Whether you notice it or not, the planet we call home also supports incalculable life, ranging from the tiniest of microscopic bacteria to the largest of towering trees. Each living organism occupies a specific niche in its environment. Still, survival comes at a cost in our world where resources such as food and space are often limited.
This is where adaptation -- when an organism becomes better matched with its current environment -- comes into play. In general, adaptations can be structural (meaning an organism undergoes bodily changes to survive) or behavioral (when a specific behavior increases an organism's chances of survival).
Adaptations, such as birds migrating south for the winter or animals using tools to forage for food, occur at the individual and population levels, as we'll learn later on.
So what causes organisms to adapt?
Environmental factors -- including competition for resources, predation, infectious diseases, climate and seasonality -- play a large role. As we learn more about how life adapts to its environment, we'll discover that some adaptations occur in a few generations whereas others take many generations to fine-tune. To begin, let's look at digestion.
Why do humans rely on foods like fruits and vegetables to survive while other organisms such as bacteria thrive by processing carbon? Read on to learn about adaptive digestion.
10: Differing Digestion
To us, a handful of grass symbolizes a sparse and inedible meal option. But for a cow and other grass-eaters, it's a mouth-watering feast. We can say the same about surviving solely on carbon, an energy source for some bacteria.
All forms of life eat or "break down" numerous types of foods and materials. But what marks these diets as adaptive?
For starters, look at ruminants: animals such as cattle and deer that possess multi-chambered stomachs to break down tough plant matter into nutritious food. Researchers have found that eating grass and other vegetation high in cellulose requires animals to ferment food in their stomachs, where microbes help break down the compound [source: Hofmann]. Though the evolution of a ruminant digestive system is a gradual and complex process, we know that sometime in the past when food was scarce, animals able to digest grass probably survived more often than others that didn't.
Furthermore, a handful of animals have developed special saliva to process foods that their close ancestors couldn't. For instance, deer, some insects, squirrels and other rodents can consume varying levels of tannin -- a chemical found in acorns and other plants -- in environments where the compound is common [sources: Austin et al.; Smallwood and Peters].
Teeth shape also allows certain animals to exploit food items. To hypothesize what human ancestors ate, scientists look at the shape of fossil teeth as well [source: Smithsonian Institution].
Read on to learn about the fascinating ways plants defy their environments to reproduce.
9: Seed Dispersal in Plants
Life uses unique reproductive tactics to increase the chances of passing its genetics on to the next generation.
Plants have developed interesting adaptations to spread their seeds. Some seeds possess tiny hooks that snag the fur or feathers of animals passing by, granting the seeds a free ride away from their parent plant [source: Smithsonian National Zoological Park]. Others may look irresistible to hungry animals that consume and spread the seeds to other locations as droppings.
One study found that a species of weeds growing in small patches of soil in a new urban environment increased the amount of nondispersing seeds, or ones deposited near the parent plant [source: Cheptou et al.]. Nondispersing seeds deposited near the parent plant may have a better chance of surviving than dispersing seeds that can't take root on concrete surfaces away from the parent plant. In this case, life rapidly adapted to its environment, especially considering that plants of the same species not living in urban settings don't produce more nondispersing seeds.
Next, we'll look at shells as adaptations.
8: Bodily Protection
Have you ever wondered why some types of crabs find shells to live in while others don't?
Some hermit crabs use empty shells to avoid drying up, or desiccating, between the ocean's tides [source: Reese]. Crabs that feed in these areas may get stuck in the sand during low tide, making them vulnerable to the sun or hungry beach predators.
Grabbing an empty shell to live in is a behavioral adaptation that allows hermit crabs to better survive the intertidal environment, especially if they get stranded for a few hours.
Similarly, species of snails and bivalve mollusks grow their own shells to provide protection from the elements and predators.
Scientists have found that the shape of physid freshwater snail shells depended on the type of predator present in the snails' environments [source: DeWitt et al.]. Here, researchers attributed the shell diversity to the hunting strategies of the snails' primary predators -- crayfish and fish. The differences in shells are adaptive because they are shaped to avoid the attacks of predators found in their small habitats.
