SPECIATION

Table l6.1 Two classifications of potential modes of speciation in 
sexual organisms

I.  By geography and level (after Mayr 1963)

1.	Hybridization (maintenance of reproductively isolated hybrids between two 
species)

2.	Instantaneous speciation (through individuals)

A.	Genetically (single mutation)
B.	Cytologically
a Chromosome rearrangement
b.	Polyploidy

3.	Gradual speciation (through populations)

A.	Sympatric speciation
B.	Parapatric speciation
C.	Allopatric (geographic) speciation
a.	Peripatric speciation (by evolution in an isolated colony)
b.	Vicariant speciation (division of range by an extrinsic barrier or 
extinction of intervening populations)

II.  By population genetic mode (after Templeton 1982)

1.	Transilience

A. Hybrid maintenance (selection for hybrids)
B. Hybrid recombination (selection for recombinants following hybridization)
C.	Chromosomal (fixation of chromosome rearrangements by drift and 
selection)
D.	Genetic (founder event in a colony)

2.	Divergence

A.	Habitat (divergent selection without isolation by distance)
B.	Clinal (selection on a cline with isolation by distance)
C.  Adaptive (erection of an extrinsic barrier followed by divergent   
microevolution)

In general we will follow Mayr's classification
THE GENETIC BASIS OF GENIC
REPRODUCTIVE BARRIERS

Is reproductive isolation based on a few or many genes?

How do the alleles of these genes interact together?

Dobzhansky and Muller early showed theoretically that reproductive isolation 
requires that populations diverge by at least two allele substitutions.  Interaction 
between loci is required. 

POSTZYGOTIC ISOLATION can be due to:
		differences in nuclear genes 
			(may be just a few differences at start - but more accumulate with time)
		structural differences in chromosomes
		incompatibility between cytomplasmic factors -         
				such as symbiotic intracellular bacteria

GENIC DIFFERENCES THAT YIELD HYBRID STERILITY OR INVIABILITY
		differences at at least two loci that interact disharmoniously in the 
		hybrid - complex epistasis (Insert example)

STRUCTURAL CHROMOSOME DIFFERENCES 
	expected to reduce fertility of hybrids because aneuploid 
		gametes may be produced
	Aneuploidy can be reduced by various meiotic mechanisms
	So aneuploidy may only be severe when the hybridizing populations differ 
		be multiple chromosomal rearrangements

	Populations often monomorphic for different chromosome rearrangements 
		form narrow hybrid zones - 
		suggests that the hybrids have reduced fitness
		chromosome differences reduce gene exchange 			
		(but hard to differentiate between gene and chromosomal differences).

PREZYGOTIC BARRIERS TO GENE EXCHANGE

	Sexual (ethological) isolation important in animals -
	genetic divergence in both the signal and response mechanisms
	May be few or many differences in 
		behavioral, morphological chemical or other features


CONCEPTUAL PROBLEMS TO BE CIRCUMVENTED BY VARIOUS 
MODES OF SPECIATION:

1. How do the multiple genetic factors required for 
reproductive isolation accumulate in a diverging 
population in the face of recombination?

2. How do alleles that reduce the fitness of heterozygotes 
increase in frequency despite selection against them?

MODES OF SPECIATION

Three main modes that form a continuum:

1. ALLOPATRIC SPECIATION - most important
	
a. Vicariant speciation
	Speciation by reproductive isolation of populations that are 
divided by the emergence of an extrinsic barrier, or by the 
extinction of intervening populations, or by migration into 
geographically separated regions.
	
b. Peripatric speciation (founder effect)
	Speciation by reproductive isolation of sub-populations 
peripheral to the main body of a population.

2. PARAPATRIC SPECIATION - disputed importance 
	The evolution of reproductive isolation between populations 
that are continuously distributed in space that have 
substantial gene flow between them.

3. SYMPATRIC SPECIATION - most controversial
	The evolution of reproductive isolation within a randomly 
mating population.


ALLOPATRIC SPECIATION

Speciation occurs at least in part as a byproduct of divergence 
by genetic drift or natural selection

Abundant evidence for this mode of isolation:

1. Variation in reproductive compatibility among geographic 
populations of species

2. The parapatric distribution of many closely related species

3. The correspondence of genetic discontinuities with present or 
past topographic barriers 

4. Evolution of incipient reproductive isolation among 
laboratory populations

VICARIANT ALLOPATRIC SPECIATION

	Distributions originally contiguous and occupied by the 
ancestor of the present species
	Barriers arise
	Distribution becomes disjunct
	Differentiation occurs in the absence of interbreeding

e.g., closely related fish and other species on either side of the isthmus of Panama  
- arose ion the Pliocene separating eastern Pacific and Caribbean populations

The genetic changes that cause the evolution of reproductive 
isolation may be caused by divergent ecological or sexual 
isolation.  

In only a few cases have the effects of ecological selection been 
well documented.

Abundant circumstantial evidence suggests that sexual selection is an important 
cause of ethological (sexual) isolation in animals - 
PERIPATRIC SPECIATION
Founder effect speciation (Mayr 1954; Carson 1975)
Speciation due to a shift initiated by genetic drift and followed 
by natural selection, between adaptive peaks

This mode was suggested by observations of several bird 
populations (e.g., kingfishers and robins) with restricted 
distributions at the periphery of a parent population that are 
often highly divergent despite similar ecologies

Strong epistatic interactions may trigger genetic revolutions

Observations suggest this mode may be common, but 
experimental evidence is limited and the mechanism not clear

PARAPATRIC SPECIATION
In theory, speciation can result from divergence of spatially 
segregatedÊpopulations connected by gene flow so long as 
selection is stronger than gene flow.

