How Evolution Takes Place

The mechanisms through which evolution occurs are discussed below.

There are more living things produced than can possibly survive, because there is not enough space and food to support them. The result is competition among organisms in the struggle for survival. The individuals in each species are not all exactly alike. They have varying characteristics, and these variations may either help or hinder them in their struggle to live.

Organisms become extinct (die out), if they do not have enough of the kind of characteristics that enable them to get sufficient food and other necessities, to withstand the climate, or to compete successfully with other forms of life. The ones that survive and reproduce pass on to the next generation at least some of the characteristics that made them better adapted to their environment. In this way the successive generations improve their internal efficiency and their adjustment to their environment. Natural selection causes a gradual change in the characteristics of the species.

In some species, females prefer males with different traits. This leads to evolution of a species where males have traits that are markedly different from that of females.

Directional selection is the evolution of characteristics that assist the species in adapting to their habitat. A species that needs longer limbs in order to run fast and catch its prey will evolve longer limbs.

Stabilizing selection is the continuance of the characteristics of a species that already has all the suitable traits needed to survive in its environment. If the average population of a species has all the characteristics it needs to survive, then individuals born with different traits do not survive long enough to pass on their traits through reproduction.

Diversifying selection ensures that the population has two different characteristics. A gene causes a disorder of blood cells within the human population, which is fatal if inherited from both parents. But when inherited from only one parent, it provides carriers of the gene resistance to malaria.

To explain how plants and animals develop variations, biologists turn to genetics, the science of heredity. Genetics shows that characteristics are passed from parents to offspring by means of genes—segments of large nucleic acid molecules contained in chromosomes. Offspring inherit these genes from their parents during sexual reproduction.

When a gene is altered in any way, the offspring acquires a new characteristic not possessed by either parent. The gene is passed on to the offspring in the mutated form. This change in the gene is called a mutation. Mutations are responsible for genetic variability(the random occurrence of variations) in a given species. When a mutation introduces a harmful characteristic, the organism probably will not survive unless the harmful effect is balanced by beneficial effects. Helpful characteristics—those which enhance the chances of survival—acquired by mutation are likely to be passed on to future generations.

Mutations can be produced in laboratories by the action of X rays, other forms of radiation, or chemicals. Mutations occur frequently in nature, often from random errors in chromosome replication. Radiation and chemicals present in the environment can also cause the genes to mutate.

The combining of genes from two parents is called sexual recombination and controls the rate at which variations occur. Sexual recombination permits enough variation to allow progressive evolution but not so much variation that the changes would be chaotic. Most organisms reproduce through sexual recombination. Related to sexual recombination is genetic drift, where many of the parents' genes are not passed on to the offspring. During sexual recombination, only half of the chromosomes from each parent are passed on to the progeny. The lesser the number of offspring, the more the chance of some genes of the parents not being passed on.

When different populations of the same species become isolated from each other (as by a physical barrier such as a mountain, desert, or ocean) each population adapts to its own environment. When a physical barrier separates two populations of the same species, it can give rise to a new species. When the isolated population is subjected to genetic drift or other mechanisms of natural selection, it causes the evolution of a new species. In time, the isolated populations may diverge into different species. A physical barrier is not always necessary for populations of a species to diverge. For example, different populations sometimes eat different foods in the same general area. Such populations will have little interaction with each other and may eventually diverge into different species.

The various species of finches, known as Darwin's finches, that live on the Galápagos Islands provide an example of both forms of divergence. It is thought that these finches evolved from members of a single species blown by the wind from the mainland of South America, some 600 miles (965 km) away. The expanse of ocean isolated the displaced birds from the rest of their species. Different groups of finches found different types of food on the islands. Each group developed adaptations (such as a particular beak shape) for efficiently eating its own type of food; eventually, the groups diverged into more than a dozen species.

When unrelated species adapt to the same kind of environment, they independently develop similar characteristics—that is, they converge. For example, though whales are mammals and penguins are birds, they each share certain characteristics with fish (such as finlike appendages and streamlined bodies).

The evolutionary lineage of a species is the traits individuals of the species share that helps them survive and reproduce in their environment. For the species to continue, it must maintain its evolutionary lineage. Therefore, nature has certain barriers that prevents the species from producing offspring without the traits that are part of the evolutionary lineage. These barriers include unique courtship rituals that prevent mating between species, or inability of the offspring produced through inter-species mating to produce their own offspring.

Sometimes, the barriers to reproduction between species or populations of the same species are physical, such as creation of a new mountain range or river that happen over millions of years. This leads to divergence.

Other evolutionary principles include the following:

1. The pace of evolution differs from species to species. Some species of brachiopods, for example, have not changed in hundreds of millions of years. Gradual evolution occurs over a long period.On the other hand, several species of horses have appeared and become extinct within a few hundred thousand years. In addition, the pace of evolution may vary throughout the history of a particular species. The fossil record indicates that many species have had short periods of rapid change followed by long periods of virtually no change, a process called punctuated equilibrium.

2. Highly complex species evolve from relatively simple ones. Simple species, however, may evolve from complex ones—a process called degeneration—if a simpler structure helps a species to adapt better to its environment.

3. When different species interact closely, as in predator-prey or parasite-host relationships, they coevolve—that is, they evolve together. For example, through evolution, rabbits have developed keen hearing, which helps them escape predators, such as owls. Owls, on the other hand, have developed feathers that permit virtually silent flight.