69. Variation and Evolution

Variation refers to the differences between individuals of the same species. These differences are caused by a combination of genetic and environmental factors, which can eventually lead to evolution.

1. Causes of Variation

An individual's observable characteristics, or phenotype, are determined by two main factors:

• Genetic Variation: Apart from identical twins, every person has a unique genome. Mutations (changes in DNA) create new alleles, leading to different traits.

• Environmental Variation: The conditions in which an organism grows also affect its characteristics. For example, diet, sunlight exposure, and lifestyle choices can alter physical appearance regardless of genetics.

Most characteristics, such as height or skin tone, are a result of the interaction between many genes and various environmental factors.

2. Natural Selection

Charles Darwin developed the theory of natural selection in the 1800s to explain how species change over time.

• Survival of the Fittest: Individuals with characteristics best suited to their environment are more likely to survive.

• Reproduction: Surviving individuals are more likely to breed successfully.

• Inheritance: The beneficial alleles that provided a survival advantage are passed on to the next generation.

Over many generations, these advantageous traits become more common in the population.

3. Evolution and Speciation

Evolution is the gradual change in the inherited characteristics of a population over time.

• Common Ancestry: The theory suggests that all living species evolved from simple life forms that first developed over 3 billion years ago.

• Speciation: If two populations of the same species become so different due to natural selection that they can no longer interbreed to produce fertile offspring, they are considered two distinct new species.

4. Evidence for Evolution

While it took a long time for Darwin's theories to be accepted, they are now supported by significant evidence:

• Fossil Records: Show how organisms have changed over millions of years.

• Antibiotic Resistance: We can observe evolution in real-time as bacteria develop resistance to drugs through natural selection.

Summary Table: Key Concepts