20.1.1 - Evolution Rates

Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization

First Edition

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

At least three different "rates of evolution" have been studied by Earthly zoologists. First there is the "morphological rate" -- the speed at which the size and shape of organisms belonging to a given species evolves over time. For instance, it has been shown that the average dimensions of horses’ teeth have increased at a rate of about 0.1% per 1000 years.⁴⁴⁰ Another measure of the velocity of evolution is the "taxonomic rate." As shown in Table 20.1, the taxonomic evolution rate measures how fast new subspecies, species, genera, and so forth arise naturally. Finally, the "genetic rate" of evolution specifies the speed at which alterations in genes are occurring in the subject population. Measured genetic rates generally confirm the values given in Table 20.1.*

Taxonomic Class	Class	Order	Family	Genus	Species	Subspecies
Table 20.1 Average Taxonomic Evolution Rate, in Millions of Years per Unit of Taxonomic Classification (after Rensch⁴⁴⁹)
Mammals	190	65	12-25	15	0.1 - 10	0.001 - 0.1
Reptiles	310	185	85	50
Fishes	450	270	80	50
Insects	350	180	160	12
Crustaceans	540	410	160	~12

A wide variety of different factors operate collectively to increase or decrease the rate of evolution of species on any given planet. It is true that modern geneticists recognize evolution is primarily a function of genetic variation stored in the species’ general gene pool. Evolution advances by reshuffling previously accumulated gene types by a process known as "recombination." (See the discussion of the benefits of sexual reproduction in Chapter 12.) Nevertheless, the primary and ultimate source of all genetic variation is mutation.³²⁰⁸ If the mutation rate is too low little variability is retained, leaving a smaller inventory of possible adaptations for natural selection to act upon in response to environmental changes. If the mutation rate is too high, desirable characteristics are mutated away or are selected out before they have a chance to be assimilated into the gene pool. Thus there exists an optimum range of mutation rates for any class of organisms.³⁰⁵

The average background level of radiation at the Earth’s surface is about 0.12 roentgens/year. About one-third of this is cosmic rays from space. The other two-thirds comes from terrestrial sources such as natural crustal radioactivity and deposits of potassium-40 in our bodies.³⁹⁰ What effect on the mutation rate -- and on evolution -- does this background radiation have? Among smaller organisms, often as much as 100-1000 times the background is required just to double the natural mutation rate. Among larger organisms, as little as 3-10 times above background may produce a similar effect over the whole body. It would appear that on Earth today natural radiation accounts for only a small fraction of all spontaneous mutations.

The situation may have been much different in the past. I.S. Shklovskii and V.I. Krassovskii, two distinguished Russian astrophysicists, have calculated that Earth may have passed within 10 parsecs of a supernova event perhaps a dozen times since its formation 4.6 eons ago. In each case, the scientists believe, the intensity of cosmic radiation must have risen at least by a factor of 30. This should have caused an increase of one order of magnitude in the natural background, which would at least double the mutation rate (and so the maximum rate of evolution) for the largest creatures on Earth. The effects could have persisted for more than 10,000 years.²⁰

Extraterrestrial creatures inhabiting a planet in the outer Core regions of the Galaxy should experience such a supernova event far more often -- perhaps once every 10 million years. Over the course of geological history more than 500 local supernovae might occur. This would double mutation rates at regular intervals and keep the pace of evolution high -- especially during the very early stages in the evolution of life on the planet when gene inventories were still small. We might hypothesize that species "turnover rates" may be significantly higher near Core regions than in the Disk of the Galaxy.

Many other factors may influence the rate of evolution on other worlds. For example, stellar class of the primary sun may be important. The hottest stars for which habitable planets are thought to exist are the F5 suns. These objects radiate more strongly in the blue part of the spectrum than our Sol, emitting about four times as much ultraviolet radiation. Because of this, many xenologists suspect that the early evolution of alien life on a world circling such a star should be considerably faster than on the primitive Earth. More energy could penetrate the oceanic surfaces, creating more complex nutrients faster and thus speeding the origin of life. F5 suns will also have stronger solar winds, which may lead to increased atmospheric ionization and greater climatic variability (and hasten evolution as well).

