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
|
|
Course-external links are
in brackets Click [index] to access site index Click here to
access text’s website Vocabulary
words
are found below |
|
(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
|
What is a Subspecies?
(supplemental discussion) |
|
Subspecies
are morphologically distinct from other subspecies of the same species |
|
Members
of subspecies are more likely to breed within their own subspecies than with
other members of their species |
|
Subspecies
are geographically localized |
|
Some
researchers argue that the subspecies concept is sufficiently flawed as to be
irrelevant |
|
It
doesn't really matter because apparently the rallying cry of humanity goes
something like: "Prosperity before subspecies!" |
|
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), |
|
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:
|
Hybrid Inferiority |
Increasing Fitness Cost to Would-Be Hybridizers (going from bottom to top) |
||
|
Hybridization Attempted |
|||
|
Has Genetic Component (right
& above) |
|||
|
|
(k)
Increasing fitness costs of postzygotic barriers select for prezygotic barriers and, ultimately, reproductive isolation, i.e., speciation
(l)
[reproductive barriers
(Google Search)]
[index]
(a)
Prezygotic barriers include
(ii)
Habitat isolation
(iii)
Behavioral
isolation
(vi)
Gametic isolation
(b)
See Figure 24.5, A summary
of reproductive barriers between closely related species
(c)
[prezygotic barriers
(Google Search)]
[index]
(a)
Two organisms that are not able to meet cannot mate, period (well, at
least, two individuals whose gametes are not able to meet cannot fertilize)
(b)
The most profoundly important mechanism to speciation
(particularly allopatric speciation) is some kind of
mechanism whereby individuals from different populations fail to
meet and thereby fail to mate
(c)
We'll return to this concept
(d)
All of the barriers that we will subsequently discuss are ones that
function within geographically overlapping populations
(e)
See Figure 24.6, Two modes
of speciation
(f)
See Figure 24.7, Allopatric
speciation of squirrels in the Grand Canyon
(g)
[geographical isolation
(Google Search)]
[index]
(a)
Two individuals living within overlapping ranges may nonetheless never
meet if both avoid the same spots within their ranges
(b)
For example, high up in a tree is a long way from the ground though
both share geographical ranges; an individual that sticks to the canopy will
only rarely contact an individual who sticks to the ground
(c)
Other examples of habitat isolation include preferences for habitat on/within
different species
(d)
Habitat isolation probably explains, at least in part, why the world
has so darn many beetles (i.e., a given range has a lot of distinct habitats
for such a small and versatile creature; those are Beatles to the right ŕ)
(e)
[habitat isolation (Google Search)]
[index]
(a)
Two individuals who meet, who are very similar, will nevertheless fail
to progress to mating if behaviors (especially mating behaviors) are
incompatible
(b)
Basically, the problem is that individuals belonging to two populations may not "speak the same language"
(though note that "language" deserves ironic quotes since we are
referring particularly to nonverbals)
(c)
This is typically observed in courtship rituals
(d)
An individual in a courtship ritual simultaneously is justifying his or
her worthiness to mate as well as indicating status as a conspecific
(e)
See Figure 24.3, Courtship
rituals as a behavioral barrier between species
(f)
See Figure 24.16, Mate
choice in two species of
(g)
[behavioral isolation
(Google Search)]
[index]
(a)
Two individuals who breed only at certain times and not at overlapping
times are effectively reproductively isolated
(b)
[temporal isolation
(Google Search)]
[index]
(a)
Mechanical isolation refers to an inability to mate even given a
willingness to mate by the two participants, due to morphological
incompatibility
(b)
Note that with mechanical isolation we start down a road toward an
increased costliness of lack of reproductive isolation
(c)
Particularly, attempting to mate is not without cost, e.g.,
susceptibility to predation for animals during the mating process
(d)
An important example of mechanical barriers is found in flowering
plants that may be adapted to pollination by different insects
(e)
[mechanical isolation
(Google Search)]
[index]
(a)
Given successful mating, both male and females bear the costs of
mating, but so far only the male has managed to waste gametes
