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EVOLUTION: Unifying Concept in Biology

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Title: EVOLUTION: Unifying Concept in Biology Author: Carol Eunmi Lee Last modified by: Carol Eunmi Lee Created Date: 4/8/2013 10:53:10 PM Document presentation format – PowerPoint PPT presentation

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Title: EVOLUTION: Unifying Concept in Biology


1
Speciation
2
Todays OUTLINE
  • (1) Geographic Mechanisms of Speciation
  • (What circumstances lead to the formation of new
    species?)
  • (2) Species Concepts
  • (How are Species Defined?)

3
Mechanisms of Speciation
Last Time Genetic Models The roles of
Mutations Natural Selection Genetic
Drift This Time Geographic Models Allopatric
Model (difference place) Sympatric Model (same
place) Parapatric Model (adjoining)
4
(1) Mechanisms of Speciation
  • Last Time Genetic Models
  • How do Genetic Drift, Natural Selection,
    Mutations, etc. create new species?
  • Are there speciation genes?
  • This Time Geographic Models
  • How does speciation occur in Nature?
  • Is geographic isolation required?

5
Mechanisms of Speciation
Geographic (Ecological) Models Allopatric Model
(different place) Disperse to Another
Location Vicariance a barrier is formed This
geographic split could lead to Dobzhansky-Müller
incompatibilities Sympatric Model (same
place) Polyploid speciation Mate Choice Niche
Partitioning (e.g. different food source, Host
Plant) Parapatric Model (adjoining)
6
Geographic Models of speciation
  • Allopatric speciation geographic isolation
  • Sympatric speciation no geographic isolation
  • Parapatric speciation geographic separation (or
    gradient), but not isolation

7
Allopatric Models
Involves Geographic Isolation Dispersal Vicar
iance
8
Allopatric Models
  • Following geographic separation between
    populations,
  • Dispersal
  • Vicariance
  • This geographic separation provides the setting
    that allows speciation at the molecular level to
    occur (last lecture)

9
Allopatric Models
Dispersal Vicariance
  • Random Mutations would arise in the separated
    populations, and then selection or genetic drift
    would lead to fixation of those mutations
  • If different mutations are fixed in the different
    populations, reproductive isolation could arise
    through Dobzhansky-Müller incompatibilities (last
    lecture)

10
Allopatric Speciation
  • Examples (see book)
  • Dispersal
  • Colonization of islands
  • Colonization of lakes
  • Vicariance
  • Highway going through a forest
  • Fragmentation of habitats
  • Formation of Panama splitting the Caribbean
    Pacific Oceans

11
Sympatric models
Speciation with no geographic separation
Speciation despite gene flow
  • Formation of polyploids
  • (discussed in previous lecture)
  • (2) Natural Selection due to
  • Niche Partitioning
  • Sexual Selection

12
Sympatric Model
(1) Formation of Polyploids Polyploidy (extra
chromosomes) Important mechanism for plants Also
occurred possibly in vertebrates, some
crustaceans (covered in previous lecture)
13
Sympatric Model
  • (2) Selection in the face of gene flow
  • Niche Partitioning
  • Strong assortative mating and sexual selection
    (disruptive selection)

14
  • Example of Niche Partitioning
  • Soapberry bugs have adapted to two different host
    plants

15
Selection drives beak length apart
16
Evolutionary change in beak length on the new
small fruit
trend toward smaller beaks on smaller fruit
17
Niche Partitioning
Soapberry bugs mate on different host plants
  • The populations are unlikely to encounter each
    other
  • Reduces gene flow
  • Isolation
  • Disruptive Natural Selection
  • Adaptation to alternative hosts leads to
    reproductive isolation (through the genetic
    mechanisms discussed earlier, such as
    Dobzhansky-Müller model)

18
  • But, sometimes hybrid zones do form between
    populations that are in the process of speciating
  • Sometimes hybridization between different species
    results in vigorous new species or populations,
    especially in plants (hybrid vigor, or
    heterozygote advantage)
  • The effects vary depending on how distant the two
    species or populations are and whether the
    different alleles at different loci are able to
    work together (coadapted gene complexes)
  • Hybrids between different populations within a
    species do tend to have an advantage
    (Heterozygote advantage). However, mating between
    very distant populations (different species) can
    lead to hybrid breakdown.

