Evolution Requires Genetic Changes
Definition
Evolution
Evolution is defined as a change in the heritable characteristics of a population over time.
Only traits that are genetically inherited (passed from parent to offspring via DNA) contribute to evolution.
Heritable vs. Acquired Characteristics
- Heritable traits: encoded in DNA, subject to mutation, recombination, and natural selection.
- Acquired traits: result from environmental influence or learning during an organism’s lifetime.
- Heritable traits: darker fur color in moths due to a genetic mutation.
- Acquired traits: a tennis player develops stronger bones and muscles in their dominant arm.
Acquired traits cannot be passed to offspring.
Darwinian Evolution (Natural Selection)
Definition
Natural selection
Natural selection is the process where individuals with traits better suited to their environment have higher reproductive success, passing these traits to the next generation.
Charles Darwin’s theory of evolution by natural selection explains how populations change:
- Variation exists in populations due to random mutations, crossing over, independent assortment, and random fertilization.
- Overproduction of offspring means competition for resources (food, space, mates).
- Selection pressures (predation, disease, climate, competition) affect survival chances.
- Individuals with advantageous heritable traits survive and reproduce more successfully.
- These traits (alleles) become more common in the population over time.
Lamarckism and Its Falsification
- Jean-Baptiste Lamarck proposed that acquired characteristics (traits gained during an organism’s lifetime) could be inherited.
- No mechanism exists for changes in muscle use, behavior, or environment-driven traits to alter DNA in gametes.
Giraffes stretched their necks to reach higher leaves, and this lengthened neck was passed to offspring.
NoteModern genetics, which was unavailable during Darwin’s time, has confirmed that evolution occurs through changes in DNA within populations, not through the inheritance of acquired traits.
TipIf asked to compare Darwin and Lamarck, always emphasize inheritance of heritable vs. acquired traits.
Mechanisms of Evolution
- To understand how heritable characteristics change over time, we need to explore the mechanisms that drive evolution.
- The most important of these is natural selection.
1. Natural Selection: Survival of the Fittest
- Natural selection occurs when individuals with certain traits are better suited to their environment
- This leads them to having higher reproductive success.
- Over time, these advantageous traits become more common in the population.
- In a population of moths, darker-colored moths may be better camouflaged against predators in a polluted environment, giving them a survival advantage.
- Over generations, the population becomes dominated by darker moths.
2. Genetic Variation: The Raw Material for Evolution
- Natural selection can only act on traits that vary within a population.
- This variation arises from:
- Mutations: Random changes in DNA that can create new traits.
- Sexual Reproduction: The mixing of genes during reproduction creates unique combinations of traits in offspring.
- Without genetic variation, evolution would not occur because there would be no differences for natural selection to act upon.
- Remember, evolution acts on populations, not individuals.
- An individual cannot evolve during its lifetime, but a population can change over generations.
Nature of Science (NOS) - The Theory of Evolution
- Darwin’s theory of evolution by natural selection explains a vast range of observations (fossil record, biogeography, homologous structures, DNA evidence).
- It is extremely well supported and widely accepted.
- However, science cannot provide “absolute proof.”
- The theory of evolution is therefore called a theory, but in science this means a well-substantiated explanation, not a guess.
- Define evolution in terms of heritable characteristics.
- Why does the inheritance of acquired traits not lead to evolution?
- Compare Darwin’s theory of evolution with Lamarck’s theory.
- Explain why the theory of natural selection is described as a theory in science.
