12 Differential rate of Somatic Evolution

Ms. Sangeeta Dey and Prof. A.K. Kapoor

Contents:

1. Introduction

2. What is Evolution?

3. Punctuate equilibrium

4. Differential evolutionary rate

5. Homology and Homologous Organs

6. Analogy or Analogous Structures

7. Convergent evolution

8. Parallel evolution

9. Adaptive radiation

10. Vestigial organs

11. Summary

Learning Objectives:

To understand the rate of evolution
To understand the differential evolutionary rate
To describe the phenomena of punctuated equilibrium
To know about factors that leads to adaptive radiation
To understand homologous, analogous and vestigial organs in the context of evolution rate.
To understand the importance of convergence and parallelism

1. Introduction

The understanding of evolutionary history of any group of animals is of fundamental importance and also tells us the time of events in evolution. Thus it is essential to determine the sequence of events in the past. Similarly the rate of evolution is also important. The rate at which evolutionary changes occur cannot be determined directly. However, evolutionary events fit in a chronological frame work provided by the sequence of geological and paleontological events. Therefore in order to determine the rates of evolution of animals we must clearly understand as to what is meant by evolutionary rates. In the literature three definitions of evolutionary change are available. According to the first definition, evolution may be viewed as the change in the genetic composition of the primate populations. The second definition suggests that evolution is the morphological differentiation exhibited by a set of animals. The third definition of evolution is the progressive diversification of texa in a larger taxonomic set (Buettner – Janusch, 1969). Each of the above definitions implies a different criterion for determining the rate of evolution.

2. What is Evolution?

Evolution is an incredibly common phenomenon and may occur between every generation for every group of organisms in the world, including humans. Over a period of time, the relative proportions of alleles in the population changes, some may increase and some may decrease and still others may remain the same. Over a short run of just a few generations, such changes in inherited traits may be very small, if further continued and elaborated, the results can and do produce spectacular kinds of adaptation and whole new varieties of life emerges due to these changes. These short term effects and variation in allele frequency in a population from one generation to the next is referred to as microevolution whereas the long term effects through fossil history are sometimes called macroevolution. However, the basic evolutionary mechanisms are similar.

Darwin saw evolution as the gradual unfolding of new varieties of life from previous forms over long periods of time due to the evolutionary process. But these long term effects can only come out by accumulation of many small evolutionary changes occurring in every generation. Today we study evolutionary changes occurring between generations and are able to demonstrate how evolution works. We define evolution from the modern genetic perspective as a change in allele frequency from one generation to the next generation.

Allele frequencies are numerical indicators of the genetic makeup of a population, and the population is referred as an interbreeding group of individuals. An inherited trait may be present in slightly different form in different individuals. The variant genes that underlie these different forms in inherited trait are called alleles. The different expression of the inherited traits is the result of genetic variation within a population. The frequencies for combination of genes represent the proportions of a total and hence allele frequencies refer to only the whole groups of individuals i.e. populations. Individuals do not have the allelic frequencies; they have the genes or the combination of these genes. Therefore an individual cannot evolve, only a group of individuals can evolve over time.

Genetic variation is the morphological or physiological changes in an organism that takes place due to change in the genetic composition or in the environment. The environment never remains the same but it changes due to several factors. So, in order to adapt with the changing environment, the variations in an organisms are necessary to combat the harsh environmental conditions and thus survival. If there are no variations in an individual, then they cannot able to adapt accordingly with the changing environment and ultimately die. The presence of variability ensures the fitness of the individuals who are well adapted to the environment due to their heritable qualities in a population. The interaction of variation and natural selection causes evolution to occur.

How do allelic frequencies change? Or how does evolution occur? The modern theory of evolution isolates general factors that can produce alterations in allele frequencies. These factors are responsible for the introduction of variant genotypes within a population and due to this, different phenotypes evolve and depending on the suitable phenotype, the population adapt and compete for survival. Thus, survival of the fittest occurs and they able to produce more offspring who possess adaptable phenotypes and thus those phenotypes are selected over generations by natural selection.

