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E Evolutionary Change evolution, such as random genetic “drift,” or random changes in genes and traits over time, artifi- Dakota E. McCoy cial selection by humans for human-desired traits Harvard University, Cambridge, MA, USA in other organisms, and sexual selection for traits that increase an individual’s chance of mating. All of these mechanisms operate by impacting Synonyms heritable variation to drive evolution. Evolutionary change requires that individuals Descent with modification vary in ways that are passed on (in whole or in part) to their offspring. Although the observation of heritability in nature (“like begets like”)is Definition sufficient for some evolutionary inquiry, scientists now have a rich understanding of the primary Evolutionary change is the heritable change in mechanism of heritability: genetics. All life on populations and species over time, due to mecha- earth is based on genetic code that is carried nisms such as natural selection, random genetic within cells and translated into observable traits. drift, and sexual selection. Humans, sea horses, mushrooms, the common cold virus, and more all reproduce by passing genetic material in the form of deoxyribonucleic Introduction acid (DNA) or ribonucleic acid (RNA) on to their descendants. Every organism is the sum of inter- Evolution is heritable change over time, through actions of their genes (functional stretches of which species change, diverge, and sometimes DNA or RNA) with their environment, resulting create new lineages. “Evolution” and “natural in their phenotype, i.e., the set of all traits that are selection” are often used interchangeably, but the expressed in a given environment and are seen by two are distinct: evolution is the pattern, and nat- other individuals. In addition to genetics, “epige- ural selection is one of many mechanisms that netic” modifications (changes to the ways genes cause evolution. Under natural selection, organ- are expressed, such as conditional switches that isms which are better adapted to their environ- turn genes “on” or “off ”) can also be heritable and ment have more (or healthier) offspring, so their thus can play a role in evolution. traits are more often passed on to future genera- Mutation generates variation in the genetic tions. In this manner, natural selection drives evo- code due to (i) random errors in our cellular lution. However, other mechanisms can also drive machinery or (ii) environmental influences # Springer International Publishing AG, part of Springer Nature 2018 T. K. Shackelford, V. A. Weekes-Shackelford (eds.), Encyclopedia of Evolutionary Psychological Science, https://doi.org/10.1007/978-3-319-16999-6_2094-1 2 Evolutionary Change (induced errors). Genetic mutation can translate suggests, not all phenotypic characteristics are into heritable differences in an organism’s observ- heritable! Eye color is fully heritable from able traits, or phenotype, with neutral, positive, or known genetic variants, but interest in computer negative effect on the organism’s fitness. While science is determined in part by inherited talent mutation, and thus differences between organ- but also in large part by one’s environment grow- isms’ genetic code, is the basis of variation, sev- ing up. This is one complication of evolutionary eral higher-level factors contribute to variation. inquiry: there is not a one-to-one correspondence Gene flow between separated populations of the between traits we can observe and measure same species can also induce variation, and (phenotypes) and heritable units (genes) coded recombination during sexual reproduction reshuf- in DNA. fles genes and their associated traits. DNA, deoxyribonucleic acid, is a long thin Scientists research evolutionary change by molecule that contains the “code” which is trans- examining the fossil record, interpreting genetic lated into proteins, the building blocks of cells, evidence in the present, inferring genetic path- tissues, organs, and ultimately our bodies (some ways over time, modeling change, and performing viruses use RNA, rather than DNA, as their unit of real-time evolutionary experiments with model heredity; it is debated whether viruses are truly organisms such as flies, yeast, and mice. This alive). DNA has an iconic “double helix” struc- wealth of approaches has contributed greatly to a ture, with chemical bases in the center surrounded scientific understanding of the causes and persis- by phosphate–sugar chains. The chemical bases tence of variable life on earth. are of four kinds, represented by the four letters A, Evolution is popularly conceived as a stairway T, C, and G (which refer to adenine, thymine, of progress – from simple to complex, from bad to cytosine, and guanine, respectively). DNA is good, and from animals to man. This is shaped roughly like a spiral ladder, and these completely untrue. Rather, it is more like a tangled four bases pair up with one another (A with