Populations

Population: A aggregate of organisms that freely interbreed - a species may consist of one population or many Metapopulation: A network of populations united by migration (immigration/emigration)

Population

Metapopulation

Species Range

Metapopulations

range

What makes populations diverge?

• Local adaptation• Genetic drift

– frequencies of alleles fluctuate because of chance variation if survival/reproductive success among members of a populations

– in small populations, fluctuations lead to extinction of some alleles by chance

What keeps populations similar?

Migrationmovement of genes via movement of breeding

individuals from one population to another

Speciation

• Genetic Isolation– little or no interbreeding

– typically geographical isolation, but could be behavioral or other mechanisms

• Divergence– Natural selection, sexual

selection

– genetic drift

divider

Population 1

Population 2

Allopatry: Populations or species whose ranges do not coincide Sympatric: Populations or

species whose ranges do coincide

When Two Sibling Species Become Sympatric

Scenario 1

Freely interbreed: become homogenized back into one species

When Two Sibling Species Become Sympatric

Scenario 2

Post-zygotic (post-mating) isolationno offspring produced, or offspring less fit when

they cross

Either (1) They will interbreed until one (the rarer) goes extinct(2) They will evolve pre-zygotic (pre-mating) isolating mechanisms, called reinforcement

Mules (Horse X Donkey)

Gopher Chromosomal Species

When Two Sibling Species Become Sympatric

Scenario 3

If effective pre-zygotic isolating mechanisms exist, they are two good species

Either (1) One species will out-compete the other for limiting resources, driving the poorer competitor extinct(2) They two will evolve mechanisms to partition resources - they will become ecologically different

Galapagos (Darwin’s) Finches

Speciation between Large Populations

Some barrier appears that splits a large population into two (or more) rise of mountain ranges splitting of river

drainages climate change

fragmenting habitat continental drift

EuropeanBison

AmericanBison

Speciation in Small Populations

A species may primarily consist of small populations, or (more typically) small populations exist on the periphery of a species range e.g. on isolated patches of appropriate habitat, like islands.

Small, Isolated Populations Rarely Persist

• Small populations that fluctuate in size are at great risk of going to zero = extinction

• Small populations have lowered genetic diversity– caused by genetic drift & inbreeding– can result in lower fitness: (1) loss of favored

heterozygotes, (2)increased incidence of deleterious recessive alleles

If Small, Isolated Populations Persist

• Rapid evolution because of genetic drift and local adaptation

• If there is little immigration from other populations, the potential for speciation is high, if the population doesn’t go extinct first (most do go extinct)

When do you see lots of similar species?

Taxa that are speciosethose with low dispersal ability (mainland) or high

dispersal ability (islands)

Geographic regions that are specioseArchipelagos (chains of islands)Complex landscapes (mountains & hills etc.)Places where there are repeated cycles of

habitat fragmentation followed by reconnection (cyclic climate change)

Hawaii

Santa Monica MtnsCalifornia

Smoky MtnsNC & Tenn

ChocoColombia

High Species Diversity

Where is endemism highest?An endemic species is a species that occurs in only one

geographic location and nowhere else

• Refugia from climate change

• Isolated islands – especially if large, old

• Isolated habitat – disjunct mountain

ranges, karst (cave) regions

• High diversity areas

Rifting Island ‘Continents’

• Pieces of former continents that move by continental drift independently of others

• Often have old, unique flora and fauna

• Examples: Madagascar, New Caledonia, New Zealand

Oceanic Islands• Usually formed by volcanism (hotspots, mid-

ocean rifts) – most maintained as coral atolls

• Many organisms have ranges across many island systems (coconut, fruit bats), but isolated island clusters develop unique flora & fauna

• Examples: Polynesia, Hawaii, Galapagos, Lesser Antilles

Landbridge Islands• Connected to nearby continents during the

Pleistocene, when the ocean level was lower

• Most organisms similar to the mainland, but species richness is lower

• Examples: Trinidad, Sumatra, Britain, Ireland

Island Effect• ‘Island effect’ is tendency for islands to have

fewer species than mainland areas, unique species

• Depends on dispersal ability– Greatest IE: freshwater fish, amphibians– Intermediate: reptiles, mammals– Lowest: birds, plants, insects

• Greatest IE on large, old, remote islands– Immigrants rare, persist long enough to speciate