Tempo and Mode of Evolution
The fossil record tells us a great deal about the evolutionary history of life.
For example, from fossils we can often determine relationships among species, how
characters change over time within a group of species, the rate at which these characters
change, and how species diversity changes over time.
In the fossil record, one pattern is very common: a species appears suddenly, persists for
a time, then disappears, presumably because of extinction.
This pattern posed a major problem for Darwin and subsequent evolutionary biologists
because it does not show smooth evolutionary transitions.
Darwin blamed this on the incompleteness of the fossil record.
If evolutionary change is gradual, but most of the record has not been preserved, the
result would be a pattern in which there is little change within species and the sudden,
rather than gradual emergence of new species.
The incompleteness of the fossil record is a fact:
1. Many of the species that existed have likely not been preserved.
2. Second, only hard body parts are preserved.
3. The probability of preservation varies over geological time.
4. he probability of preservation varies geographically within any time period.
After the modern synthesis, most paleontologists accepted the notion that evolutionary
change is gradual, and that the reason we often don't see gradual change in the fossil
record is because the fossil record is incomplete.
PHYLETIC GRADUALISM - slow, gradual change within species, and that speciation
involved the gradual divergence of populations.
Although the fossil record often isn't detailed enough to necessarily capture gradual
changes, a number of examples have been found.
In 1972 S. J. GOULD and N. ELDREDGE proposed that the pattern we see in the fossil
record isn't necessarily an artifact of the fossil record being incomplete.
They proposed that most species actually change little during their history, a state called
stasis.
In addition, they proposed that most evolutionary changes occur at speciation; not
gradual change, but sudden change.
They called this pattern PUNCTUATED EQUILIBRIUM.
Gradualism vs. Punctuated Equilibrium
Evolutionary Change
Within species
During speciation
Phyletic Gradualism
usually gradual
usually gradual
Punctuated Equilibrium
little (stasis)
rapid (punctuated)
The logical basis for phyletic gradualism should be obvious.
There are changes within species due to natural and sexual selection.
Populations diverge and speciation occurs as different populations respond to different
environments.
The logical basis for punctuated equilibrium may not be as straightforward
Why does most evolutionary change occur at speciation?
To explain rapid evolution at speciation Gould and Eldredge drew on the PERIPHERAL
ISOLATE model of speciation.
This model proposes that speciation occurs in small populations separated by a barrier
from other populations.
Allopatric event in Peripheral Isolates
A small population on the edge of a species range may experience extreme conditions
And may evolve rapidly - because of drift and selection
Speciation may be accompanied by a genetic revolution - an extensive reorganization of
the gene pool
Gould and Eldredge argued that rapid evolution is most likely to occur in small
populations. Isolation eliminates the homogenizing influence of gene flow.
Genetic drift and selection can thus cause rapid divergence of the small, isolated
population from the original population.
These small, isolated populations may diverge so much that they become different
species.
Mechanisms for Stasis
To explain stasis two ideas have been invoked: STABILIZING SELECTION and
EVOLUTIONARY CONSTRAINTS.
Stabilizing selection - Under stable environmental conditions intermediate phenotypes
may be selected and traits may be kept from changing over time
It would seem unlikely that the environment and therefore selection would remain stable
over vast periods of time
However, an organism may be buffered from large environmental changes through
habitat selection.
Evolutionary constraints are factors intrinsic to the organisms that prevent it from
responding to selection (e.g., lack of genetic variation, non-heritable traits, etc.)
One approach to answering the question of stasis in the context of evolutionary
constraints has been to focus on “living fossils” – species or clades that show little or no
measurable morphological change over extended periods
Why have these species failed to change?
One possibility is that it is simple due to a lack of genetic variation
Another explanation for the pattern of stasis has been put forth by Stanely and Yang
(1987) - “zig-zag” evolution
Little net change within a species, but a great deal of fluctuation
Due to changes in response to environmental changes over time
Because these changes tended to fluctuate about a mean value, the overall pattern was
one of stasis
This pattern has also been referred to a dynamic stasis
Points to Ponder
Time and Scales of Observation
In the case of distinguishing stasis from gradual change in the fossil record, the finer the
time scale, the better off we are
Rules for Testing the Pattern
Our goal is to follow changes in morphology in speciating clades through time and
determine: 1) whether change occurs in conjunction with speciation events or
independently and 2) whether rapid change is followed by stasis or continued change
The theory of punctuated equilibrium can become tautological
We define fossil species on the basis of morphology, so it might be trivial to observe a
strong correlation between speciation and morphological change
To avoid circularity, an acceptable test requires that 1) the phylogeny of the clade is
known, so we can identify which species are ancestral and which descendant and 2)
ancestral species survive long enough to co-occur with new species in the fossil record
Note:
If the second criterion is no fulfilled, it is difficult to know whether the new
morphospecies is indeed a product of a splitting event or the result of rapid evolution in
the ancestral form
Q. Example of punctuated equilibruim?
One group in which evolution does appear to have been punctuated is snails in Lake
Turkana in Kenya in the Pliocene and Pleistocene
The fossil record is extraordinarily complete for these snails, and it appears that for long
periods of time snails show no change in morphology.
These periods of stasis are punctuated by periods of rapid evolutionary change.
These rapid evolutionary changes are usually, though not always, associated with
speciation. There is thus some evidence for the punctuated equilibrium idea.
Summary
In reality these two models for the pattern of evolutionary change are not mutually
exclusive. Instead, they represent two ends of a continuum, and patterns of evolutionary
change may fall anywhere in between.
For example, some species may show little evolutionary change over time while others
may show substantial evolutionary change.
Some speciation events may involve rapid evolutionary divergence while others may
involve slow evolutionary divergence.
Stasis may be associated with gradual divergence or gradual change within a species may
be associated with rapid divergence at speciation.
Which patterns are more common, or if all are nearly equally common, is currently
unknown.