BIOGEOGRAPHY AND SPECIATION
• Because of geographic barriers and thermal
gradients the ocean can be divided into
provinces - biogeographical regions with
characteristic spp. Assemblages.
• Classifications of groups of spp. belonging to a
province usually based on qualitative
judgements.
• A between-locality similarity index can be
calculated and similar locales can be grouped
into provinces.
•
Jaccard’s Coefficient:
• Mij=Cij/[Ni + Nj - Cij]
• Cij = # of spp. the 2 samples have in
common
• Ni = # of spp. in sample i
• Nj = # of spp. in sample j
• Matrix
of coeff. (locality & spp.) can be clustered
hierarchically
• Based on Mij’s can use unweighted-pair group
method
• 27-28° N. latitude is transition from Pacific
equatorial water mass and colder California current
• 34-35° at Pt. Conception cool water from north and
warm water from south
• Imbrie & Kipp, 1961 - Factor analysis to construct
assemblage of Atlantic planktonic forams - they fit
very well with water mass distribution. - surfacethermal structure of oceans - same for diatoms
Adaptability and Biogeographical Range:
• Species living in variable environments should
also have the flexibility to occur over a broad
biogeographical range
• Capitella capitata - worldwide
• Mulinia lateralis - New Brunswick Yucatan
• Jackson, 1974 - Biogeographical range for
shallow-water bivalves is much broader than for
deep-water spp.
• The
significance of biogeographical range lies
in its possible inverse relation to extinction rate
• Species over a wide area might be less prone to
extinction
• Bivalve genera with cosmopolitan ranges go
extinct much later
• Levinton, 1974 - cosmopolitan bivalves of the
Paleozoic and Mesozoic have much longer
geological time ranges than genera with more
restricted ranges
Dispersal and Range Extension:
• Biogeographical range can be enhanced
through chance migration across barriers
• New colonizations of marine organisms often facilitated by humans - provide
insight on the rapidity of spread and effects
3 Types of Colonization of Marine Organisms:
a) planktonic larvae may traverse great
oceanic expanses and colonize new
coasts
b) attached forms (i.e., barnacles) may raft
across oceans - logs, ships, etc.
c) species introduced by humans for culture
• Littorina
littorea - periwinkle - was noticed in
Nova Scotia in the 19th century
• Since that time it has spread southward and
become the dominant gastropod of New
England
• Probably facilitated by shipping - down to
MD, limited by temp., has probably displaced
all other local spp. of Littorina
• propagule - min. # of individuals that can
establish a reproducing population
• If
colonization target is small - the absence of a
suitable habitat or the chance of extinction due
to predation or competition by residents will
increase
• If immigrants of a species invade a target area
devoid of the species at a constant rate (i), the
expected # of individuals, Nt, at time t is:
E (Nt) = i/(b-d)[e(b-d)t-1]
b = birth on the target area
d = death on the target area
• If
b is less than or equal to d, population size is
maintained or increased (if b=d) only through
immigration.
• If we have an exponentially growing pop. with
initial size, a (# of propagules) - the probability
of extinction by some time t is
P0(t) = [[d(e[b-d]t-1)] / be [b-d]t -d]a(#prop.)
• With b=0.7, d=0.2, the limiting probability of
extinction with 1 propagule is 0.29; with 2
propagules it is 0.08; with 4 it is only 0.006.
• The
significance of this calculation applies to
the study of gastropods living on both sides of
the Atlantic - many whose pelagic larvae are
found in the open-ocean
• Thus, a is probably high (Scheltema, 1971)
Island Biogeography:
• Schoener 1974 followed the colonization of
marine invertebrates onto plastic mesh sponges.
Large and small were placed near and far from
an algal bed (a presumed source of colonists).
The experiment was performed at Bimini
Lagoon, Bahamas; so P (spp. pool) was
probably large but indeterminate. The following
results were obtained:
1) A stabilization of species number did
occur with time
2) There were more species and individuals
on large than on small sponges
3) There was no difference in species
number on “near” or “far” sponges,
probably because the source area was not
the algal bed
4) Early colonization species were
suspension or detritus feeders and other
types followed
Origin of New Species:
Speciation - isolation of daughter pop. from parent
pop. - genetic divergence prevents
interbreeding
- isolation of a pop. followed by allopatric,
genetic divergence
• Isolation in the sea requires barriers that are of
common occurrence
• Open sea is an effective barrier for most shelf
benthic invertebrates
• North-south
provincial boundaries are often
locations of between-water mass isolation (Midocean Ridges)
• Isthmus of Panama
• Although allopatric speciation is an obvious
explanation for speciation on either side of a
geographic barrier, some species’ ranges end at
thermal discontinuities
• Point Conception CA (N-S temp. breaks)
• Cape Hatteras, NC
• It
is possible that genetic divergence can occur
on either side of such a discontinuity - even with
some gene flow; thus, spp. might originate
because of strong divergent selection in two
geographically adjacent but environmentally
different habitats. This model of divergence and
speciation is referred to as parapatric speciation.
Species Problems - spp. recognition;
morphology etc.
