  Prezygotic and postzygotic barriers

Prezygotic barriers prevent mating or egg fertilization if members of different species try to mate. Examples:

a.  Habitat isolation - Two species that live in the same area, but occupy different habitats rarely encounter each other.

b.  Behavioral isolation - Signals that attract mates are often unique to a species.  (e.g., different species of  fireflies flash different patterns)

Courtship ritual as a behavioral barrier between species.

c.  Temporal isolation                                    - Two species breed at different times of the day or during different seasons.   d.  Mechanical isolation                                    - Closely related species attempt to mate, but are anatomically incompatable.  (Example:  flowering plants with pollination barriers; some plants are specific with respect to the insect pollinator, often occurs with butterflies/moths) e.  Gametic isolation                                    - Gametes must recognize each other.  (Example:  fertilization of fish eggs, chemical signals between sperm and egg allows sperm to “recognize” the correct egg)

- Postzygotic barriers

a.  Reduced hybrid viability- Abort development of hybrid at some embryonic stage. b.  Reduced hybrid fertility- Meiosis doesn’t produce fertile gametes in vigorous hybrids. donkey + horse = mule (sterile hybrid)                        

c.  Hybrid breakdown- First-generation hybrids are fertile, but they cannot produce fertile offspring in the next generation (e.g. different species of cotton).

A summary of reproductive barriers between closely related species

The problem with the idea of biological species concept --> How do you get organisms to breed to see whether viable offspring are produced?  There are… 3.  Alternative concepts of species                        a.  Ecological species concept

- Species are defined by their use of environmental resources; their ecological niche (e.g. species that are defined by their food source such as butterflies with certain flowers).

b.  Pluralistic species concept

- Factors that are most important for the maintenance of individuals as a species vary.  These factors include reproductive isolation, i.e. not being in the same area so mating is precluded.                 

c.  Morphological species concept

- Species are characterized according to a unique set of structural features.  This is still the most widely used concept by taxonomists.

d.  Genealogical species concept

- Recognizes species are sets of organisms with unique genetic histories. This idea is based often on molecular analyses such as DNA sequences.

B.  Modes of speciation            

1.  Allopatric speciation

- Allopatric speciation describes speciation that takes place in populations with geographically separate ranges.  Gene flow is interrupted and new species evolve.

2.  Sympatric speciation

- Sympatric speciation describes speciation that takes place in geographically overlapping populations.  Chromosomal changes and nonrandom mating reduce gene flow.

Remember: Species arise when individuals in a population become isolated one from the other.

 Examples of Allopatric speciation:

            Allopatric speciation of squirrels in the Grand Canyon. Animals like birds do not show speciation like those animals that are barred from breeding by the canyon.

Key question about allopatric species is whether they are indeed different enough that viable, fertile offspring would not be produced by mating. This can be tested sometimes as in…

           

Another place where adaptive radiation is apparent is on island chains. This example is illustrative of what happened on the Hawaiian islands.

Would this example be allopatric or sympatric speciation?

Remember Once geographic isolation has occurred, there still must be changes that reproductively isolate populations of individuals. If the populations evolve so that they are now new species, they cannot interbreed to produce fertile, viable offspring.

2.  Sympatric speciation

- Speciation that takes place in geographically overlapping populations. 

- This can occur by chromosomal changes and nonrandom mating. Both can reduce gene flow between organisms and cause populations to evolve to new species.

Example: Polyploidy (greater than 2 sets of chromosomes) - most common in plants.

Sympatric speciation by autopolyploidy in plants.

-Animals diverge mostly due to reproductive isolation. 

-Reproductive isolation is a result of genetic factors that cause offspring to rely upon resources not used by previous generations.  (Example:  switch to a new food source)

-Example: Lake Victoria has 200 closely related species of Cichlids (fish) which probably all arose from one ancestor with the driving force for speciation being:

Competition for a limited resource (food) within the lake, and adaptation to new food sources. This gave rise to different species that are kept from breeding with each other by distinctive coloration pattern.

C.  From speciation to macroevolution

How then do we get from the mechanism of speciation to evolution on a grand scale; i.e., macroevolution?

Two models:

-The Gradualism model suggests that change is gradual with the accumulation of unique morphological adaptation.

- The Punctuated Equilibrium model suggests that rapid change occurs, with a new species “erupting” from the ancestral lineage and then staying the same thereafter.

However it does occur, we need to remember that

Speciation occurs when divergence leads to reproductive barriers between the new and the ancestral population. And this probably takes vast amounts of time to occur.

But how do evolutional novelties emerge? For example, how did something as complex as the eye first evolve? We need to remember that:

Most evolutionary novelties are modified versions of older structures.

A range of eye complexity among mollusks.

Lesson from the eye example: 

Existing structures can be modified for brand new functions.  These are called Exaptations: structures that evolve for one purpose but become useful for another function. Finally, we should…

Remember that evolution is not goal oriented.  Differential reproduction is only a reaction of individuals to their environment. Example:  The branched evolution of horses. This figure can give the illusion of goal-oriented evolution of the horse, but it is only an illusion.