Natural selection is the motor behind evolution. It was the insight of both Darwin and Wallace, and it provides a mechanism for how descent with modification from a common ancestor may occur. Natural selection is a biological law that can be summarized as follows: 1) all populations vary; 2) all populations produce more offspring than can survive; and 3) individuals within populations with traits that allow them to successfully mate and reproduce viable offspring will be selected for. Having viable offspring means having children that can themselves successfully reproduce. Natural selection is sometimes, unfortunately and mistakenly, referred to as “Survival of the Fittest,” a statement that ignores the chance involved in survival. Many of the great vertebrate extinction events have had less to do with “fitness” than with the effect of rapid environmental change on organisms that are well-adapted to the previously stable environment.
Within a given population, different individuals will possess different traits or characteristics. Those animals with traits that allow them to survive to reproductive age in a given environment, and to therefore mate and pass on these inheritable traits, are “selected for.” In other words, natural selection is simply the differential survival of animals with any combination of traits that allow them reproduce viable offspring.
We now understand, as Darwin, Wallace, and their contemporaries did not, that descent with modification is due to the inherited mutations (modification) of genes, the molecular units of heredity. Genes ultimately produce the physical traits we see in animals. A brief review of genetics is in order here. We now know that DNA is the universal code stored in the nucleus of animal cells, and it acts as a library which can be read. The books in the library are genes, and they must be read (transcribed) by messenger RNA (mRNA). The transcribed gene message is then translated from the mRNA by cellular machinery (such as ribosomes and other forms of RNA) into an amino acid sequence that becomes modified into a protein. When we say a gene is “expressed,” we mean a protein has been generated from a gene’s code. If you remember nothing else from this genetic lesson, remember this: it is the proteins that cause cells, tissues, and organs to take on their characteristic features. Mutations in genes, therefore, ultimately lead to the expression of different or modified proteins that can cause subtle to major changes in an organism’s anatomy and future success in passing on its genes to the next generation. This means that the shape of the skeleton is impacted by which genes are and are not expressed.
Recently, research into embryonic development has given us an even better insight into how major structural changes might occur in a given population of organisms. We now understand that there are two major types of genes: developmental and “house-keeping” genes. Developmental genes are those that are expressed during embryonic development, and their proteins control the symmetry, skeletal development, organ placement, and overall form of the developing animal. In contrast, “house-keeping” genes are expressed during the animal’s daily life to generate proteins which keep the cells, tissues, and organs in the body functioning properly. As you might suspect, mutations in developmental genes can have radical consequences for body form and function, whereas mutations in “house-keeping” genes tend to affect the health and reproductive success of the post-embryonic animal.
Let us reiterate natural selection, now with genes in mind. Within a given population, different individuals will possess different gene combinations. Those with genes that, when expressed, allow them to survive to reproductive age in a given environment, and to therefore mate and pass on their genetic inheritance, are “selected for.” Again, natural selection is simply the differential survival of animals with any combination of developmental and “house-keeping” genes that allow them reproduce viable offspring. Only genes which are passed on in the gametes of vertebrates effect evolution: the sperm or eggs must carry the modified, duplicated, or mutated developmental or “house-keeping” genes.
It should be emphasized that evolution occurs at the level of the population, not the individual. The individual is born into the world with a particular mixture of traits, but that individual does not and cannot evolve. For example, if a predator attacks a population of a particular species, the individuals that survive are those who already have traits to fight or escape – an individual without these traits cannot “evolve” them in response to this threat. In our hypothetical population attacked by a predator, faster animals with long legs may survive more often than slower animals with stubby legs. Thus, more of the faster individuals will pass on their traits than the slower ones. However, the slower individuals cannot “evolve” longer legs or faster speeds – they will simply be selected against. Over time, the population will evolve so that the average individual is relatively fast simply because individuals with this trait tend to survive and reproduce whereas slow individuals tend be eaten before reproduction. Thought of yet another way, biological evolution and natural selection boil down to sex and time against a given environment. In a particular environment, inherited combinations of genes will allow some to survive to reproduce viable offspring, whereas other individuals will be less successful at these tasks. The shape and form of the vertebrate skeleton are a testament to all of these processes.