Important words and concepts from Chapter 24,
Campbell & Reece, 2002 (4/6/2004):
by Stephen T. Abedon (abedon.1@osu.edu)
for Biology 113 at the Ohio State University
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Course-external links are
in brackets Click [index] to access site index Click here to
access text’s website Vocabulary
words
are found below |
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(1) Chapter title: The Origin of Species
(a)
"It is not enough to explain how adaptations evolve in populations… Evolutionary theory must also explain the
multiplication of species, the
radiation of an existing species that gives rise to two or more new
species."
(b)
"Status as a peripheral isolate merely gives a lottery ticket to a
small population. A population can't win (speciate) without a ticket, but there
are very few winning tickets." Stephen Jay Gould, p. 443,
(c)
[origin of species (Google Search)]
[population evolution and
speciation (BSC Courseware)] [index]
PATTERNS OF SPECIATION
(MACROEVOLUTIONARY)
(a)
Speciation is the formation of a new species from an
older, immediately ancestral species
(b)
[speciation (Google Search)]
[observed instances of
speciation (Talk.Origins)] [some more observed speciation events (Talk.Origins)] [index]
(a)
Anagenesis is the transformation of a single ancestral species
into a single descendant species; anagenesis is a type of speciation
(b)
Anagenesis involves the extinction of the older, ancestral species
(c)
Anagenesis is converse to "branching" evolution; that is, it
is "non-branching" evolution
(d)
Contrast anagenesis with cladogenesis
(e)
See Figure, 24.1, Two
patterns of speciation
(f)
[anagenesis (Google Search)]
[index]
(4)
Cladogenesis (adaptive radiation)
(a)
Cladogenesis is the transformation of one ancestral species
into more than one descendant species; cladogenesis is a type of speciation
(b)
Cladogenesis does not (or, at least, does not necessarily) involve the
extinction of the parental species
(c)
Cladogenesis is branching evolution
(d)
Only via branching evolution can species increase in number
(e)
The “evolution of many diversely adapted species from a common ancestor
is called adaptive radiation.” (p. 471, Campbell & Reece, 2002)
(f)
Cladogenesis is probably more common than anagenesis
(g)
For whatever it is worth, anagenesis is probably just a special case of
cladogenesis where the parental population either
(i)
goes extinct coincident to the formation of the progeny species, or
(ii)
the parental species is driven to extinction by the progeny species
soon after the latter's genesis
(h)
(there two scenarios are effectively the same thing so far as the
fossil record is concerned)
(j)
See Figure, 24.1, Two
patterns of speciation
(k)
[cladogenesis (Google Search)]
[index]
SPECIES CONCEPTS
(a)
Just what the heck is a species?
(b)
Organisms do not exist on a genotype/phenotype continuum
(c)
Instead, the genotypes and phenotypes of populations of organisms appear to be constrained to discrete
types
(d)
Populations of types which satisfy certain criteria are termed species
(e)
Key to understanding species as well as speciation is the
concept of reproductive isolation
(a)
The idea of reproductive isolation is a squishy one
(b)
Absolute reproductive isolation means that genes (alleles) do not pass
from one population to a second population, one with which the first population
is reproductively isolated
(c)
Note that reproductive isolation does not mean that individuals
within two populations are not mating nor producing offspring within
populations; instead, if there are offspring, those offspring are not
contributing their alleles to either of the parental populations (e.g., because
these hybrid offspring are sterile and/or do not survive to reproduce)
(d)
Also note that reproductive isolation need not be 100%; it is possible
for two populations to maintain a large degree of reproductive isolation with
some small amount of gene exchange still occurring (a.k.a., introgression)
(e)
Thus, the phrase "reproductive isolation" describes some
point along a spectrum ranging from something greater than a total lack of
reproductive isolation (free gene exchange between populations) to complete
reproductive isolation (no gene exchange between populations)
(a)
There is more than one way to define just what a species is
(b)
That is, there are various species concepts
(c)
Species concepts include:
(i)
Biological species concept
(ii)
Morphological species concept
(iii)
Recognition species concept (mating recognition)
(iv)
Cohesion species concept (phenotype space, e.g., as applied to
bacteria)
(v)
Ecological species concept
(vi)
Evolutionary species concept
(d)
We will emphasize in particular the first two of these species concepts
(e)
[species (Google Search)]
[index]
(8) Biological
species concept
(a)
The biological species concept
is a way of defining species, one that employs as its numero uno (i.e.,
number one) criteria the concept of reproductive isolation
(b)
A biological species is a "… population or group of populations whose members have the potential to interbreed with one another in nature to produce
viable, fertile offspring, but who cannot successfully interbreed with members
of other species. In other words, a biological species is the largest unit of
population in which genetic exchange is possible, and that is genetically
isolated from other such populations." (emphasis mine)
(c)
"Put still another way, each species is circumscribed by reproductive barriers that preserve its integrity as a species
by blocking genetic mixing with other species."
(d)
"Remember that biological species are defined by their
reproductive isolation from other species in natural environments. In the laboratory or in
zoos, hybrids can often be produced between two species that do not interbreed
in nature."