19
Increasing genetic distance
Mating between different species (Lions x
tiger, Horse x donkey)
Fitness
Will not mate or Produce inviable or sterile
hybrids
Mating between relatives
Populations within a species
Outbreeding Depression Hybrid Breakdown
Inbreeding Depression
Hybrid Vigor (due to Heterozygote advantage)
20
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21
Reinforcement
  • So, when hybrids are formed between different
    species, they are often costly and maladaptive
    because of hybrid breakdown

22
Reinforcement
  • So, when hybrids are formed between different
    species, they are often costly and maladaptive
    because of hybrid breakdown
  • In such cases, you would predict that mechanisms
    to avoid mating would evolve to avoid the
    production of maladaptive hybrids (
    Reinforcement)

23
  • Reproductive isolation could occur at many
    different levels
  • Prezygotic (before the egg is fertilized)
  • Genetic drift and divergence in bird song-wont
    mate
  • Selection on coat color-dont recognize each
    other
  • Postzygotic (after the egg is fertilized)
  • DM incompatibilities cause embryo to not develop
    (enzymes dont work together)

Postzygotic barriers
Prezygotic barriers
Reduced Hybrid Viability
Reduced Hybrid Fertility
Hybrid Breakdown
Gametic Isolation
Viable, fertile offspring
Fertilization
24
Reinforcement
  • So, the prediction is that in sympatry (when two
    different species are in the same place),
    mechanisms to avoid mating (prezyotic isolation)
    would be strong
  • Whereas in allopatry, prezygotic isolation would
    not be needed because the different species would
    not come into contact

25
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26
(2) How are Species Defined?
27
How are species defined?
  • So, what criterion should be used?
  • Historically, the most common criteria had been
    using morphological characters (how an organism
    looks)

28
Speciation is a messy process
  • Rates of molecular, phenotypic (morphological)
    evolution and reproductive isolation are not
    necessarily concordant, but often discordant
  • Speciation is a jagged messy idiosyncratic
    process, where species boundaries are often
    difficult to define

29
  • Problem Populations are in the process of
    speciating from one another, and species
    boundaries are often difficult to define until
    the populations are sufficiently divergent by all
    measures

30
  • So then, how do you define species???

31
Species are dynamic rather than static entities,
with boundaries changing constantly Many groups
are in the process of speciation
Darwins view Species are arbitrary constructs
of the human mind imposed on a continuum of
variation
32
  • Three Main Species Concepts
  • Biological Species Concept
  • Phylogenetic Species Concept
  • Morphological (Phenetic) Species Concept

33
1. Biological Species Concept (Ernst Mayr, 1942)
  • A group of interbreeding populations that are
    evolutionary independent of other populations

34
1. Biological Species Concept (Ernst Mayr, 1942)
  • Example all human populations belong to the
    same biological species

35
Biological Species Concept
Strengths
  • An unambiguous empirical criteria which is
    clearly linked to speciation (if populations
    cant intermate they cant belong to the same
    species)
  • Using reproductive isolation as the criterion is
    meaningful as it confirms the lack of gene flow
    between groups

36
Biological Species Concept
  • PROBLEMS
  • Many species are asexual and do not intermate
    (viruses, bacteria, protists)
  • Many highly divergent species can hybridize
    (plants)
  • Only applicable to present (not fossil taxa)
  • Ability to intermate sometimes drops off
    gradually (ring species)

37
Ring Species
38
2. Phylogenetic Species Concept
  • The smallest group that is monophyletic is called
    a species

39
2. Phylogenetic Species Concept
There are several monophyletic groups here
  • Monophyletic group
  • A group with a shared derived (descendant)
    character
  • A group that contains a common ancestor and all
    its descendents

40
Phylogenetic Species Concept
  • Typically, a phylogeny is constructed using DNA
    or other types of data (proteins, morphological
    traits)
  • The phylogeny reveals hierarchical relationships
    among groups
  • The smallest group that has a shared derived
    character and is monophyletic is called a species

41
Phylogenetic Species Concept
There is a derived character that is shared by
the 4 populations
  • Monophyly

The smallest monophyletic group is called a
species
42
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43
Phylogenetic Species Concept
Strengths
  • Easy to see evolutionary relationships on large
    and small taxonomic scales
  • It can be used on any species (sexual, asexual)
    for which there is phylogenetic information
    (molecular, morphological, biochemical data) on
    extant or fossil species

44
Phylogenetic Species Concept
  • Problems
  • Need a good phylogeny time consuming and can be
    expensive
  • Not recognize paraphyletic groups (a monophyletic
    group that does not include all the descendents
    reptiles are paraphyletic, as they do not include
    birds, because birds emerged from within
    reptiles)
  • A trivial trait (single mutation or trait) can
    make a group monophyletic, and may not warrant
    calling a group a new species

