"The view generally entertained by naturalists is that species, when intercrossed, have been specially endowed with the quality of sterility, in order to prevent the confusion of all organic forms." Charles Darwin, Origin of Species 1st edition (on Kindle)
The most interesting part of this passage is that I just talked about it today in my Population and Evolutionary Biology (BIO 225) course. You have lower fitness if you breed with close relatives (inbreeding depression) and distant relatives (outbreeding depression). I'll assume that the sweet spot of highest fitness is not a point but a range - related enough that there's coadaptation (you're parts work together) but not too close that you're breeding with sibs. If we were to graph fitness against relatedness it would look like a plateau with cliffs at close sibs on one end and different species at the other. The sides are steep but it's not a step, that is, the fitness increases quickly but not instantly. So sibs have the lowest fitness, cousins higher fitness, second cousins higher than that then I suspect at 3rd cousins fitness is equal to the general population. At the other end you have different, but related, species breeding but the offspring are not well suited to the local environment or are sterile. This falls apart when hybrids do better though this tends to be in sites where neither parent does well.
Skunk seen yesterday that popped down a hole on the property line
"The view generally entertained by naturalists is that species, when intercrossed, have been specially endowed with the quality of sterility, in order to prevent the confusion of all organic forms." Charles Darwin, Origin of Species 1st edition (on Kindle)
The most interesting part of this passage is that I just talked about it today in my Population and Evolutionary Biology (BIO 225) course. You have lower fitness if you breed with close relatives (inbreeding depression) and distant relatives (outbreeding depression). I'll assume that the sweet spot of highest fitness is not a point but a range - related enough that there's coadaptation (you're parts work together) but not too close that you're breeding with sibs. If we were to graph fitness against relatedness it would look like a plateau with cliffs at close sibs on one end and different species at the other. The sides are steep but it's not a step, that is, the fitness increases quickly but not instantly. So sibs have the lowest fitness, cousins higher fitness, second cousins higher than that then I suspect at 3rd cousins fitness is equal to the general population. At the other end you have different, but related, species breeding but the offspring are not well suited to the local environment or are sterile. This falls apart when hybrids do better though this tends to be in sites where neither parent does well.
|
| Skunk seen yesterday that popped down a hole on the property line |
"At the other end", and albeit outside your area of specialization, brown bears and polar bears make an excellent and well studied example of two closely-related species, with very different environmental adaptations, that produce occasional hybrids. A recent study also establishes that there is a population of "brown bears" in Alaska which consists entirely of bears with brown bear patrilines and polar bear matrilines. (See http://mbe.oxfordjournals.org/content/31/6/1353.full)
ReplyDeleteThat's an interesting example. I can't access the article (from home) but I wonder what the fitness is (and the variation) among hybrids. Also make one question either species concepts or those bears being different species. Thanks for the article - I'll check it out when I get back to the office.
ReplyDelete