Download Lecture #14 -Isolation and the Origin of Species Fall 2001

Lecture #14 -Isolation and the Origin of Species
Fall 2001
** Isolating Mechanisms - are phenomena that prevent inbreeding of closely related taxa
(plants or animals). They are based on the following factors: (a) spatial distance, (b)
environmental factors and (c) reproductive biology. Reproductive Isolating Mechanisms are
usually placed in one of two categories:
** Prezygotic Mechanisms & ** Postzygotic Mechanisms
Prezygotic mechanisms include:
(a) geographical isolation = spatial distance prevents inbreeding - taxa of plants do not occur in
the same area ... e.g., Plantanus occidentalis and P. orientalis occur in the northeastern U.S. and
the Mediterranean respectively ... and are distinct. However, when you put them together, they
produce fully fertile hybrids (remember allopatric ... when ranges do not overlap). Often
geographical isolation is promoted by barriers such as mountains, rivers, oceans, deserts, roads,
etc..
(b) ecological isolation - plants live in the same region but grow in distinctly different habitats.
Vernonia (ironweed)
meadows
one species lives in sunny, moist environments along roadsides and in
the other in shady, well-drained mature hardwood forests. Sympatrically, they
rarely form hybrids; however, in the greenhouse they cross.
(c) seasonal (temporal) - plants occur in the same region but flower at different times. Some
ironweeds and sunflowers bloom in midsummer and others in late summer and fall.
(d) ethological - plants are isolated from one another by pollinator behavior patterns. Certain
hummingbirds, bees and moths are species specific with regard to the flower they visit.
(e) mechanical - pollination is prevented by structural differences in flower parts. In the
Asclepiadaceae, the proper pollinia must be inserted by the pollinator into the slits in the
gynoecium (similar phenomenon in orchids).
Postzygotic Mechanisms (mechanisms that kick in after fertilization) include:
(a) hybrid inviability - the hybrid zygote dies, the endosperm disintegrates, the seed fails to
germinate, seedlings die, anthers abort etc....
(b) hybrid sterility - first generation hybrids are produced .... which flower, but they have
aborted stamens, or more typically, infertile pollen.
(c) hybrid breakdown - occurs in second or later-generation progeny. In some interspecific
crosses in Vernonia, the first generation hybrids are completely fertile, but the 2nd generation
progeny exhibit continuous variation from full fertility to complete inviability.
Speciation - the evolutionary divergence and differentiation of a formerly homogeneous
population system into two or more separate species. Speciation requires genetic variation ...
which is common among most populations, plus reproductive isolation.
** - Most evolutionary scientists have stressed a need for spatial distance as a prerequisite for
speciation .... i.e., allopatric speciation. IN plants, this may not be necessary .... and sympatric
speciation occurs from time to time .... mainly via polyploidy but not always.
** - Two modes of Evolutionary Change: (a) Phyletic Evolution - a change from one state to
another through time in which a species might evolve into something different from its
ancestor, and (b) Divergent Evolution - a single population system that differentiates into two
distinct evolutionary lines.
** - As an ecotype (a group that is separated from the parent population) becomes specialized
and adapts to its environment, isolating mechanisms build up, reducing gene exchange. As
divergence increases, the elements of the populations become less and less able to exchange
genes. Divergence leads to reproductive isolation ... and this isolation promotes eventual
speciation.
** - The question is whether sympatric speciation can occur (speciation from populations that
overlap). The answer is yes in plants .... especially in those species that are self-fertilizing,
because this tends to restrict gene flow (and in those species that produce polyploids).
** - In some plants (Clarkia for example) local populations acquire a set of distinctive features
through a complex variety of evolutionary mechanisms and then the adaptive novelties spread
over certain areas and replace the less adaptive genotypes.
Darwinian Gradualism Versus Punctuated Equilibrium
** Darwin believe that organisms were in a constant struggle for survival throughout the
earth s history. IN each generation, the fittest produce the most offspring ... and thus were
evolutionarily the most successful. Basically, this was seen (and still is by many) as a
constant gradual process resulting eventually in speciation.
** - Another view to evolution and speciation was put forth by Stephen Jay Gould and
Niles Eldridge in 1977
in which they claim that the fossil evidence frequently suggests
that for many taxa organic evolution is not a gradual unfolding process but a process of
homeostatic equilibria, punctuated on occasion by rapid and episodic events of speciation
via flooding, temperature changes, meteorite impact, etc... followed by long periods of
stasis or non-change.
** - In the big scheme of things, the fossil record seems to support Punctuated Equilibrium
Theory
showing that indeed there were long periods of stasis punctuated by shorter
periods of rapid change. The association of rapid speciation of plants with such events as
mountain uplifts and climate changes .... is compatible with this theory.
** - Hybridization ..... the role of hybridization in plants is controversial. Helianthus and
Quercus species readily form hybrids, which contradicts the biological species concept.
