Fields of biology, hypotheses, and theories

Biologists observe the natural world, identify what they find, test their ideas with experiments, and build explanations. Together, those activities make up science: the observation, identification, experimental investigation, and theoretical explanation of natural phenomena (events and processes in nature).

Research at every level

Biologists study life at every level of organization. The level they choose depends on their interests and on the tools available to them.

To place a research question in a field, ask what level it measures. A study of how the structures inside liver cells change after a meal is cell biology. A study of how the chambers of the heart work together to pump blood is anatomy and physiology.

Many researchers also focus on the same few species so they can compare results and find principles that apply to other species, including humans. These shared study species are model organisms. The bacterium Escherichia coli (E. coli), for example, is simple and easy to grow in the lab. Others include a yeast (Saccharomyces cerevisiae), a fruit fly (Drosophila melanogaster), a nematode worm (Caenorhabditis elegans), the house mouse (Mus musculus), and a small flowering plant (Arabidopsis thaliana).

Parts or whole systems

Over the 20th century, more and more biologists studied life by breaking complex systems into simpler parts and examining each part on its own. This approach is reductionism. Molecular biology, which focuses on single genes and the proteins they code for, grew up with it.

More recently, new tools have let researchers study large groups of genes and proteins at once. That has fueled systems biology, research aimed at understanding how emergent properties (properties that arise only when parts interact) come about. A systems biologist might try to characterize an entire cell process driven by dozens of different proteins. The idea isn’t new, though: physiologists have long studied organ systems, and ecologists have long studied whole ecosystems.

Hypotheses

When scientists want to explain something they’ve observed, they propose a possible explanation based on earlier observations or experiments. That proposed explanation is a hypothesis. Suppose you notice that maple trees drop their leaves in autumn. One hypothesis is that shorter days cause it; an alternative hypothesis is that colder temperatures do.

A useful hypothesis makes predictions, expected outcomes that could turn out to be right or wrong. That makes it testable: experiments can produce data that either agree or disagree with it. It should also be falsifiable, meaning that if it’s wrong, further observations or experiments can show that it’s wrong.

A hypothesis is never proven. When the data agree with it, researchers say the data support the hypothesis, or that they failed to reject it. The hypothesis stays provisional, because a better explanation may not have been thought of yet.

Theories

In everyday speech, a theory is little more than a guess. In science, the word means something far stronger. A broad explanation of some aspect of the natural world, backed by a large body of evidence, is a scientific theory. A theory pulls together many observations and tested hypotheses, and it builds on laws from other sciences, such as chemistry and physics.

One example is the theory that DNA is the genetic material and is organized into genes. Every one of the thousands of species studied so far uses DNA and has genes that produce proteins. The theory’s predictions, such as that mutations in genes can change traits and that genetic material passes from parents to offspring, have been confirmed by experiments. A scientific theory has two key features:

1. It is consistent with a vast amount of known data.
2. It makes many correct predictions.

Like a hypothesis, a theory can never be proven. Because theories are so thoroughly supported, though, biologists consider them extremely likely to be true and treat them as knowledge, the awareness and understanding of information.

Check yourselfAfter months of watching a songbird, a researcher proposes that males sing at dawn to defend their territory. Is this a hypothesis or a theory? Why?

A hypothesis. It’s a single proposed explanation based on observations, and it still needs to be tested. A theory is a broad explanation backed by a large body of evidence and many correct predictions.