Like the physid snails, natural selection can also produce other adaptations in organisms. Learn how pesticides can lose their effectiveness on the next page.
7: Acquired Resistance
As natural selection has shown us, many environmental pressures shape the genetics of a specific population. In these cases, organisms that survive to reproduce are the ones credited with adapting.
Look at pesticide use on insects, for instance. We initially thought using pesticides on insects would do the trick in getting rid of them. But a more complex phenomenon was at play. Since not every insect is genetically identical, there's a chance that some insects will express resistance to the insecticide, survive encountering it and reproduce offspring with similar resistance.
After using the same pesticide over and over, we learned that each new generation of insects became increasingly tolerant to the pesticide. Before you know it, an entire population of insects is resistant. Some call this process artificial selection [source: PBS].
Although we may not think of the human body as an "environment," it's surely home to living microorganisms set on survival. Adaptations can occur within us as well. In efforts to prevent creating antibiotic-resistant strains of bacteria, doctors prescribe antibiotics to patients with bacterial infections only when necessary.
Next, the presence of sunlight, especially the heat it produces, is a constant hurdle for some organisms living on Earth. How does life adapt to beat the heat?
6: Avoiding the Sun
In environments with extreme heat and little water, organisms including elephants, desert plants, reptiles and even humans, adapt to survive.
Staying cool is vital. To achieve this, elephants adapt by flapping their thin ears to dissipate heat and cool the blood running through them [source: Simon].
To avoid drying up in a desert environment, organisms often store water in their bodies. The Gila monster, a desert lizard found in the southwestern United States, stores diluted water in its bladder after a several-minute drinking binge [source: Davis and DeNardo]. We can also look to cactus plants and camels that use their bodies as vessels for water storage, too.
Believe it or not, our skin has an adaptation of its own -- tanning. Humans adapt to the sun's ultraviolet rays through the production of melanin, the pigment that gives skin its color [source: O'Neil]. Melanin shields deeper layers of the skin and prevents the sun's harmful rays from breaking down folic acid, an important vitamin that repairs blood cells in the body.
As a short-term adaptation, most people develop a tan when exposed to the sun. In the long-term, human skin color reflects where your ancestors evolved, with the ancestors of darker-skinned humans having evolved closer to the equator and the ancestors of lighter-skinned humans having evolved in places with limited sunlight.
So far, we've learned that adaptations are flexible across life. Read on to explore how organisms use tools to adapt to their environments.
5: Tool Use
Although animals don't use computers to complete tasks like humans, they still create innovative solutions by using tools. Usually, organisms use tools, or modified natural objects, to adapt to environmental constraints that may get in the way of accessing food.
Some species of hungry, dedicated birds fit this mold. The Egyptian vulture, for instance, is well-known for using its mouth and a stone to crack open ostrich eggs. Another type of bird, the woodpecker finch, has been observed placing a cactus spine in its mouth to prod delicious grubs out of tree holes. In addition, green herons have been observed "fishing" with bait by dropping small objects on the surface of water and waiting for fish to approach the object thinking it's food [source: Kosseff].
These adaptations don't have to be geographically widespread for them to take hold, either. Tools can be used for self defense as well. In one study, scientists reported a group of octopuses off the coast of Australia that suctioned coconut shells under their bodies in the event they'd need to use them for hiding from predators later [source: Kaplan].
Humans and other organisms rely on oxygen to breathe. But what happens when there's too little? Read on to find out.
4: Surviving With Little Oxygen
Much like humans, many organisms need oxygen to survive. Whether a marine organism holds its breath for several minutes or a llama travels high into the mountains, life can adapt to environments with little oxygen.
For instance, marine mammals such as walruses have larger amounts of oxygen-rich blood circulating through their bodies in order to go on long dives without breathing [source: U.S. Fish & Wildlife Service]. Animals also slow down their heart rate and blood circulation to conserve oxygen, too.
