Darwin’s Theory of Evolution and Natural Selection
Darwin’s Theory of Evolution
Darwin’s theory of evolution explains how species of living things have changed over geological time. The theory is supported by evidence from fossils and by the rapid changes seen in microorganisms such as antibiotic-resistant bacteria. Many species have become extinct in the past, and extinctions continue to occur.
Darwin’s Finches
Darwin studied the wildlife on the Galápagos Islands—a group of islands on the equator almost 1,000 kilometers west of Ecuador. He noticed that the finches (songbirds) on the different islands were fundamentally similar but showed wide variations in their size, beaks, and claws from island to island. For example, their beaks differed depending on the local food source. Because the islands are so distant from the mainland, Darwin concluded that the finches that had arrived there in the past had changed over time. Darwin studied hundreds more animal and plant species. After nearly 30 years of research, in 1858 he proposed his theory of evolution by natural selection.
Natural Selection
The theory of evolution states that evolution happens by natural selection. Here are the key points:
- Individuals in a species show a wide range of variation.
- This variation is due to differences in genes.
- Individuals with characteristics best suited to the environment are more likely to survive and reproduce.
- The genes that allowed the individuals to be successful are passed to their offspring in the next generation.
Antibiotic-Resistant Bacteria
Microorganisms such as bacteria and viruses reproduce very rapidly and can evolve in a relatively short time. One example is the bacterium E. coli. Its DNA can be damaged or changed during replication, and most of the time this causes the death of the cell. But occasionally, the mutation is beneficial for the bacteria. For example, it may allow resistance to an antibiotic. When that antibiotic is present, the resistant bacteria have an advantage over the bacteria that are not resistant. Antibiotic-resistant strains of bacteria are an increasing problem in hospitals.
Extinction
Individuals poorly adapted to their environment are less likely to survive and reproduce than those that are well adapted. Similarly, a species poorly adapted to its environment may not survive and could become extinct.
Factors that can cause a species to become extinct:
- Changes to the environment, such as a change in climate
- New diseases
- New predators
- New competitors
Human Impact on the Environment
Human beings compete with other living things for resources and space. As the world’s population increases and standards of living improve, there is a serious danger of permanent change to the global environment. Human activities have led to environmental pollution and a reduction in the land available for other animals and plants, making it difficult for some species to survive. There is a need to achieve a level of development that also sustains the environment for future generations.
Population Growth
Like all living things, humans exploit their surroundings for resources. Before the beginning of agriculture about 10,000 years ago, small groups of humans wandered across large areas, hunting and gathering just enough food to stay alive. Population numbers were kept low because of the difficulty of finding food. The development of agriculture led to a population explosion that has accelerated enormously during the past 500 years. Unlike other species, humans can adapt to and survive in almost all habitats and climates.
Pollution
Land Pollution
Most rubbish is buried in landfill sites, and not all of it comprises safe materials. Even common household items can contain toxic chemicals such as poisonous metals. Many smoke alarms contain radioactive americium. Industrial waste is also discharged onto the land. Many farmers apply pesticides to improve their crops, but these can damage living things. Toxic chemicals can be washed from the land into rivers, lakes, and seas.
Water Pollution
Water pollution is caused by the discharge of harmful substances into rivers, lakes, and seas. Many aquatic invertebrate animals cannot survive in polluted water, so their presence or absence indicates the extent to which a body of water is polluted.
Air Pollution
Factories can cause air pollution. The most common source of air pollution is the combustion of fossil fuels. This usually happens in vehicle engines and power stations.
Organisms in Their Environment
Animals and plants have features that allow them to adapt to their environment. The organisms in a community compete with each other for limited resources, including water and space. Plants also compete with each other for light and minerals.
Competition
Animals: food, water, space
Plants: light, water, space, mineral salts
Definitions
Environment: All the conditions that surround a living organism
Habitat: The place where an organism lives
Population: All the members of a single species that live in a habitat
Community: All the populations of different organisms that live together in a habitat
Ecosystem: A community and its habitat