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"Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such group."
When does speciation occur?
evolution altered physical or behavioral characteristics of populations long enough that they no longer interbreed
How do new species arise?
- Branching evolution occurs when a single "parent" species diverges into two species
- -parent species continuing while second species arises from it
based on reductions in gene flow between populations of the same species
when gene flow is drastically reduced for a long period, populations will evolve separately
most important factor in reducing gene flow between populations
- -from change in environment
- -or migration of part of population
When geographic separation plays a part in the evolution of a new species.
Extrinsic isolating mechanism
- Geographical separation:
- individuals of two populations cannot interbreed if they live in different places (the first step in allopatric speciation)
What are the 6 intrinsic isolating mechanisms?
- Ecological isolation,
- temporal isolation,
- behavioral isolation,
- mechanical isolation,
- gametic isolation,
- hybrid invability or infertility
Even if they live in the same place, they can't mate if they don't come in contact with one another.
Even if they come in contact, they can't mate if they breed at different times
Even if they breed at the same time, they will not mate if they are not attracted to one another
Even if they attract one another, they cannot mate if they are not physically compatible
Even if they are physically compatible, an embryo will not form if the egg and sperm do not fuse properly.
Hybrid inviability or infertility
even if fertilization occurs successfully, the offspring may not survive, or if it survives, may not reproduce (e.g., mule)
- Speciation occurs in absence of geographical separation
- (much rarer than allopatric speciation)
Speciation more likely when species introduced to an environment in which few other species of its kind exist
- such an environment has many niches available
- -leads to rapid specialization, a new which in turn leads to speciation.
rapid emergence of many species from a single species that has been introduced to a new environment
The pace of speciation
Gradualist model: evolution proceeds at a slow, steady rate
Punctuated equilibrium model: organisms undergo long periods of stasis or little change, followed by relatively brief episodes of rapid speciation