Descent With Modification: A Darwinian
View of Life
30 November, 2005
Text Chapter 22
Descent with modification is the unifying principle of biology.
The genetic changes that occur due to the agents of microevolution
(genetic drift, gene flow, mutation, sexual selection, and natural
selection) mean that no population of organism remains the same from
generation to generation.
This mechanism answers many of the “Why…” questions in biology.
The idea that populations change over time is simple, obvious, and
controversial, because it contradicted or seemed to contradict other
explanations of life.
Science had little difficulty accepting the idea that populations change
over time, because of the scientific tradition of embracing the best
possible explanation for the observed data. Philosophy and religion do
not have this kind of tradition, and tend to resist alternative explanations.
Darwin in Historical Context
Charles Darwin proposed the idea that populations change over time, and
suggested that individuals with advantageous heritable traits have more
offspring than average (natural selection).
Although Darwin and Mendel were contemporary, Mendel’s work was
not widely accepted for more than fifty years after the publication of The
Origin of Species.
Darwin did draw on nineteenth-century progress in the field of geology.
The idea that the Earth could change over time led to biologists
wondering if life could also change.
Lamarck proposed inheritance of acquired characteristics, but there was
no evidence for this model of evolutionary change. Darwin’s model of
descent with modification was based on extensive field work and
observation.
Darwin noticed that a new species could arise from an ancestral form
by gradual accumulation of adaptations to a different environment.
This is descent with modification.
Ernst Mayr’s summary of Darwin’s observations and inferences:
O1. All species have the reproductive capacity to increase exponentially
in number.
O2. Populations tend to remain relatively stable in size.
O3. Environmental resources are limited.
I1. Not all individuals will reproduce, and not all offspring will survive
to reproduce in turn.
O4. Individuals in a population vary extensively in characteristics.
O5. Much of this variation is heritable.
I2. Individuals best suited to the environment will be the ones most
successful at producing offspring.
I3. Unequal reproduction will lead to gradual change, with favorable
characteristics accumulating over time.
Excess
Reproductive
Capacity
Variation Within a Species
Adaptive Phenotype
Selection can cause substantial change in the genotype and
phenotype of a population. Darwin drew on artificial selection as a
prime example.
Selection is an interaction
between an individual and its
environment, but it is the
population, not the
individual, that evolves.
Selection can only amplify or
diminish heritable variation.
Selection is situational:
characteristics that are
selected for in one
environment may be selected
against in another.
Evidence of evolution comes from:
Biogeography - Closely related species tend to be geographically close.
Fossil record - Shows gradual change and descent with modification.
Comparative anatomy - Anatomical similarity in diverse structures.
Comparative embryology
- developmental
pathways are among the
most conserved.
Structures are adapted for
diverse purposes.
Molecular biology - all
living things share a
common ancestor. The
degree of relatedness of
two species (or even two
individuals) can be
determined from DNA
sequence. This is the
strongest and least
refutable evidence for
evolution.
Convergent Evolution
Island Biogeography