45
Phylogenetic Species Concept
  • Problems
  • A trivial trait (single mutation or trait) can
    make a group monophyletic, and may not warrant
    calling a group a new species
  • The cut off for a species is often arbitrary.
    For example, 3 sequence divergence is often used
    for bacteria

46
Phylogenetic Species Concept
  • Monophyly

Sometimes a trivial trait, like a single point
mutation could make a group monophyletic, and a
species according to the phylogenetic species
concept
The smallest monophyletic group is a species
47
3. Morphological (Phenetic) Species Concept
  • Identifying species using overall similarity (but
    not in a phylogenetic context no hierarchy no
    branching pattern, no ancestral-derived
    relationships)
  • Most often morphological traits are used, but any
    phenotype could be used

48
Morphological (Phenetic) Species Concept
Strengths
  • Most intuitive the way we recognize species
  • Easiest. Easier than constructing phylogeny or
    intermating

49
Morphological (Phenetic) Species Concept
  • Problems
  • Different species can look similar due to
    convergent evolution
  • Populations that look distinct sometimes belong
    to the same species
  • Speciation can occur without changes in
    morphology or other traits (cryptic species)

50
Which species concept to use?
  • When we discuss animals we often use the
    biological species concept
  • Plants it depends, since very distant plants
    can hybridize phylogenetic species concept is
    often used.
  • Bacteria difficult problem.
  • Bacteria do not interbreed (? Biological Species
    concept). In some cases massive exchange of
    genetic material (horizontal gene transfer) leads
    to phylogenetic confusion.
  • Often a combination of the Phylogenetic and
    Phenetic Species Concepts (biochemical and
    morphological like cell wall traits) are used.

51
Species are dynamic rather than static entities,
with boundaries changing constantly Many groups
are in the process of speciation
Darwins view Species are arbitrary constructs
of the human mind imposed on a continuum of
variation
52
  • However, concept of species is still useful
  • Species are considered the largest group with a
    common evolutionary fate

53
Concepts
  • Geographic Models
  • Allopatric
  • Sympatric
  • Reinforcement
  • Problems with the concept of Species
  • Species
  • Biological
  • Phylogenetic
  • Phenetic (Morphological)
  • Monophyly

54
  • 3. Which of the following is a species according
    to the biological species concept?
  • (A) All hominin species (most are fossil
    species).
  • (B) A population of bacteria for which 80 of
    their DNA sequences are identical.
  • (C) All allopolyploid plants.
  • (D) A group of beetles that can intermate and
    produce offspring for multiple generations.

55
  • 4. Which of the following is NOT a reason that
    Species are difficult to define?
  • (A) Many plants that are genetically divergent
    are able to mate
  • (B) Many organisms that are morphologically
    similar are genetically distinct
  • (C) Many organisms are asexual
  • (D) Sometimes groups split off from within a
    monophyletic group (such as birds splitting off
    from the reptiles)
  • (E) Sometimes sexual populations that are unable
    to interbreed could still be the same biological
    species

56
  • 3. Which of the following is most likely to be a
    "species" according to the Phylogenetic Species
    Concept?
  • (a) A population of bacteria that has a gene that
    allows glucose metabolism
  • (b) Bird populations, which share a unique
    heritable feather structure
  • (c) Spider populations that can interbreed and
    produce fertile offspring
  • (d) Crustacean populations that form a clade
    (genetically-related group), except for one
    population within the clade that colonized land
    and became insects
  • (e) Populations of deer that share similar antler
    shape

57
  • 4. Under which of the following scenarios is
    reinforcement most likely to evolve?
  • (a) Different fish species, with each living in a
    separate pond
  • (b) Two snail species, where each lives on
    opposite sides of a freeway
  • (c) Different species of crickets living together
    in a park
  • (d) Different insect species, each living on a
    different species of fruit in a forest
  • (e) Different species of allopolyploid plants
    living in a field

58
  • 5. Which of the following scenarios is likely to
    lead to the most rapid formation of new species?
  • (a) Two populations become geographically
    separated, and there is continued migration
    between the populations
  • (b) Two populations become geographically
    separated, and then new mutations arise in each
    population that become fixed due to genetic drift
  • (c) Two populations that are in the same location
    diverge due to sexual selection for different
    traits in the two populations
  • (d) Two populations become geographically
    separated, and then new mutations arise in each
    population that become fixed due to selection
    favoring different egg coat proteins in the
    different habitats
  • (e) All of the above would on average lead to
    equivalent rates of speciation

59
answers
  • 3D
  • 4E
  • 3B
  • 4C
  • 5D
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