Evolution and Systematics

Convergence, Divergence and Parallel Evolution

Distantly related taxa can come to resemble one another through the process of convergence

Closely related taxa may quickly develop very different morpholgies through divergence

Species may have diverged in the distant past can maintain similar morphologies through parallel

evolution

Adaptations

An adaptation is a character or suite of characters that helps an organism cope with its environment

A preadaptation (or exaptation) is an adaptation that performs a function other than previously held

e.g. the lower jaw of odontocetes is used to transmit high frequency sounds underwater but first

evolved to transmit low frequency sounds from the ground

Adaptive Radiation

Rapid diversification of a lineage into many forms

Can obscure relationships due to rapid evolutionary change if in distant past

If recent, may be hard to detect differences: what is a species??

Biological Species Concept

Inability to interbreed

Studying evolutionary relationships

Systematics – the study of defining evolutionary relationships among organisms both extinct and extant

A phylogeny is a hypothesis about evolutionary relationships

Often shown on a tree

Can never be “proven” only strongly supported!!!

Phylogenetic Trees(Cladograms)

Tree representing best estimate of phylogenetic lineages

Lines are clades or lineages (groups of related taxa from a common ancestor)

Nodes = branch points = speciation events

Cladistics

Organisms can be deemed related based on shared derived characters (synapomorphies)

Characters

any feature useful in phylogenetic analysis

May be ancestral (primitive) or derived (apomorphy)

Characters may be primitive or derived but taxa are not

Taxa are all endpoints of evolution

Character State

Condition of the character

Homology and Analogy

Cladistics relies on finding synapomorphies

Homology

Characters that arise from similar ancestry

Bats’ wing bones and human fingers

Analogy

Similar characters that do not share evolutionary history

Bird wing and bat wing

Do analogies help in resolving evolutionary relationships?

Determining Character States

It is critical to determine which character states are ancestral and which derived

Can use outgroups or closely related lineages; often use sister group – the most closely related

lineage

Character states shared with outgroup likely are ancestral

Types of groups on cladograms

Monoplyletic

includes hypothetical ancestor and all descendents

Paraphyletic

does not include all descendants of an ancestor

Polyphyletic

Collection of descendants from >1 ancestor not including all ancestors

Types of characters

Behavioral

Physiological

Mophological

Molecular

Molecular vs Morphological Characters

Molecular

Huge number of possible characters (down to each nucleotide)

Can find parts of genome not under environmental selection

Long time periods can obscure due to saturation (problems with parallel evolution)

Time to saturation depends on rate of evolution at each locus

Morphological

Evolve more slowly (little saturation)

Can include extinct taxa

Can have problems with convergence

Defining characters can be difficult

Use of both types of data best!

Fossil Taxa

Contribute most when they help plug holes in long divergent lineages

Can complete morphological series, help determine homologies

Can help determine earliest occurences

Can’t Use many characters – results in poloytomy (unresolved nodes)

Constructing a Cladogram

Select group, define all taxa

Select and define characters and character states

Create data matrix

Use outgroup comparison to determine ancestral and derived states

Construct all possible cladograms

Select best cladogram using parsimony

Principle of Parsimony – the best cladogram is the one involving the fewest evolutionary transitions

(steps)

Uses of phylogenies

Character mapping

Pinniped Evolution and Systematics

The pinnipeds

Monophyletic group with 3 monphyletic families

18 phocids, 14 otariids, walrus

Diversity was once much greater (13 species of walrus are extinct)

First pinnipeds arose in Oligocene (27-25mya)

Much speciation in last 2-5 million years

Poor fossil record generally

Major pinniped synapomorphies

Large infraorbital foramen (hole below eye to allow vessel and nerve passage) (1)

Short, robust humerous (6)

Digit I on hand emphasized (7)

Digit I and V on foot emphasized (8)

Mono or diphyly?

