GEOL2301: Palaeoecosystems

Evolution – the concept that life has changed through time – is incontrovertibly demonstrated by various disparate lines of data: genetics, comparative anatomy, biogeography, and of course the fossil record.

The mechanism behind evolution has attracted more discussion. The modern view was established in 1859 by Charles Darwin in On the Origin of Species by means of Natural Selection. Darwin was trying to make sense of biological variation observed whilst serving as Ship’s Naturalist on the Beagle (1831–36) – a challenge to the prevailing view of the ‘fixity of species’. Darwin’s thoughts were inspired by Rev. Thomas Malthus’s famous essay on population growth, and Charles Lyell’s theory of uniformitarianism.

Natural selection – the mechanism of evolution – is the logical consequence of three testable premises. Watch each video and define each concept.

• These are 'extended' videos, to help you appreciate each concept. What's important is that you appreciate why natural selection is inevitable in any setting where all three concepts can be demonstrated to be met (see video 3E).
• List evidence in support of each premise.

• Heritability
• Briefly: parents resemble their offspring

• Variability
• Briefly: offspring may differ from their parents in ways that may influence their reproductive potential

• Superfecundity
• Briefly: Resource availability places an upper limit on population size

• Why do these three principles necessarily give rise to natural selection?
• Briefly: Resource availability places an upper limit on population size

Modes of selection

The principle of uniformitarianism asks us to imagine what would happen if processes that we can observe in the present day – such as selection – were to persist over long time scales.

Selection describes the pattern by which traits that improve the fitness of individuals (i.e. how well they can convert resources into offspring) increase in frequency within a population.

The three modes of selection, stabilizing, directional and disruptive selection, if they persist over geological time periods, might be expected to set a species on one of three very different evolutionary trajectories, stasis, anagenesis and cladogenesis.

Identify the modes of selection associated with the trait optima (orange) sketched below. For each mode of selection, sketch how you would expect the frequency of trait occurrence (red line) to change over time:

• Double click the white "×" button to reset the canvas.

Describe each mode of selection, and explain how it might lead to the associated species-level pattern over evolutionary timescales.

• Stabilizing selection
• Selection against variation maintains 'average' morphology in a stable environment.
• Directional selection
• Selection for an extreme leads to consistent directional change
• Disruptive selection
• A population diverges to form separate species adapted to each of two non-overlapping optima

Case study on Anagenesis: Change through time

Anagenesis does not increase diversity by the genesis of additional species. Rather, it describes change in a single lineage through time to an extent to which palaeontologists would identify the end-members as belonging to different species.

• What makes the trilobites of the Builth inlier a good case study of anagenesis?

• Writing an essay on this question would be a great way to revise this topic!

  You might mention:

  • High sedimentary/temporal resolution – can 'see' evolution in progress
  • Potential link between easily-measured morphological feature (pygidial ribs) and environment (oxygen levels)
  • Stable environment inferred from sediments – allows specialisation and thus precise response to even small environmental changes
  • Specimens readily ascribed to one of many lineages, each displaying the same overall morphological trend
  • Recognition that multiple morphologically–defined 'species' in fact belong to a single lineage

Cladogenesis: Branch creation

Lineage splitting – which leads to one species becoming two – can occur through sympatric (‘same land’) or allopatric (‘other lands’) processes.

Sympatric speciation is generally thought to be rare, as it requires populations that share a space to become reproductively isolated. Allopatric speciation enforces reproductive isolation by the establishment of a geographical barrier, whether temporary or permanent – often at the periphery of a population (where it is given the more specific term peripatric speciation). If sufficient evolution has occurred (whether by natural selection or neutral genetic drift), the populations will be unable to interbreed when (if) the barrier is removed.

• What is peripatric speciation?
• Peripatric speciation is allopatric speciation that occurs at the boundaries of a species' range.
• Why might peripatric speciation be a common form of allopatric speciation?
• • Margins of ranges might be easily isolated from the main population, allowing reproductive isolation
  • Range margins may be at the limits of a species' environmental tolerance -- leading an isolated population, cut off from genetic mixing, to adapt to the sligtly different local environment
  • If range margins are range margins because they are stressful environments, there may be vacant niches into which a lineage can slide itself despite not being perfectly adapted, owing to a lack of incumbent competitors.
• Why might it be difficult to identify peripatric speciation from the fossil record?
• Bottom line: The fossil record is too incomplete. You might specifically mention:
  • Small geographic range
  • Short temporal duration
  • Difficult to identify the limits of a species' range

Tempo and mode of evolution

Allopatry and sympatry speak to where evolutionary change occurs. Palaeontologists may also be interested in when it occurs. An enthusiastic uniformitarian such as Darwin might anticipate that the fossil record would indicate continual, gradual evolution. Such phyletic gradualism describes the tendency of a single lineage to change gradually yet consistently over time. In contrast, punctuated equilibrium suggests that most evolutionary change is concentrated in short bursts, separated by extended periods of stasis.

