Biological evolution accounts for diversity over long periods of time. Through billions of years of evolution, life forms have continued to diversify in a branching pattern, from single-celled ancestors to the diversity of life on Earth today. Life forms of the past were in some ways very different from living forms of today, but in other ways very similar. Evolution is still continuing today. Humans directly impact biodiversity, which may then impact future evolutionary potential. Present-day species evolved from earlier species; the relatedness of organisms is the result of common ancestry. Life on Earth 3.8 billion years ago consisted of one-celled organisms similar to present-day bacteria. There is evidence of eukaryotes in the fossil record from about one billion years ago; some were the precursors of multicellular organisms. The early evolutionary process of eukaryotes included the merging of prokaryote cells. Geological change and biological evolution are linked. Tectonic plate movement has affected the evolution and distribution of living things. Living things have had a major influence on the composition of the atmosphere and on the surface of the planet. During the course of evolution, only a small percentage of species have survived until today. Background extinctions are a normal occurrence. Rates of extinction vary. Mass extinctions occur. Extinction can result from environmental change. Human influence may be causing a modern mass extinction. Extinctions may create opportunities for further evolution in other lineages to occur. Rates of evolution vary. Rates of speciation vary. Evolutionary change can sometimes happen rapidly. Some lineages remain relatively unchanged for long periods of time.

Evidence of Evolution concepts for undergraduates

The patterns of life’s diversity through time provide evidence of evolution. Evolution can sometimes be directly observed. An organism’s features reflect its evolutionary history. There is a fit between organisms and their environments, though not always a perfect fit. There is a fit between the form of a trait and its function, though not always a perfect fit. Some traits of organisms are not adaptive. Features sometimes acquire new functions through natural selection. The fossil record provides evidence for evolution. The fossil record documents the biodiversity of the past. The fossil record contains organisms with transitional features. The fossil record documents patterns of extinction and the appearance of new forms. The sequence of forms in the fossil record is reflected in the sequence of the rock layers in which they are found and indicates the order in which they evolved. Radiometric dating can often be used to determine the age of fossils. There are similarities and differences among fossils and living organisms. Similarities among existing organisms (including morphological, developmental, and molecular similarities) reflect common ancestry and provide evidence for evolution. Not all similar traits are homologous; some are the result of convergent evolution. The geographic distribution of species often reflects how geologic change has influenced lineage splitting. Artificial selection provides a model for natural selection. People selectively breed domesticated plants and animals to produce offspring with preferred characteristics.

