|
Random Mutations and Evolutionary Change: Ronald Fisher, JBS Haldane, & Sewall Wright
For 70 years after the publication of the Origin of Species, it seemed as if
Lamarck's shadow would loom
forever over Darwin. On the one hand, most biologists came to the reality of evolution — that
living species shared a common ancestry and had been transformed over time. But
natural selection — the
engine of evolution, according to Darwin — remained controversial. Many biologists argued that
there must be some built-in "direction" to the variation that arose in each generation,
helping to push each lineage towards its current state.
Many of these first geneticists who rediscovered
Mendel's insights around 1900
also opposed natural selection. After all, Darwin had talked of natural selection gradually altering
a species by working on tiny variations.
But the Mendelists found major differences between traits encoded by
alleles. A pea was smooth or wrinkled,
and nothing in between. In order to jump from one allele to another, evolution must make giant
jumps—an idea that seemed to clash with Darwin.
Natural selection in a Mendelian world
But in the 1920s geneticists began to recognize that natural selection could indeed act on
genes. For one thing, it became clear that
any given trait was usually the product of many genes rather than a single one. A
mutation to any one of the genes
involved could create small changes to the trait rather than some drastic transformation. Just as
importantly, several scientists — foremost among them Ronald Fisher (above left), JBS
Haldane (above right), and Sewall Wright (below left) — showed how natural
selection could operate in a Mendelian world. They carried out breeding experiments like previous
geneticists, but they also did something new: they built sophisticated mathematical models of evolution.
Small, not drastic, changes
Known as "population genetics," their approach revealed how mutations arise and, if they
are favored by natural selection, can spread through a population. Even a slight advantage
can let an allele spread rapidly through a group of animals or plants and drive other forms extinct.
Evolution, these population geneticists argued, is carried out mainly by small mutations, since drastic
mutations would almost always be harmful rather than helpful.
Wright introduced the most compelling metaphor in population genetics, known as the
"adaptive landscape"
(see figure, below). You can imagine the varying fitness
of different combinations of genes as a hilly landscape, in which the valleys represent less-fit combinations
of genes and the peaks represent the fitter ones. Natural selection tends to move the populations towards the
peaks of the hills. But since the environment is always changing, the peaks shift, and the populations follow
after them in a never-ending evolutionary journey. |
Natural selection in the wild
Population genetics became one of the key elements of what would be called
the Modern Synthesis. It showed that natural selection could produce evolutionary
change without the help of imaginary Lamarckian forces. Scientists have
used the mathematical tools developed by Fisher, Wright, and Haldane to
measure evolutionary change in the wild with exquisite precision. Their
insights have even allowed medical researchers to decipher the puzzle
of some hereditary diseases. Sickle-cell anemia, for example, is caused
when children inherit two defective copies of a gene involved in making
hemoglobin. But a single copy of this allele can give some protection
against malaria (see figures, right). Natural selection finds a balance between the reproductive
disadvantage of being born with two copies of the allele and the advantage
of having one. Genetic disorders such as sickle-cell anemia are actually
the agonizing byproduct of natural selection acting on our ancestors.
|
