Darwin’s ‘Origin’ (ch.4-6): Evolution by Natural Selection, and some difficulties

Every day this week I will be publishing excerpts from my distilled version of On the Origin of Species. If you never have, or never will read it, hopefully this will give you a sense of what an amazing book it is.

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Chapter 4: Natural Selection

Given the power of human selection to change domestic animals and plants, it is surely of little surprise that the vast array of wild forms we see in nature might have been developed over many thousands of generations through the action of Natural Selection.

If there is no variation between individuals, Natural Selection can do nothing. Given vast time scales and many individuals, however, Natural Selection can cause considerable change with even small amounts of variation. While humans change only evident characteristics, every aspect of an organism and every new variation is continually tested and shaped by nature.

Sexual selection

Some characteristics are very strikingly displayed in only one sex and not in juveniles, and these are the result of a Sexual Selection, operating alongside Natural Selection. Just as all organisms struggle for life in order to reproduce, males compete with each other for possession of females. Males must not only be vigorous, but have the most powerful weaponry or most attractive displays in order to succeed. Though failure in sexual competition does not entail death, if an individual does not reproduce, its traits will not be passed on. Sexual Selection is therefore no less powerful than Natural Selection.

Peacock feather engraving

Circumstances favouring the action of Natural Selection

The more variable a group of organisms is or the more individuals there are in a population, the more likely it is that favourable variations will arise by chance for Natural Selection to act on. If distinct races of a species develop in a region with variable conditions, the degree to which these races differ will depend on the length of time for which they have been diverging and how much the region varies. Interbreeding reduces the accumulation of differences between groups by blending characteristics.

Isolated regions and islands prevent interbreeding with other populations, preventing new variations being diluted. They might therefore be expected to nurture new varieties. Continents, however, have a greater array of physical conditions, more complex organic interactions and vast numbers of individuals: they are more fertile grounds for the production of species. 

Divergence of Character 

Competition is most intense between very similar species using the same limited resources. If one considerably outstrips the other in some respect, the losing species may be replaced by the other. The more diversity between and within species, the more individuals can be supported.

For example, should different hunting habits arise in a population of wolves, the region would be able to support more wolves. Those hunting deer would be likely to meet those hunting the same prey to reproduce, and deer-hunting features, such as speed, would develop in that group, while sheep-hunters might become stronger. Intermediate forms would be less successful at hunting either prey, and the populations would diverge.

Darwin's branching diagram

The figure shows ten related species. After ten thousand generations, species A has developed into new races a1 and m1. Each new race carries new beneficial variations as well as the successful characteristics of species A. They may continue to change, as a1 has into a2 or further branch into new forms, as m1 has into m2 and s1. Continued divergence could result in not just new species, but even new genera, families and classes. New varieties may seamlessly replace their parental varieties, or out-compete closely related forms and cause them to go extinct:  m1 continues through a chain of offspring forms while s1 has gone extinct.

In some rare cases, forms that remain successfully competitive will continue unchanged for a long period of time without going extinct, as f1 has done. Change is eventually inevitable, however, and the geological record shows that innumerable forms have existed without leaving any descendents at all. Life is endlessly being reworked by Natural Selection, as fresh new branches continually bud and ill-adapted limbs are deftly pruned back.


‘It may be said that natural selection is daily and hourly scrutinizing, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life.’


Chapter 5: Laws of Variation

The source of  variation upon which natural selection acts is largely a mystery. I believe that some key sources are the effects of use and disuse, organisms’ habits and laws of growth on reproductive elements. The reproductive system is highly susceptible to external influence. Some variation must also be due to the direct effects of the conditions of life and environment on developing individuals, but this is not inherited.

One cause of inheritable variation results from the use and disuse of different physical parts. Heavily-used parts increase in size, while little-used parts tend to decline and decrease in size over generations as changes are inherited, eventually becoming mere rudiments. For example, I believe the legs of ostriches increased in size due to natural selection, allowing them to run and defend themselves against predators, while their wings shrank with lack of use.

Rhodedendron Brookeanum plate

Another source of variation is the fantastic ability of organisms to acclimatize to different habitats. For example, rhododendrons thrive at many different altitudes in their Himalayan ranges, as well as growing vigorously in English gardens. Organisms have innately flexible constitutions, and changing characteristics such as behaviours can allow rapid acclimatisation. It is the natural selection of underlying constitutions that drives the greatest changes long-term, however.

