View
251
Download
0
Category
Preview:
Citation preview
8/13/2019 Variation Chap 10
1/28
CHAPTER 10
VARIATION
8/13/2019 Variation Chap 10
2/28
Variation
Outline
The nature of species
Prezygotic isolating mechanisms
Postzygotic isolating mechanisms
The geography of speciation
Darwins finches
Problems with the biological species concept
8/13/2019 Variation Chap 10
3/28
The nature of species
knowledge in species is the fundamentals importance
to evolutionary biology
any concept of species must account for 2 phenomena:
i. distinctiveness of species that occur together at a
single locality
ii. connections among population of the same
species that are geography separated
8/13/2019 Variation Chap 10
4/28
The distinctiveness of sympatric species
species that occur together (sympatric) are distinctive
entities that are phenotypically different, utilize
different parts of the habitat and behave separately
sympatric species that appear to be nearly identical
are called sibling species.
The nature of species
8/13/2019 Variation Chap 10
5/28
8/13/2019 Variation Chap 10
6/28
The nature of species
The biological species concept
if sympatric species exchanged genes, such species are
rapidly lose their distinctions
- gene pools of different species become homogenized
if the geographically distant populations share genes, it
will keep the populations integrated as members of the
same species
8/13/2019 Variation Chap 10
7/28
The nature of species
The biological species concept
Ernst Mayr coined the biological species concept
which define species as:
.. groups of actually or potentially interbreeding natural populations
which are reproductively isolated from other such groups.
species - individuals that are capable of interbreeding
and producing fertile offspring
member of different species that cannot produce fertile
offspring are said to be reproductively isolated
8/13/2019 Variation Chap 10
8/28
The nature of species
The biological species concept
hybridization - member of different species interbreed,
results in unhealthy and sterile
offspring or no offspring at all.- genes from one species unable to enter
gene pool of another species
8/13/2019 Variation Chap 10
9/28
The nature of species
Problems with applying the biological species concept
effective way of understanding the existing of species
but difficult to apply to populations that do not occur
naturally together (allopatric)
many organisms undergo asexual reproduction,
reproductive isolation is not a matter.
8/13/2019 Variation Chap 10
10/28
Factors involved in the formation of new species
reproductive isolating mechanisms fall in 2 categories:
i. prezygotic isolating
- prevent the formation of zygotes
ii. postzygotic isolating
- prevent the proper functioning of zygotes
8/13/2019 Variation Chap 10
11/28
Prezygotic isolating mechanisms
Ecological isolation
even two species occur together in the same area, they
may utilized different portions of environment and not
hybridize because they do not encounter each other because of differences in behavioral and ecological,
two species rarely came into direct contact
eg. : lions stay in open grassland, tigers in forestvalley oak can achieve 35 m tall, scrub oak only
1-3 m tall
8/13/2019 Variation Chap 10
12/28
Prezygotic isolating mechanisms
Behavioral isolation
related species of organisms often differ in their
mating rituals, make them distinct in nature even share
the same places eg. : mallard and pintail duck share the freshwater in
North America, but nest side-by-side
8/13/2019 Variation Chap 10
13/28
Prezygotic isolating mechanisms
Temporal isolation
different breeding seasons become the factor that
prevent hybridization
hybrids between two species can easily madeexperimentally, but in nature rarely occurs
eg. :L. graminifoliaflowers in early spring,L.
