Download
1 / 34
340 likes | 472 Views
Speciation. The Biological Species Concept. based primarily on the actual or potential ability to produce fertile offspring by members of a group of organisms and results in reproductive isolation from other such groups (by Ernst Mayr).
E N D
The Biological Species Concept based primarily on the actual or potential ability to produce fertile offspring by members of a group of organisms and results in reproductive isolation from other such groups (by Ernst Mayr). Dependent on the existence of reproductive isolating mechanisms
Prezygotic isolating mechanisms are those that prevent breeding between members of two different species or prevent the fertilization and include habitat isolation some geographic barrier (different continents) or when microhabitat partitioning occurs within the same geographic behavioral isolationMating behaviors that are used to attract or signal potential mates are often different in the various species temporal isolationmating at different times of day or seasons mechanical isolationdue to structural incompatibility gametic isolationit is possible that the gametes will not form a zygote
Postzygotic reproductive isolating mechanisms are those that occur after fertilization and include reduced hybrid viability. Because of genetic incompatibilities the embryo may cease development. reduced hybrid fertility. Many hybrids between two different species are not fertile or are largely sterile. For examples mules. hybrid breakdown. Even if the hybrid is not sterile and is able to produce offspring, those offspring are often sterile or feeble (often affects the F2).
major problems with the BSC the BSC is not applicable for those organisms that reproduce asexually or those that produce fertile hybrids (e.g., some plants). It also does not apply to extinct species or to those that are geographically separated (or allopatric)
Examples of other species concepts Ecological Species Concept identifies a species in terms of its ecological niche (the role that it plays in its environment).
Pluralistic Species Concept defines a species based on some key unifying factor(s) that vary; for some the unifying factor is the niche, for others it is reproductive isolation, and for some a combination of the two.
Morphological Species Concept identifies evolutionarily independent lineages by differences in size, shape, and other morphological features. The logic behind this species concept is that distinguishing features are most likely to arise if populations are independent and isolated from gene flow.
The Phylogenetic Species Concept is based on reconstructing the evolutionary history of populations
Paleontological Species concept is the only way that some species (e.g., some extinct) can be classified and involves the use of the fossil record
Speciation or the origin of new species is the outcome of isolation and divergence. Isolation is created by reductions in gene flow and divergence is created when mutation, genetic drift, and selection act on the separated populations
Allopatric speciation A population becomes geographically separated, then as selection pressures change for the 2 populations, there are changes in allele frequency that can ultimately lead to formation of organisms that are different enough to be considered to be of different species.
Sympatric speciation A new species arises without any geographic separation of a population. An example in plants would be the evolution of a new species due to polyploidy occurring. This can happen in a hybrid plant resulting from the interbreeding of two parental plant species.
Phylogeny is the evolutionary history of a species or group of species. Phylogenies are based on: fossil records morphological similarities molecular homologies (homologies are similarities that are due to shared ancestry whereas analogies are similarities that are due to convergent evolution instead of shared ancestry).
Systematics is an analytical approach to understanding the diversity and relationships of organisms (extant as well as extinct). Now the use of molecular systematics (using comparisons of DNA, RNA, proteins etc.) allows for more realistic evolutionary relationships.
Recall that scientists strive to categorize living organisms into increasingly inclusive taxa. The goal is to show the evolutionary relationships of the various species. One of the common methods for studying phylogenetic systematics (classification based on evolutionary history) is cladistic analysis
Cladistics A phylogeny is a phylogenetic diagram that shows patterns of shared characteristics based on cladistics (the analysis of how species may be grouped into clades).
Each branch of the phylogeny is called a clade but note that clades are like the taxa (genus, family etc.) and thus can be nested within larger clades. A clade is defined as a group of species that includes an ancestral species and all of its descendents.
Identifying shared primitive and shared derived characters also helps with cladistic analyses. An outgroup is useful for the comparisons. Note that data on the taxa being studied are input into computer programs which often generate multiple cladograms and that the cladogram or cladograms that is/are most parsimonious are the ones that are considered to be the ones that are most likely to reflect the phylogeny.