Evidence for diphyly

Biogeorgaphy and morphology

Otarrids and odobenids close to bears; phocids close to mustelids

Evidence for monophyly: the best explanation

Molecular, karyological, morphology

All support close ties to ursids, mustelids, otters (sister group unclear)

Diving behavior and breeding patterns suggest eared seals evolved first (Costa 1993)

Phocids are most aquatically adapted (diving, breeding, body plan)

Early Pinnipeds

Find describe in 2009 sheds new light on early evolution

Pujila darmwini was “walking seal” ~24 mya

Otter-like body, webbed feet, lived in freshwater lakes of Canadian Arctic

Suggests pinnipeds went through a freshwater phase

High productivity associated with cold water upwelling probably supported prey base early pinnipeds

exploited

First found from cool waters and rocky coasts of eastern N. Pacific during late Oligocene

Pinnipedimorpha clade

Show ancestral, heterodont, dentition

Many similarities to archaic bears

Later forms show derived homodont dentition

Early Pinnipeds

• Pinnipedimorpha clade

– Lateral and vertical movement of vertebral column possible

– Both sets of flippers modified for aquatic locomotion

– Still very capable on land, probably spent more time there than modern forms

Modern Pinnipeds: Otariidae

• Seal lions and fur seals

• Shallow divers often targeting fast-swimming fish

• Monophyletic group first appeared late Miocene (11 mya) but all modern forms in last 2-3 my

• Two subfamilies

– Otariinae (seal lions)

– Arctocephalinae (fur seals)

Some Otariid synapomorphies

– Frontals extend anterior between nasals (9)

– Uniformly spaced pelage units

– Trachea subdivides close to voicebox (13)

– Secondary spine on scapula (11)

– External ear flaps “pinnae”

– Can turn hindflippers forward; use to walk

Otariid systematics

– Otariinae (sea lions) monophyletic, not Arctocephalinae (fur seals) which are still poorly resolved

– Hybridization and Introgression may cause problems

– aggressive sexual behavior of male sea lions directed at other species

Modern Pinnipeds: Odobenidae

• Current 2 subspecies relicts of once diverse group

– Modern walrus large-bodied, shallow diving mollusk feeder

• Monophyletic family, origin middle Miocene (16-9 mya) eastern North Pacific

Odobenid synapomorphies

• Five synapomorphies

• Modern walrus distinguished by squirt-suction feeding

• TUSKS ARE NOT A SYNAPOMORPHY

– They evolved in only one lineage leading to modern walrus

– Many ancient odobenids did not have tusks

Where do the odobenids fit?

• Molecular evidence points to otariids, but morphological data suggests a close association with phocids

– Middle earbone enlarged

– No pinnae

– Well-developed thick subcutaneous fat

– Abdominal testes

– Similarities in hair and venous system

• What gives?

– Still unclear where walrus fit in pinniped clade

– Odobenids probably branched off from basal pinnipeds very early leading to a long branch

– Subsequent long-branch attraction causes molecular similarities

Odobenid movements

• Origin in eastern North Pacific

• Invaded Atlantic through Carribean

• 600,000 ya modern walrus reinvades Pacific through Arctic and diverge into subspecies

– Middle earbone enlarged

– No pinnae

– Well-developed thick subcutaneous fat

– Abdominal testes

– Similarities in hair and venous system

• What gives?

– Still unclear where walrus fit in pinniped clade

–Modern Pinnipeds: Phocidae

– “True” seals, lack ear flaps

– Generally larger than otariids

– Some fantastic divers

– Weddell and elephant seals over 1000m

– Late Oligocene origin (29-23mya) in N. Atlantic

– Monphyletic family with two subgroups

– monoachines and phocines

Some phocid synapomorphies

• Unable to turn hindflippers forward

• Inflated entotympanic bone (21)

• No supraorbital process (10)

Subspecies, hybridization and a misplaced genus

– Five subspecies of harbor seal recognized based on morphological, molecular, behavioral differences

– Eastern and western sides of Atlantic and Pacific, lakes of northern Quebec

– Harp seal Phagophilus groenlandicus x hooded seal Cystophora cristanta hybird – what does this mean

for biological species concept

– What is the status of the gray seal genus?

Phocid systematics

• Are traditional subgroups monophyletic?

