Principles of Evolution (part II) 

Natural Selection – the mechanism of evolution as told by Wallace and Darwin

**Natural Selection - differential reproductive success that results from the interaction of organisms with their environment.** 

By way of example, we illustrated the process of natural selection with the peppered moth story.   

I.  Some Background 

Remember three important subtle points:  1)  Natural selection can amplify or diminish only heritable variations.  Only heritable variations are affected; acquired characteristics play no role in evolution.2)  Populations evolve, not individuals--see p. 184)  Natural selection is situational (the direction it takes depends on the environment, i.e., the selective forces). 

To illustrate these subtle points using the case of the peppered moth… 1) moth color is inherited perhaps under control of a single gene; 2) the changes in nature over time involve whole populations, that is, in pre-industrial times the dark moth form was quite rare and perhaps in the distant past the dark mutation had not yet appeared.  With time and natural selection at work, the dark moth became the most common form within the population.  3) the 3^rd subtle point is quite important, for what is advantageous in one environment may not be in another.  In the saga of the peppered moth, under polluted tree bark conditions following the industrial revolution the dark from became most common, yet a return to a predominance of light colored forms follows restoration of healthy, non-polluted tree bark.  (The story of the peppered moth has additional complexities but we will ignore them). 

A. About heritable variations
Heritable variations are governed by genes, and as we have seen, genes are mere chemicals containing coded information in the form of DNA.  DNA’s ability to mutate is the fundamental basis for evolutionary change.  While this primary source of variation is the result of random, chemical events (i.e. gene mutations), other aspects of the process of evolution (e.g. natural selection) are very nonrandom, as we will see.    

B.  About Populations
Natural Selection (evolution) is a population level phenomenon, i.e. populations evolve, not individuals, and furthermore, the variations upon which natural selection operates exist within populations (variations exist only in the context of populations). 

Population - a) a group of individuals of the same species, b) live together in same area at same time, c) exchange genes (reproduce sexually). Variations important in natural selection are held within a population.

Sources of Genetic Variation w/in populations:
     1) Mutation - creates new alleles/new traits/new phenotypes- The fundamental basis of change
     2) Sexual reproduction - recombines alleles (recall Punnett-squares)
          Benefit of sexual reproduction: genetic variation in offspring allows some to survive in changing environment (e.g. clonal algae turn sexual
         during unfavorable conditions)
     3) Gene Flow - gain or loss of alleles (genes) from a population  
          gain alleles - immigration (arrival of new individuals)    lose alleles - emigration or death 

C.  About Situations or Circumstances
The type of change natural selection can bring about depends on the situation or circumstances of the environment.  For example, for a population of domesticated, pedigree dogs the continued existence of each breed is dependent on an environment in which owners continue to breed pure lines for the survival of the breed.  Perhaps of more interest is the situation (environment) in which breeders desire new combination of characters and experiment with hybridization between breeds.  Breeders hope to develop new lines of pure breeding dogs in this way.  Each decade, new breeds of dogs, cats, rabbits, etc. are created in this process (=artificial selection) that is not unlike the process of natural selection.   

Expounding on the situational aspect in which evolution operates, Stephen Jay Gould, Harvard paleontologist and prolific writer popularizing evolutionary concepts, puts fourth an interesting idea that he calls the “contingency of evolutionary history” in his 1989 book, Wonderful Life: The Burgess Shale and the Nature of History.  The title was inspired by the 1946 movie It’s a Wonderful Life in which George Bailey learns how his own life was a contingency for many other events.  One such contingency that became a prerequisite for human evolution was the extinction of the dinosaurs that made way for mammal diversification.  Apparently, dinosaur extinction was contingent on an asteroid that impacted Earth 65 million years ago.  Had these events not occurred, humans may never have evolved.  (The question of why humans evolved (to what purpose) is not a question science can address; the question of how humans evolved is explored through the process of science.  See section 16.13 if you are interested in human evolution.) 

II.  Modes and examples of Natural Selection (see p. 185-187) 

Genetic variation within a population creates, in part, a diversity of phenotypes, phenotypes that have a strong genetic component (as opposed to environmentally induced phenotypic differences).  Depending upon the situation/environment or circumstances affecting survival, certain variant phenotypes may enjoy greater reproductive success.  Those aspects of the phenotype that correlate with greater success we think of as adaptations, e.g. the shell of reptilian eggs is an adaptation to life on land.  Adaptations result from natural selection.  

The following examples illustrate differential reproductive success (i.e., natural selection) as driven by different scenarios of selection pressures. 

1.  Stabilizing selection - favors intermediate phenotype
                          - does not lead to change
                          - extremes selected against
                          ex. Birth weight in humans  and   gall fly gall size on golden rod where woodpeckers prefer larger galls to prey upon and a wasp prefers to prey upon smaller galls.

2.  Directional selection - important when environmental change occurs
            ex. resistance to pesticides/antibiotics as seen in evolving populations of pathogenic bacteria and other pests.
                        - favors an extreme phenotype, leads to changes in populations & eventually (if you accept theory of uniformity, uniformitarianism)  to new species formation.

The origin of the hard, reptilian shelled egg can also be thought of in terms of directional selection coupled with genetic mutations leading to ever more protective egg coverings evolving from jelly-like frog egg-ish beginnings.  Selection pressures would favor those eggs that survive greater desiccation as experienced in the ever more terrestrial lifestyles of the early reptiles.

