You should have a working knowledge of the following terms:
- allometric growth
- homeotic genes
Introduction and Goals
Previously we examined microevolutionary events that account for changes in allele frequencies between generations within a population. For example, microevolutionary events can account for changes in the beak size of finches, but what about more pronounced changes? How does evolution explain the appearance of wings, lungs, or for that matter, bird beaks themselves? The answer to how major changes come about is complex and involves developmental events. By the conclusion of this tutorial you should have a basic understanding of:
- The distinction between a microevolutionary event and a macroevolutionary event
- How genes can affect the timing of developmental events and the misplacement of organs
This figure shows the similarities between embryos of a dog, an alligator, and a human. Note how similar they are at this early stage of development. Can you identify the human? Obviously, later on in development you wouldn't have this difficulty.
This sequence of embryos shows that all vertebrates share a common set of genes that are expressed similarly during early development. As development progresses, however, different genes are expressed in different ways; the net result at birth is three very different phenotypes, each having the unique characteristics of their respective species.
Figure. Similarities between vertebrate embryos. Which is the dog, the alligator, the human? (Click image to enlarge)
Simply stated, those genes whose expression distinguish one species from other species are genes that affect one or more macroevolutionary processes. In other words, microevolution describes changes in the genetic makeup within a species, whereas macroevolution describes changes in the genetic makeup between species.
Macroevolution, as the name implies, describes big changes, hence they are of great interest to evolutionary biologists. However, it would be a mistake to think that major structures (e.g., wings, legs, beaks) arose at a given instant in time. Fossil records support the idea that these modern structures have their ancestral roots in far more primitive structures. For example, many changes likely contributed to the form of bird wings, and there were many macroevoutionary events that gave rise to the modern bird wings observed today.
Many evolutionary biologists think that modern birds evolved from a dinosaur lineage. There are many lines of evidence to support this idea. One such line of evidence is the presence of wings (and feathers) on some dinosaur fossils. However, the ancestral function of feathers probably did not include flight. Structures that arise and are used in one context but in another context have different or additional functions, are termed exaptations. Go to the following site and return prepared to answer what function these primitive feathers might have served. The little feathered dinosaur fossil
Development and Evolution (aka Evo-Devo)
Changes in the genes that control development are very significant in macroevolution. At first this may seem counter intuitive because development describes irreversible changes in an organism from fertilization to death, whereas evolution describes changes in populations between generations. How then do evolutionary biologists attribute developmental events to explain macroevolution?
The answer is that some genes that affect critical developmental processes, after mutation, result in dramatic phenotypic changes. As you will learn, mutations in these critical genes, which are expressed early in development, can profoundly affect the adult phenotype.
Initially it was thought by evo-devo researchers (those who study the role that development plays in evolution) that these mutations resulted in "hopeful monsters." It was proposed, that in a single step, a mutation in a developmental gene could result in a new species. This idea is no longer held by most workers in the field. Instead, available data indicate that a series of changes, in one or more developmentally significant genes, can gradually lead to macroevolutionay events that give rise to a new species.
Different classes of developmental genes can have major affects on an organism. Keep in mind that none of these events necessarily result in macroevolution; rather, they may contribute, in various ways, to the appearance of a new species.
Homeotic genes determine the placement of body parts. Most insects have two pairs of wings, however, flies have one set of flying wings and one set of small balancing wings (halteres). A single mutation in the gene bithorax will result in a fly with two complete sets of flying wings (shown here). This mutation results in an organ appearing in the wrong place. The general term for any mutation that results in a misplaced organ is homeosis. Another homeotic mutation in flies includes the trait of legs growing out of eyes.
All animals, including humans, have homeotic genes. One of the most evolutionarily conserved homeotic genes is the hox gene. The various alleles of this gene can have a profound influence on development. In the next two questions you will explore how mutations in the human hox gene can affect human development.
Figure. Homeosis. (Click image to enlarge) Normally, the second thoracic segment of the fly produces a pair of wings on the dorsal side and a pair of legs on the ventral side of the animal. The third thoracic segment produces just a pair of legs on the ventral side of the animal. A mutation in the bithorax locus causes a homeotic transformation of the third thoracic segment into the second thoracic segment so that the fly now has two pairs of wings.
Development is a progressive process, therefore, it should not surprise you that certain developmental genes regulate the timing of certain events. There are a variety of genes whose mutant alleles alter the timing of various developmental events. Just as homeotic genes alter the position of an organ, these developmental genes alter when things happen. Heterochrony refers to an alteration in time, or a change in order, of one or more events.