This mode is hard to document and few examples are known

SYMPATRIC SPECIATION
Most models envisage the evolution of assortative mating 
(sexual isolation) between two phenotypes that are favored 
by disruptive selection (intermediates have low fitness).

This is unlikely because recombination is expected to break 
down the association between genes for the disruptively 
selected phenotypes and those for the assortative mating

However, if disruptive selection favors preference for two 
distinctive resources or habitats and mating occurs within 
the habitats, then the problem is much reduced 

Assortative mating is then a direct, rather than indirect, 
consequence of disruptive selection
THE ROLES OF NATURAL SELECTION 
AND DRIFT IN SPECIATION

Controversial questions:
1.	Can pre- or post-zygotic barriers evolve by direct selection 
on the responsible genes?   OR

2. Does incompatibility mostly evolve in allopatry as a 
byproduct of divergent selection (close linkage to - hitch 
hiking-, or pleiotropic effects of, selected loci)?

3. 	Can prezygotic (sexual) isolation evolve, or be enhanced by, 
natural selection due to higher fitness of hybrids produced 
through secondary contact (reinforcement of isolating 
mechanisms - reproductive character displacement)? 

A. SPECIATION BY NATURAL SELECTION

Dobzhansky (1936) and Muller (1939) introduced a two-
locus model that may be generalized to multiple loci 
or to multiple alleles at one locus
Ancestral population is A2A2 B2B2

Insert Fig. 16.5
















B. SPECIATION BY PEAK SHIFT

Also known as PERIPATRIC SPECIATION 
               or FOUNDER EFFECT SPECIATION

Involves a combination of genetic drift and natural selection in a colony that 
has a bottle neck of small population size

At certain loci, rare alleles that lower the fitness of heterozygotes increase in 
frequency by genetic drift and become fixed.

This may create epistatic selection for allele frequency changes at other loci

The colony may later become incompatible with the ancestral one

INSERT FIG. 16.6
Comparison of models of adaptive divergence and peak shift:

Fig. 16.6A. adaptive divergence of one subpopulation in altered environment
Fig. 16.6B. evolution of one subpopulation by genetic drift and selection without a change in 
environment.

RATES OF SPECIATION

Rates vary widely (between a few thousands to 20 MYs)
	
Mean = about 3 MYs (but see below)

FACTORS FAVORING HIGH RATES:
 1. Abundant topographic barriers that provide opportunities for allopatric 
divergence

2. Low dispersal rates

3. Strong sexual selection

4. Ecological specialization
	Persistence of newly formed species is favored by ecological opportunities 
(vacant niches)
5. Bottlenecks in population size
6. Speciation in sympatry 
	
Coyne and Orr estimated approximately 200,000 years for taxa speciating in 
	sympatry vs 2.7 MY for those in allopatry    (Insert figure here).
SPECIAL CONSIDERATIONS IN 
PLANT SPECIATION

Several phenomena play a greater role in plant than animal speciation:

1. Asexual reproduction by vegetative reproduction or apomixis - 
development from an unfertilized egg

2. Self fertilization provides the potential for reproductive isolation of self 
fertilizing inbred lines

3. Ecological isolation and adaptation - Evolution of ecotypes or 
subspecies associated with different habitats

4. Hybridization is very frequent between closely related plant species.  
Hybrids also have the potential to form new species

5. Polyploidy is much more frequent in plants than in animals. It is in fact a 
major mode of speciation in plants

SPECIATION BY POLYPLOIDY

A mechanism that can result in instant speciation

Results from the doubling or other increase in the 
chromosome complement

Polyploidy is common in plants especially in hybrids between genetically 
divergent populations

The polyploid is postzygotically isolated from  its diploid ancestor because 
backcross progeny have aneuploid gametes with low fertility

Establishment of a polyploid population probably requires ecological or 
spatial segregation

Polyploids often differ physiologically from their progenitors - they often 
colonize different, often stressful, environments

Polyploid species can have multiple independent origins and can often be 
recreated by deliberate artificial breeding

As a consequence of all these properties, plant taxonomic species correspond less 
well to biological species than in animals

CONSEQUENCES OF SPECIATION

Except for sympatric speciation by disruptive selection, speciation in itself is not 
usually considered to be adaptive

Rather it is considered to be a by-product of genetic differentiation due to 
selection and drift.

The most important consequence of speciation is the production of diversity 
because once speciation has occurred lineages evolve independently of one 
another.

Speciation stands at the border between microevolution and macroevolution.

MODELS OF PUNCTUATED EQUILIBRIUM VS 
PHYLETIC GRADUALISM

Some paleontologists argue that speciation may be required for and significant 
morphological evolution to occur at all

Punctuated equilibrium

Eldredge and Gould (1972); Stanley (1979), Gould and Eldredge (1993)

Hypothesis: Broadly distributed well-established species may be incapable of 
evolving substantially due to internal constraints imposed by complex 
epistatic interactions

Rather (based on Mayr's (1954) proposal that peak shifts in small populations can 
give rise to morphologically divergent new species  -
it is argued that most evolutionary changes in morphology are triggered by, and 
associated with, speciation.
(Insert Fig. 16.32).

However, population geneticists are skeptical of punctuated evolution theory 
because the patterns of variability seen in contemporary natural populations 
differs from the observed in the fossil record
Futuyma has proposed a simple hypothesis that may reconcile  the conflict 
	how speciation might facilitate long-term evolutionary change in 
morphological and other phenotypic traits

(Insert Fig. 16.33).