3.  Disruptive Selection - the intermediate phenotype selected against.
            ex. predator favors medium sized prey

In all 3 modes:  Survival of the Fittest where fitness=reproductive success (not strength)
 

4.  Sexual selection - Selection for mate attracting traits and thus an advantage in reproductive success.
                                  ex. Male’s bright plumage in birds, males greater strength, larger antlers

Sexual selection provides an explanation for sexual dimorphism - male & female differ  beyond gonads and genitalia.  Recall the extreme case of the dwarf male angler fish.  Based on a morphological species concept alone, male angler fish may have been named as a separate species when originally captured from the depths of the ocean.  Only after the realization that the small angler fish were simply  males which spawned with the much larger female angler fish would a single species rather than two be recognized.  The knowledge of the capacity for interbreeding is of course the basis for the biological species concept. [A morphological species is defined by the similarities in morphology (form) between members of the same species.  A biological species is defined by the capacity for sexual reproduction to occur between and within populations.]

Evidence of Evolution 

Before presenting some of the major lines of evidence for the fact that evolution has taken place, it might be useful to partition the scale of evolution into two categories:  1) the changes readily observed by population biologists today (microevolution), and 2) the profound changes which have accumulated from these population level processes (macroevolution).  Both scales of evolution provide evidence that evolution has occurred. 

Microevolution – the following three statements are meant to express the same basic concept and   define microevolution as: 1) change in the genetic makeup of a population, 2) change in a population’s allele or genotype frequencies, 3) change within the gene pool of a population.  Gene pool is simply a term created to express the concept of the collective total of all the genes in a population.  Basically, microevolution is evolution within a species such that gene frequencies change over time. 

Given an accumulation of population level changes, it is possible to look backward into history and see the larger scale of change, including the formation of new species, genera, families, etc. (macroevolution).  Evidence for microevolution comes from studies in population genetics.  Evidence for the larger scale of change is presented below.   

Macroevolution - origin of groups at and above the species level.  Macroevolution is said to be the “Grand View of Evolution” and includes the origin of new designs.  New designs are often reflected in the taxonomic hierarchy, e.g. the notochord is a new design unique to the phylum Chordata.  New designs are also reflected in phylogenies, e.g. a shared, derived character common to all deuterostomes is the new formation of the mouth from a 2^nd embryonic derived opening into the animal body. 

Evidence of Macroevolution: 

1.  Fossils -- Paleontology- the study of fossils.  Fossils are the visual remains of once living organisms preserved by the earth.  Fossils are formed when sediment deposition buries dead bodies and the body remains undergo preservation via mineralization or an impression of body remains is preserved.  Fossils show: 1) change over time; & 2) continuity of change.

            ex. Archaeopteryx  - has teeth and long bony tail like reptiles but has feathers like birds.  Archaeopteryx is one of many known "missing links" between reptiles & birds (see page p. 259, "Interpreting and misinterpreting the past").  

2.            Biogeography - study of the geographical distribution of plants and animals; it includes both the historical distributions as revealed by paleontology and present day distributions. 

            Mammal Distribution From Their origin 150 Million Years Ago To Today.  The concentration of diverse marsupial (pouched) mammals in Australia and the restriction to Australia of the only remaining monotremes (egg laying mammals) is thought to be the result of evolution within these groups in the absence of placental mammals on this island continent.  Placental mammals, having evolved on northern continental masses, did not reach what is now Australia, thus leaving the marsupials to diversify without placental mammal competition.  (Bats are an exception, they are indigenous to Australia, other placental mammals did not reach the continent until the recent introductions by man—the rabbit, a placental mammal, is especially successful in Australia today as an exotic pest).  Presumably, placental mammals would have lead to the mass extinction of marsupial species.  At least in South America, the introduction of placental mammals from the north coincides with the extinction of many marsupial species known from the S. A. fossil record.  

3.  Comparative morphology - study of body parts, esp. internal body parts.  The evolutionary view that mammals and birds are descendents
of ancient reptiles is supported by the anatomy of vertebrate forelimbs (see fig. 13.8, p. 201).  The forelimbs of vertebrates have undergone morphological divergence and are recognized today as homologous structures.

Homologous structures – structures in different species that are similar because of common ancestry; structures in different species that have undergone morphological divergence becoming modified for different functions; Homologous structures have the same basic structure but may appear somewhat different depending on the degree of modification produced by evolution, ex. Whale flipper, bat wing, & human arm are all homologous (have same bones, are descended from common ancestor).  But a bat’s wing & a fly’s wing are analogous-similar in function but not origin or structure.  Analogous structures result from morphological convergence where dissimilar body parts become similar through evolution (ex. exoskeleton forms fly wing, feathers form bird wing, and skin forms bat wing--each wing type is analogous to the others).

Comparative anatomy reveals Vestigial organs – structures reduced in size & of little (or modified) function, e.g., in some snakes, such as pythons, evidence of their shared ancestry with legged creatures is found in their vestigial pelvic girdle and leg bones; some modern-day whales also have vestigial hind leg bones. 

4.  Comparative Embryology -  ex. Humans, like all vertebrates, have a notochord, post anal tail and gill pouches as embryos.  Such evidence does support the view that all vertebrates share a common ancestor and that the differences between vertebrates today is the result of “descent with modification” from this ancestral type.  A replay of the changes from the ancestral type was once thought to exist in the changes during embryonic development.  The well-worn phrase "ontogeny recapitulates phylogeny" (or stated more clearly, embryonic development recapitulates evolutionary history) is a gross overstatement.  The point is, that useful features to unraveling the evolutionary relationships among animals can be learned from comparative embryology. 

5.  Molecular Biology – The genetic code is universal.  Also, degrees of relatedness are reflected in nucleotide base sequence (DNA) similarity and in amino acid sequence similarity in proteins.   For example, horse hemoglobin amino acid sequence is more similar to the sequence in hedgehog hemoglobin (both the horse and hedgehog are mammals) than to the sequence in hummingbird hemoglobin.