Lecture 11 Origin of Species (speciation) Flashcards
How do new species originate from existing species?
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How Do Species Attain and Maintain Separate Identities?
speciation, microevolution, macroevolution, reproductive isolation
Speciation
one of several processes by which
new species arise
microevolution
changes over time in allele
frequencies in a population
macroevolution
the broad pattern of evolution
above the species level
Reproductive isolation
– Absence of gene flow between populations
– Always part of speciation
The Biological Species Concept
- 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
Gene exchange
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
Prezygotic isolating mechanisms
Mechanisms that prevent formation of a zygote
Postzygotic isolating mechanisms
Mechanisms that prevent development into an adult
Reproductive isolation
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
Seven mechanisms of reproductive isolation
Temporal isolation
Mechanical isolation
Ecological isolation
Behavioral isolation
Gamete incompatibility
Hybrid inviability
Hybrid sterility
Temporal isolation
Some populations cannot interbreed because the timing of
their reproduction differs
Mechanical isolation
Size or shape of an individual’s reproductive parts prevent
it from mating with members of another population
Ecological isolation
Populations adapted to different microenvironments in the
same region may be physically separated
Behavioral isolation
In animals, behavioral differences can stop gene flow
between related species
Gamete incompatibility
- 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
Hybrid inviability
- If genetic incompatibilities disrupt development, a hybrid embryo may die, or hybrid offspring that
survive may have reduced fitness (e.g., ligers)
Hybrid sterility
- Some interspecies crosses produce robust but sterile offspring (e.g., mules)
B-Reproductive isolating mechanisms
Prezygotic isolating mechanisms
* Ecological isolation
* Behavioral isolation
* Temporal isolation
* Mechanical isolation
* Prevention of gamete fusion
Postzygotic isolating mechanisms
* Hybrid inviability or infertility
Barriers to reproduction
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Ecological isolation
– Different habitat
– Land iguana vs marine iguana
Temporal isolation
– 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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Behavioral isolation
– 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
Mechanical isolation
– 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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Gametic isolation
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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Postzygotic isolating mechanisms
– Hybrids that do survive
may be weaker
– Hybrids may be sterile
– mules
* Abnormal sex organs
* Failure to form gametes
Limitations of the Biological Species Concept
- 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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Other Definitions of Species
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
The geography of speciation
Speciation can take place with or without
geographic separation
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Speciation Is Influenced by Geography
- 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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Allopatric (“Other Country”) Speciation
- 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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Sympatric (“Same Country”) Speciation
- 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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Allopolyploidy
- 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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Autopolyploidy
- 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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Sympatric speciation by disruptive selection
- 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
Sympatric
without geographical separation
Allopatric
geographically isolated populations.
- Polyploidy and disruptive selection are two ways by which a single species may undergo
sympatric speciation.
