Transitional Forms
Background Information
Evolution predicts that there should be a great many fossils of transitional forms, intermediate between ancestors and descendents. Creationists often argue that these transitional forms do not exist, and therefore, this disproves evolution. Indeed, they often quote Darwin, who was puzzled at the lack of transitional fossils, and Gould, who has said that there really are not very many transitional forms in the fossil record.
Why would Darwin have thought that there are no transitional forms? Why would creationists continue to argue that there are none? Why would Gould suggest that there are not a great many, yet agree that there certainly are some? To assess these issues, it is necessary to look into the actual biological mechanisms involved.
Genetic Change
At each new generation, offspring inherit their genes from their parents. Occasionally, an individual receives a mutated copy of a gene. When that individual passes on its genes to its offspring, that mutated copy of the gene becomes part of the genetic diversity of the population. If individuals with that particular genetic character are more successful than their fellows, then that genetic variation may become the norm for that population.
As this process occurs over the course of many generations, different genetic variations, involving different genes, arise in the population. Those which are advantageous become common as individuals with them reproduce more effectively than others. It is this process that accounts for evolutionary change. The consequence of this is that:
- each genetically-coded trait follows the same "rules" (because all genes are subject to the same chemistry, and are inherited the same way)
- it is unlikely that several traits will change at one time (because mutations occur at random, so there is no method for making mutations in many genes occur in unison)
- therefore, an intermediate between an ancestor and a descendent is likely to have some characteristics that are like those of the ancestor (mutations have not occurred to change those characteristics), and some characteristics that are like those of the descendant (mutations have occurred to change those characteristics).
The Common Misconception
The common misconception is that adults can mutate--that individual, full-grown organisms somehow change from one form into another form. Perhaps, this idea derives from scenarios such as the Teenage Mutant Ninja Turtles, or Spiderman, in which something happens to an individual, causing them to change into something else. A variation of this misconception is that mutations occur in DNA, and are inherited genetically, but they change everything about the organism rather than affecting just one trait. These misconceptions may have their roots in an incomplete understanding of genetics and molecular biology.
According to this misconceived notion, evolution from ancestor to descendent should occur by the gradual change of all characteristics in unison. It may be that each individual changes independently, or that every individual in a population shows the same characteristics over time, as generation after generation slowly change. One way or another, the vague notion is that somehow, everything changes at once.
What
Transitional Forms Must Be Like
In terms of transitional forms, the difference between these two models is illustrated here. The genetic model predicts that intermediate forms will always be "mosaics." Each individual will have some ancestral characteristics, and some descendent characteristics. The numbers of each type of characteristic depends on which mutations occurred by that stage in the evolution from ancestor to descendent. By contrast, the model in which adults can mutate predicts that every characteristic will be equally intermediate between the characteristics of the ancestor and the characteristics of the descendent.
Because evolution proceeds according to the rules of normal genetics, "transitional forms" must always be this sort of "mosaic." They will always look like individuals that display part of the genetic diversity of the population--because that is exactly what they are.
When people suggest that "there are no transitional fossils," however, they have based their expectation on the "adults-can-mutate" model. While it is true that there are no such transitional forms, it is not because evolution "doesn't happen." It is because it doesn't happen that way. That particular transitional form is an impossibility.
When Darwin first formulated his model of evolution by natural selection, he did not know how genetic inheritance worked. Mendel's papers were still largely unknown. Instead, Darwin thought it likely that Lamarck might somehow be correct, and that acquired characteristics might somehow be inherited. Now that we understand genetics reasonably well, and understand how mutations in DNA affect the traits of organisms, we know that acquired characteristics are not inherited. We know that there is no mechanism to cause mutations to occur in response to any kind of perceived need. We understand that the genetic mechanism described above is a reasonably accurate description.
