Natural selection, a concept first theorized by Charles Darwin and Alfred Russel Wallace, is the adjustment of genes (DNA) throughout generations based on factors that help living organisms survive and reproduce. Sometimes this is known as survival of the fittest or the adaptation of organisms that are better suited to the environment.
Natural selection can be defined as the process through which species adapt to their environment in order to survive. Natural selection occurs when traits that predispose organisms to survive in an environment are passed from parents to their offspring.
- Organisms that are unable to adapt to changes in the environment are unlikely to survive or reproduce.
- Those that can adapt are much more likely not just to survive, but also to thrive and reproduce.
- As offspring are born, they will have the advantageous genetic traits passed on from their parents.
- As a result, the species will change over time.
- Why? Because offspring will develop traits or characteristics that predispose them to do well in their natural environment.
This idea that species change over time in order to survive in their environment is what natural selection is. This idea is the cornerstone behind Charles Darwin's theory of evolution. Practically speaking, natural selection occurs when mutations (changes) to an organism's DNA are advantageous. Such mutations giving the organism a competitive advantage.
There are many real-world examples of the impact of natural selection in animals throughout the animal kingdom.
- Galapagos finches have different types of beaks. During times of drought, the finches with the larger beaks survived better than those with smaller beaks. During rainy times, more small seeds were produced and the finches with smaller beaks fared better. Since the environment supports both types of beaks, both remain in the population.
- Peacock females pick their mate according to the male's tail. The ones with the largest and brightest tails mate more often. Today, because most breeding males have large, bright tales, it is rare to find males that do not have bright feathers. This is because tail characteristics are passed from adult breeding peacocks to their offspring.
- Most peppered moths used to be a light color with black spots. When London's atmosphere became soot-filled due to burning coal during the Industrial Revolution, the white trees got darker. As a result, light-colored moths were eaten by birds more readily. Within months, most moths became darker. After factories had to reduce soot output, light-colored moths increased in number.
- Deer mice that migrated to the sandhills of Nebraska quickly changed from dark brown to light brown, a change that allowed the species to survive in this environment. Scientists say this change came about as a result of just one genetic change. As a result, the deer mice quickly became better able to hide from predators in the sand, allowing for survival in the new environment.
- Most eels eat fish and cephalopods, which does not require a particularly powerful bite. Moray eels evolved to grow a second set of jaws and teeth that allowed them to eat hard-shelled prey, such as crabs and snails.
- Warrior ants have a chemical signal that tells other ants in the family not to attack. Some have adapted and learned to imitate the chemical signal from other colonies. This allows them to invade and take over another colony without being detected by the workers.
- The disease malaria is among the leading causes of death around the world. Those who are predisposed to be more resistant to malaria than others are less likely to be affected by this disease. The malaria parasite cannot live in sickle-shaped blood cells. People with sickle cell anemia have a better chance of surviving in an area with malaria, reproducing and sharing this trait with their offspring.
- Disease resistance as a whole, not just malaria, is a strong natural selection factor for human beings. For example, the way SARS-CoV-2 evolved in bats allowed it to become the type of pathogen particularly capable of wreaking havoc on the human population. Researchers are still trying to identify what factors determine whether or not a person is able to fight off COVID-19.
- As humans moved from rural areas to urban areas and started living in cramped quarters, disease spread quickly through the population. Only those humans with strong immune systems survived. As they reproduced, their offspring tended to be born with immune systems that made it possible for them to survive in a densely populated environment.
- Scientists suspect that the shape of human hands evolved over time. Those with better manual dexterity had an advantage in tossing a spear, making tools or throwing a rock in order to get the food they needed to survive.
While natural selection tends to occur much more slowly in plants than animals, it does definitely happen.
- If plant seeds find their way into a compost pile, they'll often take root and produce seedlings without being planted or receiving any nurturing. Due to their ability to take root and grow in inhospitable conditions, these seedlings are often particularly strong. When transplanted into a garden, these particularly resilient plants often outperform seedlings that were intentionally grown.
- When seeds are saved from plants grown in a particular geographic area, the plants grown for future generations of seeds tend to be particularly well-suited for that climate, and so will produce better crop yields when grown in similar environmental conditions. This is why it's common for gardening enthusiasts to seek seeds from companies local to their areas.
- The field mustard plant survived drought in southern California because of genetic changes that reduced the growing season length. The original mustard plants had a longer growing season and died from lack of water in the long California summers. Only plants that produced flowers early in the season produced seeds. Thus, field mustard quickly evolved to have a short growing season.
- There are two species of Encelia: Asteraceae in Baja, California, with one type living in the desert region and the other living in the coastal dunes that border the desert. Even though these two species could interbreed, they do not. This is likely due to maintaining selection that has to take place to prepare each of the species to thrive in its distinct environment.
- Key factors related to plant appearance and characteristics impact natural selection in flowering plants. For example, flower height, stem length and other traits will vary based on factors like what pollinators are present while flowers are growing and environmental factors like snowmelt and other things that impact temperature and access to water.
Natural selection depends on the environment. Traits that are helpful in one environment will not be helpful at all in others. Organisms that have the ability to adapt to their environment are the ones that will survive and pass on their mutated genes to their offspring.
- A biological ecosystem may be home to giraffes with different length necks. If something caused low-lying shrubs to die out, the giraffes with shorter necks would not get enough food. They would not survive to produce offspring. After a few generations, the surviving giraffes would have longer necks, because that body type is more suited to survive in the environment.
- If a hypothetical species of rats live in a certain type of tree with the branches evenly spaced, natural selection would lead the rats to be right-sized for that type of tree. Smaller rats could not reach from branch to branch and larger rats would break the branches and fall. Right-sized rats would survive and reproduce. Soon, most rats would be just the right size for the tree branches.
- If there are red bugs and green bugs in a hypothetical environment where predators (such as birds) prefer the taste of the red bugs, the green ones are more likely to survive. Soon there will be many green bugs and few red bugs. The green bugs will reproduce and make more green bugs, leading to a reality in which nearly all of the bugs born into this area will be green.
- In a hypothetical ecosystem that is prone to flooding, lizards with long legs could climb better to avoid floods and reach food. As a result, eventually, most of the lizards in that type of ecosystem would have long legs. This trait would be passed to them from their parents, who survived because they had long legs.
- Insects can become resistant to pesticides very quickly, sometimes in one generation. If an insect has a mutation that makes it resistant to a certain chemical, then some of its offspring will also be resistant. Insect generations can be just a matter of weeks, so insects in an area can become immune to a chemical very quickly. Before long, the pesticide might not affect newly born offspring at all.
- Bacteria can become resistant to antibiotics. This can happen very quickly since bacteria can produce several generations within one day. The few with mutations that make them able to withstand treatment from antibiotics will survive and reproduce. This leads to antibiotic-resistant strains of bacteria.
Charles Darwin demonstrated that organisms with inheritable traits that help them survive and reproduce will become more common in a habitat over time. This is the mechanism for evolution. Now that you better understand how natural selection works and how different species today have been shaped by natural selection, consider some examples of evolution in biology and beyond. To learn even more, explore examples of macroevolution across time and species.