Lecture 11 Origin of Species (speciation)

Question 1

How do new species originate from existing species?

Answer 1

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Question 2

How Do Species Attain and Maintain Separate Identities?

Answer 2

speciation, microevolution, macroevolution, reproductive isolation

Question 3

Speciation

Answer 3

one of several processes by which
new species arise

Question 4

microevolution

Answer 4

changes over time in allele
frequencies in a population

Question 5

macroevolution

Answer 5

the broad pattern of evolution
above the species level

Question 6

Reproductive isolation

Answer 6

– Absence of gene flow between populations
– Always part of speciation

Question 7

The Biological Species Concept

Answer 7

• Ernst Mayr defined species as …
• “… groups of actually or potentially interbreeding natural
  populations which are reproductively isolated from other such groups”

Species composed of
– populations whose members mate with each other
– produce fertile offspring

• Reproductive isolation – do not mate with each other or do not
  produce fertile offspring

Question 8

Gene exchange

Answer 8

Focus on the ability to exchange genes
– Prezygotic isolating mechanisms
Mechanisms that prevent formation of a zygote

– Postzygotic isolating mechanisms
Mechanisms that prevent development into an adult

Question 9

Prezygotic isolating mechanisms

Answer 9

Mechanisms that prevent formation of a zygote

Question 10

Postzygotic isolating mechanisms

Answer 10

Mechanisms that prevent development into an adult

Question 11

Reproductive isolation

Answer 11

Gene flow does not occur between populations
○ Different genetic changes accumulate
○ Reinforces differences between diverging
populations
○ If pollination or mating cannot occur, or if zygotescannot form, the isolation is prezygotic
○ If hybrids form but are unfit or infertile, the
isolation is postzygotic

Question 12

Seven mechanisms of reproductive isolation

Answer 12

Temporal isolation

Mechanical isolation

Ecological isolation

Behavioral isolation

Gamete incompatibility

Hybrid inviability

Hybrid sterility

Question 13

Temporal isolation

Answer 13

Some populations cannot interbreed because the timing of
their reproduction differs

Question 14

Mechanical isolation

Answer 14

Size or shape of an individual’s reproductive parts prevent
it from mating with members of another population

Question 15

Ecological isolation

Answer 15

Populations adapted to different microenvironments in the
same region may be physically separated

Question 16

Behavioral isolation

Answer 16

In animals, behavioral differences can stop gene flow
between related species

Question 17

Gamete incompatibility

Answer 17

• Even if gametes of different species meet, they often have molecular incompatibilities that prevent them from fusing
• Primary speciation route of animals that release
  free-swimming sperm in water

Question 18

Hybrid inviability

Answer 18

• If genetic incompatibilities disrupt development, a hybrid embryo may die, or hybrid offspring that
  survive may have reduced fitness (e.g., ligers)

Question 19

Hybrid sterility

Answer 19

• Some interspecies crosses produce robust but sterile offspring (e.g., mules)

Question 20

B-Reproductive isolating mechanisms

Answer 20

Prezygotic isolating mechanisms
* Ecological isolation
* Behavioral isolation
* Temporal isolation
* Mechanical isolation
* Prevention of gamete fusion

Postzygotic isolating mechanisms
* Hybrid inviability or infertility

Question 21

Barriers to reproduction

Answer 21

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Question 22

Ecological isolation

Answer 22

– Different habitat
– Land iguana vs marine iguana

Question 23

Temporal isolation

Answer 23

– 2 species of wild lettuce grow along roadsides in the SE U.S.

– Hybrids can be made experimentally and are fertile

– Rare in nature because one flowers in early spring and the other in summer

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Question 24

Behavioral isolation

Answer 24

– Blue-footed boobies select mates after an elaborate
courtship display - Will not mate with other boobies

– Lacewings rely on auditory signals to attract mates
– Females are able to distinguish calls of different species

Question 25

Mechanical isolation

Answer 25

– Structure of the male and female copulatory organs may be incompatible
– Mating Bradybaena snails with shells that spiral in the same direction ) and unaligned genitals of snails with shells that spiral in
opposite directions)

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Question 26

Gametic isolation

Answer 26

Prevention of gamete fusion in mechanical isolation

– In animals that shed gametes directly into water, the eggs and sperm
derived from different species may not attract or fuse with one another
– In plants, the growth of pollen tubes may be impeded in hybrids
between different species

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Question 27

Postzygotic isolating mechanisms

Answer 27

– Hybrids that do survive
may be weaker
– Hybrids may be sterile
– mules
* Abnormal sex organs
* Failure to form gametes

Question 28

Limitations of the Biological Species Concept

Answer 28

• The number of species to which the biological
  species concept can be usefully applied is limited cannot be applied to fossils or asexual organisms (including all prokaryotes), because mating cannot be observed
• emphasizes absence of gene flow, but gene flow occurs between many morphologically and ecologically distinct species
  For example, grizzly bears and polar bears are distinct species, but they can occasionally mate to produce “grolar bears”

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Question 29

Other Definitions of Species

Answer 29

The morphological species concept distinguishes a species by its structural features

The ecological species concept defines a species by its ecological niche, the sum of its interactions with the nonliving and living parts of the environment

Question 30

The geography of speciation

Answer 30

Speciation can take place with or without
geographic separation

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Question 31

Speciation Is Influenced by Geography

Answer 31

• Speciation is a 2-part process
  – Initially identical populations must diverge
  – Reproductive isolation must evolve to maintain these differences
• Homogenizing effect of gene flow erases differences
• Speciation more likely in geographically isolated populations

Populations can become geographically isolated for a variety of reasons

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Question 32

Allopatric (“Other Country”) Speciation

Answer 32

• Geographically separated, or allopatric, populations appear much more likely to have evolved substantial differences leading to
  speciation
• Ex. Little paradise kingfisher varies little throughout range
  – Isolated populations are strikingly different from each other and maintain

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Question 33

Sympatric (“Same Country”) Speciation

Answer 33

• In sympatric speciation, speciation occurs in
  populations that live in the same geographic area
• Sympatric speciation is less common than
  allopatric speciation
• It occurs if gene flow is reduced by factors such as
  – Polyploidy
  – Sexual selection
  – Habitat differentiation

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Question 34

Allopolyploidy

Answer 34

• Two species hybridize
• Resulting offspring have one
  copy of the chromosomes of
  each species
• Infertile: cannot reproduce with
  either species – can’t produce
  gametes
• Can reproduce asexually
• Can become fertile if
  chromosomes spontaneously
  doubled (polyploidy)

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Question 35

Autopolyploidy

Answer 35

• All the chromosomes arise from a single species
• Error in cell division produces
  tetraploids
• DNA is replicated, NO cytokinesis
• Cannot produce fertile
  offspring with normal diploids

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Question 36

Sympatric speciation by disruptive selection

Answer 36

• Sympatric speciation may occur over the course of multiple
  generations through disruptive selection
  – Cause a population to contain individuals exhibiting two different phenotypes
• Two phenotypes would have to evolve reproductive isolating
  mechanisms
• Two phenotypes could be retained as polymorphism within a single
  population

Question 37

Sympatric

Answer 37

without geographical separation

Question 38

Allopatric

Answer 38

geographically isolated populations.

• Polyploidy and disruptive selection are two ways by which a single species may undergo
  sympatric speciation.