There are various types of heterochronic alterations. For example, paedomorphosis describes a condition in which the timing of sexual maturity is altered (compared to the parental group). The result is a morphologically juvenile individual that is sexually mature and capable of reproduction. Some salamanders are paedomorphic; the axolotl (shown here) can grow to full size and reproduce even though it looks larval. (It retains its gills and other juvenile features, including living in the water.) In this way, adult axolotls can look very different from closely related salamanders that undergo complete maturation.
Figure. An axolotl. (Click image to enlarge) Axolotls retain juvenile features into adulthood. External gills are a juvenile character.
Growth is the irreversible increase in volume. In some cases the growth of different organ parts is isometric, resulting in a mature organ that has the same shape as it had in the juvenile state. However, many organs show disproportionate growth (allometric growth). In allometric growth, not all parts of the organ and/or organism grow at the same rate. The result is an adult structure that is shaped differently than the juvenile structure, exemplified by many human secondary sex characteristics. The genetic basis of allometric growth involves differential timing of genes; therefore, allometric growth is a form of heterochrony.
Allometric growth can occur at the cell, tissue, organ, and organismal level. The first left figure depicts growth proportions of the human body. As one can see, many features exhibit allometric growth. For example, at birth the head is about 25% of the body's length, but at maturity it is about 12% (or less) of its length. Also, note that extremities grow disproportionately. The other figure depicts changes in the shape of human and chimp skulls during development.
Allometric Growth in a Human Female
Allometric Growth in the skull of a human and a chimpanzee
Earth's Life History is Marked by Macroevolutionary Changes
These figures display the eras in the Earth's history (the geological time scale will be discussed in future tutorials). The Cambrian period (570 - 505 million years ago) represents a relatively small window of time (geologically speaking), but most modern groups of animals appear in the fossil record in the beginning of the Cambrian period. Indeed, the period is defined by a tremendous "explosion" of new species. In this period (545-525 million year ago), animals diversified rapidly. Fossil evidence suggests that some of these animals were unlike anything roaming Earth today. Below is an artist's rendition of what the ancient oceans might have looked like during the early Ordovician, which followed the Cambrian explosion.
Early Geological Eras
Recent Geological Eras
A display from UC Berkeley's Museum of Paleontology.
It is not clear why this incredible diversity of animals appeared at this time in Earth's history. Some evolutionary biologists think that this period marked the beginnings of complex predator/prey relationships. The predator's motility and sensory apparatuses became more complex, and the adaptive "response" of prey involved a similar increase in sophistication to counter these measures.
Although the reason for such a great diversification is debated, it is clear that during the Cambrian explosion macroevolution was taking place on a grand scale. Bilaterally symmetrical animals were prominent for the first time, as were segmented, highly mobile organisms. As a matter of fact, the half-billion years since have shown morphological changes that are simply refinements of these Cambrian body plans.
One of the most abundant sources of fossils from the Cambrian period was discovered in British Columbia and is known as the Burgess Shale. This site, now designated as a World Heritage Site, provides the best examples of fossils from the Cambrian explosion.
In some rare instances, macroevolutionary events occur abruptly and give rise to a new species. However, many evolutionary biologists think that the majority of speciation events occur over extended time periods. Hence, many, if not most species, arise gradually as microevolutionary events accumulate until such a time as new opportunities cause isolating events (e.g., adaptive radiation).
Although macroevolutionary events play a defining role in speciation, microevolutionary events, acting over extended periods of time (especially with strong selection), may contribute to macroevolutionary changes. The underlying genetic processes of microevolution and macroevolution are the same. However, some genetic changes (e.g., alteration in pivotal developmental processes) are more likely to result in a rapid and dramatic change in the phenotype, which will result more quickly in a new species. For example, a heterochronic mutation event, in which certain members who carry the altered allele can reproduce earlier in life, may confer a selective advantage and the allele will increase in frequency over time (microevolution). At some later point an isolation event may separate these early reproducers from late reproducers and, over time, the two populations may drift apart and fill separate niches; over hundreds or thousands of generations, the two populations may become incapable of interbreeding and a new species arises (macroevolution).
The data indicate that macroevolution rarely happens in one giant step. Wings likely arose over an extended period of time, in a stepwise fashion. Lastly, evolution has no predestined pathway. For example, the fossil evidence indicates that feathered limbs appeared before flight; only later did feathers appear on wings. This example of exaptation, in which feathers arose in one context and later became adapted for an additional function, is common. In other words, evolution largely works on existing structures and processes. Through the process of natural selection, new ways of adapting old structures to new conditions allow certain populations to be come better adapted to a new environment, and hence become more fit. Over time, with repeated selection, these changes can become significantly different, so as to appear as major features of a new species.