(b)
The female does not lose gametes to hybridization until conception has
occurred
(c)
Keep in mind that a female's eggs are typically a lot more expensive
than a male's sperm, especially when the female (but not the male) is charged
with the brunt of the cost of raising the offspring, and when the female is
much more limited in reproductive opportunity
(d)
Thus, a female typically has more incentive to avoid conception than
does a male
(e)
Mechanisms whereby conception is avoided following mating are termed
gametic isolation
(f)
This can involve either a destruction of sperm prior to their reaching
the egg, or an incompatibility between sperm and egg such that the sperm is
unable to penetrate and thereby fertilize the egg
(g)
Gametic isolation additionally occurs when pollen is excluded
by flowers
(h)
[gametic isolation (Google Search)]
[index]
(a)
Postzygotic isolation is very costly, to be avoided if possible
(b)
This is because postzygotic isolation basically represents the
formation of an offspring with reduced Darwinian fitness
(c)
Postzygotic reproductive isolating mechanisms include
(iii)
Hybrid breakdown
(d)
Viability and fertility, of course, are what define Darwinian fitness
(e)
Postzygotic isolation mechanisms select for prezygotic isolation mechanisms (which, in turn, leads
effectively to speciation)
(f)
See Figure 24.5, A summary
of reproductive barriers between closely related species
(g)
[postzygotic reproductive
isolation, postzygotic isolation
(Google Search)]
[index]
(a)
Reduced hybrid viability means that the hybrid basically dies, either
before successfully reproducing or before reproducing at a rate that is as high
as that experienced by the non-hybrid progeny of the parent species
(b)
The earlier during the period of parental care (if any) that the
offspring becomes inviable, in general, the better for the caring parent
(c)
Thus, ideally inviability, if it is going to occur, occurs soon after
conception, especially if this frees up the female for subsequent mating and
reproduction
(d)
For the parents, the most costly time for inviability to occur is after
parental care is over but prior to the occurrence of successful reproduction by
the hybrid
(e)
[reduced hybrid viability
(Google Search)]
[index]
(a)
(b)
Hybrids can display reduced fertility, that is, relative to the
fertility displayed by either parent
(c)
Often this reduced fertility occurs as a consequence of problems during
meiosis
(d)
Hybrid infertility selects for prezygotic isolation
mechanisms (which, in turn, leads effectively to speciation)
(e)
See Figure 24.4, Hybrid
sterility, a postzygotic barrier
(f)
[reduced hybrid fertility
(Google Search)]
[index]
(a)
Hybrids may display neither reduced viability nor reduced fertility,
but the offspring of hybrids may still go on to display reduced viability or
fertility; this is hybrid breakdown
(b)
With hybrid breakdown it is thus the grandchildren of a mating
between species
that display the costs resulting from failures to display prezygotic reproductive isolation
(c)
[hybrid breakdown (Google Search)]
[index]
(27) Selection for
reproductive isolation
(a)
Two populations that fail to achieve prezygotic reproductive isolation display a reduced average
fitness as a consequence of reduced hybrid viability and reduced hybrid fertility
(or hybrid breakdown)
(b)
Individuals who can avoid mating such that hybrids are therefore not
produced can enjoy a Darwinian fitness
advantage over individuals who fail to avoid such matings
(c)
Thus, selection often will favor the evolution of prezygotic isolating mechanisms among populations whose ranges
overlap and that produce hybrids sporting reduced Darwinian fitness
(d)
[selection for reproductive
isolation (Google Search)] [index]
UNDERMINING BARRIERS
(a)
Two populations that are producing low-fitness
hybrids may yet achieve a more-complete level of prezygotic reproductive
isolation
(b)
While this movement toward prezygotic reproductive isolation is
occurring, gene exchange may be occurring between the two populations such that
genetic differences are lost
(c)
Thus, from the point in time at which two distinct populations overlap
in range there exists a race between the evolution of the mechanisms of
prezygotic reproductive isolation and the reformation of a single, not
internally reproductively isolated population
(d)
Additionally, one population may out-compete the other, driving the
latter to extinction
(e)
See Figure 24.8, Has
speciation occurred during geographic isolation?
