Evidence for Evolution

Falsifiability in science

The principle that scientific hypotheses need to be evidence-based and testable in a way that could disprove them; with accumulated evidence and lack of disproof, hypotheses become widely-accepted theories

How theories become accepted

Biologists develop hypotheses for how and why change happens; when tested repeatedly against evidence and not disproven, these hypotheses become widely-accepted theories

Artificial selection

Selective breeding of domesticated species by people to promote desired traits

Artificial selection example: dogs

The diverse range of dog breeds with a diverse range of traits were all bred from gray wolf ancestors

Artificial selection example: Brassica oleracea

Breeding individuals of Brassica oleracea with different morphology creates distinct vegetable varieties (e.g., cabbage, kale, broccoli, Brussels sprouts, kohlrabi) all from one wild mustard ancestor species

Comparative anatomy

A line of evidence for evolution based on comparing anatomical structures across species

Homologous traits

Similar characteristics resulting from common ancestry (e.g., the similar bone arrangement in tetrapod front limbs across whale, frog, horse, lion, human, and bird, despite very different functions)

Mitochondria as homologous structures

Mitochondria are homologous across all eukaryotic cells (plants, fungi, animals, protists) because these organisms share a common ancestor that acquired mitochondria via endosymbiosis

Ontogeny

The study of the developmental history of an organism through its entire life

Ontogeny as evidence for evolution

Individuals early in development (like embryos) exhibit homologies — shared traits with their common ancestor and other species — that often can't be seen in adult organisms (e.g., pharyngeal arches and a post-anal tail shared by chick and human embryos)

Analogous traits

Similar traits that serve a similar function but do not result from shared common ancestry

Convergent evolution

The evolution of similar features in independent evolutionary lineages, typically because they are subject to similar selection pressures (similar environments)

Convergent evolution example: myrmecophagy

The aye-aye and the striped possum (and separately, giant anteaters and aardvarks) independently evolved unique finger/body anatomy adapted for eating ants, despite not sharing a recent common ancestor with that trait

Convergent evolution example: starch storage organs

Potatoes (underground stems) and sweet potatoes (underground roots) have both evolved organs to store starch, derived from different anatomical structures

Fossils as evidence for evolution

Fossil remains demonstrate changes in groups of organisms over time

Relative dating

A method of estimating the age of a fossil by comparison to known rock layers around it

Absolute dating

A method of estimating the age of a fossil through measuring the decay of radioactive elements

Transition fossil

An "intermediate" organism whose fossil contains traits from both an ancestral group and a descendant group, especially useful evidence when the descendant group is very modified from the ancestor

Rhynia fossils

Transition fossils showing traits from earlier bryophyte plants (like mosses) as well as vascular plants that can more efficiently transport water, marking a transition between water-based and land-based plants

Tiktaalik fossils

Transition fossils that include scales and a jaw like fish ancestors, as well as the head, neck, and wrists of tetrapods living on land — evidence of the fish-to-tetrapod transition

Biogeography

The study of the geographic distribution of living things; combined with fossils, genetic evidence, and continental drift to explain how close relatives live in different parts of the world today

Biogeography example: Hawaiian honeycreepers

The Hawaiian Islands have had dozens of types of grosbeak birds (honeycreepers), all closely related but adapted for a variety of food sources and habitats — newly-formed environments like islands often show many closely-related species pointing to major evolutionary change from a single common ancestor

Blending hypothesis (old inheritance theory)

An outdated 1800s theory that parent genetic material mixes like paints, leading to intermediate offspring traits; does not match how traits actually reappear across generations

Gregor Mendel

A scientifically-minded monk, alive during Darwin's time but only widely recognized much later, who built a greater understanding of inheritance through experiments breeding (crossing) pea plants with variation in different traits

Allele

An alternative version of a given gene that leads to variation in traits

Phenotype

The form of a trait that an organism expresses (e.g., purple or white flower color)

Genotype

The set of alleles an organism possesses for a gene; offspring inherit two alleles for each gene, one from each parent, through sexual reproduction

Dominant allele

An allele that dictates the trait an organism presents when paired with a different (recessive) allele

Recessive allele

An allele whose trait is masked when paired with a dominant allele, and is only expressed when both inherited alleles are recessive

