Genetics and Human Evolution
Genetics is the engine behind human evolution. Understanding how genes work, how they're passed on, and what forces change them over time gives you the tools to explain why humans look, function, and adapt the way we do. This section covers the core genetic concepts you'll need for the rest of the unit.
Key Genetic Terms in Human Evolution
DNA (deoxyribonucleic acid) is the molecule that carries all of your genetic information. It's organized into tightly coiled structures called chromosomes. Humans have 46 chromosomes (23 pairs).
Along those chromosomes sit genes, which are segments of DNA that code for specific traits. Humans have roughly 20,000–25,000 genes. Each gene can come in different versions called alleles. For example, the gene for blood type has three alleles: A, B, and O. Which alleles you inherit determines your blood type.
Two more terms you'll see constantly:
- Genotype is your genetic makeup, the specific combination of alleles you inherited from your parents. It determines what traits you could express.
- Phenotype is what actually shows up: your observable characteristics. Phenotype results from the interaction between your genotype and your environment. Height is a good example. Your genes set a range, but nutrition and health during childhood affect where you end up in that range.
Mitosis vs. Meiosis in Genetic Variation
These are two types of cell division, and they do very different things for genetics.
Mitosis produces two identical daughter cells. It's how your body grows and repairs itself. Every somatic (body) cell divides this way. Because the copies are identical, mitosis does not create genetic variation.
Meiosis is the type of cell division that produces gametes (eggs and sperm). It reduces the chromosome number by half, from 46 (diploid) to 23 (haploid), so that when two gametes combine at fertilization, the offspring ends up back at 46. Meiosis is a major source of genetic variation through two mechanisms:
- Independent assortment — Each pair of chromosomes lines up randomly during division, so the combination of maternal and paternal chromosomes in each gamete is different every time.
- Crossing over — Homologous chromosomes physically exchange segments of genetic material, creating allele combinations that neither parent had.
Together, these processes mean that every gamete a person produces is genetically unique, which is why siblings (other than identical twins) look different from each other.
Mendel's Laws and Human Diversity
Gregor Mendel's experiments with pea plants in the 1860s established the basic rules of inheritance that still apply today.
Law of Segregation: Each individual carries two alleles for each gene (one from each parent). During gamete formation, those alleles separate so that each gamete carries only one. This explains how two parents who both carry a recessive allele can produce a child who expresses the recessive trait.
Law of Independent Assortment: Alleles for different genes are passed on independently of each other during gamete formation. This means inheriting one trait (like hair texture) doesn't determine what you inherit for another trait (like eye color). This independent sorting is a big reason human genetic diversity is so vast.
Punnett squares are simple diagrams that let you predict the probability of offspring genotypes and phenotypes based on the parents' genotypes. For a single-gene trait, you set up a 2×2 grid with one parent's alleles across the top and the other's down the side, then fill in the combinations.
Heredity is the broader term for the passing of traits from parents to offspring through genetic material. Mendel's laws describe the specific patterns heredity follows.
Evolutionary Forces on Genetic Variation
Four main forces drive changes in the genetic makeup of populations:
- Mutation is a change in a DNA sequence. Mutations are the only source of entirely new alleles. Most are neutral, but some matter a great deal. The sickle cell allele, for instance, arose from a single point mutation but has major effects on health and survival. Lactase persistence (the ability to digest milk into adulthood) is another mutation that became common in populations with a long history of dairy farming.
- Gene flow is the transfer of alleles between populations, usually through migration and interbreeding. When people move and reproduce with a new population, they introduce alleles that may not have been present before. The spread of the Duffy-null allele in African populations is one example: this allele provides resistance to a particular form of malaria and its distribution reflects historical patterns of gene flow and selection.
- Genetic drift refers to random changes in allele frequencies due to chance, not because certain alleles are more advantageous. Drift has a much stronger effect in small populations. Two classic examples: the founder effect (a small group breaks off and starts a new population with limited genetic diversity) and the bottleneck effect (a disaster drastically shrinks a population, randomly eliminating alleles).
- Natural selection is the differential survival and reproduction of individuals based on their traits. Individuals with alleles that help them survive and reproduce in a given environment pass those alleles on more frequently. Resistance to malaria in regions where the disease is common and physiological adaptations to high altitudes in Tibetan and Andean populations are well-studied examples.
Population Genetics and Evolution
At its core, evolution is the change in allele frequencies within a population over time. That's the definition you need to know. Evolution doesn't happen to individuals; it happens to populations.
Population genetics is the field that studies how and why allele frequencies shift within and between populations. The genetic variation already present in a population provides the raw material that natural selection, drift, gene flow, and mutation act on.
When these forces operate over many generations, they can shift allele frequencies enough to produce significant biological differences between populations. Over long enough timescales, this process can lead to speciation, the emergence of entirely new species. That's the link between the genetics you're learning here and the broader story of human evolution.