Transcript Population Change

Population Change

Populations can change and diverge when they are isolated
from one another.
• Speciation is a splitting event that creates two or more
distinct species from a single ancestral group.
Figure 23.1
Large ground finch
Small ground finch
Population Change

Speciation is the outcome of isolation and divergence.
• Isolation is created by reductions in gene flow.
• Divergence is created when mutation, genetic drift, and
selection act on populations separately.
Defining and Identifying Species

Species are distinct types of organisms because they
represent evolutionarily independent groups.

Lack of gene flow makes a species independent.
Figure 23.2
A
B
C
D
E
F
G
H
I
J
Defining and Identifying Species

Species are defined based on several different concepts:
• The biological species concept is based on reproductive
isolation.
• The morphospecies concept is based on identifying
evolutionarily independent lineages.
• The phylogenetic species concept is based on reconstructing
the evolutionary history of populations.
Table 23.1
Willow and Alder Flycatcher
Isolation and Divergence in Sympatry

Sympatric speciation occurs in populations that
occupy the same geographic area. Gene flow is
occurring.
• Natural selection can cause this process to occur in
spite of gene flow.
• Polyploidy, a type of mutation, can also cause sympatric
speciation.
Soapberry bugs
Figure 23.5a
Beak length correlates with fruit size.
Balloon vine
(native species)
Flat-podded
golden rain tree
(non-native species)
Short-beaked population
growing on non-native
plants
12
Long-beaked population
growing on native plants
8
Frequency
4
0
2
3
6
7
8
9
Beak length (mm)
10
11
12
11
12
8
Non-native plant
(small fruit)
4
Native plant
(large fruit)
0
2
3
6
7
8
9
Fruit radius (mm)
10
Figure 23.5b
This correlation appears to be a product of natural selection.
9
Beak length (mm)
8
7
6
5
1880
1900
1920
1940
Date
1960
1980
Figure 23.6
Diploid parent
Tetraploid parent
(Two copies of
each chromosome)
Meiosis
(Four copies of
each chromosome)
Mating
Haploid gametes
Diploid gametes
(One copy of each chromosome)
(Two copies of each chromosome)
Triploid zygote
Meiosis
(Three copies of
each chromosome)
When these gametes combine, most offspring
have incorrect number of chromosomes.
Isolation and Diversion in Allopatry

Allopatric speciation occurs when populations become
physically separated from each other.
• A population can colonize a new habitat.
• A new physical barrier can split a widespread population
into two or more isolated groups.