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The hierarchy of selection

selection hierarchy
We most typically think of natural selection working at the level of the individual, favoring those better at leaving behind more individual descendants. However, with a little imagination, we can see how natural selection might work at other levels of biological organization as well. Moving down the hierarchy, natural selection could act on the cells within an individual, favoring those cell lineages better at leaving behind descendent cells. Moving up the hierarchy, natural selection could act on species, favoring those species better at diversifying into descendent species.

Selection at the individual level
To understand how selection works at different levels, first consider the requirements of natural selection in the most familiar situation, at the level of the individual organism, using a beetle population as an example:

1. Variation in traits.
Some beetles are green and some are brown.
2. Differential birth and death.
Since the environment can't support unlimited population growth, not all individuals get to reproduce to their full potential. In this example, green beetles tend to get eaten by birds and survive to reproduce less often than brown beetles do.
3. Heredity.
Because this trait has a genetic basis, the surviving brown beetles have brown offspring.

In the end, the advantageous trait, brown coloration, becomes more common in the population.

Selection at the cellular level
This process requires just three basic features to operate: variation, differential birth and death, and heredity. However, these traits are not unique to populations of individuals. As an example, imagine a group of cells within one person's liver:

1. Variation in traits.
Most of the cells are basically the same, but one has experienced a chance mutation, inactivating a gene that controls the cell's growth.
2. Differential birth and death.
Because it has lost the ability to regulate growth, the cell with the chance mutation divides more rapidly than the others.
3. Heredity.
The normal cells pass on normal DNA when they divide, and the mutant cells pass on mutant DNA when they divide.

In the end, the cells carrying mutant DNA become much more common in the liver and may even take over, causing the liver to malfunction. In this case, the result of selection is a process better known as cancer. Selection at the cellular level is constantly operating within all multicellular organisms (including humans), but we rarely notice it except when it leads to such detrimental effects. Interestingly, cellular selection can work against selection at the level of the individual: what's advantageous for a cell lineage (e.g., replicating out of control) can be disadvantageous for the whole organism (e.g., causing an early death from cancer).

Selection at the species level
Moving up the hierarchy, we can also see how this same process would work on entire lineages of organisms. As an example, imagine a group of closely related species:

1. Variation in traits.
Some species have patchy distributions and others have uninterrupted distributions. Patchy distributions often result in isolated populations evolving into new species.

species 1 has patchy distribution and species 2 has uninterrupted distribution

2. Differential birth and death.
Species with patchy distributions are more likely than others to undergo speciation — the analog of "birth" for a species.

species 1 undergoes more speciation than species 2

3. Heredity.
Species with patchy distributions tend to speciate into lineages with similarly patchy distributions, and species with uninterrupted distributions tend to speciate into lineages with uninterrupted distributions.

the result is more species with patchy distribution

In the end, this process favors species with patchy distributions, and those species will eventually become more common than those with uninterrupted distributions. If the patchy lineage happens to be purple and the continuous lineage happens to be brown, the number of purple species will increase faster than the number of brown species.

Selection at the species level is much harder to observe than selection at the cellular, or even the individual, level because species selection takes place over millions of years. Nevertheless, we know that this process theoretically could happen. We even have some evidence that it does happen. Paleontologist David Jablonksi and his colleagues have found evidence suggesting that geographic range in marine mollusks may evolve this way. They collected data on the occurrence of more than 1000 different mollusk species fossilized in late Cretaceous rocks of southeastern North America and found a few key patterns:

1. Variation in traits.
Geographic range varies across mollusk species. Some have broad ranges and some have small ranges.

2. Differential birth and death.
Those mollusk species with broad ranges are extinction resistant (i.e., have a low "death" rate).

3. Heredity.
When mollusks with broad geographic ranges speciate, they tend to give rise to other species with broad geographic ranges — in other words, geographic range is a heritable trait for these species.

Marine mollusks seem to have all the basic characteristics that would result in species selection — but how important has this process been in their evolutionary history and the history of life in general? Scientists are still working out the answer to that question. While we know that selection can operate at levels above that of the individual, it is still unclear how important this process has been in shaping the history of life.