Macroevolutionary changes are complex changes notably those that can be studied in the fossil record e.g. origin of new higher taxa.

Relative rates of evolution during and between speciation events

Controversy is in part about relative rates; also whether macroevolution simply microevolution extrapolated over a much longer time scale?; or does macroevolution occur by different mechanisms?

Evolutionary rates have been measured for many characters, in many species, at many different geologic times. The rates are very variable.

Q. Are the rates of change seen in the fossil record consistent with the mechanisms of evolutionary change studied by population geneticists?
A. Rates in artificial selection experiments are far higher than those measured in fossils. Known mechanisms of population genetics comfortably accommodates the fossil data.

PATTERNS IN FOSSIL RECORD?

Doesn't show smooth evolutionary transitions. Often a species appears abruptly, persists for a period then becomes extinct. A related species may then arise, but with little sign of any transitional forms between the putative ancestor and descendant

Darwin's explanation was that the record was incomplete; if evolution really was gradual, but most of the record had been lost, then the result would have been the pattern that we see now. He claimed that where a complete fossil record could be found, it would show a gradual morphological change through time in lineages, and where lineages became split, gradual divergence of species. Evolution by Phyletic Gradualism.

G. G. Simpson and colleagues (1950's) re-emphasised the role of natural selection.

Rates of evolution varies because intensities of selection vary.
Geologically abrupt changes in morphology in fossil lineages may reflect bouts of pronounced directional selection.
Periods of little or no direct change, by contrast, may reflect bouts of very weak directional or stabilising selection.
Evolution by Punctuated Gradualism.

New species arise by splitting of lineages
New species develop rapidly
A small sub-population of the ancestral form gives rise to the new species
A new species originates in a isolated/peripheral part of the ancestral species' geographic range.

A) If a geological site preserves the record of the ancestral species, the descendant species will be evolving elsewhere and will not be preserved at the same site as its ancestor.
B) The new species will only leave fossils at the same site if it re-invades the same area. Re-invasion could happen if the descendant was outcompeting its ancestor or was sufficiently different to coexist ecologically. The transitional forms would be unrecorded not because of the incompleteness of the fossil record at that site but because evolution took place elsewhere.

1) That most new species make sudden appearances in the fossil record
2) Between speciation events there are long periods of morphological stasis.

1. High quality of stratigraphic sequence
2. Rigorous biometrical measurements

The Plio-Pleistocene Snails of Lake Turkana

Site = Lake Turkana (Northern Kenya). The sedimentary record is exceptionally complete. Williamson (1981) documented the changes in snail shell patterns. Measured between 5 and 24 characters in 3300 specimens from 13 species' lineages.

Concluded that snails in all 13 lineages showed no change for prolonged periods, with occasional periods of punctuational change. The abrupt changes were concentrated in intervals of about 5,000 to 50,000 years, much shorter than the long periods of constancy. The picture (on handout) shows the complex pattern of events. The Y-axis shows the passage of time and the level of the lake; the picture shows the changes in shell shape for a set of snails and bivalves.

Does appear to be long periods of morphological stability interrupted by fossil beds in which relatively rapid changes in shell shape take place. These newly evolved populations then persist before becoming extinct and being replaced by fossils resembling the ancestral form. The periods of transition coincide with each other in the various genera; and at times when the sea level changed.

As the water level lowered, larger lakes would have fragmented into groups of smaller lakes, and the snails would have had their geographic ranges fragmented into many smaller isolated populations. This combination of environmental change and isolated populations are, of course, the conditions required for allopatric speciation.Potentially these snails show the essential features of the punctuated equilibria model.

However this study has been criticised on several grounds. One argument is that the changes in the snails may not have been evolutionary changes but "ecophenotypic switches". A snail's adult phenotype depends upon the environment during development. If rear snails under different environmental treatments, can switch the phenotype between strikingly different forms - different enough to look like separate species. So perhaps the snails in the study haven't speciated; they just resemble the range of morphological variants seen in the modern world.

If Williamson's punctuations are ecophenotypic switches, it would hardly be surprising if (a) they took place rapidly and (b) changes were synchronous across genera. Then they would not provide support for Eldredge and Gould's theory. So proceed with caution.......

Another example: Ordovician trilobites in Wales

Sheldon's 1987 biometric study of the evolution of trilobites. These are extinct arthropods and can be classified by external morphological features such as the number of pygidial ribs - a feature on the tail section of the trilobite's body.

He examined the number of these ribs in 3458 specimens from 8 generic lineages over a period of about 3 million years. In all 8 lineages the average number of ribs increased and the evolution was gradual.

The picture (on handout) shows the pattern. Time on the Y-axis and number of ribs on the Y-axis.

• There are no abrupt changes. A population at any one time was usually intermediate between the samples before and after it. There were also a number of reversals in the different lineages.
• Moreover they were NOT synchronised in the different lineages.
• Sheldon interpreted the changes as evolutionary and certainly not ecophenotypic.
• Author described data as a fine example of gradual evolution.

EXTENSIONS TO ELDREDGE + GOULD

1. Peripheral population may live under abnormal conditions?

Role of isolated populations in causing rapid changes at speciation?
Stabilising selection will favour the evolution of homeostatic mechanisms that regulate development = not controversial. However under extreme conditions do these mechanisms break down?

2. Macroevolution proceeds via macromutations?

Suggested that most evolutionary breakthroughs could be by macromutations.

We know that there are genes that play regulatory role in e.g. developmental processes in addition to conventional structural genes. Regulatory genes can have multiple effects on the phenotype. Some argue that large scale changes can occur through drastic macromutations in regulatory genes.

Gould proposed that new species might arise in this way rather than by the conventional mechanism of selection of mutations of small effect.

The neo-Darwinian response is that developmental macromutations may arise from time to time but will always be selectively disadvantageous because they are introducing such a gross change.

Macromutations can happen: e.g. in Drosophila there are mutations with effects in early development with large phenotypic consequences (homeotic mutations). But can these "monsters" play an important role in evolution?

3. Stasis reflects developmental constraints?

The neo-Darwinist invokes stabilising selection - the alternative is that it reflects developmental constraints (defined as "limitations on phenotypic variability caused by the structure, character, composition, or dynamics of the developmental system").

In effect, such constraints mean that species do not change because they lack the necesssary genetic variation. If we go back to to Williamson's snails; He argued that developmental constraints were operating during the periods of apparent stasis.

The hypothesis that a character isn't changing because it has no genetic variation can be tested. The prediction is that the character's heritability is zero. We cannot estimate this from the fossil record but that of similar shell characters in modern species is known. Heritabilities of these modern characters are significantly positive which argues against a role for developmental constraints.