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Modern Definition of Evolution
Evolution is fundamentally about changes in the genetic makeup of populations, not individuals. Specifically, it is defined as a change in allele frequency within a population over successive generations.
An allele is a specific variant of a gene (e.g., the 'A' allele or 'a' allele for a particular trait).
Allele frequency refers to the proportion of a specific allele in a population's gene pool. For instance, if 60\% of all gene copies for a trait are the 'A' allele, then the frequency of A is 0.6.
Evolution occurs when these proportions shift over time, indicating a population-level genetic change. This means that while an individual organism's genetic code generally doesn't change during its lifetime, the genetic composition of an entire group of organisms can and does change across generations.
The Evolution of Populations and Hardy-Weinberg
The Hardy-Weinberg (H-W) theorem serves as a vital null model in evolutionary biology. It provides a mathematical framework to describe a theoretical population that is not evolving.
A population is said to be in Hardy-Weinberg equilibrium when the allele and genotype frequencies remain constant from one generation to the next. This stability indicates the absence of evolutionary change.
By comparing observed allele and genotype frequencies in real populations to those predicted by the Hardy-Weinberg principle, scientists can determine if a population is evolving and, if so, infer which evolutionary mechanisms (like natural selection or genetic drift) might be at play.
Allele and Genotype Frequencies (Hardy-Weinberg)
To understand Hardy-Weinberg equilibrium, we use specific symbols for allele and genotype frequencies:
Let p represent the frequency of the dominant allele (A) in a population.
Let q represent the frequency of the recessive allele (a) in a population.
Since these are the only two alleles considered for a given gene, their frequencies must sum to 1 (or 100\% of the alleles in the population):
p+q=1Under H-W equilibrium, assuming random mating and the absence of evolutionary forces, the frequencies of the three possible genotypes in the next generation can be predicted:
The frequency of homozygous dominant (AA) individuals is predicted by p^2. This comes from the probability of an individual inheriting an 'A' allele from two parents, each with an 'A' allele frequency of p (p \times p = p^2).
The frequency of heterozygous (Aa) individuals is predicted by 2pq. This accounts for two ways to get a heterozygous genotype: inheriting 'A' from the father and 'a' from the mother, or 'a' from the father and 'A' from the mother (p \times q + q \times p = 2pq).
The frequency of homozygous recessive (aa) individuals is predicted by q^2. Similar to p^2, this is the probability of inheriting an 'a' allele from both parents (q \times q = q^2).
When summed together, the frequencies of all possible genotypes must also equal 1, representing 100\% of the population:
p^2+2pq+q^2=1Notes on Terminology:
Allele: A variant form of a gene. Here, 'A' and 'a' are the alleles for a specific gene.
Genotype: The genetic makeup of an organism; the combination of alleles an individual possesses for a particular gene (e.g., AA, Aa, or aa).
Homozygous Dominant (AA): An individual with two copies of the dominant allele.
Heterozygous (Aa): An individual with one dominant and one recessive allele.
Homozygous Recessive (aa): An individual with two copies of the recessive allele.
Mendel’s Pea Example (application of Hardy-Weinberg)
Let's apply the Hardy-Weinberg principle to a classic example: Mendel's pea plants and their seed color.
Assume Yellow (Y) is the dominant phenotype and Green (y) is the recessive phenotype.
The genotypes corresponding to the Yellow phenotype are YY (homozygous dominant) and Yy (heterozygous).
The genotype corresponding to the Green phenotype is yy (homozygous recessive).
If we know the allele frequencies in a population, for instance:
The frequency of the dominant Y allele (p) is 0.6
The frequency of the recessive y allele (q) is 0.4
We can calculate the expected genotype frequencies in the next generation, assuming Hardy-Weinberg equilibrium:
Frequency of homozygous dominant (YY) = p^2 = (0.6)^2 = 0.36
Frequency of heterozygous (Yy) = 2pq = 2(0.6)(0.4) = 0.48
Frequency of homozygous recessive (yy) = q^2 = (0.4)^2 = 0.16
These calculations mean that 36\% of the population's individuals are expected to be YY, 48\% are expected to be Yy, and 16\% are expected to be yy.
From these genotype frequencies, we can determine the phenotype frequencies:
The frequency of the Yellow phenotype (which includes both YY and Yy genotypes) = p^2 + 2pq = 0.36 + 0.48 = 0.84 (or 84\% of the population).
The frequency of the Green phenotype (which only includes the yy genotype) = q^2 = 0.16 (or 16\% of the population).
