Darwin’s Four Conditions

Charles Darwin (1809-1882) is famous for developing a way for evolution to occur, natural selection.  It should be pointed out that when Darwin was alive there was no question that species evolved.  Scientists in general all agreed that the species that were alive around them did not remain fixed forever and ever, but rather changed over the course of long spans of time.  The problem was that no one could figure out how this took place.  Darwin made observations of the natural world and noticed four simple features that would result in species changing in response to the natural environment.  The process of change that Darwin proposed occurs as an inevitable consequence of these four conditions, and does not require any divine influence.  The four conditions that Darwin elucidated were variation, heritability, superfecundity, and non-random mortality.

Variation means that that each individual in a population is unique.  These differences may be very minor, but they are always there.  This is so obvious a fact that it almost does not need to be spelled out.  You are a unique individual who has never occurred before and will never occur again, and the same is true of every other species.

Heritability means that each individual will tend to pass on the variations it has to its offspring.  In this way, the variations that are present in a population will tend to be passed down through the generations.  In other words, short individuals will tend to have short offspring, etc.  This heritability is not perfect, in most traits, because there is mixing between the traits of each parent.

Superfecundity means that more young are produced than can possibly survive.  Each individual strives to pass its genes on into the future.  To accomplish this, the more offspring produced the better and since all organisms use this strategy, a great many offspring are produced.  This leads to competition among unique individuals for a limited number of available resources needed for life.

Non-random Mortality means that how dies matters.  Many organisms die, and this is especially true of young organisms.  This is driven,in part,  by the competition mentioned above and, in part, by factors such as harsh weather conditions and other environmental factors.  But while it is a foregone conclusion of superfecundity that some individuals will die, it is the fact that these deaths do not occur at random that allows for population-level changes to occur.  In other words, the survivors survive for a reason, and the reason is that they have some advantage, however small, over those that did not survive.  The survivors are then able to pass their advantages, whatever they are, on to the next generation.

After generations and generations of this combination of conditions, populations of individuals become evermore adapted to the environments in which they live, and so evolution by means of natural selection occurs.

 

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Birding the Sacramento Bypass

Yesterday morning I got up early and drove over to the Sacramento Bypass Wildlife Area to do some birding.  It was a lovely, and fairly cool, morning with a beautiful light layer of valley fog nestled over the low, wet areas.  I spent a couple of hours simply roaming around following whatever seemed interesting.  My very first sighting was a Swainson’s Hawk sitting in a tree right above me and calling.  It took off and flew in front of me as it crossed the bypass to begin its day.  Just a little farther along was a Lark Sparrow foraging in the open grassy area.  These large and dramatically patterned sparrows are a favorite of mine, so a day that started with finding one was sure be delightful no matter what else I found.  As I walked past the tule wetland a group of 5 Wood Ducks (1 male and 4 females) lifted off giving their whistling call, and I saw a Great Blue Heron hunting along one edge.  I stopped for a little while and watched the heron catching crayfish, of which there were apparently many!  It was impressive to watch this bird swallow the crayfish alive, claws and all.  What stops the crayfish from damaging the birds esophagus and stomach?  Something must because the heron caught and ate four over the course of about ten minutes.  Successful morning!  Another cool predator/prey interaction was a Brewer’s Blackbird that had its bill stuffed with dragonflies it had caught.  It must have had at least five dragonflies in its bill stacked like a puffin lines up anchovies.  I am not even sure how a Brewer’s Blackbird, not the most aerially adept bird, goes about catching dragonflies!  I stopped between two large cottonwood trees at the edge of the north channel for a while and listened and watched.  A family of Spotted Towhees came by working their way through the bushes and calling to one another.  the group was comprised of an adult male, and adult female, and two hatch year birds.  The young birds were doing little other than following their parents movements and begging whenever one of the adults found something worth eating.  Cute family!  As  I walked back to the car, with the sun light slanting through the trees, I got to see and hear numerous Common Yellowthroats as I passed from territory to territory.  They can be so densely packed into and area, it is rather amazing.  So, it was a lovely morning, and I am especially glad to have gotten out birding since it has been quite a while for me.

Here is the full species list for the morning.

Great Egret, Great Blue Heron, Green Heron, White-faced Ibis, Mallard, Wood Duck, Turkey Vulture, Northern Harrier, White-tailed Kite, Swainson’s Hawk, Ring-necked Pheasant, Killdeer, Mourning Dove, Rock Pigeon, Anna’s Hummingbird, Belted Kingfisher, Downy Woodpecker, Black Phoebe, Western Kingbird, Western Scrub Jay, American Crow, Tree Swallow, Barn Swallow, Bushtit, House Wren, American Robin, Northern Mockingbird, European Starling, Common Yellowthroat, Spotted Towhee, California Towhee, Lark Sparrow, Song Sparrow, Dark-eyed Junco, Red-winged Blackbird, Brewer’s Blackbird, and House Finch.

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Baby Birds!

Central California is awash with baby birds right now!  They are all over the place, out of the nest and following their parents around begging for food and learning what it is to be a bird.  All the young birds I have been seeing are in their hatch year plumage which means that they have the fully formed and functional feathers that they will keep until they molt next year (for most species, this first molt will take them into their adult plumage that they will then replace each year for the rest of their lives).  Just in the last few days in West Sacramento, I have been seeing young Western Scrub Jays, House Finches, American Crows, Bushtits, House Sparrows, Lesser Goldfinches, and Nuttall’s Woodpeckers.  Over the weekend in Berkeley, I saw a young Band-tailed Pigeon in its drab hatch year plumage following one of its parents, all be it a bit clumsily, to the bird feeders that my mom keeps.  The hatch year plumage of this species does not have the classic bronze feathers or white ring on the nap of the neck.  Young birds also lack the dramatic bright yellow bill and feet of the adults.  The population of this large and lovely pigeon species has been declining fairly quickly for unknown reasons, so to see that they are breeding in the Berkeley hills is especially exciting!  It will be very more exciting to see what young birds of other species show up around the area!

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