T, bush with connections, splits, divergences, and C with G) to form the rungs while the outer bars interlocking parts. Humans are merely a tiny tip are formed by phosphate and sugar. DNA is “tran- at the end of a tiny branch on the gigantic “Bush of scribed,” i.e., copied by the enzyme RNA poly- Life.” merase into messenger RNA, a molecule similar to DNA, which is then “translated” into protein. DNA base pairs are grouped into sequences of Heritability three called “codons” which correspond to specific amino acids (or in some cases, “stop” codons Genetics signal a stop in translation). The third position of a Genes control heritable traits and are the units of codon is called the “wobble” position because heredity. “Genotype” refers to the genetic material base pairs in that slot do not always obey the in an organism, while “phenotype” refers to pairing rules described above (A with T, G with observable characteristics whether inherited or C); further investigation of this important phe- not. Phenotypes result from genes, from environ- nomenon is outside the scope of this article. ment/behavior, or from the interaction of genes Genes are stretches of DNA, ranging between with the environment (often popularly written as less than one hundred base pairs to over two “nature plus nurture”). Selection generally acts on million, that in essence code for a specific string phenotypes, but phenotypes are not directly trans- of amino acids which in turn folds into a specific mitted from parents to offspring – genes are. protein. Almost all human DNA is the same in A single human’s overall phenotype could include every individual human (over 99%); thus, only a tall height, brown hair, brown eyes, interest in very small proportion of the genome comprises and success at computer science, preference for individual variation. neutral-toned clothing, no memory for details, and Genes sit in a specific place on “chromo- great storytelling ability. As this example somes,” which are the dense threadlike packages Evolutionary Change 3 of genetic information that cluster inside the that you can inherit either a blue or brown allele nuclei of our cells. However it is worth noting from your parents and that brown is dominant to that our nuclear DNA – the DNA on chromo- blue. If you have two blue alleles, you have blue somes in the nucleus – is not the only DNA we eyes; similarly, two brown alleles produce brown have in our bodies. Most notably, mitochondria eyes. But if you have blue and brown alleles, since have their own DNA that is inherited from mother brown is dominant to blue in this imaginary sim- to child (since mothers’ ova contain the cell plified scenario, you will have brown eyes. How- organelles). Mitochondria have DNA because ever, you could have a child with blue eyes, as they were independent organisms which our long as (i) you pass your blue allele on to the child ancestors “domesticated” and put to work inside and (ii) that child inherits a blue allele from their our cells. Chromosomes consist of DNA coiled other parent as well! This is an important princi- around “histones,” a special type of protein that ple: traits that are not expressed in a parent can allows the supercoiled structure of DNA. Humans nonetheless be genetically hidden only to be have 23 pairs of chromosomes (since we inherit expressed in later generations (Most traits, includ- 23 from our mother and 23 from our father). ing eye color, are much more complex than Twenty-two pairs are “autosomal” chromosomes, described here). which conventionally are numbered from 1 to Crucially, DNA can replicate itself. This is the 22 from largest to smallest, while the 23rd pair basis of life. When our cells divide, DNA is cop- consists of our sex chromosomes. These are ied so that each cell contains a complete copy of the X and Y chromosomes that determine our our genetic code. During cell division, which is biological sex (Other animals have different essential for growth, maintenance, and reproduc- mechanisms of sex determination, such as tion, DNA is “unzipped” and copied by molecules temperature-dependent sex determination in and enzymes dedicated to these tasks. Sometimes some reptiles (Warner and Shine 2008).). Female during this process of copying, our copying mech- humans have XX sex chromosomes, while males anisms make errors. A stretch of DNA that read have XY chromosomes. Some autosomes may be AATC might get accidentally copied as AATG. If familiar to you; for example, if an individual this mistake is not noticed and corrected by our inherits three copies of chromosome 21 instead cellular mechanisms, it will be preserved in that of the usual two copies, this is referred to as cell’s genetic code as a mutation. If that cell is a “trisomy 21” or “Down syndrome.” reproductive cell, the mistake could replicate to The place of a certain gene on a chromosome is offspring. There are many kinds of mutations, called its locus.

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