Altitude also determines the types of life that can survive in a given environment. Humans can adapt to gradual decreases in oxygen, and often people can acclimatize, or eventually adapt to high-altitude climates. To achieve this, the body creates more red blood cells and capillaries to help supply the body with more oxygen [source: O'Neil]. There is, however, still a cost to this adaptation: fatigue and not being able to perform daily activities with the same rigor.
Over time, human populations have developed more permanent adaptations for living in high altitudes with little oxygen. For example, some high-altitude natives from Bolivia and Peru produce more hemoglobin in their blood, which carries additional oxygen in the bloodstream and allows people to take in extra oxygen through their lungs [source: O'Neil].
But sometimes the most interesting adaptations are the ones you don't see. Read more about camouflage next.
3: Crafty Camouflage
Life, particularly in the animal kingdom, is known for its masterful camouflages. Animals using camouflage disguise themselves to blend in with their surroundings or, sometimes, to mimic other animals in their environment.
Camouflage is a useful adaptation because its users can avoid becoming dinner -- and steer clear of being seen while trying to catch dinner, too. The chameleon stands as one of camouflage's sterling examples, especially since the animal can change the pigmentation of its skin to match its surroundings.
But other camouflage adaptations may not be as voluntary. Stick bugs are known for, well, their stick-liked appearance, while leaf insects are masters at blending into similarly colored foliage.
Changes in an organism's appearance can also be shaped by other animals in its environment. For example, birds avoid eating the monarch butterfly because of its poisonous diet. Since birds identify monarch butterflies by their appearance, another species -- the viceroy butterfly -- has mimicked the appearance of monarch butterflies to avoid predation as well [source: West Virginia Division of Natural Resources].
How do bears and other hibernating animals make it through the winter asleep? Find out more ahead.
2: Surviving the Cold
As the seasons change, so do life's adaptations. Just as some animals adapt to heat, others must survive the cold -- whether it's permanent or seasonal. Organisms living in cold climates naturally have larger bodies and thicker skin and fur. But many organisms adapt on a seasonal basis, too.
Animals that hibernate, or enter a deep sleep of sorts, during winter use the behavior to survive periods of cold temperatures when food is not widely available. Prior to hibernating, animals will stock up on fat to burn throughout the winter. In some cases, animals such as the American black bear can go 100 days without waking up, drinking, eating, defecating or urinating [source: Tyson].
So far, multiple sources have claimed that climate change has negatively affected the cycles of hibernating animals, making them emerge from their winter slumber earlier than usual [sources: Inouye et al.; Humphries et al.]. These early arousals demonstrate how easily life can adapt to its changing environment.
Rather than slumbering in a dedicated shelter, some animals are adapted to freeze and thaw with the winter weather. The wood frog hibernates under leaves, where it can freeze and come back to life in the spring. Scientists credit the frog's stealthy adaptive response to certain proteins and glucose that act as an antifreeze for the frog's cells [source: Rhode Island Vernal Pools].
Lastly, we'll look at why senses are important adaptations.
1: Sensory Specialization
Humans and other primates heavily depend on their sense of vision to interpret the world around them. Other forms of life do this too, but often rely on other senses -- some of which humans don't even have.
Bats rely on echolocation, or by emitting and receiving high-pitched sounds, to navigate their environments in the dark. This adaptation enables bats to fill an ecological niche as night dwellers.
The same can be said of birds of prey that possess acute vision at long distances or sharks' abilities to pick up electromagnetic currents.
Yet the "Use It or Lose It" idea seems to apply to some sensory adaptations for animals. Like the ancestors of moles, which lost their sight gradually from living underground, one species of cavefish lost its eyes altogether, according to a researcher's findings [source: Borowsky]. In a lab setting, the scientist crossbred the eyeless fish from different caves and found that some of the offspring possessed working eyes. The study revealed that the fish still had the genes to produce eyes, but they were not expressed because of the dark conditions of the cave.
Explore more resources about the natural world on the next page.
Lots More Information
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