Download BIOL 317 LECTURE NOTES – WEEK 10 SUMMARY SPECIATION

BIOL 317 LECTURE NOTES – WEEK 10 SUMMARY
SPECIATION AND DIVERSIFICATION IN PLANTS
Speciation: the evolution of a new species from an existing species. In a
phylogenetic tree, the nodes, or branching points, indicate speciation events.
What are species?
Species are usually considered to be the basic unit of biodiversity, but what does
this mean? Ever since Darwin, biologists have been debating the nature of
species, even debating the reality of species. Study and debate on the nature of
species boundaries and definition are ongoing.
Examples of ways in which species have been thought about:
Typological species concept: used by early philosophers and natural
historians; the idea that all species were created immutable, in perfect form, and
that variation was the result of environmental imperfection. The concept of type
specimens and type species remains in taxonomy today.
Morphological species concept: post-Darwin; the range of variation is
measured and species boundaries are defined by gaps or morphological
discontinuities. Variation is genetic and environmental; there is no perfect ‘type’.
Ecological species concept: species are recognized based on the
environmental niche space that they occupy; different species are separated in
ecological space. Plants tend to vary geographically in their morphology,
according to local climate and environment; this type of local variation may be
recognized as an ecotype, not the same as a species.
Biological species concept: species are recognized based on reproductive
isolation between them. Reproductive isolation refers to barriers to gene flow
between groups of individuals. This may be separation in space and time, and/or
biochemical or genetic incompatibility preventing the formation of fertile offspring.
Phylogenetic species concepts: have various definitions. One is that species
should be monophyletic lineages on independent evolutionary trajectories,
recognized by evidence of phylogenetic divergence eg. synapomorphies and/or
DNA evidence.
Different species concepts use different properties of groups of organisms to
recognize criteria for defining species. These properties may arise at different
times during the process of speciation, leading to disagreement over the number
of species recognized. Eg., ecological and morphological divergence may take
place between populations before those populations become reproductively
incompatible; monophyly in DNA sequences within a population may not be
achieved until after the population has been reproductively isolated from other
populations.
Patterns and mechanisms of speciation can both be conceived to be either
gradual or sudden.
Gradualism: speciation happens slowly and constantly over time, with gradual
transition between phenotypes. This can happen by accumulation of genetic
differences over time, selection for reproductive isolation through reduced fertility
of hybrids, or mechanisms that prevent crossing (for example, pollinator
specificity).
Punctuated equilibrium: speciation takes place in sudden bursts of rapid
phenotypic change, with little change in phenotype between speciation events.
This can happen by "quantum" or "macromutational" change, also called saltation
– small changes in genotype can result in large, dramatic changes in phenotype
(the “hopeful monster” of Goldschmidt). In plants, sudden speciation may take
place via hybridization accompanied by chromosomal or ploidal change.
How does speciation occur?
Allopatric - occupying separate geographic distributions
Sympatric - occupying the same or overlapping geographic distributions
Allopatry is usually thought of as a necessary precursor to speciation, examples
of allopatric speciation are well documented. Speciation in sympatry is thought to
be rare, and good empirical examples are lacking.
Conventional (selectionist) view:
A population in allopatry is subject to a different environment. Directional
selection drives the evolution of morphological novelty as an adaptation to the
new environment. At the same time the new “species” evolves differences that
make it less compatible with its progenitor and therefore is “reproductively
isolated.” Reinforcement, through evolution of physical or genetic barriers to
crossing (reproductive isolation), may occur if the two species come back into
contact.
Alternative (release-from-selection) view: In this scenario, a population, most
likely in allopatry, is subjected to a relaxation in selection by virtue of being in a
new environment where some new resource is abundantly available, or some
new niche space is available. In this setting, phenotypic variability is greatly
expanded and new phenotypes at the extreme of the range of variability survive
and reproduce that normally would not. If one of these is initially better "adapted"
to the new environment, then selection will rapidly fix the new phenotype. If the
new environment has many available resources/niches, or if the evolutionary
novelty allows exploitation of previously unused resources, then many new forms
may arise in a short period of time, resulting in an adaptive radiation.
Examples of mechanisms facilitating speciation in plants:
Pollinator shift. Reproductive isolation and morphological differentiation via
mutations in floral form that result in attracting alternative pollinators. Eg., sister
species of Mimulus with very different floral morphology attracting visits from
either bees or hummingbirds. Differences are controlled by few genetic changes;
fertile hybrids are readily formed in the lab.
