Descent with Modification

Descent with modification refers to the passing on of traits from parent organisms to their offspring. This passing on of traits is known as heredity, and the basic unit of heredity is the gene. Genes are the blueprints for making an organism, and, as such, hold information about its every conceivable aspect: its growth, development, behavior, appearance, physiology, and reproduction.

Heredity and Evolution

According to Charles Darwin, all species descended from only a few lifeforms that had been modified over time. This "descent with modification," as he called it, forms the backbone of his Theory of Evolution, which posits that the development of new types of organisms from preexisting types of organisms over time is how certain species evolve.

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An Introduction to Evolution

By Laura Klappenbach

How It Works

The passing on of genes is not always exact. Parts of the blueprints may be copied incorrectly, or in the case of organisms that undergo sexual reproduction, genes of one parent are combined with the genes of another parent organism. That is why children are not exact carbon copies of either of their parents.

There are three basic concepts that are helpful in clarifying how descent with modification works:

• Genetic mutation
• Individual (or natural) selection
• Evolution of the population (or species as a whole)

It is important to understand that genes and individuals do not evolve, only populations as a whole evolve. The process looks like this: Genes mutate and those mutations have consequences for the individuals within a species. Those individuals either thrive or die out due to their genetics. As a result, populations change (evolve) over time.

Clarifying Natural Selection

Many students confuse natural selection with descent with modification, so it's worth repeating, and further clarifying, that natural selection is part of the process of evolution, but not the process itself. Natural selection comes into play, according to Darwin, when a species as a whole adapts to its environment, thanks to its specific genetic makeup. Say at some point in time two species of wolves lived in the Arctic: those with short, thin fur and those with long, thick fur. Those wolves with long, thick fur were genetically capable of living in the cold. Those with short, thin fur were not. Therefore, those wolves whose genetics allowed them to live successfully in their environment lived longer, bred more frequently, and passed on their genetics. They were "naturally selected" to thrive. Those wolves who were not genetically adapted to the cold eventually died out.

Furthermore, natural selection doesn't create variation or give rise to new genetic traits—it selects for the genes already present in a population. In other words, the Arctic environment in which our wolves lived did not prompt a series of genetic traits that didn't already live in certain of the wolf individuals. New genetic strains are added to a population through mutation and horizontal gene transmission—e.g., the mechanism by which bacteria become immune to certain antibiotics—not natural selection. For instance, a bacterium inherits a gene for antibiotic resistance and therefore has a greater chance of survival. Natural selection then spreads that resistance through the population, forcing scientists to come up with a new antibiotic.