Pattern, process and the evolution of meaning: species and units of selection

Many of the fundamental concepts of biology lack consensual, precise definitions. Partly, this is due to a contrast between our discrete language and the continuous character of nature. Some debates over these concepts are confounded by the use of the same terms with different specific meanings, indicating a possible need for an expanded scientific lexicon. Words have their own histories, and frequently scientific terms with a vernacular origin retain associated vestigial meanings. Even terms newly coined within science have histories and changing meanings, which can lead to confusion among debaters. Debates over concepts are further confounded when the same terms are used in different fields of biology, with distinct (even conflicting) objectives, and by biologists with different approaches and perspectives. I illustrate these issues by considering the debate over the concept of species and the unit of selection.

1. 1.

   Exons are sections of a gene that are transcribed and translated into a protein sequence. Introns are regions of DNA that occur interspaced between exons and are not translated into protein. A gene can include several interspaced exons. Some genes can potentially produce different proteins when different combinations of exons are translated. The timing, location and pattern of exon transcription is regulated via the activation and inhibition of additional DNA sequences: the enhancers and promoters.

2. 2.

   A clade is a monophyletic group, i.e., a group consisting of a single common ancestor and all its descendants. It is thus based on evolutionary relationship and not strictly on similarity. It also implies the inclusion of all the descendents of an ancestor. Thus, according to cladists, Reptiles are not a monophyletic group as they to not include all the descendents of their common ancestor, namely the Birds.

3. 3.

   As a species expands its distribution, local populations adapt to their local environment and diverge from one another, but retain the ability to interbreed with adjacent populations. A «ring species» is used to describe a species whose distribution has expanded such that the most extreme, divergent populations secondarily contact one another and are reproductively isolated, even though they can exchange genes via the remaining intermediate populations. For instance, Larus gulls have a cirum-polar distribution around the North Pole. Most adjacent populations are able to hybridize, forming a chain of interbreeding. However, the Herring Gull, from Great Britain, does not hybridise with the Northwestern European Lesser Black-backed Gulls.

4. 4.

   As speciation is a continuous process, «incipient species» is a term used to refer to populations that appear to be in an early stage of speciation, revealing some degree of reproductive isolation and/or phenotypic divergence, yet not having yet attained full isolation.

5. 5.

   During regular meiosis of a diploid cell, each copy of the two homologous chromosomes is present in half of the descendent haploid cells (e.g., sperm). Under meiotic drive, this ratio is biased in favour of one of the chromosomes.

6. 6.

   The different terms allow one to distinguish different evolutionary histories of a character and its relation to its present function. Thus, adaptation is reserved for a character that evolved specifically for its present function. A character which originally evolved for a function distinct from its present function is considered an exaptation. For instance, the bird wing originally evolved for thermal regulation and only subsequently for flight. Gould suggested that when uncertain about the original function of a character one should refer to it simply as an aptation. Disaptation is a formerly adaptive character that has lost its function. If it is presently dysfunctional or detrimental, it is referred to as a maladaptation.

I thank the promoters of the International Symposium “Evolution: Today and Tomorrow” and the Centre of Philosophy of Science of the University of Lisbon for hosting the event and organizing the publication of its proceedings. This study was funded by the scholarship SFRH/BPD/41391/2007 from the Foundation for Science and Technology (Portugal).

Affiliations

1. Unidade de Investigação em Eco-Etologia, ISPA – Instituto Universitário, Rua Jardim do Tabaco 34, 1149-041, Lisbon, Portugal
   • André Levy

Corresponding author

Correspondence to André Levy.

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