Many people who do not have a deep understanding of genetic mechanisms seem to have the unstated assumption that some kind of Lamarckian inheritance occurs, and that individual organisms can somehow change to accommodate some kind of need. Thus, it seems to make sense that transitional forms should be perfect intermediates in all characteristics. It may not be possible, or at least not easy, to cast of this misconception without first gaining a deeper understanding of the genetic and molecular underpinnings upon which evolution rests.
The Role of Small Populations
We should also consider the dynamics with which a newly-arisen mutation will spread through a population, and eventually become the norm. The dynamics depend on several variables, but the one that is most relevant to this issue is the size of the population. The larger the population, the longer it will take for any version of any gene to become common. A rather simple-minded reconstruction helps illustrate why this is so.
If an individual is born with a mutation, how many generations will it take before all individuals in the population carry that mutation (i.e. the mutation is "fixed" in the population)? If the size of the population is two individuals, then it will be necessary at least for that individual to have two offspring that carry the mutation, and for these two to out-compete other individuals, to produce a population with two mutants.
If the population size is 10 individuals, then after one generation, there may be 2 mutants and 8 non-mutants. After the next generation, there may be 4 mutants and 6 non-mutants. After the next generation, there may be 8 mutants, and 2 non-mutants. Only after the fourth generation will all 10 be mutant.
From this simple comparison, it should be easy to see that, in a population of 100 individuals, it should take longer for the mutation to become "fixed," and in a population of 1000 individuals, it should take even longer. Since most natural populations are fairly large, it is unlikely that any new mutation will become common in the population until many generations have passed.
However, if a small group of individuals migrates away from the main population, and becomes isolated, then that small group will have different dynamics than the main population. A new mutation may become "fixed" in the small group fairly easily, where it would be very slow to be "fixed" in the main population. Thus, small, isolated populations are more likely to show evolutionary change than are large populations.
It is more interesting to consider what happens if a large population becomes subdivided into several different sub-populations. Each small sub-population is subject to relatively easy "fixation" of new mutations. However, because mutations occur at random, each sub-population is likely to acquire different mutations. After some number of generations, the characteristics of one sub-population may be slightly different from the characteristics of another sub-population. After even more generations, the characteristics of the different sub-populations will be more different. Given long enough, the characteristics may become quite different--possibly different enough that one or more sub-populations is unable to breed with the others, and has become a different species.
When an individual of any species dies, it is not particularly likely that it will become buried under just the right conditions to become fossilized. Therefore, fossilization is more likely for large populations, and less likely for small populations. That is, the small populations that are most likely to produce the true intermediates from one species to another are the least likely to be fossilized.
The Role of Semantics
The only way to be truly certain that this individual is the intermediate form between an ancestor and its descendent is to know the family pedigree with complete accuracy. Strictly speaking, my father is the intermediate between my grandmother and me, while my aunt and uncle are not intermediates at all. Thus, a paleontologist looking at "fossil evidence" from my grandmother and one of her sons might be correct in saying that her son "appears to be an intermediate form" between my grandmother and me, but he would not be strictly correct in saying that the "intermediate form" is, in fact, the true intermediate.
A similar situation occurs in assessing the relationships of fossils. If we have three species from three different time periods, and they appear to form a reasonable series of ancestor to descendent, we cannot say with certainty that the intermediate is a true intermediate, or whether it is a "sister species." If the intermediate is a fossil from the true intermediate species, it may be the fossil of an individual that was not in the direct family tree. Thus, to be precise, we must say that the intermediate fossil is very much like what we expect the intermediate fossil to look like, but we cannot say, and will not say, that this particular fossil is the intermediate. It might be the true intermediate's uncle.
Thus, to use strictly precise language, it may always be possible to say "there are no transitional fossils" because we may merely have the fossilized uncles of the true transitional individuals. At the same time, we can also say "we have a great many transitional fossils" because we have many representatives of many species that show transitional characteristics between ancestral and descendent species. Brothers, sisters, aunts, and uncles all share enough characteristics that we can place them in valid groupings. This is true even when we are speaking of sister species (developed from different sub-populations of the same ancestral species), or related groups of species within a genus.
