Expected Anomalies in the Fossil Record

TL;DR

This paper analyzes how sampling conditions and phylogenetic tree structures influence the likelihood of finding intermediate fossils, providing a mathematical framework to understand gaps in the fossil record.

Contribution

It introduces an analytical null model that predicts the expected presence of intermediates based on tree shape and sampling times, clarifying why intermediates may often be absent.

Findings

The likelihood of finding intermediates depends on phylogenetic tree shape.

Sampling times significantly affect the probability of observing intermediates.

Mathematical model explains gaps in the fossil record without invoking fossilization improbability.

Abstract

The problem of intermediates in the fossil record has been frequently discussed ever since Darwin. The extent of `gaps' (missing transitional stages) has been used to argue against gradual evolution from a common ancestor. Traditionally, gaps have often been explained by the improbability of fossilization and the discontinuous selection of found fossils. Here we take an analytical approach and demonstrate why, under certain sampling conditions, we may not expect intermediates to be found. Using a simple null model, we show mathematically that the question of whether a taxon sampled from some time in the past is likely to be morphologically intermediate to other samples (dated earlier and later) depends on the shape and dimensions of the underlying phylogenetic tree that connects the taxa, and the times from which the fossils are sampled.