Evolution and the Origin of Species
18.1 Understanding Evolution
Evolution, the unifying theory of biology, describes a mechanism for the change and diversification of species over time.
Charles Darwin and Natural Selection
Charles Darwin and Alfred Wallace independently developed the theories of evolution and its main operating principle: natural selection.
Visible evidence of ongoing evolution: Darwin
The differences in shape and size of beaks in Darwin's finches illustrate ongoing evolutionary change.
Processes and Patterns of Evolution
Natural selection can only occur in the presence of genetic variation; environmental conditions determine which traits are selected.
Evidence of Evolution
Evidence for evolution has been obtained through fossil records, embryology, geography, and molecular biology.
Misconceptions of Evolution
There are many misconceptions about evolution, including the meaning of the word theory, the way populations change, and the origin of life.
18.2 Formation of New Species
Species and the Ability to Reproduce
A species is defined as a group of individuals that, in nature, are able to mate and produce viable, fertile offspring.
Speciation is an event in which a single species may branch to form two or more new species.
Allopatric speciation occurs when a single species becomes geographically separated; each group evolves new and distinctive traits.
Sympatric speciation occurs when two individual populations diverge from an ancestral species without being separated geographically.
Reproductive isolation, through mechanical, behavioral, and physiological barriers, is an important component of speciation.
18.3 Reconnection and Rates of Speciation
Over time, two species may further diverge or reconnect, depending on the fitness strength and the reproductive barriers of the hybrids.
Varying Rates of Speciation
Two patterns are currently observed in the rates of speciation: gradual speciation and punctuated equilibrium.
18.4 Evolution of Genomes
Genomic similarities between distant species
Genomic similarities between distant species can be established via analysis of genomes using advanced technology.
Accumulating changes over time
Processes such as mutations, duplications, exon shuffling, transposable elements and pseudogenes have contributed to genomic evolution.
Whole-genome duplication is characterized by an organisms entire genetic information being copied once or multiple times.
Gene rearrangement within genomes
Gene rearrangements occur via various mechanisms and produce genes causing mutation or favorable traits that can contribute to evolution.
Gene duplications and divergence
Gene duplications create genetic redudancy and can have various effects, including detrimental mutations or divergent evolution.
Noncoding DNA are sequences of DNA that do not encode protein sequences but can be transcribed to produce important regulatory molecules.
Variations in size and number of genes
The genome size does not always correlate with the complexity of the organism and, in fact, shows great variation in size and gene number.
18.5 Human Evolution
Primates evolved from arboreal ancestors and share many characteristics, including flexible skeletons, large brains, and vision reliance.
Hominin is the group which consists of modern humans, extinct human species, and all our immediate ancestors.
The Homo genus, to which humans belong, evolved from our close primate relatives, Australopithecus, and is distinguished by cranial size.
Humans (Homo sapiens) are distinct from non-human primates in their upright walking, abstract reasoning, language skills, and problem solving.
18.6 Evidence of Evolution
Fossils can form under ideal conditions by preservation, permineralization, molding (casting), replacement, or compression.
Incompleteness of the fossil record
The fossil record is incomplete because only certain types of animals are able to fossilize and most fossils have not yet been found.
Estimating the age of fossils
The age of fossils can be determined using radiocarbon dating (also known as carbon-14 dating), stratigraphy, and biostratigraphy.
What fossils tell us
Fossils tell us when organisms lived, as well as the progression and evolution of life on earth over millions of years.
Example: the evolution of horses
The highly detailed fossil record of horses has given us great insight into their evolutionary progress.
Homology, which is different from analogy, describes the relationship between similar structures that evolved from a common ancestor.
Vestigial structures have no function but may still be inherited to maintain fitness.
Convergent evolution occurs in different species that have evolved similar traits independently of each other.
Distribution of species as it relates to evolution and bi...
The biological distribution of species is based on the movement of tectonic plates over a period of time.