Speciation

Speciation
        Process of speciation
        Reproductive isolating mechanisms
        2 dimensions of evolution
        Modes of speciation

No single weed species dominates a crop production field or an agroecosystem. Usually several weed species coexist in a field to exploit the diverse resources unused by crop plants (inter-specific diversity). Within a single weed species, a diverse population of genotypes and phenotypes interfere with crop production (intra-specific diversity). Given sufficient time and other factors, new species can arise from within current weed populations. Unused resources left by homogeneous crop populations, diverse and fit weed populations, as well as crop management practices, provide strong forces for speciation.

Speciation can be defined as:
1: the formation of new species;
2: the splitting of a phylogenetic lineage;
3: acquistion of reproductive isolating mechanisms producting discontinuities between populations;
4: process by which a species splits into 2 or more species

Species can be defined as:
1: a group of organisms, minerals or other entities formally recognized as distinct from other groups;
2: a taxon of the rank of species; in the hieracrchy of biological classification the category below genus; the basic unit of biological classification; the lowest principal category of zoological classification
3: a group of morhologically similar organisms of common ancestry that under natural conditions are potentially capable of interbreeding
4: a species is a group of interbreeding natural populations that are reproductively isolated from other such groups

Process of Speciation
    The process of speciation is a 2 stage process in which reproductive isolating mechanisms (RIM's) arise between groups of populations.
Stage 1

• gene flow is interupted between 2 populations
• absence of gene flow allows 2 populations to become genetically differentiated as a consequence of their adaptation to different local conditions (genetic drift also can act here too)
• as populations differentiate, RIMs appear because different gene pools are not mutually coadapted
• reproductive isolation appears primarily in the form of postzygotic RIMs: hybrid failure
• these early RIMs are a byproduct of genetic differentiation, not directly promoted by natural selection yet

• completion of genetic isolation
• reproductive isolation develops mostly in the forms of prezygotic RIMs
• development of prezygotic RIMs is directly promoted by natural selection: alleles favoring intraspecific fertility will be increased over time at the expense of interspecific fertilization alleles

Reproductive Isolating Mechanisms
    How do species come about? How does reproductive isolation arise between groups of populations? Two types of RIMs facilitate speciation: prezygotic and postzygotic. Natural selection favors development of RIMs, especially prezygotic RIMs. Less favored by natural selection are postzygotic RIMs, which waste more energy.

Prezygotic RIMs prevent the formation of hybrid zygotes:

• Ecological isolation: populations which occupy the same territory but live in different habitats, and thus do not meet
• Temporal isolation: mating or flowering occur at different times, whether in different seasons, time of the year, or different times of the day
• Mechanical isolation: pollen transfer is forestalled by the different size, shape or structure of flowers
• Gametic isolation: female and male gametes fail to attract each other, or the pollen are inviable in the stigmas of flowers

• Hybrid inviability: hybrid zygotes fail to develop or at least to reach sexual maturity
• Hybrid sterility: hybrids fail to produce functional gametes
• Hybrid breakdown: the progenies of hybrids (F2 or backcross generations) have reduced viability or fertility

2 Dimensions of Evolution
Anagenesis
1: evolution within a lineage;
2: changes in a lineage with time;
3: progressive evolution towards higher levels of organization or specialization
Cladogenesis
1: evolutionary diversification;
2: lineage splits into 2 or more lineages;
3: fundamental process herein is Speciation;
4: a branching type of evolutionary progress involving the splitting and subsequent divergence of populations

Modes of Speciation
    The 2 stage process of speciation is realized in 2 ways, or modes: geographic and quantum speciation.

Geographic Speciation:
Stage 1:

• Starts with the geographic separation between populations.
• Geographically separated populations adapt to local conditions and become genetically differentiated over long periods of time.
• Random genetic drift adds to this genetic differentiation.
• Postzygotic RIMs develop during these long periods of time

• Isolated populations come back into contact; gene flow results in one species again.
• Development of added RIMs (prezygotic) due to natural selection: formation of a new species

Allochronic speciation: speciation without geographical separation through the acquistion of different breeding seasons or patterns

Quantum Speciation

• Rapid speciation; saltational speciation; accelerated modes of speciation, especially in stage 1; postzygotic RIMs appear rapidly
• Main mode of quantum speciation is by polyploidy, a RIM due to cytological irregularites between two populations

• Each set of chromosomes is capable of independent variation: mutation, recombination events.
• The resulting organism is in most cases incapable of forming fertile offspring with members of the ancestral diploid population.
• It has therefore acquired instant reproductive isolation.
• If it is capable of reproducing, and finding an ecological niche it can exploit, a new species has been formed.
• Example: In the foxtail species-group: ancestral diploid green foxtail hypothesized to have hybridized with unknown Setaria sp., resulting progeny were fertile, polyploid, and gave rise to yellow and giant foxtail (both polyploid), which subsequently found niche not fully exploited by green foxtail

©jdekker-2005