Dicot stem anomalous secondary growth occurs due to following reasons:

Anomalous position of cambium 

e.g.,Serjania sp. and Bauhinia sp.

Abnormal behavior of normal cambium

e.g., Bignonia, Aristolochia, Tinospora, Clematis etc.

Extrastelar cambium formation

e.g., Amaranthus sp. 

Interxylary phloem or included phloem

e.g., Entada sp (box bean), Combretum sp (Combretum), Strychnos sp.

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Hitchhiking: Change in the frequency of an allele due to linkage with a selected allele at another locus.

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Dollo’s law: A biological generalization positing that complex characters, once lost in evolution, are extremely unlikely to reappear and thus the loss of complex characters is virtually always irreversible.

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Van Valen law- he found a surprising result: rather straight curves, implying that the probability of extinction is roughly constant.

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Ockham’s principle- Principle of parsimony

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The Forces of Evolution: 

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There are four known forces that are involved in evolution:

1)mutation,

2) genetic drift

3) gene flow or migration

4) selection.