(f)
[(Google Search)]
[index]
(a)
The term for low-level movement of alleles through reproductive barriers is introgression
(b)
Note that the existence of introgression causes the biological species concept to be a very fuzzy
concept
(c)
Indeed, separate species
can remain intact despite a low level of sharing of alleles
(d)
[introgression (Google Search)]
[index]
ALLOPATRIC SPECIATION
(a)
Perhaps the best way to understand the process of speciation is to
follow a hypothetical speciation scenario
(b)
Keep in mind as always how the concepts of species, speciation,
and reproductive isolation are irretrievably
intertwined, regardless of what species concept one
is concerned with
(c)
Allopatric speciation is speciation that is initiated via the geographical isolation of populations
(allopatric = "originating in or occupying in different geographical
areas" The Random House Dictionary
of the English Language)
(d)
Key to the occurrence of allopatric speciation is the occurrence of geographical barriers—big things in the landscape that get in
the way of the movement of organisms from place to place, i.e., from one
population to another
(e)
See Figure 24.6, Two modes
of speciation
(f)
See Figure 24.7, Allopatric
speciation of squirrels in the Grand Canyon
(g)
See Figure 24.8, Has
speciation occurred during geographic isolation?
(h)
[allopatric speciation
(Google Search)]
[index]
(a)
Geographical barriers can include such things as mountains, oceans,
valleys, rivers, land between water bodies, glaciers, etc.
(b)
Key, again, is simply that the geographic barriers arise (typically via
geographical or environmental processes) thus splitting a formerly contiguous
population into a discontiguous one (note: “discontiguous” really is a word:
click here for google
search with over 8,000 hits in April of 2002)
(c)
Note that not all geographical barriers are of the same significance to
all organisms
(d)
["geographic
barriers" species (Google Search)]
[index]
(a)
"Whenever populations become allopatric, it is possible for speciation to occur
as the isolated gene pools accumulate genetic differences
by microevolution. But an isolated population that
is small is more likely than a large population to change substantially enough
to become a new species."
(b)
"The geographical isolation of a small
population usually occurs at the fringe of the parent population's range. The
splinter population, or peripheral isolate, is a
good candidate for speciation for (these) reasons:"
(i)
The peripheral population probably occurs at the extreme of the
population's range consequently potentially reflecting extremes of variation
within the parental population due to both incomplete mixing within the whole
population and environmental extremes at the periphery of a species' range (=
selection before formation of geographical barrier)
(ii)
Founding of a small peripheral population typically involves a founder effect which differentiates the
founding population from the parent population further (= founder effect)
(iii)
A new population cut off from its parent population typically will not
immediately become large (if there were space to do this in, chances are it
already would be large; exceptions are when populations are founded in new
locales by accident, such as on islands) so consequently genetic bottlenecking further differentiates
the peripheral population from the parental population (note once again that
genetic bottlenecking and the founders effect are not identical phenomena) (=
genetic drift/bottleneck)
(iv)
Environmental differences between ranges will result in natural selection following different paths in
the peripheral population relative to the natural population (= selection after
founder effect/geographical isolation)
(c)
[peripheral isolates
(Google Search)]
[index]
(a)
When two formerly isolated populations
come back into contact, the range over which this occurs typically will not
encompass the entire range of either population
(b)
The area over which the population's ranges overlap, and within which
hybridization occurs, is called a hybrid zone
(c)
Note that it is within the hybrid zone that the reproductive isolation
of two populations is tested and evolves
(d)
Two populations that come into contact at a hybrid zone will either
evolve more-robust reproductive isolating mechanisms, e.g., behavioral isolation, or will fail to, thus setting the stage
for a melding of the two populations back into one
(e)
Two populations may be able to stably retain something resembling species
status as a consequence of ecological considerations and only limited gene
exchange at the hybrid zone (i.e., some form of hybrid inviability or hybrid infertility)
(f)
"Stabilizing selection
would restrict phenotypic variation to a range narrow enough to define the
species as separate from other species… The genetic basis for this cohesion of phenotype may involve specific combinations of alleles and specific linkages between gene
loci on chromosomes."
