Descent With Modification: A Darwinian View of Life

30 November, 2005Text 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.

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.

Ernst Mayr’s summary of Darwin’s observations and inferences:

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