Taxa evolve from common ancestors and then diversify. Scientists use the phrase “descent with modification” because even though related organisms have many of the same characteristics and genetic codes, changes occur. This pattern repeats as one goes through the phylogenetic tree of life:

1. A change in an organism’s genetic makeup leads to a new trait which becomes prevalent in the group.
2. Many organisms descend from this point and have this trait.
3. New variations continue to arise: some are adaptive and persist, leading to new traits.
4. With new traits, a new branch point is determined (go back to step 1 and repeat).

Phylogenetic trees can show when certain traits evolved and which species have those traits. For example, Figure 10.1 shows a tree with several animal species. The labeled dashed lines show when different traits evolved. For example, lungs evolved within a species that was a common ancestor of antelopes, bald eagles, and alligators; this means present-day antelopes, bald eagles, and alligators have lungs. We also learn from that tick mark that lamprey and sea bass do not have lungs. Fur evolved within an ancestor of modern-day antelopes. Only antelopes have fur – all other species depicted on this phylogeny do not. We can also see that all species except lamprey have jaws. Note that tick marks may also show where a trait is lost – there aren’t any tick marks like that in Figure 10.1, but many trees do show trait losses, because traits can be both gained and lost over evolutionary time.

Ancestral and derived traits

Clades are defined as being all of the descendants of a particular ancestor. If an ancestor of a clade had a particular trait, then all of the descendants will have that trait (unless the phylogeny indicates it was lost through evolution). For this clade, this would be an ancestral trait because it evolved before the ancestor of the clade. In figure 10.2, the amniotic egg is ancestral to the Amniota clade. All of the members of the Amniota clade have an amniotic egg because the ancestor of the clade had this trait. However, the amniotic egg is not ancestral to the Vertebrata clade because it evolved more recently than (i.e., after) the ancestor of the Vertebrata clade. Thus, some of the members of the Vertebrata clade have amniotic eggs, but not all of them. This makes amniotic egg a derived trait with respect to the Vertebrata clade. Derived traits are traits that evolved after the ancestor of a clade, and thus are possessed by only some, not all, of the members of the clade.

When traits are unique to a particular clade, they are useful for identifying and distinguishing which organisms belong to that clade. This special type of trait is called a synapomorphy, sometimes called a shared derived trait. With respect to the Amniota clade, the amniotic egg is a synapomorphy because all of the members of the Amniota have this trait and no species outside of this clade have this trait. Thus, you can conclude that if a species has an amniotic egg, it belongs to the Amniota clade.

If a trait is ancestral to a clade and evolved long before the ancestor of the clade, it is called a symplesiomorphy, sometimes also called a shared ancestral trait. This is because all members of the clade have that trait, but so do members of other clades. With respect tot he Amniota clade, vertebrae would be a symplesiomorphy. The ancestor of the Amniota has a vertebra, and so all members of the Amniota have a vertebrae. However, other organisms that are not members of the Amniota clade, such as the fish and lampey, also have vertebrae. Therefore, while it is true that all Amniota have vertebrae, knowing that an organism has a vertebra does not tell you whether that species is in the Amniota clade or not.

It is important to remember that all these terms (derived, ancestral, synapomorphy, and symplesiomorphy) are relative to the clade being referenced. A trait that is a synapomorphy for one clade could be a symplesiomorphy for a different clade and a trait that is ancestral to one clade could be derived within a larger clade. For example, in Figure 10.2, the amniotic egg is a synapomorphy with respect to the Amniota clade, but derived with respect to the Vertebrata clade. These terms help scientists distinguish between clades in building phylogenetic trees.

Convergent Evolution

The evolution of species has resulted in enormous variation in form and function. Sometimes, evolution gives rise to groups of organisms that become tremendously different from each other. When two species evolve in diverse directions from a common point, it is called divergent evolution. Such divergent evolution can be seen in the forms of the reproductive organs of flowering plants which share the same basic anatomies; however, they can look very different as a result of selection in different physical environments and adaptation to different kinds of pollinators (Fig 11.1).

In other cases, similar phenotypes evolve independently in distantly related species due to those species facing similar selective pressures. For example, flight has evolved in both bats and insects, and they both have structures we refer to as wings, which are adaptations to flight (Fig 10.3). However, bat and insect wings have evolved from very different original structures. We call this phenomenon convergent evolution, where similar traits evolve independently in species that do not share a recent common ancestry. The trait in the two species came to be similar in structure and have the same function, flying, but did so separately from each other.

Homologous and analogous structures

Some organisms may be very closely related, even though a minor genetic change caused a major morphological difference to make them look quite different. Similarly, unrelated organisms may be distantly related, but appear very much alike. This usually happens because both organisms share common adaptations that evolved within similar environmental conditions, called convergent evolution. When similar characteristics occur because of environmental constraints or similar selective pressures, and not due to a close evolutionary relationship, it is an analogy or homoplasy. For example, insects use wings to fly like bats and birds, but the wing structure and embryonic origin is completely different. Thus, wings in bats and birds are a homoplasy (Fig 11.2).

Another example can be seen in arctic mammals such as foxes and snowshoe hares, which each grow white fur during the winter months. White fur allows these organisms to blend into the ice and snow that characterizes their polar home, and presumably protects them from predation. However, foxes and snowshoe hares do not share a common ancestor with white fur. Of course they ultimately share a common ancestor, as do all mammals, but the fox lineage is full of non-white animals, as is the group to which hares belong. The winter white of arctic foxes and snowshoe hares is thus a homoplasy, due to convergent evolution in a white, wintry landscape.