• Monk seals Monachus often considered most basal of phocids due to ancestral characters (some moreso

than fossil taxa)

Pinniped Evolution: Summary

• Morphologic and molecular data support monophyly

• Derived from arctoid carnivores, probably close relatives of bears

• Earliest appear 27-25mya in north Pacific

• Modern lineages diverged quickly

• Position of the walrus unclear

Cetacean Evolution and Systematics

Cetaceans

• Monophyletic group with 3 suborders

– Archaeoceti (extinct)

– Odontoceti (~76 species)

– Mysticeti (11 species)

• Earliest marine mammals (with sireneans) 53-54 mya

Cetacean Origins

• Currently some questions about origins: several competing hypotheses

• Evolved from small primitive ungulate group

– Could be from mesonychid condylarths

– Could share common ancestor with hippos

– Could be sister group of other artiodactyls (even-toed; hippos, camels, antelope, pigs, giraffes, etc)

– Could be another ancestor not closely related to moder artiodactyls

Cetacean Origins: The old favorite

• 1. Decendent of Order Condylartha, Family Mesonychidae

• Wolf-like with digitigrade stance (walk on toes), possibly hoofed

• Massive crushing dentition; early skulls suggest similarity

Cetacean Origins: close to hippos?

• 2. Some molecular data points to close affinity with hippos; recent skull finds disagree – more like

mesonychids

Cetacean Origins

3. Sister group to clade including hippos and artiodatyls; not particularly close to mesonychids

– Works well with #2 if hippo ancestors were very different morphologically

– Probably all derived from mouse-deer like ancestor

Cetacean Origins: Indohyus brings us closer to an answer

• 4. Sister group to cetaceans more primitive than other artiodactlys

– Recent finds in India suggest cetaceans closest ancestor is an ancient artiodactyl group (raoellids)

– Similarity to cetaceans based on morphology of inner ear, the arrangement of incisors, and

morphology of premolars

– Indohyus was an aquatic wader based on bone density and oxygen isotopes

– Carbon isotopes suggest feeding on terrestrial vegetation or omnivores on land but escaped to water

when in danger like modern African mouse deer

– Adaptation to aquatic habitats did not occur first in early cetaceans, but more basal species –

cetacean branch probably driven by switching to aquatic prey (unique dentition and oral skeleton)

– Early cetacean ancestors went through a hippo-like stage

– Study published in 2009 suggests that hippos are, in fact, closest living relatives of cetaceans.

Archaeocete cetaceans

• Paraphyletic group of ancient whales that gave rise to modern whales

– lack telescoped bones of the skull

– Elongate snout

– Narrow braincase

– Large temporal fossa

– Well defined sagital and lambdoidal crests

• Earliest from Early Eocene (>50 mya)

• Extinct by end of Eocene

• Pakecetoids are most ancient group (50 my)

– Pakecetus – earliest whale; India and Pakistan

– Ear morphology gives them away as cetaceans

– Lived in an arid environment with ephemeral streams and floodplains

• Always found in river deposits

• At best site, 60% of mammal remains are pakicetids!

– Quadropedal and probably mainly terrestrial but not swift runners (dense bones that may have

been for ballast)

– Long thin legs and short hands and feet suggest they were poor swimmers (quadropedal

paddling) and many deposits were rivers that were too shallow for swimming

– Teeth vary greatly – some hyena-like

• may have been scavengers or predators

• Probably ate freshwater aquatic organisms and land animals near water

• Ambulocetids

– Found in middle Eocene rocks of India and Pakistan

– Most basal amphibious marine cetaceans

• Nearshore marine (estuaries and bays) but tied to freshwater for drinking

– Abulocetus natans and others close to size of male sea lion

– Show first signs of hearing adaptations

– Eyes above profile of skull

• Ambulocetids

– Likely slow on land

– Elongated hind feet and tail that would aid in locomotion

• Probably swam like modern otter swinging tail and feet

– Probably ambush hunter like modern crocodiles

• Remingtonocetidae

– Short-lived group from Middle Eocene of India and Pakistan

– Nearshore tidal environments, but more aquatic than ambulocetids

– Long narrow jaws

– Probably swam with tail like Amazonian giant otter

– Captured fast-swimming aquatic prey

– Protocetids

– Globally distributed during the middle Eocene

• First group to leave South Asia

– Expanding niches inhabited including deep offshore waters but probably restricted to tropics

– Nasal openings more caudal than earlier species

• Could breath with much of head underwater

– No fluke

•– Lifestyle probably very similar to modern pinnipeds

– Hindlimbs may not have been able to support weight in some species

• Basilosaurids

– Middle to late Eocene/early Oligocene

– large-bodied family with elongated vertebral bodies (Basilosaurinae)

