Natural Selection


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Overview Videos


From Kurzgesagt


Stated Clearly




Natural Selection Simulators


The simplest one:  Watch how genotype and environment interact as the little critters reproduce for five generations--results depend on how you set initial allele frequencies and what environment you choose.  From McGraw Hill


The Guppies:  Based on the famous John Endler Guppy experiments.  Still the best example of actually measuring natural selection in the wild, and in simulated lab environments.  Choose the kind of guppy, the kind of predator and watch what happens.  From PBS


Full gene frequency calculator.  Let the simulation do all the math of the p's and q's. But you select initial gene frequencies, fitness, number of populations to simulate, number of individuals in the population and number of generations.  You can play with this one for hours.  Even has mutation rates and bottlenecks.  From Radford  University







"Fitness, to a geneticist, is not the same as fitness to a movie director or a sports columnist. Fitness is not measured by physical attributes, it is measured by the number of offspring produced in the next generation that survive and reproduce. In a hunting-gathering society, the most fit person may have been the near sighted male who could not go on the hunt because he would stumble and make too much noise. If he were left behind to gather fruit and berries with the women, he may have become the most fit person in the tribe. Grandchildren, great-grandchildren, etc., are the best measures of the fitness of an individual. This has always been my favorite explanation of why so many of us are near sighted, and why society changed from hunting-gathering to agriculture. It's all population genetics!


"The most fit phenotype in the population is assigned a fitness of 1. If there are two equally fit phenotypes, each is assigned a fitness of 1. Those less fit must be assigned a fitness of less than 1. The difference between 1 and the fitness value is called the selection coefficient. The relationship between fitness, w, and the selection coefficient, s, is given by the equation, w = 1-s."












Genetic Drift


Nice article from FamilyPedia on genetic drift--gives plain English and also the math



Genetic Drift as the null hypothesis.  How can we tell when evolutionary change is due to natural selection?

We can tell the story--we identify a difference in fitness and then we see a population change in a direction that the fitness difference should drive it.

But how can we be sure it's still not just random drift?  The math can help, but so many other factors complicate it.




Bottlenecks and Founder Effects--genetic drift at its extreme




Natural Selection



How do we tell this from genetic drift?




When we are the selective force




Figure 1 The continuum from laboratory model organism to a truly wild organism. Two key aspects of the continuum are the degree to which selection is anthropogenic versus natural in origin (red arrow, left to right) and the degree to which the environment is human controlled or created versus natural (blue arrow, right to left). Illustrative categories of animals with different selective histories and environments are placed very approximately along these axes with arrows (there is probably considerable variation in these categories of animals in their position on these continua, which is not shown here). The bedrock of modern immunology rests on the experimental power of working in the controlled but artificial systems at the far left of this spectrum, but much still remains to be learned about the causes and consequences of variation in immunological function from studies moving toward the wild end of this spectrum. Human communities can also be considered to vary along a limited area of the blue environmental continuum.

Continuum of deliberate human versus non-intervention natural selection 


Gene Flow


Sex can really mess things up, allowing genes to "flow" from one population to another.

In fact, gene flow is just a way of looking at the results of production of variation by sex

Even bacterial and viral gene transfers are considered "gene flow" and may be very important at population level.



Inbreeding Depression





Inbreeding Depression.  This is just the negative side of the variation produced by sex, as viewed at the population level.

What's interesting about inbreeding depression is that it takes us right back to the question of how the production of variation, and the factors that constrain that production, can lead to the parsing of organisms into species, and the parsing of biological organization into hierarchically stable components like cells, organisms, and species. 









Web Resources



Great site for getting details of math on fitness, genetic drift and selection, all for plant pathogens--fascinating topic.  Also explains all in good plain English.

Gene Flow from Nature magazine Knowledge Project--good graphics

University of Ilinois at Chicago--good review of basic genetic concepts for Mol Bio course

Evolution concepts applied to software designers

Lesson from Natural Selection for international aid workers

Nice review article on Fitness from Allen Orr (Nat Rev Genet.)



Larry M Frolich, Ph.D.     Miami Dade College     Wolfson Campus      Natural Sciences      Miami, FL  33132    Office 1504     (305) 237-7589    e-mail  Creative Commons License