Evolution is traditionally spoken of as an inherited change over generations. Do

Question

Evolution is traditionally spoken of as an inherited change over generations.

Does evolution happen one change at a time - or are there multiple changes occurring between two successive generations? Is there a known timeline for the frequency of such changes in any species?

Something like ...

20 Million years ago - Species X showed A, B, C.. Z characteristics

19 Million years ago - Species X following characteristics changed A'

17 Million years ago - Species X following characteristics changed B', C', Q', M'

16 Million years ago - Species X following characteristics changed A''

Explanation / Answer

There can be multiple changes at the DNA level generation after generation, however most of these changes are insignificant (doesn't contribute for any specific character). But rarely, there can be a significant trait/change gained by an individual because of such changes in DNA level. Unfortunately such changes gets 'lost', unless it is fixed in the population (i.e., the gained individual should be successful in breeding and passing the trait/change to large number of progenies. Those progenies should further continue to pass on the trait till about every one (or most of them) in the population has those trait/change). Only such changes contribute for the species development (continuous accumulation of such changes, over time will result in the novel population that is very different from the original population).

Most of the timelines that we know are reconstructed from the DNA/protein sequences. Usually, DNA or protein sequences from the closely related species will be compared and looking at the differences they have, using suitable models, ancestral state of those sequences are determined. Once the sequence for the last common ancestor is known, mutational rate per unit time can be used to estimate the evolutionary time for those species.

A simple example would be the blue eye color in humans: it occurred about 10,000 years ago and it is still not fixed in the population.

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