DEFINING SPECIES AND SPECIATION
While Darwin entitled his book "On the Origin of Species",
the book dealt primarily with a mechanism of evolution (natural selection)
in which variation was critical. But what will selection do with
this variation? Change the frequency of dark morphs of moths, or morphs
of snow geese, or change the mean and the variance of the distribution
of heights in human populations? What is the result of disruptive selection?
How do we decide that natural selection has actually lead to the origin
of new species? The answer to these questions depends on one's species
concept. The concept of species is an important but difficult one in
biology, and is sometimes referred to the "species problem".
Some major species concepts are:
Typological (or Essentialist, Morphological, Phenetic) species
concept. Typology is based on morphology/phenotype. Stems from the Platonic
"forms". Still applied in museum research (type method)
where a single specimen (type specimen) is the basis for defining
the species. In paleontology all you have is morphology: typology is practiced
and species are defined as morphospecies (e.g., snail shells in
fossil beds). Problems: what about sexual dimorphism: males and females
might be assigned to different species. Geographic variants: different
forms viewed as different species? What about life stages: caterpillars
and butterflies? If typology is let run it can lead to oversplitting taxa:
each variant is called a new species (Thomomys) pocket gophers with
> 200 subspecies.
Evolutionary species concept. "A species is a series of
ancestor descendent populations passing through time and space independent
of other populations, each of which possesses its own evolutionary tendencies
and historical fate" (George Gaylord Simpson). Simpson was a paleontologist
and emphasis on stability over time is best appreciated in the fossil record.
Inherently morphological, but his claim is that morphologies have genetic
bases, so it is indirectly a genetical definition. Problem: gaps in the
fossil record impose arbitrary boundaries between species, especially those
undergoing gradual size/shape evolution. Compare with Cladistic species
concept (pg. 418). How speciation affects existing taxa can alter one's
view of species.
Biological species concept. from population-level thinking of the modern synthesis.
"Species are groups of actually or potentially interbreeding populations which are reproductively isolated from other such groups" (Ernst Mayr; Museum of Comparative Zoology, Harvard).
"Species are systems of populations; the gene exchange between these systems is limited or prevented in nature by a reproductive isolating mechanism or several such mechanisms." (Theodosius Dobzhansky; Rockefeller and Columbia Universities).
Not the first to claim the importance of reproductive continuity:
"a set of individuals who give rise through reproduction to new individuals
similar to themselves" (John Ray, 1682). "A species is a constant
succession of similar individuals that can reproduce together." (George
Louis Buffon, 1707-1788). Note the characteristic Mayr: "biological"
species concept implies that all other species concepts are non-biological.
Recognition concept. species are groups of individuals that share
a common fertilization system (a "specific mate recognition system",
SMRS of Hugh Paterson, South Africa). Emphasis is on those characteristics
of species that tend to hold them together; something that members of a
species share. Biological species concept stresses that which makes
a species different from other species; cant define species without
reference to other species. Contrast isolation vs. recognition.
See figure 15.2, pg. 409.
There are other species concepts (now you know why it this has been
called the 'species problem'): Ecological, Pluralistic, etc. One philosophical
approach is to ask whether species are "individuals" or "classes".
There are some conceptual and practical problems with the Biological
Species Concept: Are species real or are they arbitrary categories
imposed by biologists? Populations: where do they begin and end; often
arbitrary and grade into other populations; Genus, Family, Order, etc.
are these human constructs? Is a genus of bees = a genus of birds in terms
of levels of organization? What are the typological grounds for the boundaries.
What about "species" that can freely mate such as species of
orchids that can mate sometimes between genera (wide cross). What
about asexual species? They don't reproduce with other species
so every individual is a species?? Mayr would hold that species
are real units. Views species boundaries as being defined by limits of
gene exchange: each species is a group of populations held together by
exchange of genes in a genetic system that allows free recombination
among the chromosomes of this system. Holds that species are real objective
units with definable limits - basic units of evolution. No mistake
that the Biological Species concept was advanced by two zoologists who
worked with organisms that did not present some of the more obvious problem
of plants and bacteria (Nevertheless, there is clear discontinuity in the
phenotypes of bacteria).
Isolating "Mechanisms" (misleading term: is it a mechanism
in that it evolved for the purpose of isolating; or did isolating "mechanisms"
evolve in one context and serve to prevent mating on another?). Premating
mechanisms prevent interspecific crosses. Temporal or Ecological
isolation (don't meet due to different time of emergence or occur in different
habitats). Ethological (behavioral) isolation (meet but don't mate)
e.g. fireflies. Mechanical isolation (can't transfer sperm, morphological
incompatibilities). See table 15.1, pg. 405.
Postmating isolating mechanisms inhibit or prevent interspecific crosses
gametic mortality (sperm transferred but does not fertilize eggs).
zygote mortality (egg is fertilized but zygote dies). hybrid
inviability (F1 hybrid has reduced viability: incomplete development).
hybrid sterility (F1 hybrid viable but sterile) e.g., mule
Premating isolation prevents wasting of gametes: highly susceptible
to improvement by natural selection. Damselflies: character displacement
of wing spot density. Rapid speciation events often involve behavioral
isolation: Hawaiian Drosophila: hundreds of species in the past several
million years. Postmating isolation does not prevent the wasting of
gametes and its improvement by natural selection is indirect. Isolating
mechanisms may work in concert; if one breaks down, another will prevent
gene exchange (e.g., bird songs and plumage patterns). This issue of the
opportunity for selection to act on pre- vs. postmating isolating mechanisms
is important in the discussion of Reinforcement in the next lecture.
Breakdown of isolating mechanisms will lead to hybridization (crickets
in eastern North America hybridize in a hybrid zone along the Appalachian
ridge). Are hybridizing "species" really species? If the hybrids
backcross to either type, introgression can occur ("the incorporation
of genes from one species into the gene pool of another species").
Many examples of hybridization in both plants and animals. Often referred
to as semispecies, i.e., not complete species.
Population structure: are populations the unit of evolution?
(Ehrlich and Raven 1969, Science 165:1288-1232) Species are just "phenetic
clusters". But why do populations cluster into "species".
Checkerspot butterfly studied on Jasper Ridge near Stanford CA by Paul
Ehrlich and colleagues (1975 Science 188:221-228). Different populations
fluctuate independently: suggests little gene exchange between populations
(But Slatkin's analysis of allele frequency data suggest otherwise.
Geographic variation in reproductive isolation. If a series of
populations can mate sequentially, but the end populations cannot, is one
species two?? Mayr would say that since they do not meet the issue is not
biologically relevant. Do you agree?
Polymorphism. Mimicry complexes of African swallowtail. Papilio dardanus
exists as one morph where no noxious species are found (Madagascar). Where
noxious models are present the same species (Papilio dardanus) takes
on different forms depending on the local model; the mimetic forms look
like completely different species but are one.
Sibling species. Morphologically indistinguishable, but are reproductively
isolated. Not always easy to test for reproductive isolation and no morphological
grounds on which to separate populations.
Descriptions of the geography of population location/overlap helps focus on how geography might influence gene flow. If gene exchange between two populations is completely stopped, what will happen? Allopatric populations/species exist in different areas (do not overlap or abut); sympatric populations/species occupy the same geographic locality; parapatric populations/species have abutting but not overlapping ranges; a peripatric distribution refers to peripheral isolates.