Allochronic speciation

[2] Modeling changes in species breeding patterns due to climate as well as understanding the genetic mechanisms that control it has proven to be important.

For example, a species with multiple breeding seasons in a year may shift those times depending on external conditions such as temperature or predation.

The time frames involving allochrony are typically divided into three categories (prevalence in nature as well as examples are provided alongside each category):[2] Other phenotypic traits are often found to co-occur with reproductive timing such as flowering number, egg-clutch sizes, reproductive lifespans, or body size—what can be defined as temporal phenotypic clines.

[4] If heritable, the same factors may be expressed as they are in a plastic explanation; however, gene flow limitations allow for adaptation to the specific conditions of the reproductive time.

[4] Adaptation by time is an extension of divergence due to limited gene flow between populations experiencing different selective pressures.

[13] Typically this is limited to spatial variation such as in ecological speciation; however, in allochrony, selection varies not just in space, but in reproductive time—giving rise to adaptive temporal clines in phenotypic traits that are heritable.

Isolation by time effectively allows adaptive temporal clines to evolve as long as the reproductive season has selective variation.

[14] Testing whether or not allochrony prevents gene flow can be difficult due to the multitude of unknown variables in wild populations and the inability to replicate and manipulate it in laboratory settings.

[1]: 203  Producing viable, and fertile offspring (or the lack thereof) is not always possible; fortunately, lake of mate tests do not necessarily indicate temporal isolation is not at play.

[1]: 206 Determining if allochrony is the source of divergence require a key pattern to be measured: isolation (and subsequently speciation) should correlate with a decrease in overlapping breeding times.

[2] This pattern indicates that daily allochrony is more prone to gene flow (closeness of breeding times can allow accidental intermixing of populations) while yearly allochrony is the least prone to gene flow (accidental intermixing is rare if large time frames exist between mating periods).

[100] In high-latitude regions, various taxa experience similar temperatures and solar radiation in cyclic patterns due to Earth's axial tilt—generating seasons that are not found at the equator.

[100] In contrast with this, latitudes near or at the equator (tropics) experience asynchrony in seasonal variation in that the regions receive similar amounts of solar radiation and maintain consistence temperature.

[100] A large scale test of the hypothesis was conducted on fifty-seven New World bird species across South, Central, and North America.

[2] The findings, using DNA, geographic and ecological distances, as well as climatic data, indicated that genetic differentiation increased in species populations where asynchrony in precipitation was present.

Breeding seasons of three populations of a species shift over time eventually causing the isolation of their genes from the other populations. This reproductive isolation can lead to speciation.
A three-dimensional space representing speciation with axes representing the factors involved in the process. [ 10 ] The temporal dimension indicates allochrony. [ 2 ] The ecological axis correlates with adaptation by time (ABT) whereas the mating axis corresponds to isolation by time (IBT). [ 4 ] Breeding time create a fourth dimension expressed as asynchrony in breeding as opposed to synchrony. [ 2 ] Speciation events are indicated by the varying colored paths that are taken.
A: Absent allochrony, only geographic and mate choice cause isolation.
B: Starts with geographic separation, mate choice furthers isolation, and is completed by allochrony.
C: Starts with mate-choice differentiation followed by allochrony.
D: Mating and ecological factors accompany allochrony.
Speciation represented as a continuum of gene flow where equals the rate of gene exchange. The three primary geographic modes of speciation ( allopatric , parapatric , and sympatric ) can exist within this continuum, as well as other non-geographic modes.