Thus, species must be more loosely defined and arbitrary in plants than in animals.
Plant hybrids are more readily seen than animal hybrids for three reasons: (a) ethological
isolation is more effective in animals, (b) zoologists may have overlooked hybrid animals
since they are more difficult to find, and (c) animals have more complex developmental
sequences and structure which makes successful hybridization less likely.
Hybrid plants are likely to have little affect in stable environments. However, they may be
at an advantage in disturbed and changing environments.
Polyploidy - the multiplication of the chromosome set
a distinct process affecting the
evolution of flowering plants. In a diploid species, n is the basic haploid set of
chromosomes, but in a polyploid species, n is a multiple of x. Some species have diploid
numbers of 18, 36, 54 and 90 (as in some composites such as Chrysanthemum in which
diploid = 18 and haploid = 9).
(Additional Information on Polyploidy) -
Cells (and their owners) are polyploid if they contain more than two haploid (n) sets of
chromosomes; that is, their chromosome number is some multiple of n greater than the 2n
content of diploid cells. For example, triploid (3n) and tetraploid cell (4n) cells are polyploid.
Polyploidy is very common in plants, especially in angiosperms. From 30% to 70% of today's
angiosperms are thought to be polyploid. Species of coffee plant with 22, 44, 66, and 88
chromosomes are known. This suggests that the ancestral condition was a plant with a haploid
(n) number of 11 and a diploid (2n) number of 22, from which evolved the different polyploid
descendants.
** - In fact, the chromosome content of most plants groups suggests that the basic angiosperm
genome consists of the genes on 7-11 chromosomes. Domestic wheat, with its 42 chromosomes,
is probably hexaploid (6n, where n (the ancestral haploid number) was 7.
Some other examples:
Plant
Probable ancestral haploid number
domestic oat
Chromosome number
Ploidy Level
7
42
6n
peanut
10
40
4n
sugar cane
10
80
8n
banana
11
22, 33
2n, 3n
white potato
12
48
4n
tobacco
12
48
4n
cotton
13
52
4n
apple
17
34, 51
2n, 3n
Polyploid plants not only have larger cells but the plants themselves are often larger. This has led
to the deliberate creation of polyploid varieties of such plants as watermelons, marigolds, and
snapdragons.
** - Polyploidy has occurred often in the evolution of plants.
** - Polyploidy and Speciation
When a newly-arisen tetraploid (4n) plant tries to breed with its ancestral species (a backcross),
triploid offspring are formed. These are sterile because they cannot form gametes with a
balanced assortment of chromosomes. However, the tetraploid plants can breed with each other.
So in one generation, a new species has been formed.
Polyploidy even allows the formation of new species derived from different ancestors.
In 1928, the Russian plant geneticist Karpechenko produced a new species by crossing a cabbage
with a radish. Although belonging to different genera (Brassica and Raphanus respectively), both
parents have a diploid number of 18. Fusion of their respective gametes (n=9) produced mostly
infertile hybrids.
However, a few fertile plants were formed, probably by the spontaneous doubling of the
chromosome number in somatic cells that went on to form gametes (by meiosis). Thus these
contained 18 chromosomes - a complete set of both cabbage (n=9) and radish (n=9)
chromosomes.
Fusion of these gametes produced vigorous, fully-fertile, polyploid plants with 36 chromosomes.
(Unfortunately, they had the roots of the cabbage and the leaves of the radish.)
These plants could breed with each other but not with either the cabbage or radish ancestors, so
Karpechenko had produced a new species.
The process also occurs in nature. Three species in the mustard family appear to have arisen by
hybridization and polyploidy from three other ancestral species:
B. oleracea (cabbage, broccoli, etc.) hybridized with B. nigra (black mustard) -> B. carinata
(Abyssinian mustard).
B. oleracea x B. campestris (turnips) -> B. napus (rutabaga)
B. nigra x B. campestris -> B. juncea (leaf mustard)
Modern wheat and perhaps some of the other plants listed in the table above have probably
evolved in a similar way.
Aneuploidy refers to instances where the chromosome number is not an exact multiple of
the basic number - Clarkia (Onagraceae) - haploid (n) = 5 in some, 6 in some, 7 in some, 8
in some and 9 in some species.
40% of plants and a higher percentage of ferns are polyploids. Polyploidy is related to
growth habit. Annuals tend to have the lowest frequency of polyploidy and herbaceous
perennials have the highest.
Finally, Apomixis, the replacement of sexual reproduction with asexual reproduction ....
occurs in some plant species. It is often associated with polyploidy. Many apomictic plant
species are adapted to particular microhabitats.
** - Vegetative propagation by buds, bulbs, rootstocks, etc... is widespread among many
perennials. This permits the multiplication of a well-adapted genotype that might
disappear in the course of sexual reproduction.