One time, a mutation occurred in a few of the animals, causing a huge change: they were born with stripes! This was so helpful to survival that out of the whole population, none or almost none of those with stripes died of hunger.
They lived to reproduce, and their striped offspring also did very well. Over only a few generations, the whole population was born striped. A combination: Here is one idea of how tigers could have gotten their stripes by both gradualism and punctuated equilibrium: A mutation had a huge affect, causing distinct, stripe-like markings. These were then gradually "polished up" into stripes.
The idea of punctuated equilibrium originated long after the idea of gradualism. Darwin saw evolution as being "steady, slow, and continuous". Later, scientists were studying fossils and they found that some species have their evolution almost "mapped out" in fossils. For others they found a few, very different species along the evolutionary course, but very few or no fossils of "in between" organisms.
Also, when dating the fossils, scientists saw that in some species change was very slow, but in others, it must have occurred rapidly to be able to produce such change over such a short amount of time. But small, peripherally isolated groups are cut off from their parental stock.
They live as tiny populations in geographic corners of the ancestral range. Selective pressures are usually intense because peripheries mark the edge of ecological tolerance for ancestral forms. Favorable variations spread quickly. Small peripheral isolates are a laboratory of evolutionary change. Species should be static through their range because our fossils are the remains of large central populations.
In any local area inhabited by ancestors, a descendant species should appear suddenly by migration from the peripheral region in which it evolved. In the peripheral region itself, we might find direct evidence of speciation, but such good fortune would be rare indeed because the event occurs so rapidly in such a small population. Thus, the fossil record is a faithful rendering of what evolutionary theory predicts, not a pitiful vestige of a once bountiful tale. Punctuated equilibrium is therefore mistakenly thought to oppose the concept of gradualism, when it is actually more appropriately understood as a form of gradualism in the strict and literal sense of biological continuity.
This is because even though evolutionary change aggregates "quickly" between geological sediments—relative to the species' full geological existence—change is still occurring incrementally, with no great change from one generation to the next. To this end, Gould later commented that: Most of our paleontological colleagues missed this insight because they had not studied evolutionary theory and either did not know about allopatric speciation or had not considered its translation to geological time.
Our evolutionary colleagues also failed to grasp the implication, primarily because they did not think at geological scales. The fossil record is evidence that supports this view. There are many transitional fossils that show structural adaptations of species as they transform into new species. Proponents of gradualism say that the geologic time scale helps show how species have changed over the different eras since life began on Earth.
Punctuated equilibrium, by contrast, is based on the idea that since you cannot see changes in a species, there must be very long periods when no changes occur. Punctuated equilibrium asserts that evolution occurs in short bursts followed long periods of equilibrium. Put another way, long periods of equilibrium no change are "punctuated" by short periods of rapid change. Proponents of punctuated equilibrium included such scientists as William Bateson , a strong opponent of Darwin's views, who argued that species do not evolve gradually.
This camp of scientists believes that change happens very rapidly with long periods of stability and no change in between. Usually, the driving force of evolution is some sort of change in the environment that necessitates a need for quick change, they argue. Strangely enough, scientists in both camps cite the fossil record as evidence to support their views. Proponents of punctuated equilibrium point out that there are many missing links in the fossil record.
If gradualism is the correct model for the rate of evolution, they argue, there should be fossil records that show evidence of slow, gradual change. Those links never really existed, to begin with, say the proponents of punctuated equilibrium, so that removes the issue of missing links in evolution.
Darwin also pointed to fossil evidence that showed slight changes in the body structure of the species over time, often leading to vestigial structures. Cookie Settings Accept. Manage consent. Close Privacy Overview This website uses cookies to improve your experience while you navigate through the website.
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