EVOLUTION – LIFE SCIENCE

Evolutionary Relationships

Common Ancestry; Lions, tigers, and cheetahs are all large cats, but they are different species. Even though these big cats live in different regions of the world today, scientists believe they all descended from a single, now-extinct ancestral species. The idea that different species evolved from a shared ancestor is called common ancestry. Scientists study various forms of evidence to support this theory. The table below (not shown here) summarizes the main types of evidence used to explain how modern species evolved from ancestral species.

Cladograms

A cladogram is a diagram that shows evolutionary relationships between organisms based on shared characteristics. Organisms are arranged on branches of the cladogram depending on which traits they have in common. For example, consider a simple cladogram:

• The lancelet is placed on the first branch because it has none of the traits being compared.
• The lamprey is on the second branch because it has a vertebral column (spine).
• The wolf is on a later branch because it has all of the compared characteristics.

The more characteristics two organisms share, the more closely related they are considered to be. Based on such a cladogram, a turtle is more closely related to a wolf than to a salmon, because the turtle and wolf share all but one of the characteristics being analyzed.

Evolution by Natural Selection

One of the main mechanisms of evolution is natural selection. Consider the example of the giraffe: Ancient giraffes had necks of varying lengths and fed on grass. During times when grass was scarce, giraffes with longer necks could reach leaves higher up in trees. These long-necked giraffes were better able to find food, so they survived and reproduced more successfully than those with shorter necks. Over many generations, the proportion of long-necked giraffes in the population increased until nearly all giraffes had long necks.

This process—where individuals with advantageous traits are more likely to survive and reproduce—is called natural selection.

Artificial Selection

Humans can also influence the traits of organisms through artificial selection, which works similarly to natural selection but is guided by human choice. In artificial selection: Humans deliberately choose which organisms will breed based on desired traits.

• For example, farmers might breed two tomato plants that are highly resistant to disease.
• Dog breeders might select two dogs with the most desirable temperaments to produce puppies with similar traits.

Evidence of Selection in Nature

You can tell natural selection is happening when a particular version of a trait becomes more or less common in a population over time. A famous example is the peppered moth during England’s Industrial Revolution: Before industrialization, light-colored moths were more common because they blended in with light-colored tree bark. Factory pollution darkened the tree trunks with soot. Dark-winged moths became harder for predators to see, giving them a survival advantage. Over time, the number of dark-winged moths increased while the number of light-winged moths decreased. This change in moth populations is clear evidence of natural selection in action.

Requirements for Natural Selection

For natural selection to occur, two main conditions must be met:

1. Variation in Traits: There must be differences among individuals in a population. For example, in peppered moths, there were two variations of wing color: dark and light. Without variation, there would be no traits for natural selection to favor.

2. Differential Survivability or Reproductive Success: One version of a trait must provide a survival or reproductive advantage. In the moth example, dark-winged moths were better able to avoid predators, survive, and reproduce. If both color variants survived equally well, natural selection would not occur. By meeting these conditions, natural selection drives the evolution of species, gradually changing populations over time to become better adapted to their environments.

Evolutionary Change

An adaptation is a trait that all members of a species share, and that helps them survive or reproduce in their specific environment. Adaptations arise as a result of natural selection. Let’s revisit the giraffe example. In the past, giraffes with longer necks had a better chance of reaching food in trees when grass was scarce. These individuals were more likely to survive and reproduce. Over generations, long necks became more common until all giraffes had them. Today, the long neck is an adaptation—it helps giraffes survive and is shared by the entire species. Other examples of adaptations can be found across the animal and plant kingdoms.

Examples of Adaptations

This table lists different species and some of their key adaptations that help them survive in their environments.

Selection Pressure

Adaptations develop in response to challenges in the environment. These challenges are called selection pressures. A selection pressure is any factor in the environment that influences which individuals are more likely to survive and reproduce. These can include changes in climate, predators, food availability, disease, and even competition from other species.

Types of Selection Pressures

This table describes various environmental pressures and gives examples of how they can influence evolution.

Speciation

The formation of a new species from an existing one is known as speciation. This process usually happens when a group within a species becomes isolated from the rest—often due to geographic separation. Once isolated, the group experiences different selection pressures in its new environment. These pressures lead to the development of new adaptations. Over time, as more differences accumulate, the isolated population may become so different that it can no longer interbreed with the original population. At this point, a new species has evolved. One of the most famous examples of speciation is found in the Galápagos finches. Originally, a single species of finch from the South American mainland arrived on the Galápagos Islands. Small groups settled on different islands. Each island had a different environment and different types of food. Over generations, each group developed a unique beak shape suited to its food source—such as large, strong beaks for cracking nuts or long, narrow beaks for picking insects.

Galápagos Finch Speciation

  As species continue to evolve, natural selection, adaptation, and speciation remain key forces shaping life on Earth. These processes help explain both the diversity of life and the relationships among all living organisms.