The Origin Of Species the Living World Is Not

the origin of species ...the living world is not a formless mass of randomly combining genes and traits, but a great array...of g

The Origin of Species

"...the living world is not a formless mass of randomly combining
genes and traits, but a great array...of gene combinations, which are
clustered on a large but finite number of adaptive peaks."
Theodosius Dobzhansky (1951)

The question that Darwin (1859) called "that mystery of mysteries".
What are the processes at work that lead to the origin of
discrete species?
In this lecture, we will examine a number of different modes by which
speciation may happen in nature.
These have been classified by Mayr (1963) as:
1. Gradual speciation
1A. Allopatric speciation (with geographic isolation)
*
Vicariant speciation (following the appearance of a barrier)
*
Peripatric speciation (by evolution in an isolated colony)
1B. Parapatric speciation (with contiguous but non-overlapping
distributions)
1C. Sympatric speciation (with overlapping distributions)
2. Instantaneous speciation
2A. Single genetic mutations
2B. Cytological changes
*
Chromosome rearrangements
*
Polyploidy
3. Hybridization (producing new reproductively isolated species)
Our primary focus will be on how reproductive isolation can evolve
under the gradual modes of speciation.
Historically, the focus has been on the geographic context of
speciation, because of its effect on gene flow between populations.
That still leaves the question:
What processes cause divergence in populations?
1.
Drift
*
Slow, weak force, but with lots of time expect different
substitutions in different populations. With enough differences
expect genetic incompatibility and post-mating reproductive
isolation.
2.
Natural selection
*
When habitats differ, natural selection will favor different
traits and populations will show different adaptations (local
adaptation). Populations are different phenotypically &
genetically.
*
As a byproduct, populations may be reproductively isolated. Either
they do not mate (pre-mating) or hybrids are inferior
(post-mating).

Four morphs of arctic charr are reproductively isolated.
a.
large benthic
b.
small benthic
c.
piscivorous
d.
planktivorous
3.
Sexual selection
*
Males compete for mating opportunities with females. Females often
prefer males with particular secondary sexual traits (a bright
color, long tail feather, complex song). If females from one
population prefer different traits than females from another
population, reproductive isolation can result.
*
Evidence that sexual selection contributes to speciation includes
the observation that closely related species often differ
strikingly in male secondary sexual traits.

Male hummingbirds differ in feathers used in courtship -- length,
color, and placement.

Male lacewings give calls to attract females & differ in the rate of
pulses. Each morph is reproductively isolated from others.
*
Additional evidence is that taxonomic groups that exhibit sexual
selection often have high species richness.

1A. Allopatric speciation

"Allopatric speciation is the evolution of genetic reproductive
barriers between populations that are geographically separated by an
extrinsic, physical barrier such as topography, water (or land) or
unfavorable habitat."
Futuyma (1998) p. 482

The extrinsic barrier halts gene flow between the isolated
populations.
Without gene flow, each population follows an independent evolutionary
trajectory, accumulating genetic changes through drift and/or
selection.

For these changes to lead to speciation, the genetic changes in one
population must be incompatible with the genetic changes in the other
population.
Allen Orr (1995) noted that as populations accumulate changes over
time (R), the number of possible ways in which an incompatibility can
occur rises at least as fast as R2.
The more changes occur in one population, the more possible ways in
which a genetic incompatibility could occur with the second
population.
Once genetic incompatibilities have arisen between two separately
evolving populations, the populations are no longer able to cross and
produce fertile offspring.
Speciation.
The genetic changes that accumulate in the two populations may be
caused by natural selection, sexual selection, or by random genetic
drift.
Which forces would you expect to be faster?
Which forces would you expect to be relatively more important in
peripatric speciation?
 In allopatry there is little or no gene flow between populations. So
a change in one population has a chance to fix independently of what's
going on in the other population.
Example: Isthmus of Panama
About 3 million years ago, North and South America merged, forming a
land bridge (the Isthmus of Panama) and isolating the marine
communities of the Carribean and Pacific.
Knowlton et al (1993) studied snapping shrimp from seven pairs of
similar species on either side of the isthmus.


A phylogeny of the seven pairs indicated that each of the species was
most closely related to a similar looking species from the other
coast.
Mating experiments in the lab indicated that, in each case, the sister
species from the Carribean and from the Pacific failed to produce
viable offspring.
The formation of the land bridge between North and South America led
to the isolation of populations on either side. With gene flow cut
off, the populations diverged, leading to new species in 7/7 cases.

