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. - surface-thermal 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 (Mid-ocean 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 96-98, 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.