Mechanisms of Evolution concepts for undergraduates

Evolution is often defined as a change in allele frequencies within a population. The Hardy-Weinberg equation describes expectations about the gene pool of a population that is not evolving—one that is very large, mates randomly, and does not experience mutation, natural selection, or gene flow. Evolution occurs through multiple mechanisms. Evolution results from natural selection acting upon genetic variation within a population. Evolution results from genetic drift acting upon genetic variation within a population. Evolution results from mutations. Evolution results from gene flow. Evolution results from hybridization. Natural selection and genetic drift act on the variation that exists in a population. Natural selection acts on phenotype as an expression of genotype. Phenotype is a product of both genotype and the organism’s interactions with the environment. Variation of a character within a population may be discrete or continuous. Continuous characters are generally influenced by many different genes. New heritable traits can result from mutations. Mutation is a random process. Organisms cannot intentionally produce adaptive mutations in response to environmental influences. Complex structures may be produced incrementally by the accumulation of smaller advantageous mutations. Inherited characteristics affect the likelihood of an organism’s survival and reproduction. Over time, the proportion of individuals with advantageous characteristics may increase (and the proportion with disadvantageous characteristics may decrease) due to their likelihood of surviving and reproducing. Traits that confer an advantage may persist in the population and are called adaptations. Complex traits can arise through the cooption of another trait. The number of offspring that survive to reproduce successfully is limited by environmental factors. Depending on environmental conditions, inherited characteristics may be advantageous, neutral, or detrimental. Natural selection can act on the variation in a population in different ways. Natural selection may favor individuals with one extreme value for a trait, shifting the average value of that trait in one direction over the course of many generations. Selection favoring an extreme trait value reduces genetic variation in a population. Natural selection may favor individuals with traits at each extreme of the range for that trait. Selection favoring individuals with traits at each extreme of a range maintains genetic variation in a population. Natural selection may favor individuals with an intermediate value for a trait. Selection favoring an intermediate value for a trait reduces genetic variation in a population. Natural selection sometimes favors heterozygotes over homozygotes at a locus. Heterozygote advantage preserves genetic variation at that locus (i.e., within the population, it maintains multiple alleles at that locus). Natural selection sometimes favors rare traits and acts against those that become too common in a population. Frequency-dependent selection preserves genetic variation in a population. Sexual selection occurs when selection acts on characteristics that affect the ability of individuals to obtain mates. Sexual selection can lead to physical and behavioral differences between the sexes. An individual’s fitness (or relative fitness) is the contribution that individual makes to the gene pool of the next generation relative to other individuals in the population. An organism’s fitness depends on both its survival and its reproduction. Fitness is often measured using proxies like mass, number of matings, and survival because it is difficult to measure reproductive success. Natural selection is capable of acting at multiple hierarchical levels: on genes, on cells, on individuals, on populations, on species, and on larger clades. Random factors can affect the survival of individuals and of populations. Smaller populations are more strongly affected by genetic drift than are larger populations. Genetic drift can cause loss of genetic variation in a population. Founder effects occur when a population is founded from a small number of individuals. Founder effects can affect the genetic makeup of a newly started population (and reduce its genetic variation) through sampling error. Bottlenecks occur when a population’s size is greatly reduced. Bottlenecks can affect the genetic makeup of a population (and reduce its genetic variation) through sampling error. A species is often defined as a group of individuals that actually or potentially interbreed in nature. There are many definitions of species. Speciation is the splitting of one ancestral lineage into two or more descendent lineages. Speciation is often the result of geographic isolation. Speciation can also occur without geographic isolation. Speciation requires reproductive isolation. Reproductive isolation can occur through mechanisms that prevent fertilization from occurring. Reproductive isolation can also occur through mechanisms that act after fertilization, when a fertilized egg (or the individual resulting from that egg) has low fitness. Occupying new environments can provide new selection pressures and new opportunities, leading to speciation. Occasionally offspring, known as hybrids, result from matings between distinct species or between distinct parental forms. Some hybrids have increased fitness relative to their parents. Some hybrids have decreased fitness relative to their parents. Evolution does not consist of progress in any particular direction.

Nature of Science concepts for undergraduates

Science focuses on natural phenomena and processes. Scientific knowledge is open to question and revision as we come up with new ideas and discover new evidence. A hallmark of science is exposing ideas to testing. Scientists test their ideas using multiple lines of evidence. Scientists use multiple research methods (experiments, observational research, comparative research, and modeling) to collect data. Scientists can test ideas about events and processes long past, very distant, and not directly observable. Scientists may explore many different hypotheses to explain their observations. The real process of science is complex, iterative, and can take many different paths. Scientific findings and evidence inspire new questions and shape the directions of future scientific research. Accepted scientific theories are not tenuous; they must survive rigorous testing and be supported by multiple lines of evidence to be accepted. Science is a human endeavor. Authentic scientific controversy and debate within the community contribute to scientific progress.

Studying Evolution concepts for undergraduates

Our knowledge of the evolution of living things is always being refined as we gather more evidence. Our understanding of life through time is based upon multiple lines of evidence. Scientists use multiple lines of evidence (including morphological, developmental, and molecular evidence) to infer the relatedness of taxa. Scientists use fossils (including sequences of fossils showing gradual change over time) to learn about past life. Scientists use physical, chemical, and geological evidence and comparative anatomy to establish the age of fossils. Scientists use the geographic distribution of fossils and living things to learn about the history of life. Scientists use experimental evidence to study evolutionary processes. Scientists use artificial selection as a model to learn about natural selection. Classification is based on evolutionary relationships. Evolutionary trees (i.e., phylogenies or cladograms) portray hypotheses about evolutionary relationships. Evolutionary trees (i.e., phylogenies or cladograms) are built from multiple lines of evidence. The principle of parsimony suggests that the phylogenetic hypothesis most likely to be true is the one requiring the fewest evolutionary changes. Evolutionary trees can be used to make inferences and predictions. As with other scientific disciplines, evolutionary biology has applications that factor into everyday life, for example in agriculture, biodiversity and conservation biology, and medicine and health. Because of common ancestry, model organisms can be used to provide insight into the biology of other organisms.