Modes of evolution

Evolution according to Darwin and Wallace

The Darwin-Wallace model of evolution goes something like this (expressed very simply):

  • In most populations, individuals show a variety of features for many biological traits (e.g. variations in height, speed of movement, ability to resist infectious disease). Many of these variable features can be passed from parents to offspring.
  • Although each pair of parents require only one offspring to perpetuate their lineage, in most species, parents will produce more than one offspring. Not all of them will survive till reproductive maturity.
  • Competition for resources, changing environmental conditions or disease act on populations. These factors will affect some individuals more than others. Those thatsurvive the challenging conditions may be considered to possess‘favourable’ traits that allow them to adapt and survive (Darwin called the forces that resulted in differential survival 'natural selection’).
  • The survivors will pass on those advantageous traits to their offspring. The frequency of those traits will increase in the population (over several generations).

When Darwin and Wallace proposed their ideas, they did not know anything about genetics (how traits are passed from one generation to the next). They also did not know why variations existed in population (how they were caused). These answers came later and as a result of the new information, they modified the Darwin-Wallace model of evolution. The modified model of evolution came to be called‘Neo-Darwinism’.

Types of evolutionary change

So, if populations change (evolve) over time, how quickly will that change occur? There two main ideas here.

Gradualism:Darwin thought that evolutionoccurred over a long period of time. During that time, individuals of a species will acquire many small changes and eventually, that species will split into a number of distinct species. This method of evolutionary change is called gradualism. It is supported by the geological and fossil records (particularly transitional fossils).

Punctuated equilibrium: Not all researchers agreed with gradualism. Instead, scientists such as Stephen Jay Gould proposed another model: punctuated equilibrium. According to this model, populations will experience a significant and dramatic selective forces. These dramatic events will happen onlyoccasionally, but have significant and devastating consequences on populations. This model is also supported by fossil record, which show abrupt changes in the anatomy of related organisms.


Image comparing gradualism with punctuated equilibrium. The former describes evolution occurring through acquiring small changes. The latter describes evolution occurring through large-scale changes that occur abruptly.
Image from

Most biologists accept that both of these models (gradualism andpunctuated equilibrium) are correct. Together, they provide a fuller account of evolutionary events.

Sham Nair 2014