Higher UV levels near F5 stars may also delay the appearance of land plants.¹⁰¹³ Since more of these rays must be filtered out by an ozone layer which necessarily must be thicker than Earth’s, sea plants of other worlds must wait longer than their cousins on Earth for the atmospheric oxygen content (which yields ozone) to build up. If the alien planet has small or shallow seas, then the total marine biomass may be too small. In such cases, oxygen would remain scarce, a sufficient ozone layer might never be built up, and land might never be colonized by plants. The larger the planet, however, the less likely is this catastrophe. All else being equal, larger planets have higher gravity and more compact atmospheres, which means a higher rate of ozone production. In either case, marine evolution should be comparatively rapid.

In contrast, K- and M-class stars peak in the red portion of the spectrum, emitting only 1-10% as much ultraviolet as Sol. This tremendous deficit should slow or greatly retard prebiotic evolution and the origin of life because less energy is available at the surface of the primitive planet for chemical synthesis. Tidal locking is more likely in habitable zones around K- and M-stars; if locking occurs the environment could become quite severe (though relatively uniform), which will also tend to retard evolution. Finally, the UV deficit may forestall the dissipation of the primeval hydrogen/helium transsolar atmosphere. In such a system, even the innermost worlds might remain large, gaseous, and quite jovian.³⁷⁶ In this case, then, evolution may proceed more slowly.

Another factor which may quicken the pace of evolution is the presence of moons. By raising tides on the planetary shores, natural satellites may assist chemical mixing and catalysis during the early phases of prebiotic evolution in alien seas. Several xenologists have even suggested that mechanical wave motion may encourage and accelerate the invasion of the land by primitive plant and animal lifeforms.²³⁶² An interesting variation on this theme occurs when the alien planet is itself a moon -- perhaps a super-jovian orbiter. While tidal locking will leave it a one-face world, the severely mutagenic radiations (such as exist near Jupiter) should provide ample genetic variation for selective forces to work with.

Planetary factors may also have a decisive effect on the rate of evolution. Perhaps the most influential of these is the relation between land mass distribution and the diversity of species -- a part of the science of biogeography. Biogeographers have discovered what they call the Species-Area Rule.¹⁷¹³ Mathematically, the Rule may be stated as follows: S = kA^0.27, where S is the total number of species present in a land area of A square meters. (k is some constant, see below.)

In plain English, the Species-Area Rule says that, all else being equal, the number of different species present on any land mass is proportional to the area of that land mass. More land means more species; less land means fewer species. Thus, the rate of evolution is indirectly correlated with land area, since the production of more species requires "faster" evolution.

The Species-Area Rule has another interesting feature. It predicts that the same land area, fragmented into pieces, can support more total species than the original.

Take Earth as an example. If we assume there are 2 million animal species (a low estimate), distributed over 6 continents with a land area totalling 1.48 x 10¹⁴ meter², then the constant k = 80.7 for Earth. Now suppose that the continents were broken into 100 pieces. With a hundred separate island continents, having the same total land area as before, Earth theoretically could support as many as 15,600,000 species -- more than a 7-fold increase.

Let’s try the Rule in the other direction. Today we have 6 continents and 450 taxonomic Families (such as the cat family, the dog family, the frog family, and so forth). But 225 million years ago there was only one global continent -- Pangea -- and only 146 Families. Using the Species-Area Rule, we would predict that Pangea could support 122 Families. By eliminating the modern continent of Antarctica (which is comparatively lifeless), the Rule predicts 139 Families, which is surprisingly good agreement with the paleontological data.

Why does the Rule work so well? One explanation is that the fragmentation of land masses provides a greater number of more heterogeneous environments for development. A variety of isolated habitats provides shelter from competition, and specialization may accelerate. The same land, linked together without barriers, permits competition and tends to eradicate specialized niches. Xenologists expect that the Species-Area Rule should be applicable in some general way to extraterrestrial ecologies located elsewhere in the Galaxy.

