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