Adaptation, Inheritance, and Evolution

A puppy looks like a dog, even when it looks different from its parents. You can compare its features to see both the likenesses and the differences.

What is passed on?

A trait is a feature you can observe or measure. Fur color, leaf shape, flower color and beak shape are traits. A trait can also be something an animal can do, such as spinning a web.

Offspring are the young that parents produce. Kittens, chicks and seedlings are all offspring. Inheritance means passing traits from parents to offspring when they reproduce. A generation is one step from parents to their young.

Living things have young of the same kind. An acorn grows into an oak tree. Cats have kittens. Reproduction keeps each kind of living thing going.

Look for family likenesses

Imagine two dogs with short fur and floppy ears. The mother has brown fur; the father has black fur. Their puppies all have short fur and floppy ears. Some are brown, one is black, and one has both colors.

1. List the traits you can see in each parent.
2. Compare those traits with each puppy's traits.
3. Notice the shared short fur and floppy ears. Notice the different coat colors too.

The puppies inherited features affecting coat color from both parents. A brown puppy resembles its mother in this trait. That does not mean all its inherited features came from her. Offspring of two parents inherit from both, even when a feature looks like just one parent's.

Plants and growing animals

Plants inherit traits too. These include leaf shape, flower color and features affecting fruit flavor. A gardener may save seeds from sweet tomato plants. Their offspring are likely to share useful traits, although they will vary.

Many seed-grown plants have two parents. Pollen from one plant reaches a flower on another plant. Both contribute to the offspring's inherited traits.

Compare animals at similar stages of growth. A tadpole has gills and a tail. Its parent frog has legs and lungs. As the tadpole grows, it develops the same kind of adult body. Its changing appearance is part of its life cycle.

Helpful inherited traits can also support survival. Thick polar bear fur keeps heat in. A cactus's fleshy stem stores water. You will meet these traits as adaptations.

Brothers and sisters can share many features without looking identical. Differences between living things are called variation. You can see variation within a family and across a whole species.

Different members of one kind

A species is a group of living things of the same kind that can breed with each other. Individual white oak trees belong to one species. Yet they differ in height, trunk width and leaf size.

Offspring of two parents receive different mixes of inherited traits. This helps explain why puppies in one litter have different patterns. Growing conditions can add differences too.

Some traits fall into separate groups, such as red or white flowers. Count the number in each group. Other traits, such as leaf length, vary across a range. Measure these with a ruler or a scale.

Turn measurements into a picture

Here are 16 leaf lengths, measured to the nearest centimeter:

7, 9, 8, 10, 8, 6, 9, 8, 11, 7, 8, 9, 10, 8, 9, 7 cm.

First, tally each length. Then count the tally marks.

The counts add to 16, so every leaf is included. Draw a bar graph with lengths along the bottom. Label the side "Number of leaves" and use equal steps. Make each bar as tall as its count.

The tallest bar is at 8 cm: that length is most common. The shortest leaf is 6 cm and the longest is 11 cm. Their difference, the range, is 5 cm. Most leaves cluster near the middle: 12 are 7, 8 or 9 cm long.

Make comparisons fairly

Measure each leaf from its tip to where its stalk begins. Keep that method the same. Measure many leaves, because two unusual leaves could give a misleading picture of the tree.

Plants grown from cuttings or strawberry runners have one parent. They usually share very similar inherited traits. Seedlings with two parents usually vary more. Their growing conditions still matter.

Variation matters when conditions change. A small difference in color or size may help some individuals survive. Being different is normal; young from the same parents do not have to match.

You can inherit eye color. You learn to read. Some features, including your height, depend on both your family and the way you grow.

Three ways traits arise

The environment includes food, water, light, temperature and other surroundings. An acquired trait develops during life through surroundings, experiences or learning. A scar, haircut or learned trick is acquired.

Food, sleep and health affect growth. A plant inherits features affecting its possible size, but light and water affect the size it reaches. Trees can grow bent on windy hilltops. Flamingos get pink color from substances in their food; their chicks hatch gray or white. Bigleaf hydrangea flowers tend to be blue in acidic soil and pink in alkaline soil.

