Lesson 8.4 Evolution Acts on the Phenotype

Would albinism be an advantage?

This rabbit is albino, meaning it lacks pigment in its skin, fur, and eyes. The same thing happens in other species, including humans. To most animals albinism would be a disadvantage since they need to blend into their environment to avoid predators or catch prey. How, then, does the gene that causes albinism stay in the gene pool?

Evolution Acts on the Phenotype

Natural selection acts on the phenotype (the traits or characteristics) of an individual. On the other hand, natural selection does not act on the underlying genotype (the genetic makeup) of an individual. For many traits, the homozygous genotype, AA, for example, has the same phenotype as the heterozygous Aa genotype. If both an AA and Aa individual have the same phenotype, the environment cannot distinguish between them. So natural selection cannot select for a homozygous individual over a heterozygous individual. Even if the "aa" phenotype is lethal, the recessive a allele, will be maintained in the population through heterozygous Aa individuals. Furthermore, the mating of two heterozygous individuals can produce homozygous recessive (aa) individuals. However, natural selection can and does differentiate between dominant and recessive phenotypes.

Carriers

Since natural selection acts on the phenotype, if an allele causes death in a homozygous individual, aa, for example, it will not cause death in a heterozygous Aa individual. These heterozygous Aa individuals will then act as carriers of the a allele, meaning that the a allele could be passed down to offspring. People who are carriers do not express the recessive phenotype, as they have a dominant allele. This allele is said to be kept in the population's gene pool. The gene pool is the complete set of genes and alleles within a population.

For example, Tay-Sachs disease is a recessive human genetic disorder. That means only individuals with the homozygous recessive genotype, rr will be affected. Affected individuals usually die from complications of the disease in early childhood, at an age too young to reproduce. The two parents are each heterozygous (Rr) for the Tay-Sachs gene; they will not die in childhood and will be carriers of the disease gene. This deadly allele is kept in the gene pool even though it does not help humans adapt to their environment. This happens because evolution acts on the phenotype, not the genotype (Figure below).

Summary

• Natural selection acts on the phenotype (the traits or characteristics) of an individual, not on the underlying genotype.
• Carriers of a trait can show no symptoms of a recessive disease and, yet, still pass it on to their offspring.

Review

1. What's the difference between phenotype and genotype?
2. Does natural selection act on the genotype or phenotype?
3. Explain how a lethal recessive gene can stay in the gene pool.