Species in Theory and Practice

Transcription

1 Species Concepts

2 Species in Theory and Practice Biologists have not been able to agree on exactly what a species is, or how species should be abstractly define - the controversy is theoretical, not practical Under most circumstances, there are no practical problems when defining species

3 Species in Theory and Practice Practical problems do arise when species are recognized and identified based on phenotypic characters If we accept that species have evolved from a common ancestor, then we would expect that there may be some conditions in which organisms are phenotypically intermediate Variation poses most of the practical problems of species recognition using phenotypic characters Geographic variation creates difficulties If a species varies geographically, then a good character for species recognition in one place may become useless in another place

4 Theoretical Considerations Practical difficulties have, in part, lead to theoretical questions regarding species Species are in practice mainly recognized by phenotypic characters - but the interest of the subject of evolutionary biology lies elsewhere What we want to determine is whether there is some deeper theoretical concept beyond individual characters that can be used to recognize individual species

5 Morphospecies Concept Species traditionally have been described and identified on the basis of morphological criteria, a classification system referred to as the morphological or typological species concept Species are groups of individuals that are morphologically similar and clearly distinguishable from individuals of other groups Species had traditionally been defined by reference to a morphological type Usually any geographic variation among members of the group was not detected or simply ignored

6 Concerns Regarding the Morphospecies Concept It became apparent that what appeared to be distinct morphological species at the local level were merely one in a series of morphologically intergrading populations on a broader geographic scale Geographic variation became commonplace, and species were viewed as multi-populational systems distributed over a broad geographic range Emphasis shifted from characterizing individuals from local populations to describing populational systems

7 Phenetic Species Concept A quantitative approach to systematics that seeks to classify organisms on the basis of their overall similarity Based on numerical taxonomy, which measures and records similarities for large numbers of characters Phenetic species concept defines a species as a set of organisms that look similar to each other and distinct from other sets More formally, it would specify some exact degree of phenetic similarity, and similarity would be measured by a phenetic distance statistic A species is a set of organisms not more than X phenetic distance units apart

8 Concerns Regarding the Phenetic Species Concept Phenetic species concept is an updated, numerical form of the earlier morphological or typological species concept Phenetic classification lacks a sound philosophical basis; it tends to necessitate only subjective and arbitrary decisions The neo-darwinians have dismissed typological classification because there is no reason to suppose that any ideal pattern of morphological types exists in nature

9 The Biological Species Concept Mayr - Species are groups of interbreeding natural populations that are reproductively isolated from other such groups. It places the taxonomy of natural species within the conceptual scheme of population genetics For example, a community of interbreeding organisms is, in population genetic terms, a gene pool (total aggregate of genes in a population)

10 Relationship between Morphological and Biological Species A justification for defining species morphologically is that the morphological characters shared between individuals are indicators of interbreeding Problems can, however, arise: Members of a species are by no means all uniform - biological species are regarded as polytypic - they have many (or perhaps no) morphological types Also, it is possible for a species to differ reproductively but not morphologically- sibling species

11 Is the Morphological Species still Applicable? A Test Involving Bryozoans Jackson and Cheetman (1994) addressed whether the fossil morphospecies they had identified were consistent with genetically distinct living bryozoans Determined that the morphological features used to distinguish bryozoans had a genetic basis Found unique allozymes in each of the distinguished morphospecies

12 The Recognition Species Concept According to Patterson (1993), species have a specific mate recognition system (SMRS) Species can be defined as a set of organisms with a common method of recognizing mates Advantages: SMRSs are easier to observe than interbreeding in nature The recognition species concept may more accurately represent what happens when a new species originates

13 Limitations of Reproductive Species Concepts The criterion of interbreeding is useless for asexual populations Reproductive species concepts cannot be applied to fossils It is difficult to know whether geographically isolated populations potentially can interbreed

14 The Ecological Species Concept Populations form the discrete phenetic clusters that we recognize as species because the ecological and evolutionary processes controlling the division of resources tend to produce those clusters The ecological species concept defines a species as a set of organisms exploiting a single niche (adaptive zone) The ESC supposes that ecological niches in nature occupy discrete zones, with gaps between them

15 Relevance of the ESC Consider an array of species exploiting resources that form a single resource axis (e.g., seed size) An individual suffers intraspecific and interspecific competition for food Selection within a species might favor individuals at the extremes, as they suffer less competition This process may lead to character displacement Character displacement often implies that the 2 species differ more sympatrically than allopatrically If the characters with respect to which the species differ more in sympatry are related to ecological competition, then character displacement will occur because of the advantage of avoiding competition with a better adapted species where it is present In a place where only one species exists, selection from interspecific competition is relaxed and the species evolves to exploit a larger niche

16 Contrasting the BSC and the ESC According to the BSC, species form discrete units because of gene flow But, the ESC emphasizes selection - selection favors certain forms and removes forms that are intermediate between species

17 The Phylogenetic Species Concept Species are defined as the smallest diagnosable monophyletic group Any population that forms an independent branch on the phylogeny is recognized as a species To be recognized as a species, populations must have been evolutionarily independent for a long enough time for diagnostic traits to emerge

18 Species Concepts and Asexual Species Gene flow can occur between bacteria taxa whose genomes have diverged up to 16%; in some cases, gene exchange can occur among species of different phyla! Sexual isolation is not an important criterion for identifying species of bacteria The primary consequence of gene flow between bacteria is the spread of specific sequences with high fitness advantages Consequently, it has been proposed that the key to recognizing bacterial species is finding strains that share a large suite of selectively neutral sequences