• 2 species of eel - Anguilla rostrata (Amer. Eel)
and Anguilla anguilla (European) - species
differences has been a subject of “hot” debate
• Both spp. migrate from freshwater rivers and
brackish water to spawn in open water of
Sargasso Sea
• Schmidt (1920s)- evidence suggesting
overlapping but non-identical spawning grounds
• Large
difference in vertebral # between the 2
spp.
• Because vertebral # in fishes is often a
function of ambient temp. during larval stage - it
is possible that each spp. originates as a single
randomly mixed panmictic population in its
spawning grounds
• Grounds may have different latitudes and
hence different temperatures
•Possible that all European eels do not survive trip to
Sargasso, this would suggest that Euro. eels are the
progeny of American eels reproducing in Sargasso
Sea
•A difference in chromosome # has been found
between the 2 spp. - but only a large allele
frequency difference at one biochemical enzyme
locus was detected in an intensive investigation
of many loci
•Furthermore - eels from Iceland have intermediate
vertebral #
•
Mussels:
• M. edulis is a species with subpopulations in boreal
and temperate waters of the N. hemisphere and
closely related representatives in S. Amer. ,New
Zealand, and Australia.
•Southern - M. galloprovincialia - English Channel
and Mediterranean, but M. edulis only in the English
Channel
• Barsotti & Melluzzi, 1968 - M. gallo. - recent derivative of
M. edulis - since geographic separation between Atlantic and
Mediterranean. Speciation likely still in progress
•
•Similar problem with Cardium edule (northern)
and C. glaucum (Med.) – these cockles have
overlapping distributions in Britain and Jutland
Genetic Variations:
• Hardy-Weinberg - ƒ(aa) - p2
ƒ(ab) - 2pq
ƒ(bb) - q2
• Heterozygotes might exhibit superior performance
- overdominance
• Homozygotes might be preferred - underdominance
• Genotypes with specific allele might be favored - directional selection
• Assortive mating - small populations increases
probability of chance deviations
•
•Random sampling of gametes from
parents to form offspring will on average
increase with decreasing population size –
results in a random genetic drift of allele
frequencies.
•Founder Principle – States that a small
isolate of a species migrating to a peripheral
location is bound to have a differing genetic
constitution than the parent population.
•Such an effect may be important when only a
few larvae survive a trip of ca, 100 km to a site,
originating from a parental population that
produced several million larvae
Type of Variation Measurable in Marine Organisms:
• Chromosome # and polymorphism - some marine
species have variable # of chromosomes per individual
among population and variation in morphological
characteristics of a given chromosomes.
• In the drilling gastropod Nucella lapillus, variation in
chrom # was observed along the French coast.
Haploid # of n=13 on wave-exposed coast, with n=18
in sheltered habitats - cause is unknown.
• Ahmed & Sparks, 1970 - Mytilus – chromosomes
differ in form, but not in #.
Color polymorphism:
• Color spot patterns - products of one of a few
loci
• Harpacticoid copepod color spots on
cephalothorax vary in size and shape –
• Mytius edulis, brown shell phenotype co-occurs with
more common black pheno. Crosses show that a
simple one-locus, two allele genetic model governs the
variation (Innes and Haley, 1977) – Brown allele is
dominant over black.
Genetic Polymorphism
The amount of polymorphism maintained in a species
is of great interest because it suggests the scope of
variation on which selection may act.
• Species of Macoma living on different varieties of
sedimentary substrate - examined for degree of
polymorphism at 2 enzyme loci.
• No variation found at leucine aminopeptidase
locus
• But it was more polymorphic at phosphoglucose
isomerase locus
Geographic Variation:
• Trends in the distribution of allele frequency
have been commonly observed for marine
species, particularly in protein polymorphisms.
Variation along a geographic distance is known
as a cline.
• Cline found for shell morphology, chrom #,
shell color, and protein allele frequency.
• Johnson
1971, showed that a 2 allele lactate
dehydrogenase locus in the intertidal crested
blenny Anoplarchus purpurescens showed
latitudinal clinal variation in Puget Sound, WA consistent with temperature variation. The A’
allele is associated with warm temperature and
increased from less than 0.05 from the Strait of
Georgia southward to over 0.25 near Tacoma,
WA (see graph in notes)
• Also for ectoproct bryozoan species - allele
frequency correlated with temperature in Cape
Cod - at 2 loci
• Lactate
dehydrogenase
• Powers et al. (1979) Nature 277 (240-241)
• Adaptive importance of catalytic efficiency
• In the LDH-B allozymes in Fundulus
heteroclitus
• North-south cline Atlantic coast
• Temperature effect on enzymes
•Temperature effects on LDH - how it affects
physiology.
• Growth rate at low temps, blood oxygen
affinity etc.
• In
M. edulis, 3 common aminopeptidase alleles:
• Lap94, Lap96, Lap98
• Catalytic properties of Lap94 different from 9698, 20% higher efficiency results in higher
accumulation rate of free amino acids
• High temp. and salinity, show increase in
aminopeptidase activity (increase in enzyme
protein concentration) – well understood for
salinity not temp.
• It is where these 2 conditions (temp and salinity)
occur that a cline with higher Lap94 exists
Fluctuation of Salinity Conditions:
• Higher rates of cellular protein catabolism
(during hyperosmotic acclimation) and excretion
of amines (during hypoosmotic acclimation)
results in depletion of nitrogen reserves.