How to define "evolution"?
What is evolution?

In a non-biological sense, evolution means change:


  "a process of [...] change"


Biological evolution (seeing as this is Biology stack exchange) then needs to be tweaked to give a biologically specific context. Many textbooks etc. give definitions of evolution and here are a few good ones from across the history of evolutionary biology:

Charles Darwin:


  "Descent with modification".


Mark Ridley1:


  "Evolution means change, change in the form and behaviour of organisms between generations. ... When members of a population breed and produce the next generation we can imagine a lineage of populations, made up of a series of populations through time. Each population is ancestral to the descendant population in the next generation: a lineage is an ancestor-descendent series of populations. Evolution is then change between generations within a population lineage."


Brian and Deborah Charlesworth2:


  "Evolution means cumulative change over time in the characteristics of a population of living organisms. ... All evolutionary changes require initially rare genetic variants to spread among the members of a population, rising to high frequency..."


All of these have a common theme. Biological information is moving through time, descending with a degree of directionality (e.g. parent $\rightarrow$ offspring), and the information is modified with time.

Personally I would define evolution as:


  Evolution is a change in gene frequencies, resulting from four mechanisms (mutation, migration, drift, and selection) which can all affect the transmission of heritable genetic information within populations. Evolution has the potential to cause phenotypic change (when genotype affects phenotype) and has the potential to cause both adaptation (microevolution) and divergence (macroevolution).


This definition captures that biological evolution occurs through changes in heritable genetic information, the prerequisites of evolution, the mechanisms of evolution, and what the consequences of those changes can be. 

I think it is important to include mutation (and drift, selection, and migration) because these explicitly state how evolution occurs. I don't think it is necessary to include copy number variation, chromosome numbers, or codon usage as they are not central to the process of evolution (though they can be important).

Whilst only the first sentence of my definition is critical, I think it useful to include the latter part (or something of a similar effect) whenever possible as it helps to clarify evolution and it's relevance.




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