– Very reduced hind limbs – fully aquatic

– Basilosaurus grew to 25m

– Throughout the tropics and subtropics

– Had fluke, but back undulations rather than the fluke provided propulsion

– Piscivorous

• Dorudontids

– Related to basilosaurids, sometimes put in the same family

– smaller-bodied with non-elongated vertebral bodies

– Throughout tropics and subtropics, often in deposits with basilosaurids

– Dolphin-like and more diverse than basilosaurids

– Had a fluke and swam like a modern cetacean

– Likely ancestors of odontocetes and mysticetes

Archaeocete trends

– Rapid evolution (few million years) from

– Quadropedal to flukes (hindlimb reduction)

– Freshwater dringing to seawater drinking

– Land animal to not able to move on land and giving birth in water

– Movement of nostrils to the top of the head

– Extinction probably tied to changes in food supply driven by oceanographic change

Modern Cetaceans

– Diverged from Archaeocetes about 37 mya

– Monophyletic clade derived from dorudontids

– Split between mysticetes and odontocetes probably 35 mya

– Synapomorphies

– Telescoping of skull: movement of blowholes to the top of skull

• Migration of premaxillary and maxillary bones forms a rostrum (beak)

– Fixed elbow joint not present in archaeocetes

Mysticetes (Baleen whales)

– Modern forms distinguished by baleen plates, but early mysticetes had teeth

– Origin probably tied to Oligocene development of Circum-Antarctic current and generation of nutrientrich

upwelling that led to huge zooplankton shoals

– Early mysticetes were small 4-5 m

– Major evolutionary transition is from raptorial predation (single prey item at a time) with teeth to batch

or filter feeding with no teeth (baleen present by Oligocene, but decomposes so record poor)

– Other trends include increased body and head size, shortening of the neck

Mysticete Synapomorphies

• Maxilla extends posteriorly to form infraorbital process

• Mandibular symphysis (lower jaw connection) unfused

Modern Mysticete Relationships

– Four extant families?

– Balaenopteridae,

– Balaenidae

– Eschrichtiidae

– Neobalaenidae

– Taxonomy not well-resolved

– Cytochrome b suggests that Eschrichtiidae is not valid

Mysticetes: in order of divergence

– Balaenidae

– Right whales and Bowhead

– First appear in early Miocene (23 mya)

– Heavy body, cavernous mouth, no throat grooves

– Head 1/3 of length

– Long baleen plates

– Only mysticetes with 5 digits on forelimb

– Monopyletic

• Support for two separate genera poor

– Neoalaenidae

– Anatomical data places as separate family outside Balaenidae

– More anteriorally thrust occipital shield

– Shorter, wider mouth for shorter baleen

– Separate from balaenids due to presence of dorsal fin, throat furrows, different type of baleen,

relatively smaller heard, four digits on hand, shorter humerous

– Eschrichtiidae

– Current species has 100,000 year fossil record (only one for family)

– North Atlantic population extinct in 17th or 18th century

– Probably falls within the Balaenopterids, but further work needed

• No dorsal fin

• 2-4 throat grooves

• Baleen is thicker, fewer in # and whiter than rorquals

– Unique paired occipital tuberosities on skull for neck muscles

– Balaenopteridae

– Fossil record extends 10-12 mya from Americas, Europe, Asia, Australia

– Hybrids occur

– Dorsal fin

– 14-22 (humpback) to 56-100 (fin) throat grooves extend beyond gular region

– Short baleen

Odontocetes

• Diverse array of toothed forms from freshwater rivers to deep-diving in pelagic habitats

• First appear in fossil record 28-29 mya

• Major Miocene radiation of pelagic forms appears to be linked to changes in currents and thermal

gradients

• Monophyly well supported despite well-publicized argument against with early genetic data

Are odontocetes monophyletic?

