Download Populations - Clarkson University

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)
Metapopulation
Population
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
Population 1
divider
• Divergence
– Natural selection, sexual
selection
– genetic drift
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
High Species
Diversity
Santa Monica Mtns
California
Choco
Smoky Mtns
Colombia
NC & Tenn
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, midocean 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