Somatic Evolution: All the evolutionary changes that occur in the stroma of the cells of an animal or plant other than the reproductive cells are termed as somatic evolution. Thus a somatic mutation is one that is not heritable. It is the evolution which an individual acquire due to adaptation to changing environment in the phenotypic or morphological appearance. The studying of the somatically adapted structures sometimes directs the selection processes to act and thus results in evolution.

3. Punctuated Equilibrium

A hypothesis, published in 1972 by N. Eldredge and Stephen J. Gould, proposing that in evolutionary history most change occurs very rapidly in short bursts lasting typically less than 100,000 years and is associated with speciation events. In between these speciation events are long periods (perhaps million of years) of relative stasis, in which little evolutionary change occurs. This hypothesis, which contradicted the orthodox Darwinian view of evolution as a gradual and continuous process, prompted controversy and often heated debate. The authors based their hypothesis on studies of various fossil lineages (e,g. ammonite molluscs) in which forms intermediate between species are absent, citing this as evidence that speciation events are often so brief as not to be represented in the fossil record. Subsequent scrutiny of the evidence supports a pattern of punctuated equilibrium for some, but not all lineages, so it cannot be regarded as universal. For example, the rodent lineage shows as much morphological change between speciation events as during speciation.

4. Differential Evolutionary Rate

An evolutionary rate is the description of dynamic changes that occur in a lineage across many generations, the rate of evolution and its dynamic changes are crucial for man because it depicts that humans rate of evolution is faster than other animal species. The dynamic changes of evolutionary factors may be in the genome itself or in the phenotypic expression of the underlying genetic events.

The problems involved in the measurement of the rate of evolution has been discussed by Simpson (1944 and 1949). One of the satisfactory methods to assess the rate of evolution is to measure the amount of genetic change. However, this is quite impractical due to obvious reasons and difficulties. Therefore the ideal method to find the rate of evolution is the measurement of the amount of structural change as is done in case of the discovered fossil remains. In studying the fossil remains of organisms, we focus the structural and phenotypic changes that occur with respect to the ancestral forms or the succeeding generations.

The genetic make-up of any individual carries the coded instructions or information pertaining to living and also the variations that occur due to the evolution. Thus, if these genetic instructions are decoded one can learn how lives are lived in the past. This is easily done by studying the phenotypes and their means of homeostasis. Thus the study of changes in the characteristics of the developed organisms in successive generations is a more direct method of study of rate of evolution than to study the changes in the DNA responsible for the change.

Thus the morphological studies on various organisms should be considered to evaluate the rate of evolution as it indicates that these are constructed on the same basic plan. The minor differences seen in some forms are the adaptive modifications to the diverse mode of living. This can be studied under the following heads –

Homology and Homologous Organs
Analogy or Analogous Structures
Convergent evolution
Parallel evolution
Adaptive radiation
Vestigial organs

5. Homology and Homologous Organs

The organs of similar structure and origin but dissimilar in function and form are called as homologous organs and this phenomena is known as homology. The presence of homologous organs implies a common evolutionary origin of amphibians, reptiles, birds and mammals from one ancient fish ancestor. The homologous structures seen in successive generations indicate actual relationship between the ancestor and the descendants as well as it depicts that the processors are the diverse descendants of common ancestry and thus signifies the rate of evolution.

Example:

The forelimbs of a frog, the wings of a bird, leg of a horse, the hand of a man and the flipper of a whale are homologous organs because all of them have similar pattern of basic plan (pentadactyl) i.e. same number of bones, muscles, nerves and blood vessels etc. but they do the different functions such as hopping (frog), flying (bird), running (horse), grasping (man) and swimming (whale).
Phylloclade of Opuntia and cladode of Ruscus are homologous organs as both are modified stems. Similarly, a thorn of Bougainvillea and a tendril of Cucurbita are homologous as both arise in axillary position.

6. Analogy and Analogous Structures

The organ that perform the same function but differ in their origin and structure are called analogous organs and the phenomenon is called analogy. The wings of an insect are analogous to those of birds and bats because they perform the same function but have dissimilar structure and origin. The wings of an insect are modified outgrowth of the body wall whereas wings of birds and bats are modified forelimbs. These organs have arisen in evolutionary process through adaptation of quite different organisms to a similar mode of life.

