Haldane, Biogeography and the ‘Incomplete’ Fossil Record


J.B.S Haldane once said that he would give up his belief in evolution if someone found fossil rabbits in the Precambrian. Although this witty comment will bring a smirk to the face of many biologists. Through my time working with many high schools, colleges and sixth forms, and even to an extent at university, I felt that the significance of this bold statement is seemingly undervalued by many pupils and even undergraduates.




The statement actually underlines and highlights the constraining predictions of the evolutionary view. There are requirements which make the theory of evolution so elegant and so un-falsafiable. One of which is the general emergence of organic complexity from simpler systems in a relatively predictable way.

For example the progression of evolution (in a very simplistic watered down way) is generally understood to be as follows:

Single celled organisms > multicellular organisms (cell differentiation)>(insert ~1.5 billion years here)> Aquatic invertebrates (no rabbits!!)> Aquatic vertebrates> lungfish like creatures(still no rabbits)> Primitive quadrupedal, semi-terrestrial vertebrates which over the next few hundreds of millions of years diversified into amphibians, reptiles and eventually mammals. (and some way down the line we finally we get rabbits). *

(*I know it’s a little more complex than how I have briefly put it but I’m just putting forward a watered down model to paint a simplified picture)

Haldane made the rabbit statement as he knew, as many of us know, that the layering of the fossils we find is non random, it’s no coincidence that we find specific fossils where we would expect to find them. There is a chronological order of fossils, implied by stratigraphic layering which is reconfirmed by radiometric carbon dating. The results of the carbon dating precisely matches the chronological order of the fossils.


The evolutionary process requires a continuous genetic flow throughout the ages. There can be no inexplicable disjunction. Additionally evolution requires the same continuous gene flow through space. It sound simple when you think about it. Gene flow must be continuous, the organism must pass on its DNA to continue its lineage. To do this reproduction must take place, which generally implies, with few exceptions, that two individuals must be within a relative proximity to each other at the same time.

Therefore we could say, in a similar way to Haldane, that we could abandon the idea of evolution if we found animals in a place were we wouldn’t naturally expect to find them. For example if we were to find elephant fossils in the canary islands, or koalas in France, Tasmanian tigers in Scotland… you get the point. Each of these occurrences would represent an unacceptable and explained spatial disjunction. A reality in which these occurrences are true would be just as problematic for evolution as the temporal disjunction presented by Haldane’s Precambrian rabbit.

The chronological order of fossils has logical constraints which allow us to make accurate predictions, as mentioned earlier. Furthermore, the continuous gene flow which validates the evolutionary theory constrains the possible location of plants and animals.

Show Me The Missing Link

People who deny the facts and therefore deny evolution like to smugly point out gaps in the fossil record. Insisting that there must be observable intermediates for the theory to be true. Proudly claiming that ‘there is no evidence for evolution without observable macro evolution’ or as its more commonly put ‘I’ve never seen a chimp give birth to a human’…

There are obviously so many problems to this argument, too many to cover today, maybe one day I shall blog about more of the problems with denying evolution in the future, but today I will attempt to keep focus.

One main issue that the evolution deniers have with the theory of evolution is, as stated the fossil record has ‘gaps’. However, I can see a flaw in this request to plug the gaps, every time an ‘intermediate’ is found it will of course require two more ‘intermediates’ for example:

Denier: The fossil record has too many gaps, too many missing links, find a fossil, until you do there is no proof…

Species A—*(‘Missing link’)*—Species B

We found a fossil!! we found your ‘proof’

Species A—*(Species I)* —Species B

Denier: Hey! where is the missing link for species A and I and the link between I and B. This proves nothing!

*repeat over and over until you headbutt the wall…*

What many of the evolution deniers fail to see or understand is fossil records are not a great example of evidence for evolution. That’s not to say we can not support evolution from this evidence from the fossil record as discussed before, Haldane, biogeography etc.

But seeing as many of these people are obsessed with seeing intermediates, many examples do spring to mind, the one I have chosen is the one of the most cited and possibly the best example of a ring species that has been documented.

 Ensatina eschscholtzii the living intermediates!

On the Western coast of America, the rich coastal forests of California to the Western reaches of Nevada there is a rich habitat for amphibian life. It is here that we find one of the most compelling evidence of evolution happening before our eyes in the ensatina salamander (Ensatina eschscholtzii ).We can see observable aesthetic change as we progress from north to south.


To put it simply, as the population of salamander moved south, it was met by a geographical barrier which divided to populations. These two populations were split and are referred to being in reproductive isolation. Two populations of the same organism divided by the central valley. Both groups were exposed to different selection pressures created from the slightly differed environments. As the populations travelled further south, over time these different selection pressures have altered the phenotypes of these animals (How they look). Although these changes do alter how these animals look, the genome is not altered beyond the point where postzygotic barriers arise. Each population can breed freely with the neighbouring populations producing hybrids, this is true except for one pair. These pairs inhabit the region located south of the geographic barrier. The yellow blotted ensatina salamander and the Monterey salamander are sub species of Ensatina eschscholtzii. Therefore the reproductive isolation of the eastern valley salamanders has resulted in the diversification of one species, into two subspecies through a process called allopatric speciation.  

I guess in summary the point I am trying to make is evolution is a wonderful, beautifully constructed and most importantly true idea. It’s thought-provoking and inspiring, one of natures great puzzles. Although we know natural selection is true, there is still a lot to learn. Progress is being made, universities and research institutes continue to make new discoveries and more religious groups are excepting the theory of evolution as fact. The phrase ‘well its only a theory!’ is becoming more of a rarity in today’s society. Science is making more of a strive, new generations of intrigued pupils enter the educational system every year. There are more younger people getting involved in science, getting excited about science. I’ve shared these ideas here as this was the type of biology which inspired me to pursue a degree and hopefully a career in zoology.


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