1B. Parapatric speciation

"Parapatric speciation is the evolution of reproductive isolation
between populations that are continuously distributed in space, so
that there is substantial movement of individuals, and hence gene
flow, between them."
Futuyma (1998) p. 498

Without an extrinsic barrier to gene flow, genetic differences that
arise within a population may be swamped by genetic exchange.
If, however, the population is distributed over space and:
*
little gene flow occurs between distant locations and
*
there is an environmental gradient favoring different genetic
combinations in different places,
selection may be strong enough to maintain genetic differences,
creating a genetic cline.
If the habitat changes fairly abruptly, there may be a sharp border
between different types: a hybrid zone.
Over time, the hybrid zone may shrink or remain stable.
Example: Heavy metal tolerance
Antonovics and co-workers studied heavy metal tolerance in grasses
growing near mines on land contaminated with lead and zinc. Heavy
metals are usually lethal to plants, but they observed populations
that had evolved tolerance.
Plants from the grass species Anthoxanthum odoratum were tolerant near
the mine but remained intolerant at distant sites.
Plants were locally adapted to heavy metal concentrations.

Heavy metal tolerance was not the only difference observed; flowering
time also differed along the transect.

Selfing rates were also higher near the mine.

The divergence in flowering time and increased selfing rates have
reduced gene flow between tolerant and non-tolerant grasses and
increased the reproductive isolation of grasses near the mine.
In this case, heavy metal tolerance has evolved recently, during which
time the species has always had a continuous distribution.
Therefore, parapatric speciation processes can be inferred. In most
other cases, however, it is difficult to know whether a species that
is currently parapatric may have been previously allopatric.
Distributions of the taxon can change over time.

1C. Sympatric speciation

"Sympatric speciation is the evolution of reproductive isolation
within a randomly mating population."
Futuyma (1998) p. 498

Sympatric speciation is the most controversial of all the modes of
speciation, since divergence must occur in the face of high levels of
genetic exchange.
Requires the evolution of mechanisms to reduce gene flow, which can
include both pre-mating and post-mating isolating mechanisms.
Also requires ecological divergence so that one species doesn't
exclude the other.
Imagine a single species utilizing a resource that could support a
wider variety of individuals.

If the extreme phenotypes in a population have the highest fitness
(disruptive selection), the population will not split apart if mating
is random.
If, however, genes arise that cause extreme individuals to
assortatively mate (= mate with similar types), then more extreme
offspring can be produced:

The problem with this idea is that it is difficult for assortative
mating to evolve in a way that is coupled with the trait subject to
disruptive selection.
One way for this coupling to be achieved is if there is tight linkage
(low recombination) between the genes causing the trait and the genes
causing assortative mating.
A second, more likely way is if the traits themselves lead to
assortative mating.
For example, if the traits involve utilizing different resources and
mating tends to happen around resources, then individuals with extreme
traits will naturally mate with similar individuals.
Over time, if sufficiently strong assortative mating evolves,
sympatric speciation will be the result.
Example: Apples and hawthorns
The classic case of sympatric speciation is that of the apple maggot
fly, Rhagoletis pomonella.

Hawthorns were the original host plant of Rhagoletis pomonella until
about 150 years ago, when the fly was observed on cultivated apple
trees (introduced to the Americas).
The apple maggot fly is now widespread in the northeastern US and
causes millions of dollars of damage each year to apple crops.
Rhagoletis courts, mates, and lays eggs on its preferred host plant.
Individuals that changed their preferred food source to apple trees (=
a host shift), consequently tend to mate with other individuals who
prefer apples.
Since the host shift, flies that prefer hawthorns and apples have
diverged genetically.
Feder et al (1988, 1990) found statistically significant differences
in allele frequencies at six loci, differences that allow the two
populations to be identified.
Changes in the timing of mating have also occurred: mating on apples
occurs ~ 3 weeks earlier.
Overall, matings between hawthorn and apple flies has been reduced to
only ~6% and the two are now considered incipient species.
The exact mechanisms by which reproductive isolation is accomplished
vary but fall into two main classes:
1. Premating (or prezygotic) isolating mechanisms: Mating and
fertilization are prevented.
Premating isolation may be accomplished by changes in: habitat
preferences, timing of reproduction, mating preferences, mating
behaviors, or physical incompatibilities.
2. Postmating (or zygotic) isolating mechanisms: Genetic
incompatiblity: Mating occurs but hybrid offspring are inviable,
infertile, or produce inviable/infertile offspring.
Allele from one population doesn't 'do well' with the genome of the
other population (Dobzhansy 1937; Muller 1942).
Ecological effects: Hybrids are viable and fertile, but are selected
against in the habitats of both parental species.
Would you expect premating isolation to play more of a role in
allopatric or sympatric speciation?
Coyne and Orr (1997) examined the causes of reproductive isolation in
a number of sister species in Drosophila, finding:

More rapid and complete prezygotic isolation between sympatric
species pairs.
Would you expect premating or postmating isolation to evolve sooner
between sympatric pairs?...between allopatric pairs?

Prezygotic isolation evolves earlier in the speciation
process, primarily due to the rapid mating isolation that evolves
between sympatric species pairs.
Although examples of each mode of speciation are known, it remains
unclear how frequently each occur.
Many evolutionary biologists suspect that allopatric speciation is
responsible for the vast majority of speciation events.
SOURCES:
*
Futuyma (1998) Evolutionary Biology. Sinauer Associates, MA.
*
Freeman and Herron (1998) Evolutionary Analysis. Prentice Hall,
NJ.