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Ancester-Descendant Relationships

Recently I have come across the same assertion both in print and in conversation, which basically boils down to the statement that co-existing groups cannot have an ancestor-descendant relationship, or to be exact in the written case; "Evolutionists have long maintained that contemporaries could not have an ancestor-descendant relationship but if related, they must have evolved from a common ancestor sometime in the past." (Gish 1985, p. 116).

So, is it true that co-existing groups (or contemporaries) cannot have an ancestor-descendant relationship? The answer is Yes and No.

This ambiguity stems from the nebulous nature of the term group or contemporaries. In other words, it dependant on the kind (:-)) of group one is talking about. The two kinds of group most commonly discussed wrt evolution are population and species. Thus inserting these groups in the statement we get:

Scenario 1

Is it true that co-existing populations cannot have an ancestor-descendant relationship?

A: Yes

Scenario 2

Is this true that co-existing species cannot have an ancestor-descendant relationship?

A: No

Scenario 1

For the purposes of this discussion, a working definition of population can be, a group of interbreeding individuals occupying a certain area, in which the influence on the gene pool from outside the population is minimal. In other words, breeding with individuals from outside is on a much smaller scale compared with breeding between individuals within the population. Now imagine that is is possible to count the number of alleles within this population (= P0) exactly, so that frequence of each can be plotted in the grid:

          A       a
      -----------------
     |        |        |     
 A   |   w    |   x    |
     |        |        |
     |--------|--------|
     |        |        |
 a   |   y    |   z    |
     |        |        |
      -----------------

For each allele in the population. This would provide a unique fingerprint for this population, at this particular time (= T0). Now, if we allow a certain period of time to elapse and then recount the alleles from the same area at T1, some of the values of w,x,y, and z will have changed. This is due to the removal of some individuals from the population by death or migration, and the addition of some individuals by birth or migration. This change in allele frequence will occur, whether or not natural selection occurs, and is, of course, the standard definition of evolution. Now the relevance of this to the original discussion is that this new population, P1, whilst it differs from P0, it is P0, plus the sum of the changes in allele frequence over the time interval T0 - T1. From this it can be seen that the ancestral population, P0, cannot co-exist with the descendant population P1, because P1 is P0, plus the accumulated changes over time.

Similarly, if the original polulation P0 subsequently diverges into two populations, as in:

    -------- P2
   |
   |
P0 --------- P1

and P2 undergoes unique selection pressure to diverge rapidly (incipient speciation), whilst P1 does not experience any divergent selection pressures and remains similar to the ancestral population P0. P1 will still have a different allele frequency that P0, purely due to random allele fluctuations. Thus the two co-existiong populations P1 and P2 do not have an ancestor-descendant relationship, because P1 and P2 are P0, plus the sum of the changes in allele frequency over time. The ancestral population P0 no longer exists. In genetic terms, a population, as defined by its allele frequencies, can only exist at a fixed time. Thus co-existing populations can have a common ancestral population but cannot have an ancestral-descendant relationship.

Scenario 2

A species can be defined here as a group of interbreeding individuals which are, for the most part, reproductively isolated. Probably the vast majority of species and not confined to a single population, so we can further refine our working definition to be, several populations of interbreeding individuals which are, for the most part, reproductively isolated. Since species are usually made up of more than one population, the concept of the species is much more tolerant to allele fluctuations and can be better defined by a range of allele frequencies, rather than any unique frequency. Most species experience stabilizing selection, which tends to confine the allele frequency within these tolerance limits. Thus species S0 described at time T0 will still be recognisable as species S0 at time T1 regardless of the elapsed time. However, the work of the Grants with Galapagos finch's (as reported in Weiner 1994), shows that local environmental perturbations can reverse this stabilizing pressure on a population very rapidly and, if the new selection pressure lasts long enough, and the local environment settles down in the perturbed state, the population diverges (rapidly) from the norm and speciation can result. Now, if this pertubation is experience by only one or a few populations of a particular species, a number of populations will remain within the allele frequency tolerance levels of the original species, whilst the populations which experienced the pertubation can now be recognised as a new species, since its allele frequency exceeds the tolerance limits of the original species and it becomes a reproductively isolated unit, thus:

    -------- S1
   |
   |
S0 --------- S0
Where S0 is the original species, and S1 represents those populations which have diverged to produce a new species.

In this case the coexisting species S0 and S1 can and do have an ancestral-descendant relationship.

An example

A good example is the Brown Bear, Ursus arctos. In the past (T0) the ancestral population of brown bears (P0) roamed from northern Europe to northern North America. After the last Ice Age, the ancestral population became divided into two main groups, one in Europe, P1a, one in North America, P1b. However, today (T1) bears from P1a do not rush across the Bearing Strait shouting "DaDa?" and bears from P1b do not rush across the Bearing Strait shouting "Mother?" As has been shown above, these two co-existing populations cannot and do not have an ancestor-descendant relationship. The ancestral population P0 no longer exists, because P1a and P1b are the ancestral population, plus the sum of the changes in allele frequency over time. The two populations are related by having a common ancestor population sometime in the past, i.e.:

     ------------ P1a
    |
P0 -|
    |
     ------------ P1b

However, sometime during one of the last major glaciations, a population of Ursus arctos became isolated above the arctic circle. Separated from other populations of Ursus arctos which remained further south, this population experienced extreme selection pressure to enable survival on or near the ice. Such traits as fur on the feet and a white coat for camouflarge during hunting, began to occur - each subsequent incremental increase in which, confered an increased advantage (some fur on the feet is better than none for short periods on the ice; a grey coat is better than a brown coat for hunting etc.). Over a relatively short period of time, this population exceeded the allele frequency tolerance levels of Ursus arctos, and speciated to produce the Polar Bear Ursus maritimus (I know, I know, it would have been better for the Polar Bear to have been named Ursus arctos, but there you are!). The reason for this severe, rapid and ultimately successful selection push, was that the bears were evolving into an empty niche. There was no (or hardly any) large carnivores hunting on the ice and in the snow and there was probably enough food for bears and wolves to co-habit without significant competition. Thus it was relativly easy for slight advantages to take hold in the population because the bears were only competing amongst themselves and in such a situation, slight advantages can make a difference.

Now, it is obvious from the preceding discussion that the current co-exiting populations ofUrsus arctos and Ursus maritimus do not have an ancestor-descendant relationship, because both are the ancestral population plus the sum of the changes over time. However as co-existing species, they do have an ancestor-descendant relationship, since Ursus arctos is the ancestral species to Ursus maritimus, i.e.:

    -------- S1
   |                 where S0 is Ursus arctos
   |                 and S1 is Ursus maritimus
S0 --------- S0
My thanks to Tom Holtz for suggesting the example and to Jonathan Weiner for writing such a good book.

Gish, D. T. (1985) Evolution: Challenge of the Fossil Record. Creation-life Publishers, San Diego. 278 pp.

Weiner, J. (1994) The Beak of the Finch. Vintage, London. 332 pp.