Speciation

1
Speciation

• Chapter 25

2
Speciation

• Speciation the creation of new species.
• May occur in a population due to
• reduced gene flow
• Mutation
• genetic drift
• natural selection
• Distinct species usually derive from one
  ancestral group

3
(No Transcript)
4
Speciation

• Species - evolutionarily independent population
  or group of populations
• Evolutionary independence starts with lack of
  gene flow.
• Once gene flow stops, mutation, selection, and
  drift can act on populations and genetic
  divergence can occur
• Genetic divergence, in turn, may lead to
  speciation

5
When is a population evolutionarily independent?
6
Biological Species Concept

• Defines a species as a population or group of
  populations whose members have the potential to
  interbreed in nature and produce viable, fertile
  offspring
• Speciation is due to reproductive isolation
• They cannot breed whether they are in the same
  area or not, biological incompatibility
• Two types of reproductive isolation, prezygotic
  and postzygotic

7
Reproductive Isolation

• Reproductive isolation is the existence of
  biological factors that impede members of two
  species from producing viable, fertile hybrids
• Prezygotic barriers
• Impede mating between species or hinder the
  fertilization of ova if members of different
  species attempt to mate
• Postzygotic barriers
• Often prevent the hybrid zygote from developing
  into a viable, fertile adult

8
Prezygotic Barriers
9
Postzygotic Barriers
10
Limitations to the Biological Species Concept

• The biological species concept cannot be applied
  to
• Asexual organisms
• Fossils
• Organisms about which little is known regarding
  their reproduction

11
Other Definitions of Species

• The morphological species concept
• Characterizes a species in terms of its body
  shape, size, and other structural features
• The paleontological species concept
• Focuses on morphologically discrete species known
  only from the fossil record
• The ecological species concept
• Views a species in terms of its ecological niche
• The phylogenetic species concept
• Defines a species as a set of organisms with a
  unique genetic history

12
Morphospecies Concept

• Morphological species concept can be applied
  easily but is rather subjective

13
Phylogenetic Species Concept

• an ancestral population plus all its descendants
  is called a monophyletic group
• A phylogenetic species is the smallest
  monophyletic group on a tree
• By definition, it is isolated from gene flow with
  other groups
• Precise and is applicable to all populations

14
(No Transcript)
15
Phylogenetic Species Concept
16
Species Definitions in Action The Case of the
DuskySeaside Sparrow
17
Seaside Sparrows

• Seaside sparrow subspecies were believed to be
  genetically isolated because
• populations are geographically isolated
• young birds breed near their hatching ground
• Biologists compared gene sequences
• found that seaside sparrows belong to two
  monophyletic groups
• Atlantic Coast and, Gulf Coast

18
(No Transcript)
19
(No Transcript)
20
Speciation
21
Speciation

• Much speciation takes place due to geographic
  separation, but some speciation can occur without
  separation
• Speciation can occur in two ways
• Allopatric speciation- when speciation occurs due
  to geographic isolation
• Sympatric speciation- when speciation occurs in a
  small population without geographic isolation

22
Allopatric Speciation

• Physical separation occurs in two ways
• A group colonizes a new habitat (dispersal)
• New physical barrier divides a population
  (vicariance).
• Once geographic separation has occurred
• One or both populations may undergo evolutionary
  change during the period of separation

23
(No Transcript)
24
Allopatric Speciation

• Example The antelope squirrel
• Ammospermophilus harrisi on the south rim
• Ammospermophilus leucurus on the north rim

25
Causes of Allopatric Speciation

• Cause speciation because the physical separation
  reduces gene flow
• Colonists are likely to experience genetic drift
  (founder effect)
• If the new habitat differs, natural selection may
  also cause divergence
• When an existing population is split by a new
  physical barrier, it is called vicariance

26
Vicariance

• Last ice age caused vicariance as a result of
  glaciation
• Continental drift separated species physically

27
Sympatric Speciation

• Takes place in geographically overlapping
  populations
• Often this happens when populations become
  isolated by habitat preference
• Can be due to chromosomal changes and non-random
  mating that reduces gene flow

28
Habitat Differentiation and Sexual Selection

• Sympatric speciation can also result from
  different niches or sexual changes
• Temporal separation in mating
• Different pollen production times
• Sexual preference or morphology
• Prettiest peacock
• Mating strategy-
• Mating calls in frogs

29
Polyploidy

• Polyploidy
• Is the presence of extra sets of chromosomes in
  cells due to accidents during cell division
• Has caused the evolution of some plant species
• Gene flow is reduced between polyploid and
  wild-type plants
• Two types autotetraploid and allotetraploid

30
Polyploidy

• An autopolyploid is an individual that has more
  than two chromosome sets, all derived from a
  single species

31
Polyploidy

• An allopolyploid is a species with multiple sets
  of chromosomes derived from different species

32
Polyploid Speciation

• Creates species because a mating between a normal
  diploid (2n) individual and a tetraploid (4n)
  individual results in an infertile triploid (3n)
  zygote.
• Tetraploid and diploid individuals rarely mate
  and produce fertile offspring, so these
  populations are reproductively isolated

33
Polyploid Speciation
34
Hybrid Infertility

• Many hybrid offspring are sterile because their
  chromosomes do not pair normally during meiosis
• Including triploid
• If mutation doubles the chromosome number, then
  homologs synapse normally
• Gametes may be able to self-fertilize to produce
  a tetraploid offspring

35
Hybrid Zones

• Sometimes if two species that are geographically
  isolated are able and willing to breed there is a
  hybrid zone between them
• A geographic area where interbreeding is common
  and there are lots of hybrid offspring is called
  a hybrid zone
• Sometimes the hybrid itself is a new species

36
(No Transcript)