Genetic Variation & Natural Selection
Different organisms, even of the same species, do not have identical genetic information. This is called genetic variation, and evolution depends on it. Variation arises from three mechanisms:
- Mutation — a permanent change in the DNA nucleotide sequence, and the ultimate source of new alleles.
- Meiosis — crossing over (in prophase I) and independent assortment (in metaphase I) shuffle alleles into new combinations.
- Sexual reproduction — random fertilisation combines genetic material from two distinct sources: a zygote inherits half its DNA from its father and half from its mother.
Where a mutation occurs matters. Mutations in somatic cells (body cells) can cause serious disease but are not passed on to future generations. Mutations in germline cells (gametes) are passed on — when the embryo's DNA replicates, the mutated gene is copied into every cell of the offspring.
Inherited variants are expressed through dominant and recessive patterns. Each individual carries two alleles per gene, so it is homozygous (two of the same allele) or heterozygous (one of each). A dominant trait is expressed with just one dominant allele; a recessive trait requires two copies of the recessive allele.
OUTCOMES OF MUTATION
| Outcome | Named example | What happens |
|---|---|---|
| Lethal | Cystic fibrosis; sickle cell anaemia | CF: faulty recessive allele — CC normal, Cc carrier, cc sufferer. Sickle cell: a single-base substitution |
| Disadvantageous | Osteoporosis — LRP5 gene | Mutations in 'low density lipoprotein receptor related protein 5', a bone-density gene, make bones brittle — harmful but generally not fatal |
| Neutral | Changes in non-coding regions | No effect on phenotype or function — but still inherited if in the germline |
| Beneficial | Sickle cell carriers | Heterozygotes have a reduced chance of contracting malaria — the parasite cannot reproduce in sickle-shaped red blood cells |
Sickle cell anaemia shows how small a lethal mutation can be. One base changes — an A to a T in a substitution mutation — giving a protein with one changed amino acid, and red blood cells that collapse into a sickle shape.
Sickle cell anaemia: a single A → T substitution changes one amino acid, and the whole red blood cell changes shape.
GENE POOLS AND SELECTION PRESSURES
A gene pool is all of the alleles of all of the genes for all of the individuals in a population at a particular time. A larger gene pool means greater diversity — a greater array of possible advantageous traits — so the population is more likely to survive a new selection pressure when the environment changes. A selection pressure is any environmental factor that affects survival and reproduction, and pressures may be biotic (living) or abiotic (non-living):
| Biotic pressures (living) | Abiotic pressures (non-living) |
|---|---|
| Introduction of a new species | Availability of water |
| Competition — own or other species | Light, oxygen, carbon dioxide |
| Predators | pH of soil |
| Disease agents | Temperature of air and soil |
| Lack of food | Nutrient availability |
| Human activities; symbiotic relationships | Weather — rainfall, wind, sunlight |
NATURAL SELECTION
Natural selection is the process whereby organisms with genetic traits better suited to their particular environment tend to survive longer and reproduce, passing these favourable alleles to the next generation. Over generations, organisms with more favourable genetic information increase in number while those with less favourable traits decline. Natural selection keeps favourable alleles in the gene pool and gradually removes unfavourable ones — and that change in allele frequency is evolution.
A standard example involves moth colouration. When industrial pollution darkens tree trunks, dark moths become less visible to predators than light moths. Dark moths survive and reproduce in greater numbers, so dark-allele frequency rises and light-allele frequency falls. The gene pool has changed: natural selection has occurred.
Pollution favours black moths over white moths, because the black moths are less visible to predators. The gene pool changes — natural selection has occurred.
'Fittest' does not mean physically fittest. It means carrying the favourable traits that increase survival chances — a coat colour, an ability to source food, an ability to evade a predator. Once a favourable allele's frequency has risen, the evolved population is better adapted to its niche and has a greater chance of surviving and increasing its population size.
EVOLUTION OF ANTIBIOTIC RESISTANCE
The development of antibiotic resistance in bacteria is a clear example of natural selection producing evolution. DNA mutations occur with high frequency in bacterial reproduction, and some mutations happen to confer resistance to antibiotics (drugs designed to kill specific strains of bacteria). Resistant bacteria survive and reproduce while non-resistant bacteria are killed, so the frequency of the resistance allele increases in the gene pool.
Note the direction of causation: mutations that confer a survival benefit increase in frequency, but natural selection does not induce the mutations. The environment selects among variants that already exist — organisms do not consciously adapt to it.
Natural selection also does not work in isolation. It operates alongside mutation, genetic variation, migration and genetic drift (random change in allele frequency, strongest in small populations), with gene flow moving alleles between populations.
In summary: Genetic variation → selection pressure → differential survival and reproduction → change in allele frequency → evolution