(g)
[hybrid zone (Google Search)]
[index]
(34) Scenario for
allopatric speciation
(a)
How, then, is speciation typically thought to occur?
(i)
Start with a single population
(ii)
Geographical barriers arise that separate that population into
two or more smaller populations
(iii)
Note that the parental population may
·
remain more or less intact while one or more peripheral populations may
form, or
·
the parental population may be broken up entirely into a number of
remnant populations
(iv)
The peripheral populations
·
may be different from the parental population before becoming separated
·
may be founded by only a small number of individuals
·
may not have an opportunity to increase in size over the medium term
·
may find themselves in environments that
differ from that of the parental population
(v)
Key is that the geographical barrier prevents gene flow between the
peripheral population and the parental population
(vi)
Thus, the peripheral population is in the position to diverge
genetically from the parental population
(b)
Note that the fate of the majority of peripheral populations is
extinction
(c)
Note that the fate of the "parental" population, if it has
been essentially broken up into a number of remnant populations, likely is
extinction
(d)
Note that regardless, speciation has not occurred until reproductive barriers are tested (at least according to the biological species concept)
(i)
Testing of reproductive barriers occurs only should the geographical barrier fail thus allowing the peripheral
population's range to come to overlap the range of the parental population
(e)
When the ranges of two isolated populations come to overlap, one of
three things can result:
(i)
The two populations evolve effective reproductive barriers thus
preventing significant allele exchange between populations—speciation occurs
(ii)
The two populations exchange genes to a sufficient extent that
speciation fails to occur and the two populations turn into one population
(iii)
One population can drive the other population to extinction
(f)
Recall that costly postzygotic isolating mechanisms will drive the
evolution of less-costly prezygotic isolating mechanisms
(g)
Note that should the formerly peripheral population succeed in driving
the parental population to extinction, then that would appear (in the fossil
record) as anagenesis
(h)
Note that should speciation occur such that the formerly peripheral population
and the parental population coexist, that would be an example of cladogenesis
(i)
Note that should the parental population be reduced to remnant
populations, two of which succeed in forming new species,
i.e., ones that differ morphologically from the parental population, then this
would appear in the fossil record as one species "suddenly" diverging
into two (or more) different species
(j)
[(Google Search)]
[index]
(35)
Parapatric speciation
(supplemental discussion)
(a)
“In parapatric speciation there
is no specific extrinsic barrier to gene flow. The population is continuous,
but nonetheless, the population does not mate randomly. Individuals are more
likely to mate with their geographic neighbors than with individuals in a
different part of the population’s range. In this mode, divergence may happen
because of reduced gene flow within the population and varying selection
pressures across the population’s range.” Evolution 101
(b)
[parapatric speciation
(Google Search)]
[index]
SYMPATRIC SPECIATION
(a)
Sympatric speciation is the idea of speciation events
being initiated without a geographical isolation
of populations
(b)
This may occur as a consequence of isolation between microenvironments
(different trees in the same forest, for example)
(c)
Alternatively it may involve the founding of new populations that are
reproductively isolated from the parent population from day one
(d)
One way this latter, sympatric mechanism of speciation occurs is via a
transition from sexual to asexual reproduction (asexual reproduction
immediately isolates populations since it is sex that ties populations together
genetically)
(e)
Another way, common among plants, occurs as a consequence of changes in
ploidy (i.e., number of sets of haploid genomes possessed by individual cells)
(f)
See Figure 24.6, Two modes
of speciation
(g)
[sympatric speciation
(Google Search)]
[index]
(a)
Mistakes during meiosis (nondisjunction) can generate polyploidies
(b)
If all of the chromosomes are derived from a single species,
this is called autopolyploidy
(c)
A polyploid individual may not be able to mate
with non-polyploid individuals with and resulting viable progeny
(d)
Autopolyploid individuals may still be able to produce gametes but can
mate successfully only within other autopolyploids
(e)
In the case of plants (where all of these goings on typically are going
on), the other autopolyploid could be the same plant
(f)
See Figure 24.14, Botanist
Hugo de Vries and his new primrose species
(g)
[autopolyploidy (Google Search)]
[index]
(a)
An allopolyploid is a polyploid involving
some combination of chromosomes from more-than-one species
(i.e., it is a hybrid)
(b)
"Interspecific hybrids are usually sterile because the haploid set
of chromosomes from one species cannot pair during meiosis with the haploid set from the other species. Though
infertile, a hybrid may actually be more vigorous than its parents and propagate
itself asexually."
