What is the process called that leads to a new species over time?

Without physically counting bacteria or viruses, scientists estimate that there are more than 8.7 million species of organisms living on Earth today. The origin and extinction of so many species has fascinated scientists for thousands of years, since the days of ancient Greece. The great diversity of living organisms on Earth is best explained by the evidence-based scientific concept of evolution by natural selection.

Evolution is the change in inherited characteristics or traits in a population of organisms over many generations. The mechanism that best explains evolution is a phenomenon known as natural selection. Natural selection is the process by which certain inherited traits—such as the color of a fish, height of a person, or shape of a leaf—are favored within a population. A population is a group of organisms that mate and reproduce with one another. In general,

• traits persist in a population because they contribute to the success of the organism, or
• traits are eliminated because they detract from the success of the organism.

Thousands of years of selective pressures have determined the shapes, colors, sizes, and behaviors that optimize the survival and reproductive success of organisms in the environment in which they evolved. In fact, it is often possible to tell a lot about where something lives by how it looks and behaves.

Evolutionary fitness and success refers to surviving long enough to pass genetic material on to offspring. Traits that are passed on to offspring because they contribute to success are “selected for continuation.” Traits that are eliminated from the population because they detract from success are “selected against continuation.”

Any force in the environment that favors or disfavors traits is a selective agent. A force can be biological, like a predator, or physical, like temperature. Over time, populations subjected to different selective agents may become so different that they are no longer able to breed with one another. The biological definition of species is a group of organisms that can successfully breed with each other. Under this definition, when populations can no longer breed with each other, they are considered to be different species.

Activity

Model natural selection in a population of bacteria.

History of Evolutionary Biology

In the 1800s, many people were trying determine why there were so many different kinds of plants and animals in the world. Charles Darwin wondered about the diversity of animals he saw while in the Galapagos Islands. This led him to develop the theory of natural selection, which is the best explanation we have for the diversity of life. Alfred Russell Wallace also hypothesized that the environment could help to shape the diversity of life by favoring certain traits over others. Wallace noticed that insects in the jungles of Africa and South America were very well adapted to unique environments. These two men, working independently of one another, developed the same basic explanation for the diversity of life: natural selection. These principles are supported by current scientific research.

Genetic Variation is Essential for Evolution by Natural Selection

For natural selection to occur, a population must have a wide variety of individuals with different traits. For example, natural selection would not influence fish body color if all individuals in a population were exactly the same color. The term phenotype is used to describe these physical traits. All phenotypes are the expression of genetic information in an individual’s DNA molecules. The term genotype describes the specific genetic code in the DNA molecular structure that produces a certain phenotype. Variations in genotypes can also produce variations in phenotypes.

New genotypes can be produced through the natural process of genetic mutation. A mutation is an error that is made during the DNA copying process. The mutation results in a change in the genetic code or genotype.

Sometimes mutations errors occur within the sections of the DNA strand that do not code for any phenotype or trait. Similarly, some mutations are minor and do not cause any significant change in the appearance, physiology, or behavior of the organism. Other times, mutations can cause changes in the phenotype (Fig. 1.7).

Over millions of generations of copying, even little errors in that influence phenotype can add up to big changes. Sometimes a single change in a regulating gene that controls other parts of an organism’s DNA can also have a big effect. To picture how this could work, think of the game “telephone.” In this game, one person whispers a phrase to their neighbor, who whispers it to their neighbor, and so on. The last person who receives the message compares the phase they think they heard to the original phrase. Often mistakes in repeating or understanding the sample phrase lead to changes in the meaning. The same thing happens when DNA is replicated again and again. The changes to the message in the game are similar to what happens when there are mutations. Changes in the meaning of the “message” of the DNA can cause changes in the appearance, physiology, or behavior of an organism.

A mutation can be an especially powerful force for change if it has a significant impact on the survival of an organism. Some mutations, for example, have provided resistance to antibiotics in bacteria. Because bacteria reproduce very rapidly, these mutations can quickly take effect in changing the population of bacteria.

It is also important to remember that mutations are random. An organism cannot pick or choose its mutation. For example, some species of animals that live their whole lives in caves without light have no pigment, or coloration. Because the caves are dark, there is no benefit to being camouflaged to avoid a predator or having coloration to attract a mate. However, not all animals that live in the dark lack color. In order for a species to lose its coloration, mutations must occur that allow the elimination of pigment. If the mutation never arises, the animals will stay pigmented.

