Carolus Linnaeus developed the Systema Naturae in the 18th century to classify the diversity of living things into a series of categories that are progressively more inclusive. He based his system on body structure, body function, and the sequence of bodily growth. Modern taxonomy retains the same structure of the Linnaean system, but takes genetics into account as well to construct the relationships among living things. Species, the smallest working units in biological classificatory systems, are reproductively isolated populations or groups of populations capable of interbreeding to produce fertile offspring. Cross-species comparisons identify anatomical features of similar function as analogies, while anatomical features that have evolved from a common ancestral feature are called homologies. When constructing evolutionary relationships, only homologies matter.
Charles Darwin‘s book On the Origin of Species accounts for change within species and for the emergence of new species in purely naturalistic terms. Darwin combined his observations into the theory of natural selection as follows: All species display a range of variation, and all have the ability to expand beyond their means of subsistence. It follows that, in their “struggle for existence”, organisms with variations that help them to survive in particular environments will reproduce with greater success than those without such variations. Thus, over time, nature selects the most advantageous variations and species evolve.
Gregor Mendel developed the basic laws of heredity. Today, a comprehensive understanding of heredity, molecular genetics, and population genetics supports Darwinian evolutionary theory. Genes are the portions of DNA molecules containing a sequence of base pairs that encodes a particular protein. Mendel discovered that inheritance was particulate, rather than blending. Mendel’s law of segregation states that pairs of genes separate, keep their individuality, and are passed on to the next generation unaltered. Mendel’s law of independent assortment states that different traits (under the control of distinct genes) are inherited independently of one another. The discovery of chromosomes provided a visible vehicle for the transmission of traits proposed in Mendel’s laws. Then in 1953, James Watson and Francis Crick discovered the mechanism for inheritance based on the structure of DNA.
Alternate forms of genes are known as alleles. Enzymes are proteins that initiate and direct chemical reactions. A karyotype is the array of chromosomes found inside a single cell. A genome is the complete structure sequence of DNA for a species. A codon is a three-base sequence of a gene that specifies a particular amino acid for inclusion in a protein. Because DNA cannot leave the cell’s nucleus, the directions for a specific protein are first converted into RNA in a process called transcription. Next the RNA travels to the ribosomes, the cellular structure where translation of the directions found in the codons occurs, producing proteins. Amino acids are strung together in different amounts and sequences to produce an almost infinite number of proteins. This is the genetic code.
Cell division begins when chromosomal DNA replicates and each chromosome becomes a pair of sister chromatids. DNA “unzips” between the base pairs and then each base on each now-single strand attracts its complementary base, reconstituting the second half of a double helix. Then, the sister chromatids separate and a new cell membrane surrounds each new chromosome set and becomes the nucleus of a new cell. This kind of cell division is called mitosis. Sexual reproduction increases genetic diversity, which in turn contributes to adaptation among sexually reproducing species. Sexual reproduction involves joining specialized sex cells produced by a different kind of cell division, called meiosis. Meiosis begins like mitosis, with the formation of sister chromatids. Cells then proceed to divide into four new cells, each of which has half the number of chromosomes compared to the parent cell. Because paired chromosomes are separated, the daughter cells will not be identical. Sometimes, the original pair is homozygous, possessing identical alleles for a specific gene. If the original pair is heterozygous, the chromosome pair bears different alleles for a single gene. The observable characteristics of an organism (the phenotype) may or may not reflect a particular genotype (the alleles possessed for a particular trait) due to the variable expression of dominant and recessive alleles. Some alleles are codominant. Multiple genes control most physical traits. In such cases, we speak of polygenetic inheritance, in which the respective alleles of two or more genes influence phenotype. Characteristics subject to polygenetic inheritance exhibit a continuous range of variation in their phenotypic expression that does not correspond to simple Mendelian rules.
A key concept in genetics is that of the population, or a group of individuals within which breeding takes place. Gene pool refers to all the genetic variants possessed by members of a population. Natural selection takes place within populations as some members contribute a disproportionate share of the next generation. Over generations, the relative proportions of alleles in a population change (biological evolution) according to the varying reproductive success of individuals within that population. In other words, at the level of population genetics, evolution can be defined as changes in allele frequencies in populations. Mutation, the ultimate source of evolutionary change, constantly introduces new genetic variation. Mutations occur randomly. Although some mutations may be harmful or beneficial to individuals, most are neutral. But in an evolutionary sense, random mutation is inherently positive. Without the variation brought in through mutations, populations could not change over time in response to changing environments. Genetic drift refers to chance fluctuations of allele frequencies in the gene pool of a population. Changes at the population level derive from random events at the individual level affecting the individual’s survival. A specific kind of genetic drift known as founder effects may occur when an existing population splits up into two or more new ones, especially if a small number of individuals found one of the new populations. In such cases, the gene frequencies of the smaller population tend not to contain the full range of variation present in the larger one. Gene flow, or the introduction of new alleles from nearby populations, brings new genetic variation into a population.
Natural selection accounts for adaptation, a series of beneficial adjustments to a particular environment. Natural selection shapes genetic variation at the population level to fit local environmental conditions. Selection by the forces of nature favors some individuals over others. In the process, the frequency of genetic variants for harmful or nonadaptive traits within the population reduces while the frequency of genetic variants for adaptive traits increases. Over time, changes in the genetic structure of the population can result in formation of new species. Ultimately, all natural selection is measured in terms of reproductive success – mating and production of viable offspring who will in turn carry on one’s genes. Stabilizing selection occurs in populations that are already well adapted or where change would be disadvantageous.
Clines are the gradual changes in the frequency of an allele or trait over space. While microevolution refers to changes in the allele frequencies of populations, macroevolution focuses on speciation – the formation of new species – and on the evolutionary relationships among groups of species. The microevolutionary forces of mutation, genetic drift, gene flow, and natural selection can lead to macroevolutionary change as species diverge. Cladogenesis is speciation through a branching mechanism whereby an ancestral population gives rise to two or more descendant populations. Anagenesis is a sustained directional shift in a population’s average characteristics. Punctuated equilibria is a model of macroevolutionary change that suggests evolution occurs via long periods of stability or stasis punctuated by periods of rapid change.
—April 2021
Lance Jones 