Example:

The wings of an insect are analogous to those of birds and bats because they perform the same function but have dissimilar structure and origin. The wings of an insect are modified outgrowth of the body wall whereas wings of birds and bats are modified forelimbs.
Potato and sweet potato are analogous organs as both perform the same function of storage of food but they differ in their structure. Potato is an underground-modified stem whereas sweet potato is a modified advantageous root.
Fins of fishes and flippers of whales are analogous organs because both perform the function of swimming but the flippers of whale are pentadactyl and fins of fishes are not pentadactyl.
Stings of honey bee and scorpion are analogous structures as both perform the same function. The sting of honey bee is modified ovipositor whereas in scorpion, it is the modified last abdominal segment.
The eye of an octopus and the eye of a mammal differ in their retinal position but both perform the same function. Similarly, the flippers of penguin (bird) and dolphin (mammal) that perform similar functions in these aquatic animals have originated from different structures of two different lineages.

7. Convergent Evolution

In some situations, similarities are not the result of being in the line of descent or of common origin, for some organisms may be alike in living habits and appearance, although of different ancestries. Such similar evolutionary development in different forms is termed as convergence. Convergence refers to the development of similar characteristics or adaptations in animals that differ in direct ancestry. The humming bird and the humming moth, for example, have converged in their flying habits as a result of their common search for nectar in flowers as a source of food. Convergence ordinarily applies to one or a few characteristics rather than to the overall makeup. Similarities in the retina, the layer of visual cells in the eyes, of some quite different nocturnal animals are an example. Of the two main types of retinal cells (rods and cones), only rods, which are more sensitive to dim light, are present in some deep-sea fish, bats, some lizards and snakes, and probably guinea pigs, whales and some lemurs. All these animals, however, differ markedly from each other in respect of other characteristics less directly related to their adaptation to low light conditions. It is improbable that any instance of evolutionary convergence has been as dramatic and complete as to hide all traces of the diversity of origins. A number of similarities between tarsier and human skulls once were thought by some to demonstrate that the tarsier, not the great apes, was the closest living primates relative to humans. This is now known to be a convergence caused by the fact that that the tarsier and we have both evolved large orbits for large eyes, along with small noses, as less dependence on the sense of smell was the trend.

8. Parallel evolution

An evolutionary development similar to convergence, but in related forms is parallelism. Parallelism implies a similarity in biological makeup of the ancestral forms, whereas convergence does not. If the common ancestor of two organisms were not very ancient, and if evolution in the descendant line followed more or less the same course, the term parallelism is used. The term is usually applied to two species of organisms that were similar in origin, and that remained similar as they evolved like having some of the same changes occurring in both of them even after they have separated and evolved into two different species. The old world and new world monkeys provide an excellent example of parallelism between groups living today, since they appear to have evolved in parallel from a prosimian ancestor that probably lived at least 35 million years ago (MYA).

The reason for parallelism as well as convergence is the same. The organisms, in order to survive in similar environment, must develop similar biological structures. Parallelism, like convergence is a matter of adaptation under the control of natural selection. The lack of tail in gibbons, on the one hand, and the great apes and humans on the other is probably a case of parallelism since their common ancestor probably has tails that were lost in a parallel fashion in the separate evolutionary lines after they diverged. All the monkeys, however, have nails. The cercopithecidae, the monkeys most closely related to humans and apes, are quite varied in tail length and those species with similar tail lengths are not most closely related to each other. The tail is a functionally important feature which members used for balance, and very diverse species of Cercopithecidae (for instance: the Colobus monkey and the vervets) are both arboreal and have long tails, probably as a parallel evolutionary adaptation to arboreal quadrupedal locomotion. Similarly, the reduction of the tail in the brachiating gibbons and the terrestrial Hominoidea is probably a parallel response to locomotor requirements.