canadensisflowers in summer
8/13/2019 Variation Chap 10
14/28
Prezygotic isolating mechanisms
Mechanical isolation
structural differences prevent mating between related
organisms
eg. : flowers of related species often differ in theirstructures, prevent the proper pollination
Prevention of gamete fusion
fertilization never takes place because of the sperm of1 species may function poorly within the reproductive
tract of another
8/13/2019 Variation Chap 10
15/28
Postzygotic isolating mechanisms
prezygotic prevent the formation of hybrid zygotes
if mating do occur and zygotes are produced, many
factors still prevent it to become fertile individuals
genetic complement of two species may be so differentmake they cannot function together normally
even if they can survive the embryo stage, if they are
weaker then their parents, they will be eliminate innature
if they strong, they may still be sterile
8/13/2019 Variation Chap 10
16/28
8/13/2019 Variation Chap 10
17/28
Selection
natural selection favor allele in parent that prevent
hybridization until the two populations completely
reproductive isolated
natural selection is a process selecting a good genesand eliminated the bad genes (make them easier to
adapt to environment)
reinforcement process - initially incomplete isolating
mechanisms, are reinforced by natural selection until
completely effective
selection also act on mating behavior
8/13/2019 Variation Chap 10
18/28
The geography of speciation
speciation is two part process
1. identical populations must diverge
2. reproduction isolation must evolve to maintain
these differences
the major problems is homogenizing effect of gene
flow between population
8/13/2019 Variation Chap 10
19/28
The geography of speciation
Speciation in allopatry
geographically separated populations appear leading to
speciation
eg. : Papuan kingfisher, varies little in main island ofNew Guinea, but strikingly different from each
other for the same species at the nearby islands
8/13/2019 Variation Chap 10
20/28
The geography of speciation
Speciation in sympatry
selection lead to formation of two difference species
before become two difference species, they must
evolve reproductive isolating mechanisms
most of the time, the two phenotypes would be
retained as polymorphisms (the existence of two
distinct hereditary types based on different alleles) in asingle population
sympatric speciation a rare event
8/13/2019 Variation Chap 10
21/28
The geography of speciation
Genetic changes underlying speciation
studies show that there are very few genes involved in
separating two species
numbers of genes involved are different for eachspecies
8/13/2019 Variation Chap 10
22/28
The geography of speciation
Role of polyploidy in species formation
polyploidy - doubles the chromosome number of the
original sterile individual
hybrid are sterile because of their chromosomes fromtwo species of parents do not pair with one another
results of doubling, they will have duplicate of each
chromosome, this chromosome will pair and fertilityof the polyploid hybrid may be restored
8/13/2019 Variation Chap 10
23/28
Darwins finches
1835, Charles Darwin visited Galapagos Islands to
study the evolution in plant and animals
evolution led to the existence of groups of closely
related species from a common ancestor ancestor of Darwins finches was the first reached the
island, many spaces was unoccupied
other species of birds moved to these vacant ecologicalniches and adopted a new lifestyle
8/13/2019 Variation Chap 10
24/28
Darwins finches
adaptation to their environment and aided by the
geographic isolation make the ancestral finches rapidly
split into a series of diverse populations
some evolved into different species, now occupyhabitats on the Galapagos Islands
13 species of Darwins finches comprise of four
groups:
i. ground finches
- six species, feed on seeds, bills is related to the
size of the seeds they eat
8/13/2019 Variation Chap 10
25/28
Darwins finches
ii. tree finches
- five species of insect-eating, four of them have
bills suitable for feeding insects, the woodpecker
finch has a chisel-like beakiii. warbler finch
- same with warblers on the mainland, has a
slender, warbler-like beakiv. vegetarian finch
- bud-eating bird, very heavy bill used to wrench
buds from branches
8/13/2019 Variation Chap 10
26/28
Darwins finches
8/13/2019 Variation Chap 10
27/28
Problems with the biological species concept
The extent of hybridization
biological species are reproductively isolated, so
hybridization should be rare
if hybridization is common, two quick outcomes:
i. reinforcement would occur
ii. two populations would merge into a single
homogeneous gene pool in recent years, hybridization is no longer a rarely
cases, either in plants or animals
8/13/2019 Variation Chap 10
28/28
Problems with the biological species concept
Alternatives to the biological species concept
distinctions among species are maintained by natural
selection
each species has adapted to its own environment
hybridization has little effects because alleles
introduced into the gene pool would be eliminated by
natural selection variety of ideas have been put forward to establish
criteria for defining species
Recommended