8.1 Responses to the Environment

Reacting to cues

Organisms respond to environmental stimuli in ways that improve survival. Plants grow toward light — a tropism called phototropism. Animals may show taxis, movement toward or away from a stimulus. Many responses are shaped by natural selection because they help organisms survive and reproduce.

Timed and seasonal behaviors

Circadian rhythms are internal ~24-hour cycles (like sleep/wake). Seasonal behaviors include migration (long-distance seasonal movement) and hibernation. Organisms also communicate using visual, chemical, and auditory signals to coordinate behavior.

Key terms

Tropism (phototropism)

A directional growth response, e.g., a plant growing toward light.

Taxis

Movement of an organism toward or away from a stimulus.

Circadian rhythm

An internal cycle that repeats about every 24 hours.

Migration

Seasonal long-distance movement of animals.

Behavioral response

An action that helps an organism survive; shaped by natural selection.

Try an AP-style question

Which type of behavior does this show, and why is it adaptive?

1. Phototropism, which increases light capture
2. Taxis toward moisture, which reduces water loss
3. Learned behavior, which is taught by parents
4. Kinesis toward dryness, which reduces water loss