21.1: Introduction

Natural Selection

In Bio 4A you studied genetics and the biological explanations for variation of traits in living things. Recall that Mendel described “particles” that were passed from parent to offspring. We now know those particles were genes, and that variation in the traits he studied was the result of inheriting different variants, alleles of each gene. It is important to have a good foundation in genetics to understand evolution.

Evolution is formally defined as a change in allele frequency over time. In other words, if you analyze a population of organisms today, and then sometime in the future, you may note a shift in the percentages of specific alleles. Since evolution is merely a change in allele frequency over time, then all life is constantly evolving. What fuels these changes?

The principal process involved in evolutionary change is natural selection, which is defined as “differential survival and/or reproduction.”

There are three requirements that must be met for natural selection to actually take place:

1. There must be variation among the members in the population for the trait in question
2. The variants of the trait must result in differences in survival and/or reproduction.
3. The trait in question must be controlled to some degree by genes (e.g., when you dye your hair, this new variant is not controlled by genes).

There is one well-known classic case of natural selection: melanism mutation in peppered moths. Prior to the Industrial Revolution in England, nearly every peppered moth had light-colored speckled wings, only some months had the mutated black peppered phenotype. This coloration enabled the moths to blend in on the lichen-covered tree trunks. The much rarer black peppered moths were so obvious that birds (the primary killer [i.e., the important "selector"] of peppered moths) easily caught and killed them. The result is that these forms differed in survival probability and thus the light-colored moths survived and reproduced. Black moths were only maintained because of the rare mutation.

The Industrial Revolution was responsible for the production of huge amounts of soot (from factory smokestacks). The soot was produced so rapidly that it covered the once-light-colored tree trunks. The now-dark tree trunks exposed the light-colored moths and the dark moths now were blending in. The birds now caught mostly light-colored moths, thus dark moths had higher survival and reproduction. Because they were producing more offspring (which had better survival probability), the dark moths now became abundant. The population now contained a larger fraction of dark moths, and this follows the definition of evolution (i.e., change in allele frequencies).

Note that if no variation exists in a population, then natural selection cannot work. Other modes of evolution exist as well (mutation, genetic drift, migration). These can work alone or in conjunction with one another:

Mutations

in a broad sense, is the only source of evolution novelties. However, most of mutations are results of somehow broken DNA and therefore probability of useful mutation is similar to probability to upgrade your smartphone with a hammer.

Migration

brings new genotypes to the population and therefore able to change the local course of evolution.

Genetic drift

is usually a result of catastrophic factors applied to small populations. The smaller is a population, the bigger is probability that it will simply die out. Another name of this thing is an evolution bottleneck: few will survive, and these few will be chosen not because they are better but simply by random.

Phylogenetics and Cladistics

In scientific terms, phylogeny is the evolutionary history and relationship of an organism or group of organisms. A phylogeny describes the organism's relationships, such as from which organisms it may have evolved, or to which species it is most closely related. Phylogenetic relationships provide information on shared ancestry but not necessarily on how organisms are similar or different.

Phylogenetic Trees

Scientists use a tool called a phylogenetic tree to show the evolutionary pathways and connections among organisms. A phylogenetic tree is a diagram used to reflect evolutionary relationships among organisms or groups of organisms. Scientists consider phylogenetic trees to be a hypothesis of the evolutionary past since one cannot go back to confirm the proposed relationships. In other words, we can construct a “tree of life” to illustrate when different organisms evolved and to show the relationships among different organisms (Figure 20.2).

Unlike a taxonomic classification diagram, we can read a phylogenetic tree like a map of evolutionary history. Many phylogenetic trees have a single lineage at the base representing a common ancestor. Scientists call such trees rooted, which means there is a single ancestral lineage (typically drawn from the bottom or left) to which all organisms represented in the diagram relate. Notice in the rooted phylogenetic tree that the three domains— Bacteria, Archaea, and Eukarya—diverge from a single point and branch off. The small branch that plants and animals (including humans) occupy in this diagram shows how recent and miniscule these groups are compared with other organisms. Unrooted trees do not show a common ancestor but do show relationships among species.