• Most morphological characters argue that they are, but one of the supposed synapomorphies has come

been disputed: presence of a single blowhole

– Odontocete facial structure serves a number of functions

– Respiration cause of much skull rearrangement

– Sound production (echolocation) and detection another major force

– Buoyancy control, at least in sperm whales

– Some of the 20 Synapomorphies

– Concave facial plane

– Asymmetric cranial vertex

– Premaxillary foramen present

– Maxilla overlays supraorbital process (frontal bone)

– Antorbital notch present

– Asymmetric skulls (except possibly most primitive)

– Asymmetric soft tissues in modern forms due to enlargement on right side

– Fatty melon in front of nasal passages for echolocation

Ziphiidae

• More than 20 species in 5-6 genera extant

– Found in Mocene and Pliocene, including one freshwater form; extant species mainly pelagic

– Trend towards loss of teeth with exception of 1-2 pairs anteriorly which become enlarged (only

Shepard’s beaked whale has full functional dentition)

• Possible sexual display/weapons

– Pair of throat grooves that converge anteriorly

– Phylogeny unclear; no rigorous cladistic review

Physeteridae

• Long fossil record (29-21 mya), once diverse but only one extant species

• Loss of one or both nasal bones

• Deepest known divers

• Have spermaceti organ

– not homologus to melon; “junk” below organ probably melon homolog

– in supracranial basin

– may occupy 30% of length and 20% of weight

– May control buoyancy but still unclear

Kogiidae

• Linked into a superfamily with sperm whales because of supercranial basin and spermaceti organ

• Lack both nasal bones

• Have short rostrum and are much smaller than sperm whales (<4m; <2.7m)

• Oldest known from late Miocene (8.8 mya)

“River Dolphins”

• Once put into a single family, but similarities (reduced eyes, elongated snouts) are due to convergent

evolution

– freshwater/estuaries have been invaded at least 4 times

Platanistidae

• Asiatic river dolphins

– Ganges and Indus Rivers

– Reduced eyes in Ganges form

– Long narrow beak with numerous narrow pointed teeth

– Broad paddle-like flippers

– No known fossil record, time of freshwater invasion unknown

– Bony facial crest

Pontoporiidae

• Fransiscana

– Coastal waters of western S. Atlantic

– Long rostra, tiny teeth

– Close relative of Iniidae

Iniidae

• Amazon river (botu)

– Reduced eyes

– Extremely elongated rostrum and mandible

– Conical front teeth, molariform rear teeth

– Greatly reduced orbital region

– Maxilla forms crest

– Fossils from late Miocene originated in Amazonian basin

Lipotidae

• Yangtze Tiver (baiji)

– Narrow, upturned beak

– Triangular dorsal fin

– Broad round flippers

– Reduced eyes

– One fossil, one extant species from China

– One fossil, one extant species from China

Delphinidae

– Most diverse cetacean family 36 sp, 17 gen

– Open ocean to some into freshwater (Orcella brevirostris, Sotalia fluvatilis)

– Most small to medium 1.5-4.5m, killer whale to 9.5m

– Loss of posterior sac of nasal passage

– Reduction of posterior end of left premaxilla: does not contact the nasal

– Oldest from late Miocene (11 Ma)

• Systematics are still a mess

– Some genera are not monophyletic

– Diversity likely to increase (e.g. Tursiops)

– Stenella is polyphyletic

Phocenidae

– Six small extant species

– Synapomorphies

– Raised rounded protuberances on premaxillae

– Premaxillae do not extend beyond anterior half of nares

– Spatualte (not conical) teeth

– Sister taxa of delphinids

– First appeared in late Miocene, eastern Pacific

Monodontidae

• Delphinoids with flat or convex facial planes in profile

• Extant species in Arcitc

• Miocene/Pliocene some species found in E. Pacific to Baja California

Sirenians, Sea otters, Polar Bears, and other marine mammals: Evolution

Sirenians

• Monophyletic group with two extant familes

– Trichechidae (manatees)

– Dugongidae (dugongs)

• Unique in strictly herbivorous diet

• First appear in early Eocene (50 mya)

Sirenian Origins

– Monophyly strongly supported

– Syapomorphies

– External nares retracted and enlarged reaching beyond the level of the anterior margin of the orbit

– Premaxilla contacts frontal

– Lacks sagital crest

– Bones dense and compact (for buoyancy regulation)

– Closest living relatives are proboscideans (elephants)

– Teeth and skull morphology unite the groups

– Extinct Desmostylians form clade with sirenians and elephants (monophyletic “Tethytheria”)

– First arose in Old World, but quickly spread to New World 50 mya

Ancient Sirenians

• Prorastomus (50mya) first (Jamaica)

– Had functional hindlimbs

– Dense bones, swollen ribs and presence in marine deposits suggest partially aquatic; riverine and

estuarine selective browser

• Protosiren (middle Eocene) (Egypt)

