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? Migration movement 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) isolation no 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

European Bison American Bison

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 speciose those with low dispersal ability (mainland) or high dispersal ability (islands) Geographic regions that are speciose Archipelagos (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 Mtns California Smoky Mtns NC & Tenn Choco Colombia 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