Sort Maya's traits

Maya has brown eyes, a knee scar and two languages. She can swim and is tall like her parents.

1. Brown eyes are inherited.
2. The scar came from a fall. It is acquired.
3. Speaking two languages and swimming are learned skills.
4. Height belongs in "both": inheritance and growing conditions affect it.

Ask whether a feature passes from parents to offspring. Also ask whether different surroundings or experiences can change it. Do not force every trait into just one of the first two groups.

Test a growing condition

Take similar cuttings from one parent plant. Give them the same pots, soil, water and temperature. Put one in bright light and one in darkness. Their inherited traits are alike, so the different light helps explain differences in growth. Keeping other conditions alike makes the comparison useful.

Learning is not inheritance. Children of musicians still learn to play. Some behaviors are inherited instincts, such as a spider spinning its first web. A trained dog's trick is learned.

Lamarck suggested that giraffes stretched their necks and passed longer necks to their young. Stretching during life does not pass on a longer neck. In the same way, a parent's scar or practiced skill does not appear in a newborn as an inherited trait.

A duck's feet, a cactus's stem and a penguin's huddle look very different. Each helps its owner meet a need in its habitat.

Match a feature to a need

A habitat is the place where a living thing lives. Animals need food, water, air, shelter and a suitable temperature. Plants need light, water, air, nutrients and space.

An adaptation is an inherited feature or behavior that helps a living thing survive and reproduce. To survive is to stay alive. To reproduce is to have offspring. An adaptation can help with feeding, staying safe or producing young.

There are several ways adaptations work:

Body functions can also help save water. A camel can go many days without drinking. A kangaroo rat gets almost all its needed water from the seeds it eats.

A learned action is different. A dog may fetch a ball after training. That behavior developed through learning during its life. It was not passed to it as an inherited trick.

Build a clear explanation

Name the feature. Explain what it does. Then connect that job to the habitat.

For example: "A polar bear has a thick layer of fat. The fat slows heat loss. This helps the bear stay warm in the cold Arctic air and sea."

Each sentence adds a link. "It helps the bear survive" alone misses the job the fat does. Explaining the job makes the connection clear.

Where adaptations come from

Adaptations pass through generations. They build up as inherited traits that help survival and reproduction become common. An animal cannot choose a new inherited feature when it needs one. Taking a pet rabbit somewhere icy would not give it a polar bear's body.

Some inherited abilities allow seasonal changes. An Arctic fox grows a white winter coat and a brown or gray summer coat. Its ability to grow the right coat at the right season is inherited. The changing colors help it blend with snow, then rocks and plants.

The usefulness of a feature depends on the habitat. A thick covering helps hold heat in a cold place. The same covering can make cooling difficult in a hot place.

A desert and a pond offer very different living conditions. Look at the problems in each habitat, then connect body features and behaviors to those problems.

Save water and avoid heat

Deserts receive little rain. Many have hot days and cold nights. Camels have wide feet that spread their weight on sand. Long eyelashes and closing nostrils keep blowing sand out. Their humps store fat, which supplies energy when food is scarce. They do not store a tank of water.

A fennec fox gives off heat through its very large ears. Those ears also help it hear prey underground. Resting in a cool burrow by day and hunting at night helps it avoid daytime heat.

Cacti store water in thick, fleshy stems. Their waxy covering slows water loss. Spines replace broad leaves, helping reduce water loss and keeping animals away. Wide, shallow roots take up rain quickly.

Hold heat in

Polar bears have thick fur and fat that slow heat loss. Small ears expose little surface to the cold. Fur under their feet helps with warmth and grip. Emperor penguins huddle during the Antarctic winter. They shuffle around so birds take turns in the warmer center.

Compare two foxes:

Neither ear shape is best everywhere. Each suits different conditions.

Live with plentiful water

Many rainforest leaves have pointed drip tips. Water runs off quickly after rain. Water lilies have broad, floating leaves and air spaces that help hold them at the surface. There they can reach light.