(c)
See Figure 24.15, One
mechanism for allopolyploidy speciation in plants
(d)
[allopolyploidy (Google Search)]
[index]
ISSUES RELATED TO SPECIATION
(a)
This idea that populations do most
of their evolving in small, isolated populations forms the basis of a concept
known as punctuated equilibrium
(b)
The idea is that because most morphological change occurs in small
populations and this change occurs over "only" a few 10s, 100s, or
1000s of generations, there is a reduced likelihood that fossilization will document these morphological changes step
by step as they occur
(c)
Instead, one would expect the fossil record to be represented by
dominant morphotypes (i.e., those represented by the parental population) and
then for the fossil record to suddenly record a change in morphotype should the
parental population be replaced by a peripheral population (i.e., a population
that has a different morphotype)
(d)
The "equilibrium" of the concept of punctuated equilibrium
refers to the persistence of stable morphotypes in the fossil record over long
periods (millions of years) while the "punctuated" part of the
concept refers to the "sudden" appearance of morphological change
over a period of "only" a few tens of thousands of years
(e)
In other words, the likelihood of fossilization is directly
proportional to population size and population duration:
(i)
small, short-lived populations should result in proportionately fewer
fossils
(ii)
population biological theory predicts that the majority of
morphological change/speciation events may occur as a consequence of
evolution occurring in small/short-lived populations (e.g., allopatric speciation)
(f)
"Suppose that a particular species
survives for 5 million years, but most of its morphological changes occurred
during the first 50,000 years of its existence. In this case, the evolution of
the species-defining characteristics was compressed into just 1% of the
lifetime of the species [and probably less-than 1% of the cumulative population
of the species]. On the time scale that can generally be determined in fossil
strata, the species will appear suddenly in rocks of a certain age and then
linger with little or no change before becoming extinct. During its formative
millennia, the species may have accumulated its modifications gradually, but
relative to the overall history of the species, its inception was abrupt (short
time scales/small populations)… If the species is adapted to an environment
that stays the same, then natural selection
would counter changes in the gene pool. In this
view, the tendency for stabilizing selection
to hold a population at one adaptive peak results in long periods of
stasis."
(g)
(Note that the idea of punctuated equilibrium may be to some extent
artifactual. This is because the grouping of fossils into a coherent
"species" is a very human and imperfect process. How much variation
does one allow before declaring that one fossil is a different species from
another group of fossils? The very concept of grouping fossils together as
morphotypes of a certain similarity and abundance may artificially result in an
idea that species tend to persist "unchanged" over relatively long
periods. Clearly paleontology and population biology continue to be very dynamic
sciences.)
(h)
See Figure 24.17, Two models
for the tempo of speciation
(i)
[punctuated equlibrium
(Google Search)]
[punctuated equilibrium (Talk.Origins)] [index]
(a)
A number of processes we've studied in microevolution have (sort of) analogous
processes in macroevolution (as follows, "microevolutionary
process" » "analogous
macroevolutionary process")
(i)
Birth of an individual » speciation (birth
of a species)
(ii)
Death of an individual » extinction of a species
(iii)
Genetic drift » species extinction due to
random (not foreseeable) events
(iv)
Natural selection » differential species birth and extinction
(i.e., species selection)
(b)
All else held constant, those species that survive the longest will
give birth to more new species, so any characteristic of a species that tends
to prevent extinction (large populations, broad range, generalist rather than
specialist) will tend to increase the representation of that species'
descendant species among the total number of species present on Earth
(especially given changing environments)
(c)
Other characteristics of species tend to increase the likelihood of
speciation events. These include an ability to disperse to new locations and an
ability to adapt to new environments
(d)
Note that these characteristics are not necessarily the same that will
assure a maximum short-term reproductive success by individuals in specific
environments (i.e., why do generalists persist if specialists are so much
better at doing what they do? Answer: because specialist are more susceptible
to extinction than are generalists)
(e)
Certain characteristics of a species might also make that species less
susceptible to random changes in the environment (e.g.,
asteroid impact); such characteristics might include small size, wide range, a
lack of specific dietary needs, etc.
