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Chapter 19: Mechanisms of EvolutionIntroduction
21.1 What Is the Relationship between Fact and Theory in Evolution?
Evolution is the change in genetic composition of populations over time.
Evolutionary change is observed in lab experiments, natural populations, and the fossil
record.
These genetic changes drive the origin and extinction of species and the diversification of
life.
Biologists have also accumulated evidence on how evolutionary changes occur.
Evolutionary theory is the understanding of the mechanisms of evolutionary change.
Evolutionary theory has many useful applications:
Understanding and treating diseases
Developing better agricultural crops and industrial processes
Understanding the diversification of life and how species interact
Allowing predictions about the biological world
In everyday speech, “theory” means an untested hypothesis, or a guess.
Evolutionary theory is not a single hypothesis, and it is not guesswork.
A vast array of geological, morphological, and molecular data all support the factual
basis of evolution.
Even before Darwin, several biologists had suggested that species change over time.
But no one had proposed a viable mechanism for evolution.
The young Charles Darwin was passionately interested in geology and natural history.
In 1831 he was recommended for a position on the HMS Beagle, for a five-year survey
voyage around the world.
Darwin often went ashore to collect specimens and make observations.
In the Galápagos Islands he observed that species were similar to, but not the same as,
species on the mainland of South America. He also realized that species varied from
island to island.
Darwin postulated that species had reached the islands from the mainland, but then had
undergone different changes on different islands.
He began to think about what could be a mechanism for such changes.
These observations, and many others, led Darwin to propose an explanatory theory for
evolutionary change based on three propositions:
© 2014 Sinauer Associates, Inc.
, Species change over time.
Divergent species share a common ancestor, and species have diverged gradually
through time (descent with modification).
The mechanism that produces the change is natural selection: the differential
survival and reproduction of individuals based on variation in their traits.
Darwin amassed evidence to support his ideas until 1858, when he received a letter from
another naturalist, Alfred Russel Wallace.
Wallace proposed a theory of natural selection almost identical to Darwin’s.
A paper with the work of both men was presented in 1858 to the Linnaean Society of
London.
Darwin published his book, The Origin of Species, in 1859.
The book provided exhaustive evidence from many different fields to support evolution
and natural selection.
The Origin of Species spawned a great deal of research documenting the history of life
and testing evolutionary ideas.
But the genetic basis of evolutionary change was not understood until after 1900.
In the twentieth century, Gregor Mendel’s publications were rediscovered, the roles of
chromosomes and mutations were discovered, and population genetics was
established.
A “modern synthesis” of genetics and evolution took place over the period 1936–1947.
In 1953, James Watson and Francis Crick published their paper on the structure of DNA.
In the late 1970s, DNA sequencing techniques were developed, allowing study of gene
structure and determination of amino acid sequences in proteins.
For a population to evolve, its members must possess heritable genetic variation.
The phenotype is the physical expression of an organism’s genes.
Features of a phenotype are the characters (e.g., eye color); the specific form of a
character is a trait (e.g., blue).
A heritable trait is at least partly determined by genes.
Genetic makeup of an organism is the genotype.
A population evolves when individuals with different genotypes survive or reproduce at
different rates.
Different forms of a gene are called alleles.
Alleles exist at a locus (a particular site on a chromosome).
The gene pool is the sum of all copies of all alleles at all loci in a population.
It contains the genetic variation that produces the phenotypic traits on which natural
selection acts.
© 2014 Sinauer Associates, Inc.
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