The Limits of Microevolution

Author: Stephen Caesar, ABR
Subject: Biology
Date: 1/31/2004

As discussed often in this column, intraspecific microevolution occurs frequently in nature. When a species endures outside stresses, such as environmental change or parasitic attack, its genetic code will flip an “on-off” switch, causing it to undergo minor changes that will give it a greater ability to deal with these stresses. Evolutionists claim that “macroevolution is microevolution writ large,” meaning that, over millions of years, microevolutionary changes will lead to the species evolving into a completely new, more advanced species (such as fish evolving into four-footed animals).

However, as has been shown in this column, intraspecific microevolution does not result in transpecific macroevolution for two reasons: 1) Microevolutionary changes only go so far before hitting a “ceiling,” above which no changes occur. 2) When a species undergoes microevolutionary change in one area, it undergoes decline in another. If an organism microevolves an improved ability, say, to resist parasites, it may wind up living a shorter life, or being weaker or smaller, or having fewer offspring. This is called “fitness cost” or “fitness trade-off” (Buckling et al. 2003: 2107).

In the wild, many species undergo microevolutionary changes when they fan out from a common starting point and occupy different ecological niches (deserts, jungles, tundra, woodlands, etc.). Over time, different on-off switches in their genetic make-up will cause them to develop into different strains. However, one niche species is not an improvement over the other: each one is merely adapted for its specific niche. Angus Buckling and Matthew Wills (Department of Biology and Biochemistry, University of Bath, England), and Nick Colegrave (Institute of Cell, Animal and Population Biology, University of Edinburgh, Scotland), conducted experiments on the bacterium Pseudomonas fluorescens that confirmed this.

The three researchers took six cloned (thus genetically identical) batches of P. fluorescens and allowed them to colonize different ecological niches in a laboratory. Rather than witnessing the different niche species macro evolving into new, improved species, the scientists observed the following: “As predicted, populations increased in fitness through time but showed a greatly decreased ability to diversify….These results show that niche specialization may come with a cost of reduced potential to diversify” (Ibid.). This is fitness trade-off in action.

The ability of P. fluorescens to diversify into various ecological niches is a microevolutionary improvement, but it involved a cost, as always. The scientists reported that “adaptation itself is likely to limit a population’s ability to diversify, when evolution occurs in ‘rugged fitness landscapes’” (Ibid.). All six batches of bacteria were raised in different environments, and the scientists transferred each batch to a new environment six times. They reported: “In all lines, diversification after six transfers was much less than ancestral diversification, and there was an overall negative correlation between transfer number and ability to diversify” (Ibid. 2108).

Simply put, this means that with each transfer to a new environment, each batch of bacteria became less and less able (not more and more able, as Darwinism predicts) to diversify. The greater the number of transfers to an unfamiliar environment, the smaller the ability to diversify. This is microevolution hampered by fitness cost occurring before our very eyes. Yes, the batches did improve, in that they were able to adapt to the new circumstances they were placed in, but it cost them: “Fitness of genotypes did indeed increase through time in all replicates [all batches of clones],” the scientists reported, but “there was an overall negative correlation between fitness and ability to diversify…” (Ibid.).

They concluded: “Adaptation can limit the ability of bacterial genotypes to diversify genetically. This was not the result of generalist evolution or the evolution of an intrinsic reduction in evolvability, but was caused by environment-specific adaptation” (Ibid. 2109).

You can adapt or diversify—but you can’t do both. Evolutionists predict that both should happen, with no trade-off occurring. Over time, each batch of P. fluorescens should continue adapting and diversifying until they become new, improved, separate species, all having evolved out of a common ancestor. This simply has not happened, nor can it. The ceiling has been reached.


Buckling, A., et al. 2003. “Adaptation Limits Diversification of Experimental Bacterial Populations.” Science 302, no. 5653.

Stephen Caesar holds his master’s degree in anthropology/archaeology from Harvard. He is a staff member at Associates for Biblical Research and the author of the e-book The Bible Encounters Modern Science, available at

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