Similar traits can be either homologous or analogous. Homologous structures share a similar embryonic origin due to their deep evolutionary relationship. Analogous structures have a similar function, but often very different developmental pathways. For example, the bones in a whale’s front flipper are homologous to the bones in the human arm. These structures are not analogous. A butterfly or bird’s wings are analogous but not homologous. Scientists must determine which type of similarity a feature exhibits to decipher the organisms’ phylogeny.

We use developmental/embryonic origin as an indicator of homology because development is a very complex process. The more complex the feature, the more likely any kind of overlap is due to a common evolutionary past. Imagine two people from different countries both inventing a car with all the same parts and in exactly the same arrangement without any previous or shared knowledge. That outcome would be highly improbable. However, if two people both invented a hammer, we can reasonably conclude that both could have the original idea without the help of the other. The same relationship between complexity and shared evolutionary history is true for homologous structures in organisms.

Common Misconceptions about Evolution

Although the theory of evolution generated some controversy when Darwin first proposed it, biologists almost universally accepted it, particularly younger biologists, within 20 years after publication of On the Origin of Species. Nevertheless, the theory of evolution is a difficult concept and misconceptions about how it works abound.

Evolution Is Just a Theory

Critics of the theory of evolution dismiss its importance by purposefully confounding the everyday usage of the word “theory” with the way scientists use the word. In science, we understand a “theory” to be a body of thoroughly tested and verified explanations for a set of observations of the natural world. Scientists have a theory of the atom, a theory of gravity, and the theory of relativity, each which describes understood facts about the world. In the same way, the theory of evolution describes facts about the living world. As such, a theory in science has survived significant efforts to discredit it by scientists. In contrast, a “theory” in common vernacular is a word meaning a guess or suggested explanation. This meaning is more akin to the scientific concept of “hypothesis.” When critics of evolution say it is “just a theory,” they are implying that there is little evidence supporting it and that it is still in the process of rigorous testing. This is a mischaracterization.

Individuals Evolve

Evolution is the change in a population’s genetic composition over time, specifically over generations, resulting from differential reproduction of individuals with certain alleles. Individuals do change over their lifetime, obviously, but this is development and involves changes programmed by the set of genes the individual acquired at birth in coordination with the individual’s environment. When thinking about the evolution of a characteristic, it is probably best to think about the change of the average value of the characteristic in the population over time. For example, when natural selection leads to bill-size change in medium ground finches in the Galápagos, this does not mean that individual bills on the finches are changing. If one measures the average bill size among all individuals in the population at one time and then measures them in the population several years later, this average value will be different as a result of evolution. Although some individuals may survive from the first time to the second, they will still have the same bill size; however, there will be many new individuals who contribute to the shift in average bill size.

Evolution Explains the Origin of Life

It is a common misunderstanding that evolution includes an explanation of life’s origins. Some of the theory’s critics believe that it cannot explain the origin of life. The theory does not try to explain the origin of life. The theory of evolution explains how populations change over time and how life diversifies the origin of species. It does not shed light on the beginnings of life including the origins of the first cells, which define life. Importantly, biologists believe that the presence of life on Earth precludes the possibility that the events that led to life on Earth can repeat themselves because the intermediate stages would immediately become food for existing living things.

However, once a mechanism of inheritance was in place in the form of a molecule like DNA either within a cell or pre-cell, these entities would be subject to the principle of natural selection. More effective reproducers would increase in frequency at the expense of inefficient reproducers. While evolution does not explain the origin of life, it may have something to say about some of the processes operating once pre-living entities acquired certain properties.

Organisms Evolve on Purpose

Statements such as “organisms evolve in response to a change in an environment” are quite common, but such statements can lead to two types of misunderstandings. First, do not interpret the statement to mean that individual organisms evolve. The statement is shorthand for “a population evolves in response to a changing environment.” However, a second misunderstanding may arise by interpreting the statement to mean that the evolution is somehow intentional. A changed environment results in some individuals in the population, those with particular phenotypes, benefiting and therefore producing proportionately more offspring than other phenotypes. This results in change in the population if the characteristics are genetically determined.

It is also important to understand that the variation that natural selection works on is already in a population and does not arise in response to an environmental change. For example, applying antibiotics to a population of bacteria will, over time, select a population of bacteria that are resistant to antibiotics. The resistance, which a gene causes, did not arise by mutation because of applying the antibiotic. The gene for resistance was already present in the bacteria’s gene pool, likely at a low frequency. The antibiotic, which kills the bacterial cells without the resistance gene, strongly selects individuals that are resistant, since these would be the only ones that survived and divided. Experiments have demonstrated that mutations for antibiotic resistance do not arise as a result of antibiotic application.

In a larger sense, evolution is not goal directed. Species do not become “better” over time. They simply track their changing environment with adaptations that maximize their reproduction in a particular environment at a particular time. Evolution has no goal of making faster, bigger, more complex, or even smarter species, despite the commonness of this kind of language in popular discourse. What characteristics evolve in a species are a function of the variation present and the environment, both of which are constantly changing in a nondirectional way. A trait that fits in one environment at one time may well be fatal at some point in the future. This holds equally well for insect and human species.

References

Adapted from Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/books/biology-2e/pages/1-introduction