Natural Selection and the Evolution of Species
**Natural Selection**
- Over time, populations with traits less suited to the environment will decrease.
- Organisms best adapted to their environment will thrive.
- Giraffe Example: Giraffes with longer necks can reach higher leaves, giving them an advantage.
- Accumulated changes over generations can lead to the formation of new species.
**Neo-Darwinism – Synthetic Theory**
Neo-Darwinism rejects Lamarckism. The unit of evolution is the population, not individuals. Genetic mutations and natural selection drive evolutionary change. Evolution is a continuous process requiring significant time for new species to emerge.
- Example of Synthetic Theory: Consider two wild rabbits with different alleles: allele A for dark fur and allele a for white fur.
- Due to resource scarcity, predators will more easily spot and kill the white rabbits.
- Dark-furred rabbits are better adapted to a particular territory and will outcompete others.
- Over generations, individuals with allele A will become more common, while others will disappear.
**Criticism Against Neo-Darwinism**
It does not explain the *passivity* of evolutionary adaptation. It does not address macroevolution (major changes in a group), which Darwin argued occurs through allopatric speciation.
**Theories on the Origin of Species**
- Fixism: Species remain unchanged since their creation.
- Evolutionism: New species arise from a common ancestor due to various factors.
- Lamarckism: Proposed by Jean-Baptiste de Monet, Knight of Lamarck, in his 1809 work *Philosophie Zoologique*. It suggests that environmental changes lead to changes in habits, which in turn modify organs. These acquired changes are then inherited, leading to species transformation over time. The giraffe’s long neck is often used as an example.
- Darwin-Wallace Theory: Explained below.
- Neo-Darwinism: See above.
**Criticism Against Lamarckism**
Modern genetics does not support it. Changes caused by environmental factors in an individual’s genes are not transmitted to their offspring.
**Foundations of Darwin and Wallace’s Theory**
- The world is not static but constantly evolving.
- Evolution is a gradual and continuous process.
- Similar organisms share a common ancestor.
- Evolution is the result of natural selection.
Charles Darwin, during his voyage on the HMS Beagle (1831-1836), observed flora and fauna at various ports. Alfred Russel Wallace, 17 years younger than Darwin, shared a passion for nature and traveled to tropical regions of South America and the East Indies.
**Darwinism: Evolution Through Natural Selection**
In a giraffe population, individuals exhibit variation in traits like leg and neck length. During droughts, those with longer necks can reach higher branches and access leaves. Competition for resources leads to the selection of individuals best adapted to the environment. Over generations, these advantageous traits become more prevalent, leading to changes in the species.
**Wallace and Darwin’s Evolutionary Mechanism**
- Individual Variability: Individuals within a species have different characteristics.
- Competition for Resources: Limited resources lead to competition, and not all individuals can reproduce.
- Survival of the Fittest: Individuals with advantageous traits are better adapted to survive and reproduce.
- These individuals pass on their beneficial traits to their offspring.
**Artificial Selection**
- Humans select cows with larger udders for breeding because they produce more milk.
- If milk prices drop, these cows might be sent to mountainous regions where they need to escape predators. Cows with large udders are not well-suited for high mountains. Those with smaller udders will survive and reproduce, passing on this trait.