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). Unused
resources left by homogeneous crop populations, as well as crop management practices,
provide a strong force for both inter-specific and intra-specific 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
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
Index:
2 dimensions of evolution
Reproductive isolating mechanisms
Process of speciation
Modes of speciation
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
Reproductive Isolating Mechanisms
-Two types of RIMs facilitate speciation: prezygotic and postzygotic.
-Natural selection favors development of RIMs, especially prezygotic RIMs.
-Postzygotic RIMs 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 diffeent 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
Postzygotic RIMs reduce the viability or fertility of hybrids
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
Process of Speciation
The process of speciation is a 2 stage process in which RIMs arise between groups of populations. Stage 1 a. gene flow is interupted between 2 populations b. 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) c. as populations differentiate, RIMs appear because different gene pools are not mutually coadapted d. reproductive isolation appears primarily in the form of postzygotic RIMs: hybrid failure e. these early RIMs are a byproduct of genetic differentiation, not directly promoted by natural selection yet Stage 2 a. completion of genetic isolation b. reproductive isolation develops mostly in the forms of prezygotic RIMs c. 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 How do species come about? How does reproductive isolation arise between groups 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 Stage 2: 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 Allopatric speciation: geographic speciation; the differentiation of, and attainment of complete reproductive isolation of, populations that are completely geographically separated: isolation: separation into two independent gene pools; differentiation: independent evolution in two pools; -secondary merger; -competition between the new gene pools 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: Ploidy: The number of sets of chromosomes present (e.g. haploid, diploid, polyploid) Polyploidy: multiple sets of chromosomes in an organism (e.g. tetraploid, octaploid) -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. -ex. 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 Other mechanisms in quantum speciation include chromosome rearrangements (without extensive allelic differentiation): chromosome differences due to translocations, inversions, extra chromosomes, fusions, fissions stasipatric speciation: the formation of new species as a result of chromosomal rearrangements giving homozygotes which are adaptively superior in a particular part of the geographical range of the ancestral species. catastrophic speciation: rapid speciation occurs, leading to genetic isolation with little or no morphological differentiation, but without polyploidy. All due to unknown conditions: mutators; environmental stress; all of which cause drastic chromosomal rearrangements