FIGURE 6.1 Main originators of the hypothesis of natural selection, Charles Darwin (left) and Alfred Russel Wallace (right). From Ridley (1996), Evolution, 2e, Blackwell Science, Inc., Malden, MA, pp. 9 and 10.
■ Those individuals with variations favorable for survival from this struggle (the more adaptable ones) will live to produce offspring that also have these variations, thus changing the population over time with each successive inherited variation and eventually resulting in species different from the ancestral species.
A phrase associated with natural selection is "survival of the fittest," which is potentially misleading because "fitness" is not necessarily related to the popularized idea that "the strong survive and the weak perish." Fitness in this sense actually means "better adapted" or refers to the number of offspring produced by an individual, and thus has little or nothing to do with strength. Mammals of the Mesozoic exemplify this concept, as they were physically weak and small in comparison to their dinosaurian companions but clearly were better adapted than dinosaurs for surviving the environmental changes that resulted in the extinction of the dinosaurs by the end of the Mesozoic (Chapter 16).
The tenets of natural selection have been modified since the time of Darwin and Wallace but still form the foundation of evolutionary theory. The older version of the hypothesis of natural selection is Darwinism. Although Darwin and Wallace knew that certain inheritable variations in organisms translated into adaptations, they did not know the source of the variations or the exact mechanism for their inheritance. Ironically, another scientist at the time, Gregor Mendel (1822-84), was providing the answer to this question, but his results were not widely recognized by other scientists until early in the twentieth century. Mainly through cross-breeding pea plants, Mendel discovered the basic factors underlying heredity - genes, alleles, genotypes, and phe-notypes. For example, a pair of genes at a locus (comprising a genotype) is paired because each gene came from a different parent. Consequently, sexual reproduction is responsible for most of the genetic variation in
Neo-Darwinism is a modified descendant that takes into account modern genetics, the study of heredity and variations in organisms.
an organism, because one-half of its genes came from its mother and one-half from its father. This is related to the haploid nature of male and female gametes, formed by meiosis, which combine to form a diploid zygote. Dinosaurs are also presumed, with a high degree of certainty, to have reproduced sexually through male-female pairs and not through parthenogenesis (Chapter 8). This hypothesis is supported by the numerous dinosaur eggs (a few containing embryos) and nests, the sexual dimorphism interpreted from some dinosaur skeletons of the same species, and the sexual reproductive life cycles in their closest living relatives, crocodilians and birds (Chapter 8). Dinosaurs thus had a constant source of genetic variation, as with other sexually reproducing organisms.
Another discovery by Mendel was that one of a pair of genes tends to overshadow the other gene in its physical expression, which affects the phenotype of the organism, so that the dominant gene is expressed over the recessive gene. An individual with two dominant or two recessive genes at a locus has a homozygous condition, in contrast to one with dominant and recessive genes, which is heterozygous. A heterozygous condition is defined by alleles, because a pair of genes at the same locus represents variations, or alternatives, of one another. Interestingly, proportions of these dominant and recessive traits can be predicted in offspring from parents with homozygous or heterozygous conditions through probabilities. For example, the gene for brown eyes in humans is dominant over that for blue eyes, but both parents can have brown eyes and a recessive gene for blue eyes, so they will both have a heterozygous condition. The gene frequency, which is the frequency of each gene in relation to another gene at its locus, is 0.5 for each allele in a heterozygous condition, which corresponds to a 50% probability for each (otherwise known popularly as "50 : 50"). In contrast, a homozygous condition would have a gene frequency of 1.0 for the single gene, whether it is for a homozygous dominant or homozygous recessive.
Armed with probabilities, geneticists can make predictions about the genotypes and phenotypes of pairings. In the example of eye color, the probability for any one of their offspring to have blues eyes is 25%. Probability is calculated through assigning letters to both the dominant allele (B) and recessive allele (b) in the homozygous pairs and crossing them in a diagram used by geneticists, called a Punnet square:
The probability of a brown-eyed, homozygous-dominant individual (BB) is 1 in 4, or 25%. The probability of a brown-eyed, heterozygous individual (Bb) is 2 in 4, or 50%. Lastly, the probability of a blue-eyed homozygous-recessive individual (bb) is 1 in 4, or 25%. Therefore, two heterozygous individuals can produce three possible genotypes, but these genotypes can differ in their expression as phenotypes. These probabilities are related as genotype frequencies with values between 0 and 1, such as 25% = 0.25, 50% = 0.5, and so on. Notice how the gene frequencies and genotype frequencies are different from one another.
This shuffling of genes produces variation in a population that can be predicted by calculating probabilities for successive generations, based on gene frequencies and assuming random mating with no natural selection. The expected ratio of genotype frequencies in such a case is called the Hardy-Weinberg ratio. The ratio was named after its originators, mathematician G. H. Hardy and physician Wilhelm Weinberg, who independently devised a formula describing it early in the twentieth century. For example, the preceding example has two alleles (B and b), which
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