Homozygous

Describes an individual with two identical alleles for a gene

Heterozygous

Describes an individual with two different alleles for a gene

Law of Segregation

Mendel's principle that when an organism forms gametes (sex cells), the two alleles for a gene separate, so each gamete contains only one allele per gene

Law of Independent Assortment

Mendel's principle that different genes segregate into gametes independently from one another; applies to genes that are far apart on a chromosome or located on different chromosomes

Gamete

A sex cell (egg or sperm) that carries only one allele per gene, produced via the Law of Segregation

Central dogma of molecular biology

The idea that biological information is passed down sequentially and in one direction, popularly conceived as a two-step process: transcription (DNA to RNA) and translation (RNA to protein)

Transcription

The process of converting DNA into RNA, the first step in the central dogma of molecular biology

Translation

The process of converting RNA into protein, the second step in the central dogma of molecular biology

Polygenic trait

A phenotypic trait influenced by multiple genes, rather than the single-gene traits Mendel studied in peas

Gene regulation

The control of when and how much a gene is expressed, which impacts that gene's contribution to phenotypic traits; includes chemical modifications to gene packaging

Epigenetic inheritance

The passing down of chemical modifications to gene packaging (which affect gene expression) from parents to offspring, without changes to the underlying DNA sequence

Phenotypic plasticity

The phenomenon where environmental factors outside of genetic alleles impact how certain traits form and manifest in an individual organism (e.g., high-altitude living changing blood composition for months)

Evolution (modern genetic definition)

Changes in the allele frequency in a population over generations

Gene pool

The collection of all alleles present in a population

Population-level evolution

Evolution occurs at the population level — groups of individual organisms, not individuals themselves, have an allele frequency and a gene pool

Genetic variation vs. evolution

Genetic variation does not necessarily equal evolution (change); a population may be in a steady state with consistent allele frequencies despite having genetic variation

Null hypothesis of evolution

The idea that "no evolution occurring" can be thought of as a null hypothesis — an implied statistical hypothesis of no effect/difference, or a presumption of the status quo

Hardy-Weinberg Equilibrium

A state of stability in allele frequencies, meaning a population is not evolving with respect to a particular gene; calculating it requires several assumptions

Hardy-Weinberg assumption 1

No alleles change identity (no mutation)

Hardy-Weinberg assumption 2

No individuals enter or leave the population except through births and deaths (no gene flow)

Hardy-Weinberg assumption 3

The population is very large (little to no genetic drift)

Hardy-Weinberg assumption 4

Alleles do not differently impact survival and reproduction (no natural selection)

Hardy-Weinberg assumption 5

Genotypes randomly mate with one another (random mating)

Mechanisms of evolution (4 major types)

Mutation, Gene Flow, Genetic Drift, and Natural Selection

Mutation (as a mechanism of evolution)

A molecular change in the sequence of DNA building blocks; mutations are random and rare, and rates can vary between individuals, populations, and environments

Heritable mutations

In sexually-reproducing organisms, only mutations of gametes or pre-gametes (stem cells) can be inherited by offspring

Molecular clock

A method that uses comparisons of genomes (assuming mutations happen at a predictable rate) to estimate how long ago evolutionary changes between organisms occurred

Gene Flow (as a mechanism of evolution)

The transfer of alleles into, out of, or within a population; if individuals immigrate or emigrate, so do their alleles, which may alter allele frequencies in future generations

Horizontal gene transfer

A process by which some organisms (e.g., bacteria) share alleles between organisms in the same generation, which may then be inherited by offspring, rather than passing genes only from parent to offspring

Genetic Drift (as a mechanism of evolution)

Chance events that lead to fluctuations in allele frequency; especially important in small populations

Bottleneck effect

A form of genetic drift where a major chance disaster kills many individuals, leading to a new, smaller population with a gene pool skewed compared to the original larger population

Founder effect

A form of genetic drift where an isolating event splits a few individuals off from an original population, leading to a new small population with a gene pool skewed compared to the original

Natural selection (4th mechanism, from Darwin)

The mechanism of evolution proposed by Darwin (unknown to Mendel-era genetics at first) in which heritable traits that improve survival/reproduction increase in frequency in a population over time