Speciation

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Allopatric speciation

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- The evolution of reproductive isolating mechanisms between two populations that are geographically separated.
- Occurs when a physical barrier subdivides a large population or when a small population becomes separated from a species main geographic distribution
- Two stages: first geographic separation, then as the populations experience distinct mutations and different patterns of drift and natural selection they accumulate isolating genetic differences

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Behavioural isolation

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- Communication between species that signals mating may differ greatly from one species to another
- If signals used by one species are not recognized by another species mating won’t occur

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Biological species:

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- A concept that defines a species as a group of organisms that can successfully interbreed and produce fertile offspring.
- Problems: doesn’t encompass organisms that reproduce asexually well, different patterns of reproduction can blur the definition of species. Doesn’t encompass hybridization

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Ecological isolation

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- A form of prezygotic isolation
- Happens when two organisms that can produce offspring live in different habitats so they will not come in contact

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Ecological species:

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- defines a species as a group of organisms that share a distinct ecological niche.

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Gametic isolation

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- Incompatibility between the sperm of one species and the eggs of another may prevent fertilization

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Hybrid breakdown

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- Postzygotic isolating mechanism that creates a healthy, vigorous fully fertile hybrid
- Although the hybrid appears normal its offspring will exhibit reduced survival or fertility
- Eg. Fruit fly’s hybrids have offspring that experience a high rate of chromosomal abnormalities and harmful types of genetic recombination

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Hybrid sterility

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- Postzygotic isolation mechanism that creates healthy, vigorous hybrid
- The hybrid is infertile and therefore has zero fitness as it leaves no offspring
- Caused when the parent species differ in the number or structure of their chromosomes
- E.g. Mule or Zebroids

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Hybrid viability

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- Hybrid is not viable when it receives a set of developmental instructions from each parent which don’t interact properly
- These hybrid organisms die as embryos or at an early age a phenomenon called hybrid inviability
- E.g. domestic sheep and goats

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Hybrid zone

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- The area’s where prezygotic reproductive isolation is incomplete members from different separated populations can mate and produce viable fertile offspring
- Some have persisted for hundreds or thousands of years, they are generally narrow and the majority of the populations remain isolated from one another

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Hybridization:

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when two different species interbreed to produce viable offspring.

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Mechanical isolation

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- Differences in structure of copulatory organs prevent successful mating between individuals of different species

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Migration

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- The predictable seasonal movements of animals from the area where they are born to a distant and initially unfamiliar destination, returning to their birth site later

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Morphospecies:

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The idea that all individuals of a species share measurable traits that distinguish them from individuals of other species.

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Parapatric speciation

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- Speciation between populations with adjacent geographic distributions

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Phylogenetic species:

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- Defines a species as a group of organisms bound by a unique ancestry.

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Postzygotic isolation mechanisms

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- Mechanisms that exert their effects after zygote formation
- Even if fertilization occurs reproductive isolation can occur if the resulting hybrid has lower fitness
- Hybrid inviability, hybrid sterility, hybrid breakdown

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Prezygotic isolation mechanisms

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- Mechanisms that exert their effects before the production of a zygote, or fertilized egg
- Ecological isolation
- Temporal isolation
- Behavioral isolation
- Mechanical isolation

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Reproductive isolation

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- Prevents gene pools of two species from mixing even when they are sympatric
- Can be achieved in two basic ways either prezygotic or postzygotic isolation

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Ring species

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- Species that have a ring-shaped geographic distribution that surrounds uninhabitable terrain.
- Adjacent populations can exchange genetic material but distant populations only can through intermediary populations

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Subspecies

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- Geographically sperated populations exhibit dramatic phenotypic variation
- Local variants of a species, may reproduce with other subspecies where their geographic distributions meet
- Offspring of two different sub species exhibit intermediate phenotypes

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Temporal isolation

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- Reproductive isolation based on times of day or year that mating of two species occur

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Vicariance

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The separation or division of a group of organisms by a geographic barrier, such as a mountain or body of water, resulting in differentiation of the original group into new varieties or species
- e.g. snow cricket must then stay in the snow so it is different in terms of its genome from all other crickets