– Functional terrestrial locomotion but auditory, olfactory, and visual systems appear modified for

aquatic lifestyles

• Much of the spread of sirenians tied to the spread of seagrasses in the temperate Pacific

Modern Sirenians: Trichechidae

– Appear to be derived in late Eocene/early Oligocene, possibly from dugongids

– Monophyletic, united by features of the skull and reduction of neural spines on vertebrae

– Mainly freshwater/estuarine

– Ability to produce new teeth as old ones are worn down

• 3 modern species

– West Indian manatee (Trichechus manatus)

• 2 subspecies: Antillean (T.m. manatus); Florida (T.m. latirostris)

– Amazon manatee (Trichechus inunguis); freshwater only

– West African manatee (Trichechus senegalensus)

Modern Sirenians: Dugongidae

• Paraphyletic family with Caribbean/W. Atlantic origins spreading to Pacific

• More marine than manatees

• Two extinct subfamilies and one extant

– Hydrodamalinae (includes Steller’s sea cow) appears to have split from Dugonginae (dugong) in

late Eocene

• Includes Stellar’s sea cow (extant into historical times)

• Some temperate species

• Large body size

• Loss of tusks

• May have fed on kelp high in the water column

Steller’s sea cow

• Named after Georg Steller

• 7.6m long, 4-10 tons

• Lacked teeth, had bark-like skin

• Cold waters near islands of the Bering Sea

– Prehistorically from Japan to Baja California (to Montery 19,000 years ago)

• Extinct by 1768 (27 years after discovery)

– Mainly Russian hunting, but possibly exacerbated by aboriginal hunting

Dugonginae

• Currently one species, but once many genera

• Tropical and subtropical

• Once widespread; 15 mya from North and South America, Caribbean, Mediterranean, Indian Ocean,

North Pacific

• Some extinct species used tusks to dig up seagrasses

• Modern dugongs use tusks socially, not for feeding

Sirenian evolution in the Caribbean (Domning 2001)

• From Oligocene onwards, there was a great diversity of sirenians in the Caribbean, especially dugongids

• Seagrass communities were similar to extant ones but were more diverse

• Habitat could be partitioned along several axes

– Rhizome size

– Location of feeding in water column

• Morphology of sirenians reflects partitioning of seagrass resources

– Body size differences lead to differences in access to shallow waters and ability to consume more

fibrous seagrasses (bigger are better)

– Rostral deflection influences ability to feed on the bottom or on midwater or surface plants and

ability to dig

– Tusk size influences ability to dig out largest rhizomes

– Interaction of tusk size and defection can be complex

• Why so few species today?

• Close of Central American Seaway about 3 mya led to major shifts in habitats

– Dugongids died out along with large rhizome seagrasses

– Manatees were able to disperse into open marine habitats to move into North America and to West

Africa

Desmostylia: Sirenian Relatives

• Only extinct Order of marine mammals

• Confined to North Pacific (Japan through N. America)

• Late Oligocene to Middle Miocene (33-10mya)

• Hippo-like amphibious quadropeds

• More closely related to elephants than sirenians

• Probably fed on algae and seagrasses in subtropical and cool-temperate waters

• Locomotion probably like polar bears

–Thalassocnus

• Aquatic ground sloth!

• Pliocene marine rocks of Peru

• Medium to giant sized herbivores

• Aquatic or semi-aquatic grazer on seagrasses or seaweeds (well developed lip for grazing)

• Probably swam with tail

Kolponomos

• Bear-like carnivore (early Miocene)

• Massive skull, down-turned snout, broad crushing teeth

• Coastal habitat feeding on marine invertebrates on rocks and crushed their shells

• Sea otter only marine mammal that may be similar in habitat

• Relationships problematic

– Appears to be closely related to basal ursids and forms leading to pinnipedimorphs

Sea otter Enhydra lutris

• Smallest marine mammal but largest mustelids

• Three subspecies across northern Pacific

• E. lutris arose in North Pacific early Pleistocene (1-3mya)

• Several extinct species from Africa, Europe, and Eastern United States that appear to have

consumed extremely hard prey items like modern otters

Polar Bear Ursus maritimus

• Most recently derived marine mammals

• Descended from lineage of brown bears during middle Pleistocene (300,000-400,000 ya)

• Brown bears of southeast Alaskan islands closest relatives