(e)
[biological species concept
(Google Search)]
[index]
(a)
Conspecifics are two (or more) individuals who are members of the same species
(b)
[conspecifics, conspecific (Google Search)]
[index]
(10) Problems with the biological species concept
(a)
Two problems with the biological species concept
are
(i)
that it requires sex and
(ii)
it requires sex
(b)
(that is, both the concept and the act)
(c)
Thus, the biological species concept
is difficult to apply to organisms that reproduce asexually (though not impossible to
apply if gene exchange still occurs such as between bacteria via transduction, transformation, and
conjugation), and there are examples of populations
that we otherwise might want to call separate species but which nevertheless at
some low level share a gene pool (i.e., exchange genes, a.k.a., introgression)
(d)
The biological species concept is also difficult to apply to organisms
that are dead (e.g., extinct organisms)
(e)
The biological species concept also typically must be inferred;
confirming reproductive isolation is not a simple task
(f)
["biological species
concept" problems (Google Search)]
[index]
(11) Morphological species concept
(a)
A widely employed alternative to the biological species concept is the morphological
species concept
(b)
That is, two very similar organisms are more likely conspecifics
than two less-similar organisms
(c)
This is the same, familiar species concept
that all of us have been employing most of our lives
(d)
The morphological species concept is useful particularly since it is as
applicable to fossils as it is to extant, sexually
reproducing species
(e)
However, the morphological species concept is not a terribly useful in
terms of understanding processes of speciation since ultimately such processes are
intimately tied to matters of reproductive isolation
(f)
[morphological species concept
(Google Search)]
[index]
(12) Ecological species concept (supplemental
discussion)
(a)
The ecological species concept is based on ecological competition:
(i)
"A species is a number of related populations the members of which
compete more with their own kind than with members of other species." (p.
152, Colinvaux, P. 1986, Ecology.
John Wiley & Sons.
(ii)
The more similar two organisms are, the more likely their needs will
overlap, the more likely they will compete, and therefore the more likely that
they are of the same species
(b)
Caveat: intraspecific life history divergence:
(i)
Even the ecological species
concept has problems since it requires that members of individual species
not have divergent life histories (which, in practice, is not always the case)
(ii)
It also runs into a problem also seen with the morphological species concept: At what point
does one stop the process of splitting divergent forms into new species?
(iii)
It also is not necessarily trivial to determine the degree to which two
or more individuals are competing ecologically
(c)
[ecological species concept
(Google Search)]
[index]
(13)
Pluralistic species concept
(a)
While a given species concept may be preferred in a given circumstance,
that species concept probably will not have universal application
(b)
To understand all species, living at all times, should require a
broader concept of what it means to be a species than any one species concept
indicated above
(c)
The need to mix and match species concepts as applicable gives rise to
the idea of a pluralistic species concept which recognizes, essentially, that
“the factors that are most important for the cohesion of individuals as a
species vary.” (p. 468, Campbell & Reece, 2002)
(d)
[pluralistic species concept
(Google Search)]
[index]
(a)
A subspecies is a morphologically distinct population that nevertheless
enjoy incomplete reproductive isolation from another such population
(b)
Typically two members of different subspecies are more reproductively isolated than two members of the same
subspecies
(c)
"Population biologists are discovering more and more cases where
the distinction between subspecies with limited genetic exchange and full
biological species with segregated gene pools blurs. It is as though we are catching populations at different stages in their evolutionary descent
from common ancestors."
(d)
It is important to keep in mind that the concept of a subspecies is a
somewhat fuzzy one that can differ from scientist to scientist and perhaps even
from mood to mood since subspecies represents one of those catchall categories
where one throws populations that are divergent, but not too divergent, from
other populations; clearly, however, subspecies legitimately exist as
morphological distinctive populations that, however, are not enormously
reproductively isolated from other such populations
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What is a Subspecies?
(supplemental discussion) |
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Subspecies
are morphologically distinct from other subspecies of the same species |
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Members
of subspecies are more likely to breed within their own subspecies than with
other members of their species |
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Subspecies
are geographically localized |
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Some
researchers argue that the subspecies concept is sufficiently flawed as to be
irrelevant |
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It
doesn't really matter because apparently the rallying cry of humanity goes
something like: "Prosperity before subspecies!" |
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If
you really want a good cry, try doing a "subspecies and extinct"
search on the web; you will find things like, "Three tiger subspecies
are now extinct (all of them are dead): Caspian tiger (P.t. virgata), Javan tiger (P.t.
sondaica), |
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Below are the mountain zebra, the grevy zebra, the plains zebra, and the quagga (extinct), all of which are subspecies of a single zebra species: |
(e)
[subspecies, ring species (Google Search)]
[Galapagos giant tortoise
subsepecies (Discover Galapagos)]
[tiger subspecies (The Tiger Information Center)] [index]
REPRODUCTIVE ISOLATION
(15)
Reproductive barriers (reproductive
isolating mechanisms)
(a)
Key to speciation is the formation of reproductive barriers
between populations of otherwise similar organisms
(b)
Reproductive barriers may be classified into two general categories
(ii)
Postzygotic barriers
(c)
See Figure 24.5, A summary
of reproductive barriers between closely related species
(d)
The term "zygotic" refers to the product of conception
(e)
Thus prezygotic barriers prevent conception while postzygotic barriers
interfere with the Darwinian fitness
of the hybrid progeny
(f)
Note that key to understanding the speciation process is the cost to
potential parents as increasing levels of prezygotic barriers are breached,
as well as increasing levels of postzygotic barriers are breached
(g)
The ultimate Darwinian disaster is to invest in the raising of an
offspring that never succeeds in contributing to the gene pool
(h)
The earlier such an offspring may be aborted or prevented, the greater
the Darwinian fitness of the potential parents
(i)
Keep these ideas in mind as we walk through various reproductive
barriers
(j)
Below is a tabular summary of reproductive isolating
mechanisms/barriers:
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Hybrid Inferiority |
Increasing Fitness Cost to Would-Be Hybridizers (going from bottom to top) |
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Hybridization Attempted |
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Has Genetic Component (right
& above) |