Variation, natural selection and genetic drift.

Evolution requires genetic variation.

Yet the traditional view, still reported in texts is that directional evolution and genetic drift will act to decrease variation.

How much variation actually exists in a population for selection to work on?

Mid sixties biologists used electrophoresis to measure variability.

Technique of choice separates protein on the basis of mobility through a gel under the influence of an electric current.

Generated new estimates of "genetic diversity" as the probability that two alleles chosen at random from all the alleles at that locus in the population are different.

Under random mating this equals the population heterozygosity and is equal to the number of heterozygous individuals in classical Hardy Weinberg populations. H, or genetic diversity defined this way, for a population consisting of 25 AA, 50 Aa and 25 aa individuals is 0.5.

Electrophoresis also allowed a new approach where genetic diversity could be expressed as the percent of polymorphic loci found in the population.

For example, if 20 loci are studied by electrophoresis and 16 show no variation and 4 have more than one band on the gel, then the percent polymorphism for that individual would be 4/20 X 100 = 20%. Researchers can determine these for several individuals and obtain an average for a population or even a group.

In animals, a broad range in average heterozygosity was found and was more than expected.

Birds 15%, Insects 50%, mammals 20%, fish 30%

Scientists were astonished at the variability shown even at the protein level. These findings lead to several ideas.

I just want you to be able to describe these ideas, essentially provide a definition for neutrality, soft selection, lateral transfer, total phenotype selection and their positive effects on maintaining variability.

1. Neutrality.

Neutrality (formally near neutrality) is considered the primary reason that so much variation is maintained in populations.

The proposal that most of evolutionary change at the molecular level occurs as a consequence of random genetic drift, because most mutations at this level are essentially neutral. Assuming neutrality would allow populations to maintain substantial levels of variation. Also neutral alleles will not be exposed to selection in some sense and so any changes in frequency would be due to genetic drift.

Now near neutrality is proposed for many alleles. That is there are many alleles whose consequences with regard to fitness, positive or negative, are small.

Individuals vary in position and numbers of such alleles. Thinking very simply about this, for just one small effect on a trait, I have ++++++------, you may have -++++--++--- or some other combination of small positive and negative effects that essentially are not "seen" by selection. This is because such effects are small and widespread in the population and so in total behave as neutral alleles.

Paper with more information for interested students, class will not be responsible for content on exams.

2. Adaptive landscape.

Already introduced by Dr. Thorne.

http://evolution.berkeley.edu/evolibrary/article/history_19

 

2. Soft and hard selection.

First coined by Wallace to in part address load and cost of selection, Evolution vol 29, No. 3 Sep. 1975.

Natural selection can operate without running up an impossible genetic load if it is soft selection rather than hard selection

* Soft selection: selective deaths are substituted for non selective background mortality

* Hard selection: selection occurs as extra mortality, which occurs on top of the background mortality

Figure I. Representations of hard and soft selection. (a) The concept of hard and soft selection (with respect to population density only, for simplicity not showing frequency dependence). A population of N adults with average fecundity of 2F per female produces FN eggs. As maximum adult population size is limited to N by some external factor, such as availability of space or food, 2F–N eggs must die each generation. This mortality can be random with respect to genotype and, therefore, non-selective (in yellow), or selective (soft selection, in red), but cannot depress population size below N. Hard selection (in black) introduces an additional, density independent source of mortality, which reduces population size below N. (b) An early representation of hard versus soft selection in the context of a metapopulation. In(i), the small-grain breeder saves all heads bearing 60 or more seeds for planting and future selection; some experimental plots are entirely discarded under this scheme. In (ii), the breeder first samples a few heads from plants of each experimental plot, determines the statistical distribution of seeds per head for each plot, and then harvests what is estimated to be the best 5% of all heads of each plot for planting and further selection. Under this scheme, a few heads are saved from each plot regardless of its average number of seeds per head. Soft selection resembles the second scheme, hard selection the first. Black shading indicates those individuals that were selected to reproduce, no shading those that were not selected. selection incorporates the idea that most populations appear to be at K, and mortality becomes more selective as get nearer to K.

Read this paper if you are interested in more information on this topic.

Again using simple stories to illustrate the differences, think about being the biggest, "baddest" copepod in the ocean. You are carrying a zillion eggs, etc. A whale come up to feed and takes you, and thousands of your comrades, before you get to lay those eggs. That is a clear example of background mortality not linked to phenotypic fitness.

We can also think of more relevant examples, such as birds arriving to set up nests and an unusual storm wipes out most of them. The few birds that arrive after the storm get to nest, although in most years, nesting sites are limited and would already be taken. The storm is a random occurrence, and whose nests were destroyed is random with regard to fitness, especially if there is a random component to who arrives first from year to year.

Think of the same situation but now the population arriving after and there during the storm has a few phenotypes that happened to prefer storm resistant nest sites. Also assume the weather in this location has more storms for the next few years. The storm resistant nesting is selected for, although the number of nest sites stays the same. This is an example of soft selection.

4. Heterozygote advantage (already examined--- the sickle-cell trait) and Frequency dependent selection. More and more traits are presumed to have fitness advantages in the heterozygous state.

5. For prokaryotes, lateral/horizontal transmission of genes, especially for resistance genes, is important.

https://evolution.berkeley.edu/evolibrary/news/080401_mrsa