There are of course compromise positions. Some lineages of planktic foramanifera, for example, exhibit ‘punctuated gradualism’ – gradual change in normal conditions, but rapid change when major environmental perturbations occur.

Plus ça change

Sheldon’s ‘Plus ça change’ model suggests that phyletic gradualism characterises ecological specialists, whose morphology is precisely optimized for a very specific environment that changes slowly, if at all; whereas punctuated equilibrium is the norm in ecological generalists that are adapted to an unstable environment, and only undergo change when the environment shifts beyond its usual (broad) range of variation.

The incompleteness of the fossil record can make it a little tricky to distinguish the two, but nevertheless many instances of long-term stasis can be recognized – suggesting punctuated equilibrium as the dominant means of evolutionary change. Punctuated equilibrium is precisely what one would expect to see if peripatric speciation – characterized by the isolation of small populations in fringe environments, with correspondingly marginal preservation potential – is a dominant contributor to life’s increasing diversity.

• What sorts of ecosystem might be found in environments that favour punctuated equilibrium?
• Frequently changing environments are often populated by generalists and may be characterised by r-selected life histories, as opportunitists may grab favourable conditions whilst they last, and there's not time for K-selected supercompetitors to get established before conditions change again.
• Does this provide an argument for whether punctuated equilibrium or phyletic gradualism might be the dominant mode of species generation?
• This is an invitation to speculate: you may well disagree with my ruminations!
• Recall that intermittent disturbance has been claimed to contribute to maximum-diversity environments. If most of this diversity arose in intermittently disturbed settings, in which punc eek is the norm, and phyletic gradualism characterizes low-diversity (stable, deep, resource-poor? and boring?) environments, then perhaps most species arose through punctuated equilibrium. (Is the assumption that most species occur in high diversity settings a valid one?)

Punctuated equilibrium is the friend of systematic palaeontologists, who are tasked with the job of delimiting the continuum of the evolutionary tree into separate and rigidly defined quanta. To define a species one must provide a ‘diagnosis’: a descriptive statement that distinguishes that species from any other.

Palaeontologists are usually restricted to morphological criteria; using time to discriminate species is expressly prohibited. This might be problematic in cases where species are reproductively (and genetically) isolated but morphologically identical – though palaeontologists can only speculate as to how often this occurs.

The morphological species concept applied by palaeontologists defines members of a species as those that are morphologically indistinguishable from a designated reference specimen. A species definition is founded on a description of this specimen, and its diagnostic differences by which it can be distinguished from similar-looking species. Each distinct species has a distinct specimen defined as its holotype, which serves as the canonical member of that species. This works well where species are easy to distinguish on morphological grounds…

A practical problem arises when there is morphological diversity within a species. If such a morphological continuum is overlooked, palaeontologists may erect more species than truly exist and thus over-estimate diversity. So that it will be possible to resolve cases where a diagnosis transpires to be too vague, incorrect, or unsuitable for the distinction of new material, a type specimen (holotype) must also be designated: this single specimen will become the canonical member of a species (and should be kept safely in a museum for future reference!).

• We'll revisit this concept in the synchronous session.

Extension: Different definitions lead to different conclusions

The morphological species concept has its problems. There are alternatives; you may like to think about how you might go about applying them to fossil (and living) taxa, and whether the different concepts might allow you to answer different questions.

• Biological species concept: species are “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups”
• Cladistic species concept: a species is an evolutionary lineage between branching points in an evolutionary tree (more on trees next time)
• Ecological species concept: a species is a set of organisms adapted to a single niche (more on niches next time)

That was a lot of ground to cover! Hopefully you have absorbed some key principles:

• Natural selection provides a mechanism by which new species can emerge
• Species may or may not be clearly distinct, readily defined entities, depending on the mechanism by which they came about
• Palaeontologists have little option but to define species based on morphology. Often, this works; sometimes, it may not tell the whole story.

If it's helpful, you might want to revisit the key principles that have been introduced in this session. Have a stab at defining them without referring to your notes. When you submit the form, you'll be able to see your classmates' (anonymized) definitions, and to edit your own.