Thirdly, variation may arise by chance, though we know little of how this occurs. Apparently disparate characteristics may also be linked in some way, like blue eyes and deafness in cats, so that both change when only one is acted on by natural selection. The growth of some structures can affect the shape of others. For example, the development of varied pelvis shapes in birds probably causes their correspondingly varied kidney shapes.

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The way characteristics vary reflects their evolutionary history. Rudimentary organs are of little importance, so variations that appear are not acted on by natural selection. Highly specialized characteristics are often not possessed by other closely related species, so will have developed relatively recently. Too little time will have elapsed for all but the best variations to have been discarded by natural selection.

Even if characteristics possessed by ancestral species have been absent for a long time, they may appear in descendents after hundreds of generations. For example, the myriad domestic pigeon breeds occasionally display the black wing-bars and slaty colouration typical of the ancestral rock dove. Conversely, distinct species sometimes vary in analogous ways, but not because of common heritage.

Thus, generic characters, or those possessed by all members of a set of closely related species, inherited long ago from a common ancestor, will be fairly uniform, while specific characteristics, or those that have been recently uniquely developed by individual species, will be very variable, full of potential and pitfalls.


‘it is the steady accumulation, through natural selection, of such differences, when beneficial to the individual, that gives rise to all the more important modifications of structure, by which the innumerable beings on the face of this earth are enabled to struggle with each other, and the best adapted to survive.’


Chapter 6: Difficulties on Theory

I am sure by now that a variety of objections to my theory will have occurred to most readers. While some of these are difficult to overcome, I am confident that none are fatal.

Firstly, if all species have developed from parent species, why don’t plentiful transitional forms exist? How come species appear to be such well-defined entities rather than existing in a confusion of half-forms? Though all species have developed gradually, transitional stages no longer exist because varieties bearing new advantageous adaptations will out-compete and gradually replace older varieties. These intermediates leave no fossilized remnants because the geological record is extremely incomplete.

A second challenge offered to my theory is that it is hard to imagine how extreme transitions have occurred, such as from land-dwelling mammals to aerial bats. In my opinion, this is not problematic, for many groups of species contain forms that have widely different modes of life. For example, there are species of crustacean that can breath only air, those that breathe only water or others that can breathe both mediums.

Flying squirrels plate

Though I cannot offer the direct transitional stages of dramatic shifts in styles of life, substitute examples exist for us to model the idea on. The squirrel family has species that can only climb, and others with flattened tails and extended skin flaps for gliding between trees. In the mouse-like ancestors of bats, increasingly developed skin flaps would have allowed their bearers to glide further to escape predators, and eventually full wings may have developed.

Critics of Natural Selection may doubt that extremely complex organs could have formed without the involvement of a creator. I suggest that the wonderful complexity of the eye could have developed through numerous small steps, in the same way humans might improve an instrument like the telescope. Across even a single group, the molluscs, different versions of eyes exist, ranging from mere nerve fibres coated in optic pigments to complex structures with double corneas.

ridley- mollusc eyes

In some cases, organs can be utilised for dramatically different uses. The buoyancy-maintaining swim bladder of fish has been coopted for use as both an air-breathing device, similar to a lung, or as an auditory organ in different species. The steps to the lungs of terrestrial animals are not hard to imagine.

Conversely, it is asked whether apparently unimportant characteristics could have been acted on by natural selection, having little bearing on organisms’ survival? We would do well not to underestimate the importance of even simple traits. The giraffes’ tail may be just a fly whisk, yet, the distribution of domestic cattle in Africa is dictated by whether the animals can resist the onslaught of flies.

Some structures are indeed not directly beneficial to organisms, such as the bones in the seal’s flipper, inherited from their quadrupedal ancestors. If no longer of use, these appendages diminish gradually. Natural Selection will never develop characteristics that disadvantage their bearers. It strives to perfection, yet never attains it, constrained by the raw material that it is given and by the ingrained forms of the past.


‘natural selection can act only by taking advantage of slight successive variations; she can never leap, but must advance by the shortest and slowest steps’


Previous:

Darwin’s Origin: Introduction

Darwin’s Origin, chapters 1-3: variation and struggle

Next:

Darwin’s Origin, chapters 7-8: instinct and hybridism

Darwin’s Origin, chapters 9-10: the geological record

Darwin’s Origin, chapters 11-12: geographies

Darwin’s Origin, chapter 13 and conclusion

 

dodo rose

 

© Natalie Lawrence 2018

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