9. Adaptive radiation

The process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation or in other words the evolutionary spread and differentiation of one type of animal, of whatever level of classification is called adaptive radiation. Unlike parallelism and convergence which refers to the way a particular species evolved progressively dissimilar environments and opportunities, and rapid changes in the external environment may cause new form of animals to develop from a single ancestral form. The evolution of a trait that opens up many new possibilities may also give rise to adaptive radiation.

Adaptive radiation is well exemplified by the history of the mammals. With the geological revolution that marked the end of the Mesozoic era (the age of the reptiles) and the start of the Tertiary, the previously stable climates became more changeable. The dinosaurs did not adapt and so became extinct, while the mammals evolved in many distinct lines. The rodents specialized for gnawing, the carnivores for hunting, the hoofed animals for grazing, the primates and sloths took to the trees; the whales, seals and sea cows adapted for life in the oceans, and the bat took to the air. Further, each of these mammalian orders in turn gave rise to sub lines that colonized new environments by acquiring new modes of life. Many of today’s mammals are far different from their primitive common ancestors of the Paleocene epoch.

In addition, various orders and suborders of mammals have undergone further differentiation, branching or “radiating” into types adapted to different habitats. Thus, all the chief groups of primates today include species with contrasting dietary habits. Insect eating, seed eating, leaf eating, and more or less omnivorous genera recurred in different branches of the primates as these branches departed more or less from the ancestral line. This adaptive radiation within the branches is thus accompanied by parallelism between the branches and by convergence of adaptations towards those of some non-primate lines.

10. Vestigial Organs

The vestigial or rudimentary organs are the reduced and functionless remnants of structures or organs which are of no use to the processor but they still persist from generation after generation in reduced form in an individual. They were complete and functional in the ancestors e.g. appendix in man is considered as remnant of large intestine (caecum) but it is considered to be storage organ for cellulose digestion in herbivorous mammals. The vestigial organs reveal strong evidence of evolution. They are the remnants of organ which used to perform a normal function in the ancestor but during the course of evolution, they have been reduced to vestiges.

Examples:

Vermiform appendix in man
Auricular muscles of external ear in man
Nictitating membrane
Vestigial tail vertebrae
The lobe of ear
Wisdom teeth
Canines in man reduced in size
Mammary glands in males
Body hair.

11. Summary

The evolution of the species tends to be in constant and asymmetrical over successive generations. That is, it may be rapid at one time and slow at another. In rare cases, it may even virtually stop altogether. At one time, evolution may affect the limbs, at another it may affect the jaws. This variability in the tempo of evolution of different anatomical structures in the same line makes it unwise to draw conclusions concerning the relationship of two fossil forms on the basis of a single characteristic. Instead it is necessary to follow the evolution of the whole functional systems. Since the systems themselves evolve at different rates. However, information can be drawn by following the history of the single by which it is possible that the course of change of each feature may be different and not comparable even in related lineages. In other circumstance one may relate the rate of change to the variance of character. But the variance of different characters is itself variable and is probably one of the factors controlling evolution. Thus if the number of new species or genera or families increases progressively over a given period, it may be assumed that the rate of evolution is also increasing. It is matter of considerable importance to recognize that the progressive modifications of different systems of the body (body size, brain, teeth and jaw, limb proportion) during the evolutionary development of any one group may proceed at differential rates. This is well recognized by the paleontologists. On the basis of a qualitative study of the evolution of the Equidae, Simpson has postulated the following two theorems concerning the rates of evolution:

The rate of evolution of any character or combination of characters may markedly change at any time during the phyletic evolution, even though the direction of evolution remains the same.
The rates of evolution of two or more characters within a single phylum may change independently.

Such differential rates of somatic evolution may result into structural contrasts which may be regarded as disharmonies because they do not confirm with similar correlations as observed in the context of living species. In such circumstances the true affinities of such fossil forms may thus be overlooked and misinterpreted. It is therefore necessary that differential rates of somatic evolution must be taken into account while selecting characters for their taxonomic relevance in the assessment of the phylogenetic status of the fossil types.

you can view video on Differential rate of Somatic Evolution

Suggested Readings