Another influential planetary factor is ecological complexity. Structurally complex habitats usually can support a wider diversity of species and thus a higher rate of evolution. This observation helps to explain the existence of "latitude gradients" in species diversity.²⁸⁶ That is, more species are found in equatorial tropical regions on Earth -- where habitats are more plentiful -- than in temperate or northern climes, where niches are comparatively few. Similarly, a fluctuating unstable environment favors the survival of "generalized", species with high adaptability (and presumably higher intelligence as well), whereas stable barrierless environments produce slower evolution and favor the survival of "specialized" species.¹⁷¹²

There are many other planetary factors which may affect the rate of evolution. For instance, smaller planets generally may have higher mutation rates because the levels of background radiation should be higher. There are various reasons for this. First, a diminutive world may experience less intense gravitational fractionation of rocky materials during its formation. Thus the proportion of heavy minerals (including radionuclides) should be higher in the crust. Also, and especially if it condensed in the solar nebula far from the central star, the planet may have a smaller metal-poor core and thus a weaker magnetic field.²⁸⁷⁶ With less shielding from the solar wind, flares, and cosmic particles generally, the level of mutagenic radiation reaching the surface will be higher and evolution may proceed at an accelerated rate.²¹⁴

Another major factor that is often overlooked is planetary surface temperature. For any given biochemical basis, the reactions involved in life chemistry should proceed at faster rates on warm worlds than on colder ones.^1132,1171 But life processes also depend upon the complexity of molecular structures. As a general rule, chemical species are more stable and more complex at lower temperatures.⁷⁵ Xenologists who have considered the problem believe that life of a given biochemical type will tend to evolve faster on hot worlds, but will be more complex on cold worlds. Presumably the faster evolution rate may be sufficient to compensate the lack of biochemical stability on hot planets, and vice versa. Says astronomer Michael W. Ovenden:

(Note: The effects of higher planetary surface pressure are biochemically similar to the aforementioned effects of elevated temperature.)

In addition to prebiotic and early biotic evolution, planetary temperature may also significantly affect the rate of evolution of macroscopic animal life. For example, consider a world where the emergence of life from the sea has been swift and warm-blooded species have evolved. A hot environment will selectively favor smaller lifeforms, whose high surface-to-volume ratio helps to slough off excess heat. Further, since the planet is hot, presumably more energy is available to drive the ecology (see below). More biomass can therefore be supported; since animals are generally smaller the total population will be large. Large populations can store more variability in the gene pool (all else being equal), and mutant traits are more likely to accumulate in single individuals. Hence, the rate of evolution should be somewhat faster. Finally, evolution should proceed even faster on large hot worlds, since the greater planetary surface area permits a bigger population to be sustained.²⁹

On the other hand, a generally cold environment should selectively favor larger lifeforms,⁶⁰³ whose low surface-to-volume ratio helps to retain body heat more effectively. Colder worlds should have less energetic ecologies, so less total biomass can be sustained. Since less biomass must be apportioned amongst generally larger creatures, the population should be small and evolution comparatively slow (especially on smaller worlds with reduced land areas).^718,440

* Change in gene frequency per generation Dq = v + q(s-v-u) - q²s, where q is the frequency with which the gene occurs in the original population, q + Dq is the gene frequency in the next generation, s is the gene’s selective advantage, v is the mutation rate favoring the gene, and u is the mutation rate opposing the gene.¹⁷⁰⁹ Subspecies often differ by only a single gene. The most reason able choices are v = u = 10^-6 (unstable genes may have spontaneous mutation rates as low as 10^-2, but 10^-5-10^-6 is more usual) and s = 10^-3 (e.g., 0.1% more of those organisms possessing the new gene will survive than those without it). If the frequency of the new gene is to increase from q = 1% up to q = 99% in the general population, about 9,200 generations will be required (about 10,000-100,000 years for most mammals.