Transcript CH 17 evolution of populations

VOCABULARY
Gene pool – all genes available in a
population
 Allele frequency – how many times one allele
is present compared to all the other alleles for
the same gene
 Evolution – change in allelic frequency

Genetic drift – loss of some alleles in a
population
 Genetic equilibrium – allelic frequencies
remain constant; no evolution
 Speciation – formation of a new species
from an existing species

HOW COULD IT HAPPEN?
Lets use Darwin’s Finches as out Example
Start with a group of finches in Ecuador.
 A small group makes its way to the Galapagos
Islands, possibly blown in a storm or on a land
raft.
 The finches can’t get back to the mainland –
they don’t like open water.
 Darwin found 14 different species of finches,
none identical to the ones on the mainland.

Galapagos
Of relatively recent
volcanic origin most of
animal species on the
Galápagos live nowhere
else in world, but they
resemble species living
on South American
mainland.
500 miles west of mainland
Darwin’s Finches
Differences in beaks
 associated with eating different foods
 adaptations to foods available on islands
Warbler finch
Cactus finch
Woodpecker finch
Sharp-beaked finch
Small insectivorous
tree finch
Large
insectivorous
tree finch
Small ground
finch
Cactus
eater
Insect eaters
Seed eaters
Vegetarian
tree finch
Bud eater
Medium
ground finch
Large
ground
finch
1. Founding Fathers and Mothers
Only one type of finch on mainland
 Small group arrives in Galapagos on
Island A

 Can’t return to mainland
 Doesn’t like open water

Survive and reproduce
2. Separation of Populations


Birds from Island A
cross to Island B
Two populations
isolated from each
other
ecological
isolation
 Don’t like to cross open
behavioral
isolation
water

Same species, but
isolation means can’t
blend gene pools
 Ecological
 Behavioral
 Geographic
geographic
isolation
3. Changes in the Gene Pool
Island B has different vegetation (large, thickshelled seeds) than Island A (small, thinshelled seeds)
 Birds on Island B with larger, heavier beaks
can open seeds better (natural variation,
mutation)
 Birds with large beaks survive better; smaller
beaks disappear

4. Reproductive Isolation
A few birds from Island A go to Island B
 Birds like mates to have same size
beaks
 No mating between two groups
(reproductive isolation)
 No mixing of gene pools – two separate
species

5. Sharing the Same Island
 Three
possibilities:
 Coexist – different niches
 Extinction – of least-adapted
species
 Further evolution of second group –
if enough variation, could have beak
change to fit new environment
Correlation of species to food source
Seed
eaters
Flower
eaters
Insect
eaters
Darwin’s finches

Finches with beak
differences that
allowed them to…
 successfully compete
 successfully feed
 successfully
reproduce
○ pass successful traits
onto their offspring
http://www.hhmi.org/biointeractive/originspecies-beak-finch