IB Biology Topic A4.1.1: Evolution as Change in the Heritable Characteristics of a Population Notes

Evolution Is a Change in a Population's Heritable Characteristics, Not a Change in Individuals

1. Evolution is a change in the heritable characteristics of a population over generations.
2. Populations evolve, but individual organisms do not: an individual keeps the alleles it was born with for life.
3. What changes over time is the allele frequency within the population.

Only Heritable Variation Can Drive Evolution

1. Heritable characteristics are encoded in DNA and passed from parent to offspring through gametes.
   1. Example: darker fur colour in a mouse caused by a genetic mutation.
2. Acquired characteristics develop during an organism's lifetime through use, environment, or learning.
   1. Example: a tennis player builds stronger bones and muscle in their dominant arm.
3. Only heritable variation can drive evolution, because only DNA passes to the next generation.

Natural Selection and Other Mechanisms Shift Allele Frequencies

1. Natural selection is the main mechanism that changes allele frequencies in a population.
   1. Individuals whose heritable traits suit the environment tend to survive and reproduce more.
   2. Their alleles therefore become more common over generations, so the population changes.
2. Selection is not purposeful: no organism chooses to change and no trait appears in order to be useful.
3. Other mechanisms, such as genetic drift and gene flow, can also change allele frequencies.

Lamarckism Was Falsified: Acquired Traits Are Not Inherited

1. Jean-Baptiste Lamarck proposed that characteristics acquired during life could be passed to offspring.
   1. He suggested giraffes stretched their necks to reach high leaves and passed the longer neck on.
2. This idea was falsified: there is no mechanism for use or environment to change the DNA in gametes.
3. Modern genetics confirms that evolution occurs through heritable DNA changes in populations, not through inherited acquired traits.

Falsification: How Lamarck's Idea Was Tested and Rejected

1. A scientific claim must be falsifiable: open to being disproven by evidence.
2. Lamarckism predicted acquired traits would appear in offspring, and this prediction failed under testing.
3. The Darwinian model, based on heritable variation, survived testing and replaced it.

A Scientific Theory Is a Well-Substantiated Explanation, Not a Guess

1. The theory of evolution by natural selection explains a vast range of observations.
   1. These include the fossil record, biogeography, homologous structures, and DNA evidence.
2. In science, a theory is a well-substantiated explanation supported by large amounts of evidence.
3. Science does not offer absolute proof, so a strongly supported explanation is still called a theory.