Concept of Evolution: MDCAT Biology notes

Evolution and its rivals

Evolution is the process by which new species arise from existing species over long periods. It happens through innumerable small changes. Special creation holds that each species was created separately and unchanged. It relies on inspiration and faith rather than on testable evidence. Abiogenesis (spontaneous generation) was the old idea that life arises directly from non-living matter. Biogenesis states that life comes only from pre-existing life.

Origin and early history of life

• Alexander Oparin (and J.B.S. Haldane) proposed that organic molecules formed in a "primordial soup" on the early Earth.
• The oldest known fossils are prokaryotes, which arose about 3.5 billion years ago.
• Endosymbiotic theory (Lynn Margulis): mitochondria came from ingested aerobic bacteria and chloroplasts from photosynthetic prokaryotes. Both are in effect ancient bacteria now living inside larger cells. It has also been suggested that eukaryotic flagella came from spirochete-like prokaryotes.

Species and populations

• A species is a group of organisms with similar morphology and physiology that can interbreed in nature and produce fertile offspring.
• A population is the members of one species living in one area.
• The gene pool is all the genes (alleles) in a population.
• Polymorphism means two or more clearly different phenotypes exist in the same population.

Hardy–Weinberg principle

In a large population, the mechanism of inheritance does not by itself change allele frequencies. Frequencies stay constant from generation to generation unless something other than Mendelian segregation and recombination acts on them. Those other agents are mutation, migration (gene flow), genetic drift, non-random mating and natural selection.

$$p + q = 1 \qquad p^2 + 2pq + q^2 = 1$$

Here $p$ is the dominant allele frequency, $q$ is the recessive allele frequency, and $p^2$, $2pq$ and $q^2$ are the genotype frequencies.

Worked example: if 16% of a population shows a recessive trait, then $q^2 = 0.16$. So $q = 0.4$ and $p = 0.6$, and carriers make up $2pq = 0.48$, or 48%.

Agents of evolutionary change

• Mutation is the ultimate, major source of new variation. It is the only agent here that adds new alleles to the gene pool.
• Genetic drift is a change in allele frequency that happens by chance. Its effect is strongest in small populations.
  • Bottleneck effect: a natural disaster sharply cuts the population, and the survivors carry only some of the alleles.
  • Founder effect: a few individuals colonize a new area.
• Variation that leads to evolution must lie in the genes. Acquired characters are not inherited.

Isolation and speciation

New species form when populations become isolated and diverge. Geographical barriers include oceans, rivers, mountains and deserts. The atmosphere is not a barrier.

Evidence of evolution

• Homologous organs have the same basic structure but different functions, showing common ancestry (for example the forelimbs of human, whale and bat).
• Analogous organs have the same function but a different origin (for example insect and bird wings).
• Vestigial organs were functional in ancestors but are reduced now (for example the human appendix and tail vertebrae).
• Haeckel's biogenetic law: "ontogeny recapitulates phylogeny". An embryo's development repeats stages of its evolutionary history.

Common MDCAT traps

• Haeckel's law is ontogeny recapitulates phylogeny, not the reverse.
• Genetic drift is chance, not selection, and it matters most in small populations.
• A disaster that shrinks a population causes the bottleneck effect; colonization by a few causes the founder effect.
• Mutation, not drift or adaptation, increases variation in the gene pool.
• Vestigial organs are the ones that were functional in ancestors.

Quick revision

• Prokaryotes appeared about 3.5 billion years ago.
• Oparin proposed the primordial soup.
• The Hardy–Weinberg equation calculates genotype and allele frequencies.
• The gene pool is the total genes in a population.
• Endosymbiosis explains mitochondria and chloroplasts.