1. Expose students' pre-conceptions. The educational literature indicates that misconceptions must be recognized and confronted before they can be discarded. It is usually necessary to juxtapose the pre-conception and a situation that cannot be explained by the pre-conception. Only when we recognize that our pre-conception is inadequate do we discard it, and replace it with a new conception.
One way to start may be to solicit student descriptions of how a new species might evolve from a current species. Perhaps, if students discuss this issue in small groups, they will see that they do not all have the same ideas about how this might work, and they may need to think more deeply in order to develop a story that all members of the group agree upon. Following this with a class-wide comparison of student descriptions should reveal the extent to which students understand the basic ideas, as well as the breadth of alternate explanations.
2. Evaluate alternatives. Are any of the students' explanations "right"? It might be best to set aside the notion of announcing who, if anyone, "got the right answer." Instead, consider each of the alternatives as valid hypotheses for evolutionary change. Ask what predictions each hypothesis makes, and then give students the task of searching for data that address those predictions. If one hypothesis is that individual animals turn into the new species, then we predict that, once in a while, we should observe this happening.
Ask, as well, what biological mechanisms could cause the changes that each hypothesis describes. Is there any way an adult animal can change into something else? It is unlikely that our students have heard of any such mechanism in their prior studies. What mechanisms are known that can cause changes in living things? Mutation is the only one that can produce heritable changes.
3. Help students visualize the processes. The animations on the OSO website may help illustrate the possibilities. Perhaps, when students see a "movie" of an animal changing into a different kind of animal, they will recognize it as silly. This silly movie, juxtaposed against their written statement of how evolutionary change occurs, may suggest to them that their pre-conception seems moderately unlikely.
To illustrate the actual process is more difficult, since we have to visualize a number of different events occurring over the course of many years. Those events are:
i. A mutation occurs in the DNA of a gamete or gamete-producing cell, resulting in an animal being born (or hatched) that carries the mutation.
ii. The mutation affects the ability of the individual animal to compete with its fellows. [Depending on whether the mutation is dominant, or recessive, this may or may not occur in the first individual that carries the mutation. Several generations may need to pass before two individuals carrying this gene version happen to mate, and produce homozygous offspring. Either way, however, there is a likelihood that the mutation will eventually affect the characteristics of individuals that carry it.]
iii. Over the course of many generations, individuals with the new genetic variation out-compete others in the population, eventually resulting in the new genetic variation becoming the norm in that population.
iv. Repetition of steps i, ii, and iii, with different genes and different characteristics.
This multi-step, multi-generation process is difficult to picture, especially if students have an imperfect grasp of how genes affect characteristics, and therefore how mutations can change those characteristics. However, understanding this at some level is critical, and this will depend on the grade level of the students. At a minimum, students must understand that:
i. Our cells contain genetic information that determines how we are built.
ii. The genetic information is chemical in nature, not some vague "life force."
iii. The genetic information can be changed by different kinds of chemical reactions--but only in one cell at a time. Changes are called mutations.
iv. To affect an entire organism, it is necessary that the information in a fertilized egg is changed. One must be born with a mutation; one cannot acquire a mutation later in life.*
v. The occasional mutations that occur are the ultimate source of the genetic diversity that exists in populations.**
vi. A genetic change can be passed to other members of a population only by transmission from parents to offspring. Therefore, a whole population cannot change at once. Rather, the rate of change depends on how many offspring different individuals have.***
* Of course, cancers result from single-cell mutations, so one actually can acquire mutations later in life. But, these mutations are not passed on to offspring.
** This is important, because it enables us to look around us, and see that there really is a great deal of genetic diversity. Mutations cannot always be "bad."
*** The transmission of plasmids between bacteria is an exception to this statement.
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Last updated: 31 December 2005
Comments: Jose
Bonner, OSO
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