Figure \(\PageIndex{1}\): Both of these phylogenetic trees show the relationship of the three domains of life—Bacteria, Archaea, and Eukarya—but the (a) rooted tree attempts to identify when various species diverged from a common ancestor while the (b) unrooted tree does not. (credit a: modification of work by Eric Gaba)

In a rooted tree, the branching indicates evolutionary relationships (Figure \(\PageIndex{2}\):). The point where a split occurs, a branch point, represents where a single lineage evolved into a distinct new one. We call a lineage that evolved early from the root that remains unbranched a basal taxon. We call two lineages stemming from the same branch point sister taxa. A branch with more than two lineages is a polytomy and serves to illustrate where scientists have not definitively determined all of the relationships. Note that although sister taxa and polytomy do share an ancestor, it does not mean that the groups of organisms split or evolved from each other. Organisms in two taxa may have split at a specific branch point, but neither taxon gave rise to the other.

Figure \(\PageIndex{2}\): A phylogenetic tree's root indicates that an ancestral lineage gave rise to all organisms on the tree. A branch point indicates where two lineages diverged. A lineage that evolved early and remains unbranched is a basal taxon. When two lineages stem from the same branch point, they are sister taxa. A branch with more than two lineages is a polytomy.

The diagrams above can serve as a pathway to understanding evolutionary history. We can trace the pathway from the origin of life to any individual species by navigating through the evolutionary branches between the two points. Also, by starting with a single species and tracing back towards the "trunk" of the tree, one can discover species' ancestors, as well as where lineages share a common ancestry. In addition, we can use the tree to study entire groups of organisms.

Another point to mention on phylogenetic tree structure is that rotation at branch points does not change the information. For example, if a branch point rotated and the taxon order changed, this would not alter the information because each taxon's evolution from the branch point was independent of the other.

Many disciplines within the study of biology contribute to understanding how past and present life evolved over time; these disciplines together contribute to building, updating, and maintaining the “tree of life.” Systematics is the field that scientists use to organize and classify organisms based on evolutionary relationships. Researchers may use data from fossils, from studying the body part structures, or molecules that an organism uses, and DNA analysis. By combining data from many sources, scientists can construct an organism's phylogeny. Since phylogenetic trees are hypotheses, they will continue to change as researchers discover new types of life and learn new information.

Limitations of Phylogenetic Trees

It may be easy to assume that more closely related organisms look more alike, and while this is often the case, it is not always true. If two closely related lineages evolved under significantly varied surroundings, it is possible for the two groups to appear more different than other groups that are not as closely related. For example, the phylogenetic tree in Figure \(\PageIndex{3}\): shows that lizards and rabbits both have amniotic eggs; whereas, frogs do not. Yet lizards and frogs appear more similar than lizards and rabbits.

Figure \(\PageIndex{3}\): An organism that lacked a vertebral column roots this ladder-like phylogenetic tree of vertebrates. At each branch point, scientists place organisms with different characters in different groups based on shared characteristics.

Another aspect of phylogenetic trees is that, unless otherwise indicated, the branches do not account for length of time, only the evolutionary order. In other words, a branch's length does not typically mean more time passed, nor does a short branch mean less time passed— unless specified on the diagram. For example, in Figure 20.4, the tree does not indicate how much time passed between the evolution of amniotic eggs and hair. What the tree does show is the order in which things took place. Again using Figure 20.4, the tree shows that the oldest trait is the vertebral column, followed by hinged jaws, and so forth. Remember that any phylogenetic tree is a part of the greater whole, and like a real tree, it does not grow in only one direction after a new branch develops. Thus, for the organisms in Figure 20.4, just because a vertebral column evolved does not mean that invertebrate evolution ceased. It only means that a new branch formed. Also, groups that are not closely related, but evolve under similar conditions, may appear more phenotypically similar to each other than to a close relative.