Breeding system shift. Reproductive isolation and morphological differentiation
via shift from outcrossing to selfing. Selection for reproductive assurance in
small, peripheral populations leads to selfing being adaptive, with associated
traits becoming fixed (loss of self-incompatibility, reduction in flowers, reduction
in pollen:ovule ratio). Eg., sister species of Capsella, also in rare neoendemic
Stephanomeria malheurensis.
Hybridization. Sometimes, hybridization between species leads to chromosomal
changes in the F1 generation which result in hybrids forming new species. These
are usually allopolyploids: with multiple copies of each chromosome, derived
from two different parent species. Hybrids are interfertile, but are prevented from
backcrossing with parent species by differences in chromosome number or
structure.
Polyploidy. Whole-genome duplications may take place within a species,
without any hybridization occurring. This is thought to be common in plants, and
can result in reproductive isolation between the polyploids and their diploid
ancestors, leading to divergence and speciation.
Speciation is the process by which new species arise, and is responsible for the
dramatic diversity of biological organisms on Earth today. But speciation doesn’t
occur equally across all species, and species are not equally likely to persist.
Diversification is a function of both speciation and extinction. The net
diversification rate is the speciation rate - the extinction rate, and provides a
measure of how quickly a lineage accumulates species; or of the rate at which a
lineage grows (or shrinks) in terms of its species richness. Different lineages of
organisms have had different rates of diversification over time.
Angiosperms are an example of an exceptionally species-rich, recently originated
lineage; ie. with a high rate of diversification. Darwin famously referred to the
sudden origin and rapid diversification of angiosperms as an “abominable
mystery”.
Origin of angiosperms: the first unequivocal fossil flowering plants are
approximately 90 million years old. According to the fossil record, flowering
plants underwent a rapid taxonomic diversification in the early – mid Cretaceous,
with many of today’s most species-rich lineages appearing during this time. This
took place either just before, or together with, an ecological expansion, with
flowering plants going from a minority to the majority of the diversity in terrestrial
plant assemblages.
Today, flowering plants number approximately 300 000 species, and are
arguably the largest and most important component of terrestrial ecosystems.
Compare to mammals, which originated approximately 200 million years ago,
and now number only around 5 500 species, or birds (the most numerous
tetrapod vertebrates), of similar age to mammals, with around 10 000 species.
How did flowering plants diversify so rapidly?
There are numerous hypotheses about the origin and radiation of angiosperms,
and research and debate on this topic is ongoing. One popular hypothesis is that
angiosperms co-radiated with insect pollinators, with speciation in both groups
driven by the evolution of specificity and specialization between plants and
pollinators. Another hypothesis which has recently become popular is that wholegenome duplication (polyploidy) has played a major role in the evolution and
diversification of angiosperms, leading to sudden speciation and the evolution of
new functions from redundant gene copies. In reality, there is unlikely to be a
simple answer; many interacting factors many millions of years in the past led to
the patterns of diversification that we observe today.
Particular traits, and suites of traits, are often hypothesized to have an effect on
how likely species are to speciate or to become extinct. Eg., biotic pollination
appears to be generally correlated with increased diversity in flowering plants.
Key innovations: traits which theoretically lead to increased diversification rates.
Eg., nectar spurs in Aquilegia. Following the origin of the nectar spur, speciation
in this group was rapid, probably due to increased specificity and specialization
for pollinators. There are many other examples of traits correlated with increased
diversity in specific lineages, including annual habit, bilateral floral symmetry, and
fleshy fruits, but these patterns are not generally observed across all
angiosperms.
Evolutionary “dead-ends”: traits which theoretically lead to decreased
diversification rates. Selfing has been hypothesized to be a dead end: transitions
from outcrossing to selfing are commonly observed across angiosperms, and are
thought to be irreversible, but around half of angiosperms are SI, whereas only
around 10-15% are predominantly selfing. All large, old lineages of flowering
plants are predominantly outcrossing, there are no large, old clades of habitual
selfers. This suggests that outcrossers must have a higher diversification rate
than selfers. But this evidence is circumstantial, there have been no clear
correlations documented between shifts to selfing, and decreased speciation
rates and/or increased extinction rates.
Diversification rates, and shifts in diversification rates, can be estimated by
phylogenetic analysis. Precise rates are difficult to measure because past
speciation and extinction events can not be directly observed. Approximations of
background diversification rates in angiosperms are on the order of 1 species per
10-20 million years, with a great deal of variation through time, and across
lineages. Angiosperm lineages which appear to have unusually high rates of
diversification include Lamiales and Asterales (what families have you learned
that belong to these orders?)