Speciation and the Fossil Record

Abstract

The process of the initiation, formation and establishment of descendent species from existing ancestral species is called speciation. Several aspects of this process can be studied using the fossil record, including how and why rates of speciation have changed through geologic history and how the morphology of lineages change over time as descendents gain phylogenetic independence from their ancestors. The fossil record also provides ancient deoxyribonucleic acid (DNA) which allows genetic analyses of extinct populations and species that give insight into genetic differentiation, among other processes. Organisms are neither all equally likely to be preserved in the fossil record, nor do they have similar rates of achieving reproductive isolation and morphological differentiation from their ancestors. We know more about the fossil record of speciation in groups such as marine bivalves, gastropods, plankton and bryozoans, which have comparatively better fossil records, than groups such as mammals or plants.

Key Concepts:

  • Speciation is the process of the evolution and establishment of new species from existing species: parts of this process can only be studied using the fossil record while others are better studied using extant organisms.

  • Rates of speciation and the temporal sequences of morphological change associated with speciation are two aspects of speciation that are tractable using the fossil record.

  • The Evolutionary Species Concept (ESC), which states that species are temporal populations that maintain their evolutionary cohesiveness and uniqueness from other such populations, is the most natural and commonly used species concept when dealing with species in the fossil record.

  • To be suitable for being subjects of the study of speciation using the fossil record, organisms have to be well fossilised in sufficiently large quantities for time intervals during which speciation processes took place, and beyond.

  • Morphology is one of the most conveniently and accurately measured traits in fossilised organisms.

  • Studies on living organisms with fossilised representatives tell us that morphological differences can be a good proxy for genetic differences and potential reproductive isolation.

  • Speciation can occur very rapidly (a few generations) or slowly over extremely long periods of time (millions of years).

  • The temporal resolution of the fossil record has to be suited to the particular speciation process one is interested in observing, in order for sound inferences to be made.

Keywords: punctuated equilibrium; temporal resolution; morphology; rates of speciation; modes of speciation; stasis; species concepts; fossilisation; ancient DNA

Figure 1.

A phylogenetic tree of Metrarabdotos with stratigraphic information. The x‐axis represents morphological distances but these are not to scale. The solid dots are samples while dotted lines are inferred presences of species or lineages. Inset is n.sp 10. Reproduced with permission from Cheetham .

Figure 2.

An illustration of how allopatric versus sympatric speciation might be observed from fossil samples. (a) shows a cartoon case of allopatric speciation where the descendent closed circles evolve after being physically isolated (brown bar) from the ancestral open circles, via grey circles. When the physical barrier is weakened or removed (beige bar), the descendent is able to migrate back into the ancestral range. (b) shows sympatric speciation where the descendent evolves in situ, also via an intermediate grey circle. In the panels below, we show how sampling locations and mode of speciation may affect what we observe, keeping in mind that the illustrations above are also necessarily temporally discrete, even though biological reality is continuous.

Figure 3.

Contrasting phyletic gradualism (anagenetic evolution) and cladogenesis and punctuation. (a) Anagenetic evolution is illustrated and the numbered species are continuously gradually evolving directionally (directional evolution). (b) Cladogenesis occurs when a new species is formed and morphological changes are clustered at the time of branching in a punctuated fashion. During the rest of their duration, species maintain their morphological integrity. Note that the number of species in (b) is not arbitrary, unlike in (a). (c) Reproduced with permission from Hunt , where example fossil morphological time series data best fit to a random walk, directional change, and stasis are shown. Note that the directional change and random walk time series are difficult to distinguish by eye. (d) illustrates how sampling can affect our perception of true evolutionary scenarios. Here, anagenetic speciation is interpreted as punctuated and vice versa due to the spacing of the temporal samples.

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Further Reading

Benton MJ and Harper DAT (2009) Introduction to Paleobiology and the Fossil Record. Chichester, West Sussex: Wiley‐Blackwell.

Erwin DH and Anstey RL (1995) New Approaches to Speciation in the Fossil Record. New York: Columbia University Press.

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Liow, Lee Hsiang(Dec 2010) Speciation and the Fossil Record. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001666.pub2]