(f)
"The species that endure the longest and generate the greatest
number of new species determine the direction of major evolutionary
trends."
(g)
Thus, to impact greatly on the evolution of the diversity of life, an
organism must possess qualities that go beyond simply being highly adapted to
life within a specific environment
(h)
["species selection"
and macroevolution (Google Search)]
[index]
(41)
Adaptive landscapes (supplemental
discussion)
(a)
An adaptive landscape consists of a graphical representation of the Darwinian fitness associated with all of the
genotypic combinations available to a species
(b)
Because Darwinian fitness is environmentally dependent, an adaptive
landscape is valid only for one given environment
(c)
Because allele combinations are approachingly infinite in their permutations
(and certainly vary over more than two dimensions), the graphical
representation of an adaptive landscape is itself merely a metaphorical
representation
(d)
That is, look at Figure 24.13 not as a lumpy plane which shows the
Darwinian fitness of all of the genotypes possible
in a population (higher peaks represent graeter Darwinian fitness); instead
think of the plane as a two-dimensional compression of the idea of representing
all of these genotypes within a single plane
(f)
Nevertheless, the idea here is that certain genotypes display greater
fitness in a given environment than do others
(g)
A variety of genotypes may display similar Darwinian fitnesses, e.g.,
there exists more than one peak (combination of alleles) on this
adaptive landscape
(h)
Natural selection will drive a population
up an adaptive peak; that is, the effect of selection is to eliminate those
genotypes which are found at lower elevations in this metaphorical landscape
(i)
Note that a population will tend to congregate around a single adaptive
peak, and the peak chosen will be chosen not because that peak is the highest
but because that peak is the easiest to attain given the alleles which are
present in the population
(j)
Once natural selection has driven a population up an adaptive peak, stabilizing selection will tend to keep that
population on that peak (this is especially true as alleles become more and
more co-adapted, adapted especially to the presence of certain alleles found at
other loci)
(k)
Only non-adaptive evolution (drift, migration, mutation) can move
a population off of an adaptive peak thus allowing that population to explore
another, perhaps taller adaptive peak (i.e., become even better adapted to its
environment)
(l)
Alternatively, environmental change will serve to completely change the
adaptive landscape thus allowing for adaptive evolution up a different peak (or
more likely, the population will fail to climb up a new peak and as a
consequence go extinct)
(m)
Thus, small, isolated populations in
novel environments essentially find themselves in new adaptive landscapes which
give them the opportunity to find and explore new adaptive peaks; and most such
populations go extinct but those which survive display rapid adaptation (and,
potentially, correlated morphological change than can be followed in the fossil
record)
(n)
[adaptive landscapes,
adaptive landscape
(Google Search)]
[index]
VOCABULARY
(c)
Allopolyploid
(e)
Autopolyploidy
(g)
Biological
species concept
(i)
Cladogenesis
(j)
Conspecifics
(o)
Hybrid
breakdown
(p)
Hybrid zone
(q)
Introgression
(s)
Morphological species concept
(u)
Pluralistic species concept
(x)
Problems
with the biological species concept
(dd)
Scenario
for allopatric speciation
(ee)
Selection
for reproductive isolation
(ff)
Speciation
(gg)
Species
(hh)
Species concepts
(ii)
Species selection
(kk)
Sympatric
speciation
(ll)
Temporal
isolation
