Macroevolution Part II: Allopatric Speciation

Species

a group of interbreeding organisms that produce viable, fertile offspring in nature; its members will interbreed with one another but not other organisms outside of this group

Macroevolution

evolution on a scale of separated gene pools; studies focus on change that occurs at or above the level of species

Asexual Species

do not exchange genes, but form recognizable groups; most have evolved from a sexual species, only those whose phenotype is best adapted to the environment will continue to survive, but this makes them less adapted to environmental change

Ring Species

a connected series of neighboring populations, each of which can interbreed with closely sited related populations, but for which there exist at least two "end" populations in the series; these end populations are too distantly related to interbreed, through there is a potential gene flow between each "linked" species--such non-breeding, through genetically connected, "end" populations may coexist in the same region thus closing a "ring"

Limited Interbreeding

ex. each dog species will interbreed with the domestic dog but not readily with one another; this is true even when given the opportunity to do so, thus, they are not the same species since they do not interbreed in nature

ex. tigers and lions will interbreed in captivity, but they do not interbreed in nature

Anagenesis

the accumulation of changes in one species that leads to another species; it is the lineage of a species; over time, a species may accumulate enough changes that is considered a species that differs from the ancestral species

Cladogenesis

the building of one or more new species from an ancestral species that continues to exist; this results in biological diversity

Allopatric Speciation

speciation occurs because a given group has been separated from the parent group, usually because of a geographic speciation as time goes by

Sympatric Speciation

speciation occurs even though the two groups are still living in the same area

Why does speciation occur after geographical isolation?

1. the population that left the original group will have a different allelic make-up than the original species, thus experiencing the "founder effect"
2. the two groups will continue to experience different mutations
3. the two groups will now experience genetic drift and different selection pressures due to living in separate and perhaps different environments

Adaptive Radiation

the diversification of a group of organisms into forms filling different ecological niches; the process one species inhabiting a new area and evolving into several new species

Example of Natural Selection

During droughts in the Galapagos Islands, larger seeds are more abundant. Finches with slightly larger beaks have an advantage since they are able to crack larger seeds. Thus, natural selection favors finches with larger beaks. These finches are more likely to survive and pass those genes on to the next generation

Ecogeographic Isolation

(prezygotic) two species have become so specialized for survival in different environments, that once the geographical barrier is removed the two species will never again interbreed as one species; the adaptations for survival in their geographical locations prevent gene flow

Habitat Isolation

(prezygotic) two species have developed a preference for two different habitats. even if the species has become sympatric, the probability that they will meet and mate is low

Seasonal Isolation

(prezygotic) two species have developed different times of year to mate; no interbreeding occurs

Behavioral Isolation

(prezygotic) if courtship behavior changes during separation, then sympatric mating will not occur and two new species are formed

Mechanical Isolation

(prezygotic) there is a physical or biological structure that prevents mating
ex. differences in size or fit of genitalia may not allow mating (tittie size)

Gametic Isolation

(prezygotic) the gametes are shed simultaneously but something physical or chemical prevents the sperm from fertilizing the egg

Developmental Isolation

(postzygotic) if fertilization occurs, the development of the embryo can be irregular and is thus spontaneously aborted

Hybrid Inviability

(postzygotic) a hybrid is produced, but often does not make it to reproductive age because it is weak, irregular, etc.

Hybrid Sterility

(postzygotic) some hybrids produce superior offspring but the offspring are sterile

Selective Hybrid Elimination

(postzygotic) occurs if two species are sympatric and can hybridize, and their offspring can reproduce; one of the two following things can happen:

1. the hybrids are as viable or as fit as the parents and gene flow will occur and the two species will become one again
2. the hybrids are weaker or have lower fitness than the parents and will be selected against

Natural selection will select for those individuals that will mate with their own species and the hybrids will die out. The competition between the two species will cause character displacement

Prezygotic Barriers

when allopatric speciation occurs, usually more than one isolation mechanism also occurs and more than one trait will change between the two populations

Postzygotic Barriers

keeps two populations distinct, thus they are no longer the same species and can no longer interbreed to produce, viable, fertile, offspring in nature; when two populations are allopatric and changes occur, most likely more than one of the 10 barriers will occur in the populations leading to speciation