Sexual reproduction can also increase genetic variation in a population. Many single-celled microorganisms reproduce simply by duplicating their genetic material (i.e., DNA) and dividing themselves in half. This process is called asexual reproduction. The new cells produced by asexual reproduction are genetically identical to the original parent cell unless some mutation occurs. Sexual reproduction is the production of offspring through the combination of specialized sex cells. These sex cells (also called gametes) contain only a half-copy of an individual organism’s genetic material. When the two halves—one from the male and one from the female—are combined, the offspring ends up with a brand new combination of genes.

Mutations and sexual reproduction increase genetic variation in a population. Individual organisms with unfavorable traits (e.g. malformed wings in fruit flies or bright white color patterning in male peacocks) are less likely to survive and reproduce. These individuals and the genes they carry are “selected against” or disfavored by natural selection (Fig. 1.8). In contrast, individuals with beneficial genes are more likely to survive and reproduce.

In summary:

1. Natural selection requires variation between individuals.
2. Mutations and sexual reproduction increase genetic variation in a population.
3. Natural selection occurs when environmental pressures favor certain traits that are passed on to offspring. The “big prize” in natural selection is passing on genetic information.
4. Natural selection acts on populations. Individuals do not evolve in genetic evolutionary terms. Individuals may mutate, but natural selection acts by shifting the characteristics of the population as a whole.

Activity

Model how variation in prey color and predator foraging affects survival and reproduction of a prey population.

• The theory of evolution is based on the idea that all species are related and gradually change over time.
• Evolution relies on there being genetic variation in a population which affects the physical characteristics (phenotype) of an organism.
• Some of these characteristics may give the individual an advantage over other individuals which they can then pass on to their offspring.

• Charles Darwin’s theory of evolution states that evolution happens by natural selection.
• Individuals in a species show variation in physical characteristics. This variation is because of differences in their genes.
• Individuals with characteristics best suited to their environment are more likely to survive, finding food, avoiding predators and resisting disease. These individuals are more likely to reproduce and pass their genes on to their children.
• Individuals that are poorly adapted to their environment are less likely to survive and reproduce. Therefore their genes are less likely to be passed on to the next generation.
• As a consequence those individuals most suited to their environment survive and, given enough time, the species will gradually evolve.

Natural selection in action: the Peppered moth

• Before the industrial revolution in the mid-1700s, the peppered moth was most commonly a pale whitish colour with black spots.
• This colouring enabled them to hide from potential predators on trees with pale-coloured bark, such as birch trees.
• The rarer dark-coloured peppered moths were easily seen against the pale bark of trees and therefore more easily seen by predators.

A pale peppered moth on an oak tree.

Image credit: Shutterstock

• As the Industrial Revolution reached its peak, the air in industrial areas became full of soot. This stained trees and buildings black.
• As a result, the lighter moths became much easier to spot than the darker ones, making them vulnerable to being eaten by birds.
• The darker moths were now camouflaged against the soot-stained trees and therefore less likely to be eaten.
• Over time this change in the environment led to the darker moths becoming more common and the pale moths rarer.

A pale peppered moth on a dark tree.
Image credit: Shutterstock

What have genes got to do with it?

• The mechanisms of evolution operate at the genomic level. Changes in DNA sequences affect the composition and expression of our genes, the basic units of inheritance.
• To understand how different species have evolved we have to look at the DNA sequences in their genomes.
• Our evolutionary history is written into our genome. The human genome looks the way it does because of all the genetic changes that affected our ancestors.
• When DNA and genes in different species look very similar, this is usually taken as evidence of them sharing ancestors.
• For example, humans and the fruit fly, Drosophila melanogaster, share much of their DNA. 75 per cent of genes that cause diseases in humans are also found in the fruit fly.
• DNA accumulates changes over time. Some of these changes can be beneficial, and provide a selective advantage for an organism.
• Other changes may be harmful if they affect an important, everyday function. As a result some genes do not change much. They are said to be conserved.

Different types of evolution

Convergent evolution

• When the same adaptations evolve independently, under similar selection pressures.
• For example, flying insects, birds and bats have all evolved the ability to fly, but independently of each other.

Co-evolution

• When two species or groups of species have evolved alongside each other where one adapts to changes in the other.
• For example, flowering plants and pollinating insects such as bees.

Adaptive radiation

• When a species splits into a number of new forms when a change in the environment makes new resources available or creates new environmental challenges.
• For example, finches on the Galapagos Islands have developed different shaped beaks to take advantage of the different kinds of food available on different islands.

Sketches of the heads of finches from the Galapagos Islands showing the differences in their beak shapes due to evolution.

Image credit: John Gould (14.Sep.1804 – 3.Feb.1881) – From “Voyage of the Beagle”; also online through Biodiversity