Ducks paddle with webbed feet and have oily, waterproof feathers. Fish have streamlined bodies that move easily through water. Their gills take in oxygen from the water.

Use several clues together

Imagine an animal with white fur, small ears, fat and wide, furry paws. First, connect each feature to its job. White fur blends with snow. Small ears and fat reduce heat loss. Wide paws spread weight; furry soles help keep feet warm. Together these clues point to a cold, snowy habitat.

A helpful feature has limits. A polar bear could overheat in a hot desert. A water lily would dry out on land. A cactus suited to dry soil may rot in soggy ground.

A frog eats flies, but a heron can eat the frog. The frog is both a predator, which hunts other animals, and prey, which is hunted.

Catch food or escape

Many predators have sharp teeth, claws or talons for catching prey. Forward-facing eyes help cats and owls judge distance. Owls also have keen hearing and feathers that soften flight sounds. Wolves can hunt in packs.

Prey need to spot danger and avoid being caught. Eyes on the sides of a rabbit's head give a wide view. Fast legs help escape. A tortoise's shell and a hedgehog's spines protect the body. Herds and flocks can help animals notice danger.

Blend in or stand out

Camouflage is a color or shape that blends with surroundings. A stick insect looks like a twig. A tiger's stripes break up its outline in tall grass. Both prey and predators can benefit from being hard to see.

Camouflage depends on the background. A white hare blends with snow but stands out on brown ground.

Warning colors make some dangerous animals easy to notice. A wasp's black-and-yellow stripes can warn predators about its sting. Mimicry is resemblance to another living thing that gives an advantage. A harmless hoverfly resembles a wasp, so some predators avoid it too.

Match mouths to meals

Meat-eaters have pointed teeth for gripping and cutting. Plant-eaters such as horses grind food with broad teeth. These shapes also help scientists study fossil animals' diets.

Picture a bird wading at a lake edge. Its long legs keep its body above shallow water. Its long, sharp beak helps catch fish and frogs. Linking both features to their jobs explains how a heron feeds.

Plants have defenses too

Plants cannot run away from hungry animals. Hawthorn thorns, rose prickles and cactus spines can protect them. Nettles have stinging hairs. Some plants make bad-tasting or poisonous substances. These defenses reduce the chance of being eaten.

Plants stay rooted, but pollen and seeds can travel. Different flower and seed features help with these two journeys.

Move pollen to a stigma

Pollination moves pollen from an anther to a stigma on a flower of the same kind. The anther is the pollen-making tip of a stamen. The stigma is the sticky top of the flower's female part.

Bees visit flowers for food, including sugary nectar. Bright petals, scents and nectar guides help attract them. Some nectar-guide patterns are visible to bees in ultraviolet light. As a bee feeds, pollen sticks to its body. It can rub onto another flower's stigma during the next visit.

Flowers also suit other visitors. Hummingbird-pollinated flowers are often red or orange, tube-shaped and rich in nectar. Moth-pollinated flowers are often pale, strongly scented and open at night.

Wind-pollinated flowers do not need to attract animals. Grass flowers are usually small and dull, without nectar or scent. They make lots of light pollen. Exposed anthers release it; feathery stigmas catch it.

Send seeds away

Seed dispersal spreads seeds away from the parent plant. Crowded seedlings compete with the parent and each other for light, water, nutrients and space. Traveling can also carry seeds into new habitats.

Squirrels also carry and bury acorns. Some forgotten acorns grow into trees.

Read the travel clues

First, look at a seed's shape and weight. A light seed with a papery wing can spin through the air. Next, look for hooks, juicy flesh or a floating husk. Hooks suggest travel on fur. Sweet fruit can attract an animal that carries the seeds after eating it.

Pollination and dispersal do different jobs. Pollen reaches a stigma before seeds develop. Dispersal moves seeds afterward. The bee is collecting food; the plant benefits when that feeding visit carries pollen.

Food, water, light and safe places are limited. A habitat may support many living things, but each one still has to get what it needs.

Competition and connections

Competition happens when living things need the same limited resources. Nearby plants compete for light, water, nutrients and space. Animals may compete for food, territory or mates.

Living things also depend on one another. Bees feed at flowers, and many flowers depend on bees to carry pollen. Predators depend on prey for food. A change in one part of a habitat can affect others.

A trait that helps with a need gives an advantage. A plant with larger thorns may lose fewer leaves to animals. A slightly faster deer may escape a wolf. Better camouflage can make an insect harder to find.

Compare a beetle group

Imagine a patch of dry, brown leaves. It holds 20 green beetles and 20 brown beetles. Birds eat 12 green beetles and 3 brown beetles.

1. Find the green survivors: $20-12=8$.
2. Find the brown survivors: $20-3=17$.
3. Find the total: $8+17=25$.

More than twice as many brown beetles survived. Brown matches the leaves, so birds spot those beetles less easily. Color gives them an advantage in this habitat. On green leaves, green beetles may be harder to see instead.

The starting groups were equal. That makes their surviving counts easy to compare. A helpful color improves the chance of survival; it does not protect every beetle.

From survival to offspring

Living long enough to reproduce gives an individual a chance to pass on inherited traits. Individuals that leave more offspring can pass those traits to more members of the next generation.

In evolution, fitness concerns leaving offspring that carry inherited traits onward. It does not simply mean strength or size. A small, well-hidden insect can leave more offspring than a large, easily spotted one. Survival helps, but surviving without reproducing does not pass traits on.

Species also differ in what they can tolerate. Water lilies suit ponds. Cacti usually need drier ground, and fish need water to breathe. The same habitat cannot meet every living thing's needs.

A pond can shrink in a drought. A road can divide a forest. When conditions change, living things may lose water, shelter or routes to food and mates.

Follow the change

Physical conditions are nonliving parts of a habitat. They include light, temperature, water, soil, saltiness and wind. Each species grows and survives best within a range of conditions.

Droughts, floods and storms change habitats naturally. People also change habitats by building roads, cutting forests, polluting water or bringing in other species. Some living things cope well. Others move, become fewer, or disappear from that place. Plants mainly reach new places through seeds.

Investigate one condition

Imagine three trays, each with 20 radish seeds. Water contains 0, 2 or 4 teaspoons of salt per liter. Light, temperature, soil and the amount of water stay alike. After one week, 18, 12 and 4 seeds sprout.

Compare the pattern: more salt went with fewer sprouting seeds. Keeping the other conditions alike lets you connect that difference to salt. Many seeds give a more useful result than one seed, because individual seeds vary. The test measures sprouting after one week; it does not count full-grown plants.

Read a changing population

Imagine frog counts of 48, 52, 50, 31, 17 and 22 over six springs. A drought begins after the third count. Rain returns before the last count.

The count falls from 50 to 17: a drop of 33. Frogs need water for eggs, and tadpoles need it to grow. A shrinking pond reduces breeding habitat. The later rise from 17 to 22 fits returning water. Counts show the pattern; knowledge of the frogs' needs helps explain it.

Judge a useful solution

A wildlife tunnel may reconnect places divided by a road. Nest boxes can replace some lost nesting holes. Planting native plants can restore food and shelter.

Ask what need the solution meets. Look for evidence, such as animals using a tunnel and fewer road deaths. Compare costs and possible new problems too. A fast change cannot make individuals grow new inherited adaptations.

A fossil can be a tooth, a leaf print or a trail of footprints. These clues preserve parts or traces of life from long ago.

Two kinds of clues

A body fossil preserves part of a living thing, such as a shell, bone or leaf. A trace fossil records something it did, such as making a footprint or a burrow. Fossils are usually at least thousands of years old; many are millions of years old.

Most living things never become fossils. Their remains rot or are eaten. Soft bodies are especially unlikely to last. The fossil record, all the fossils found so far, therefore has gaps.

How remains can last

A dead animal may be buried quickly in mud or sand. Soft parts decay, while hard parts last longer. More sediment piles above it and presses older layers into rock.

Water carrying minerals may fill spaces in a bone or replace its material. This can leave rock shaped like the bone. Other fossils keep original shell or bone, flattened remains, or an impression. Much later, uplift and erosion can uncover the fossil.

Not all preservation happens this way. Amber is hardened tree resin that can trap insects. Tar pits preserve trapped animals. Frozen ground has kept parts of mammoths, including skin and hair.

Keep the place and layer

A paleontologist studies fossils. The job includes careful digging, measuring, recording and comparison. A fossil's position matters as much as its shape.

In sedimentary rock layers that have not been turned over, lower layers are older. New layers settled on top of older ones. Recording a fossil's layer helps place it in time alongside other finds.

Imagine finding a shell and a footprint in different layers. Record each location before moving anything. Without those records, a scientist loses important evidence about their ages and surroundings.

Mary Anning's discoveries

Mary Anning lived from 1799 to 1847 in Lyme Regis, England. As a girl, she and her brother Joseph found an ichthyosaur skeleton. In 1823, she found the first nearly complete plesiosaur skeleton. These were sea reptiles from the time of the dinosaurs.

Her finds helped scientists understand that many past animals no longer exist. Fossils are not all dinosaurs: shells, plants and small sea creatures are much more common finds.

A fossil tells more when you connect its shape, location and age. Different kinds of clues answer different questions about past life.

Read the body and its traces

Pointed teeth suggest gripping and cutting food such as meat. Broad, flat teeth suggest grinding plants. Bone sizes help estimate body size. Footprints show how an animal moved; several trails can suggest animals traveling together. Fossil leaves and wood reveal plants that grew nearby.

Some fossils belong to extinct groups, including trilobites and ammonites. Their fossils show that animals unlike those alive today once existed.

Read a place through time

Imagine three layers in a cliff that have not been turned over:

Start with the oldest layer at the bottom. Read upward toward the youngest. The place changed from sea to land with plants. Later, land animals left footprints there. The fossils support this sequence over a long time.

Sea fossils high in mountains show that their rock formed underwater. Later, the rock was pushed up as mountains formed. The ancient sea did not have to reach the height of today's peaks.

Scientists have also found fossil forests in Antarctica. Tree trunks, roots and leaves show that this icy land once had conditions where trees could grow.

Compare past and living animals

Early horse relatives lived more than 50 million years ago. They were about fox-sized, with four front toes and three back toes. Modern horses are larger and stand on one toe per foot. Fossils from different times show changes in feet, teeth and size. Many related branches existed; these finds are not a row of individual horses growing up.

Archaeopteryx lived about 150 million years ago. It had feathers and wings, as well as teeth and a long, bony tail. This mix of features helps connect birds with dinosaur ancestors.

A fossil gives clues, not a complete recording. Colors and sounds usually leave little direct evidence. Scientists compare many fossils and living things, and revise explanations when new finds add information.

A million years is already hard to picture. Earth is about 4.5 billion years old. One billion means one thousand million.

Put life in order

The first simple life appeared more than 3.5 billion years ago. For most of Earth's history, nearly all life was tiny and lived in water.

From oldest to newest, these events help mark the story:

• First simple life: more than 3.5 billion years ago.
• First fish: more than 500 million years ago.
• First dinosaurs: about 230 million years ago.
• First flowering plants: more than 130 million years ago.
• End of non-bird dinosaurs: about 66 million years ago.
• First modern humans: about 300,000 years ago.

A larger number of years ago means farther back in the past. "Today" sits at the newest end of a timeline.

Make a long timeline

Let 1 cm stand for 1 million years. Earth's 4,500 million years need 4,500 cm of line. That is 45 m, longer than a basketball court.

Measure 66 cm back from the "today" end for the end of the non-bird dinosaurs. Modern humans appeared 0.3 million years ago. That takes just 0.3 cm, or 3 mm, beside today. The tiny human part is hard to see on the full line.

Another picture squeezes Earth's whole history into one day. Non-bird dinosaurs die out about 21 minutes before midnight. Modern humans appear about 6 seconds before midnight.

Count generations

A generation is a step from parents to offspring. If generations take 25 years, then one million years contains:

That is 40,000 generations. Mice can have young within months, so they have many more generations in the same time. Generation length matters when comparing groups.

Scientists study rock layers and natural clocks in some rocks to work out ages. Tiny amounts of substances change at steady rates over long periods. Fossils then help show what lived at different times.

Modern humans did not live alongside the last non-bird dinosaurs. About 65.7 million years separate those dinosaurs' extinction from our species' appearance. Birds are living descendants of dinosaurs, so "non-bird" keeps the statement accurate.

A growing animal changes during its life. Evolution is different: inherited traits change in a group over many generations. Over very long times, changes can lead to new species.

Follow the inherited difference

Use the beetles on brown leaves to follow natural selection:

1. The beetles vary: some are green and some are brown.
2. Brown color makes a beetle harder for birds to spot there.
3. More brown beetles survive to reproduce.
4. Their offspring tend to inherit brown color.
5. Across many generations, brown color becomes more common.

No green beetle turns brown because it wants to hide. The group changes because different individuals leave different numbers of offspring. Inheritance connects one generation's advantage to the next.

Survival alone is not enough for this change. A beetle that leaves no offspring does not pass on its traits. Also, a helpful trait need not help in every habitat. Green beetles may have the advantage on green leaves.

People who connected the clues

Charles Darwin and Alfred Russel Wallace studied living things in many places. Each worked out the idea of natural selection. Their ideas were presented together in 1858. Darwin explained the idea in On the Origin of Species in 1859.

Darwin visited the Galápagos Islands in 1835. The islands have finches with different beaks. Thick beaks crack hard seeds; thin, pointed beaks can pick up insects. Evidence connects these finches to shared ancestors that reached the islands long ago. Different branches became suited to different foods.

Fossils and branches

Early whale fossils include animals with legs. They connect whales to four-legged land mammals living about 50 million years ago. Horse fossils also show changes across long periods.

Evolution is like a branching tree. Humans and today's chimpanzees share ancient ancestors. Humans did not develop from the chimpanzees alive today. Both branches have changed since their shared ancestors lived.

Natural selection has no plan for a final, perfect animal. It can make helpful traits common in the conditions present at the time. Some inherited changes happen by chance and do not help. Lamarck's neck-stretching idea misses the difference: a change gained through stretching during life is not passed on as an inherited longer neck.

An empty pond does not mean its frog species has vanished everywhere. Extinction means that every member of a species has died. No more offspring of that species can be born.

What puts a species at risk?

An endangered species faces a very high risk of extinction in the wild. Its numbers may be small, falling quickly, or threatened by habitat loss.

Species can lose the conditions they need through climate changes, disasters, hunting, habitat loss or arriving predators and diseases. Some individuals move or cope; others cannot. If the whole species disappears, it is extinct.

Most species that have ever lived are now extinct. Extinction happened long before people. About 66 million years ago, an asteroid struck near what is now Mexico. Fires, dust and cooling that followed were the main cause of the loss of non-bird dinosaurs and many other groups. Some mammals and birds survived.

Different routes to extinction

Dodos lived on Mauritius. They were flightless birds that nested on the ground. After sailors arrived, hunting and introduced animals put them at risk. Animals including rats and pigs ate eggs. The last widely accepted dodo sighting was in 1662; the last sighting is not an exact date for the last bird's death.

Passenger pigeons once flew in huge North American flocks. Heavy hunting and forest clearing reduced them. The last known bird, Martha, died at the Cincinnati Zoo on September 1, 1914. Even a once-abundant species can disappear.

Woolly mammoths had thick hair, small ears and fat for cold conditions. Warming after the last ice age reduced their cold grassland habitat, and people hunted them. Most disappeared by about 10,000 years ago. A small group lasted on Wrangel Island until about 4,000 years ago.

Help before the last individual is gone

Protecting habitat preserves food, shelter and breeding places. Stopping illegal hunting reduces losses. Removing predators introduced by people can protect island nests. Breeding programs can sometimes supply animals for release.

California condors fell to fewer than 30 birds. The last wild birds were brought into breeding programs in 1987. Breeding and releases increased their numbers to several hundred, with more than half living in the wild by the end of 2025. Continued protection matters even after numbers rise.