Introduction to History of Life

Biological evolution consists of change in the hereditary characteristics of groups of organisms over the course of generations. Groups of organisms, termed populations and species, are formed by the division of ancestral populations or species, and the descendant groups then change independently. From a long-term perspective, evolution is the descent, with modification, of different lineages from common ancestors. The history of evolution has two major components: 1) branching of lineages and 2) changes within lineages

Evolutionary theory is a body of statements about the processes of evolution that are believed to have caused the history of evolutionary events. Biological evolution occurs as the consequence of random and nonrandom processes.

Random Processes Mutation Variation in the characteristics of organisms in a population originates through random mutation of DNA sequences (genes) that affect the characteristics. Genetic variation is augmented by recombination during sexual reproduction, which results in new combinations of genes. Variation is also augmented by gene flow, the input of new genes from other populations. Genetic Drift Results from random variation in the survival and reproduction of different genotypes. The frequencies of alleles fluctuate by pure chance, eventually one allele will replace the others Genetic drift is most important when the alleles of a gene are neutral—that is, when they do not substantially differ in their effects on survival or reproduction—and it proceeds faster, the smaller the population is.

Nonrandom Processes Natural selection Any consistent (nonrandom) difference among organisms bearing different alleles or genotypes in their rate of survival or reproduction (i.e., their fitness) due to differences in one or more characteristics. In most cases, environmental circumstances affect which variant has the higher fitness. A common consequence of natural selection is adaptation, an improvement in the average ability of the population's members to survive and reproduce in their environment. Natural selection tends to eliminate alleles and characteristics that reduce fitness (such as mutations that cause severe birth defects in humans and other species), and it also acts as a "sieve" that preserves and increases the abundance of combinations of genes and characteristics that increase fitness, but which would occur only rarely by chance alone.

Speciation If gene flow among different geographically separated populations is slight, different genetic changes can transpire in those populations. Because the populations experience different histories of mutation, genetic drift, and natural selection, they follow different paths of change, diverging in their genetic constitutions and in the individual organisms' characteristics The differences that accumulate eventually cause the different populations to be reproductively isolated; the different populations are now different species. The significance of this process of speciation is that the new species are likely to evolve independently from then on. Some may give rise to yet other species, which ultimately may become exceedingly different from one another. Successive speciation events, coupled with divergence, give rise to clusters of branches on the phylogenetic tree of living things.

What is Microevolution and Macroevolution? Microevolution refers to any evolutionary change below the level of species (e.g., changes in the frequency within a population) Macroevolution is used to refer to any evolutionary change at or above the level of species. It means the splitting of a species into two (cladogenesis) or the change of a species over time into another (anagenesis) Any changes that occur at higher levels, such as the evolution of new families, phyla or genera, is also therefore macroevolution, but the term is not restricted to the origin of those higher taxa. Another way to state the difference is that macroevolution is between- species evolution of genes and microevolution is within-species evolution of genes.

Conclusions: Distinguishing Patterns from Processes The history of evolution—the proposition that all species have descended, with modification, from common ancestors— is supported by overwhelming evidence The body of principles that describe the causal processes of evolution, such as mutation, genetic drift, and natural selection, constitutes the theory of evolution. Many of the details of the history of evolution remain to be described Also, the theory of evolution, like all scientific theories, continues to develop as new information and ideas deepen our understanding. The major causes of evolution have been identified; however, views on the relative importance of the various processes continue to change as new information adds detail and modifies our understanding.