Prokaryotic species concept

The pervasive effects of lateral gene transfer in the evolutionary history of prokaryotic taxa are now well established (Boucher et al. 2003). This appreciation of the fundamental importance of genetic exchange in the diversification of bacterial species has been factored into some systematic treatments. For example, Rossello-Mora and Amann (2001) reflected the generality of problems inherent in all species concepts as well as the development, in their view, of a scientifically more robust...

Hybrid fertility

One of the most widely estimated components of fitness is fertility. This is especially true for hybrid genotypes. The reason for the frequent use of this factor to estimate hybrid fitness is likely its relative ease of measurement. For plants and animals numerous methodologies have been applied to define the fertility of hybrid genotypes (e.g. Arnold and Jackson 1978). Often such methods allow inferences not only of the relative fitness (in terms of fertility) of hybrids and their parents, but...

Cohesion species concept

The cohesion species concept defines species as ' the most inclusive group of organisms having the potential for genetic and or demographic exchangeability' (Templeton 1989). Templeton (1989) argued that one of the main weaknesses of the biological species concept was its lack of applicability to either asexual organisms or to taxa belonging to syngameons. In Templeton's (1989) terminology the difficulty resulted from either 'too little' or 'too much sex'. Templeton (1989) proposed the cohesion...

Genetic exchange is pervasive

Studies of natural hybridization in plants and animals indicate that this process cannot be ignored as a type of evolutionary noise. Rather, examples of introgression and reticulate evolution continue to be reported for an increasing number of plant and animal taxa. These reports take on added significance because fitness estimates for some hybrid genotypes are equal to or greater than those of their parents. However, this leads to the following, frequently asked question If hybrids are...

Pre Darwin Darwin the Modern Synthesis and genetic exchange development of a paradigm

The role of genetic exchange in evolution has been of intense interest and debate for at least a century. However, the accumulation of data sets, the consideration of new models, and the resurrection of old models has led to a paradigm shift, reflected by an appreciation of the importance of gene transfer. In spite of this, arguments emphasizing an unimportant role for genetic exchange are still reflected in recent evolutionary literature (e.g. Coyne and Orr 2004). The goal of this text is to...

Louisiana irises

As discussed in Chapter 1, a main focus for my group and associated colleagues has been the estimation of hybrid and parental fitness in various organisms in both experimental and natural habitats (e.g. Shoemaker et al. 1996 Williams et al. 1999 Promislow et al. 2001). In particular, we have drawn attention to the fact that hybrid genotypes can display a range of fitness estimates, sometimes due to environmental setting. The majority of the data used to illustrate this conclusion have come from...

Hawaiian silversword complex

It has been argued that species of the Hawaiian genera Argyroxiphium, Dubautia, and Wilkesia constitute '. . . one of the most remarkable examples of adaptive radiation known to science' (Carr and Kyhos 1981). Carr and Kyhos (1981) substantiate their claim by pointing to the 28 species belonging to these three genera that (i) have bauplans ranging from large basal rosettes that form a single inflorescence before dying, to woody shrubs that annually produce many flowers, to trees, to vines and...

Behavioral characteristics

Behavioral differences as impediments to genetic exchange have been a classical focus of studies by evolutionary zoologists (e.g. see Dobzhansky 1937 Mayr 1942 Coyne and Orr 2004). Yet, even the genus Drosophila and the order Aves, used for so many decades as paradigms for the allopatric development of reproductive barriers and thus 'biological speciation', are now seen as exemplars of evolutionary diversification in the face of gene flow (e.g. Grant and Grant 1992 Noor 1995). In this section I...

Introgressive hybridization hybrid speciation and the evolution of human food sources

. . . we demonstrate that the numerous MHC DRB alleles that are present in modern domestic mammals implies that substantial backcrossing with wild ancestors, either accidental or intentional, has been important in shaping the genetic diversity of our domesticates One of the taxa referred to by Vila et al. (2005) was the domestic dog, Canis familiaris. Though not considered such by most recent immigrants to North America, C. familiaris has not only been a beast of burden and a companion, but...

Sexually reproducing allopolyploid animals

Numerous hypotheses have been posited that address the question of why polyploidy is relatively rare in animal taxa relative to plants for example. Stebbins 1950 suggested that the barrier to establishment of WGD products in animals was caused by a more easily perturbed developmental system. More recently, Orr 1990 argued that polyploidy in animals was relatively rare due to ' . the difficulty of establishing a tetraploid line in organisms with a genetically degenerate sex chromosome although...

Biological species concept

Biological Species Concept

It is somewhat of a false dichotomy to represent the biological species concept as the only framework that is defined by the occurrence, or lack thereof, of genetic exchange. Most, if not all, concepts consider genetic exchange to be of primary concern for defining species e.g. see Sites and Marshall 2004 for a discussion of several methodologies for delimiting species that assume reproductive isolation . As illustrated above, one has only to read Phylogenetic Systematics Hennig 1966 , the...

Gamete competition

Gamete competition can occur when gametes from two or more males are vying for the same set of female gametes. However, in the context of this book, gamete competition has been defined by 'the finding that contaxon gametes are superior to het-erotaxon gametes in fathering progeny, when both gamete types are present'. Such competition can occur in animals, regardless of whether fertilization is external or internal, and can affect plant crosses regardless of the mechanism of pollen transfer. The...

Phylogenetic species concept

The origin of the phylogenetic species concept can be traced back at least to Hennig 1966 , although Darwin 1859 also presented evolutionary diversification as a bifurcating tree in the only illustration found in On the Origin of Species. However, Hennig's treatment is the basis for the modern phy-logenetic approach for studying evolutionary diversification. In the context of this discussion, Hennig's methodology can be placed within the history-based format of the phylogenetic species concept....

Bombina

In Chapter 4 I argued that the fire-bellied toad species B. bombina and B. variegata represent an excellent example of some of the evolutionary effects of hybridization. Also, as pointed out in the previous chapter, much of the work on this species complex, particularly by Szymura, Barton, and their colleagues, has emphasized the reduced fitness of some hybrid genotypes e.g. Szymura and Barton 1986, 1991 . In particular, the coincident and concordant step-clines for many unlinked markers have...

North American oaks

Inferences of introgressive hybridization and hybrid speciation are replete for oaks in general. For example, Trelease 1917 reported over 120 hybrid derivatives from natural hybridization between various North American species. The data for this study came from his examinations of herbarium specimens and from reports of hybrids found in the scientific literature. In a reanalysis of hybridization among the same species examined by Trelease, Palmer 1948 arrived at the same conclusion, that...

Phylogenetic discordance

Various processes can cause non-concordance i.e. different branching patterns and taxa associations between phylogenies derived from different character sets. However, the retention of ancestral polymorphism and reticulate events caused by introgressive hybridization and lateral gene transfers are viewed as being two of the main causal factors. In the following examples including putative cases of both horizontal transfer and introgressive hybridization within plants, animals, and an...

Darwins finches

Hybridization and the origin of species variability The most curious fact is the perfect gradation in the size of the beaks in the different species of Geospiza, from one as large as that of a hawfinch to that of a chaffinch, and even to that of a warbler Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends...