In this episode of Discovering Darwin we discussed the fifth chapter of Origin of Species titled Laws of Variation. Ironically there are no laws actually discussed in the chapter but Darwin does outline a variety of phenomenon and patterns of inheritance and variation that puzzled and intrigued him. When we consider the great variation in forms that we see in nature it is often difficult to tease apart the cause of that difference in form. For example, variation in form may be genetically determined as described by Mendel and his experiments with pea plants. Of course Darwin did not have a sense of genes and how multicellular plants (excluding mosses) and animals get two copies of each gene, one form their mother and one from their father, that codes for traits. Those two copies of the genes determine the expression of physical traits. Looking at the chart below of Mendel's pea plant traits you can see how variable the sweet pea plant could be. Imagine Darwin's confusion if he saw pea plants with yellow or green seeds, purple or white flowers and variation on where the flowers are located on the plant. Even more intriguing is the notion that yellow seeded pea plants with purple flowers could be crossed and produce offspring with green colored pea seeds and white flowers! Today, with our knowledge of genetics, we understand the concept of recessive and dominant traits and how recessive traits can be hidden in the parents but expressed in the offspring when they inherit only the recessive versions of the gene. We discussed if Darwin actually was aware of Mendel's work and Josh mentioned an interesting paper by Bizzo and El-Hani that argued Darwin was aware of Mendel's work. That paper can be found here. Acclimation versus Adaptation Sarah discussed how Darwin seemed to use the terms acclimation and adaptation interchangeably as he worked through his understanding of how organisms can exhibit variation in form. Today biologists see these two concepts as separate and unique characteristics. Some of the best examples of acclimation can be seen in arctic animals, like this arctic fox, that can dramatically change their coats from winter to summer. Acclimation is when the organism alters its form, physiology or behavior in response to local environmental conditions. Often these changes are induced by hormonal changes in the organism which themselves are triggered by environmental stimuli. These changes are typically not permanent. In contrast, if you look at a population of a species you may see variation in forms but these are adaptations to local conditions. An interesting example of local adaptations is seen in the north American rock pocket mouse (Chaetodipus intermedius) that lives in the southwest desert areas of US and Mexico. This small, bite sized rodent, lives in a variety of rocky habitats which differ in color depending upon the geological history. The brown sandy colored rocks are in contrast to the black balsaltic rock that formed from geologically recent lava flow events. The mice have strong local selection to avoid being seen by their natural predators so selection favors coat colors that blend with the background. The top two images are showing the local mouse in its native local condition, the bottom two panels are swapping mice from their native locations. All of these mice are the same species but their coat color variation is genetically determined by expression of Mc1r gene for black pigmentation and that gene has undergone local mutations and selection. http://uanews.org/story/coats-different-color-desert-mice-offer-new-lessons-survival-fittest We also discussed how creatures locally adapt to caves by losing the ability to see and often become pale colored or completely lose their coloration. Josh read a wonderful quote by Darwin where he described blind cave crabs possessing components of the eye but it was incomplete in form. Image from http://imgarcade.com/1/blind-cave-crabWe also d