This example demonstrates how allele frequencies translate into observable trait frequencies in a non-evolving population.
Assumptions of Hardy-Weinberg Equilibrium
For a population to remain in Hardy-Weinberg equilibrium (i.e., not evolve), five strict assumptions must be met. If any of these assumptions are violated, the population will experience a change in allele or genotype frequencies, leading to evolution:
Population size is effectively infinitely large (no genetic drift): This means the population is large enough that chance events (like which alleles happen to be passed on to the next generation) do not significantly alter allele frequencies. In a smaller population, random fluctuations, known as genetic drift, can cause allele frequencies to change simply by chance, violating this assumption.
Mating is random (panmixis): Individuals must mate without any preference for certain genotypes or phenotypes. If individuals choose mates based on traits (e.g., assortative mating), specific genotype frequencies will be altered, even if allele frequencies remain the same in the short term.
Natural selection is not acting on any alleles: All individuals must have equal chances of survival and reproduction, regardless of their genotype. If certain genotypes have a survival or reproductive advantage (natural selection), their alleles will increase in frequency over time.
No mutation occurs: There should be no new alleles created, nor should existing alleles change into others. Mutations introduce new genetic variation and can directly change allele frequencies, thus disrupting equilibrium.
No migration (gene flow) occurs: There should be no movement of individuals (and their alleles) into (immigration) or out of (emigration) the population. Gene flow can introduce new alleles or change the frequencies of existing ones, altering the genetic composition of the population.
The Hardy-Weinberg principle is a theoretical baseline; no real population perfectly meets all these conditions. Therefore, all natural populations are, to some extent, evolving.
Causes of Evolution
Evolution, as defined by changes in allele frequencies, occurs when at least one of the Hardy-Weinberg assumptions is violated. The primary causes of evolution are:
Genetic drift: This refers to random fluctuations in allele frequencies from one generation to the next, particularly pronounced in small populations. These changes happen purely by chance, like a random sampling error, and can lead to the loss or fixation of alleles over time.
Non-random mating: When individuals do not mate randomly, allele combinations are not formed purely by chance. This can include inbreeding (mating with relatives) or assortative mating (mating based on similar or dissimilar traits). Non-random mating primarily changes genotype frequencies, but can indirectly affect allele frequencies over longer periods if linked to selection.
Natural selection acting on alleles: This occurs when certain genotypes provide fitness advantages (better survival or reproduction) in a particular environment. Individuals with advantageous alleles are more likely to pass them on, causing those alleles to increase in frequency in the population.
New mutations accumulate in populations: Mutation is the ultimate source of all new genetic variation. A mutation is a change in the DNA sequence. While individual mutations typically have a small effect on allele frequencies, their accumulation over long periods or if they confer a strong selective advantage can drive significant evolutionary change.
Migration (gene flow) between populations: Gene flow is the movement of alleles between different populations, which occurs when individuals migrate and reproduce in a new population. This exchange of genetic material can introduce new alleles or alter the frequencies of existing alleles in a population, tending to make populations more similar genetically.
Genetic Drift
Genetic drift is a significant cause of evolution, defined as random changes in allele frequencies from one generation to the next. Unlike natural selection, which is about differential survival based on fitness, genetic drift occurs purely due to chance events, much like sampling error in a statistical experiment.
Its effects are particularly pronounced in small populations, where random events can have a disproportionately large impact on overall allele frequencies. For example, if only a few individuals reproduce, the alleles they carry may not perfectly represent the allele frequencies of the previous generation, simply by luck.
Two key phenomena illustrate genetic drift:
Founder Effect: This occurs when a new population is established by a very small number of individuals from a larger source population. The allele frequencies in this new "founder" population are likely to differ from the original population, not because of selective advantage, but because of the random sampling of alleles carried by the initial founders.
Bottleneck Effect: This happens when a population undergoes a drastic and sudden reduction in size, often due to an environmental disaster (e.g., flood, disease, habitat destruction). The surviving individuals form a random subset of the original population, and their allele frequencies may by chance be very different from those before the bottleneck, leading to significantly reduced genetic variation in the new, smaller population.
Conceptual Example: Imagine a small population of 10 individuals, 5 carrying allele 'A' and 5 carrying allele 'a'. If, purely by chance, only 4 individuals reproduce in one generation, and by coincidence, 3 of them carry 'A' and only 1 carries 'a', the allele frequencies in the next generation will be skewed towards 'A', simply due to this random event, not because 'A' conferred any advantage.
Genetic Drift – Example Illustration (simplified)
To further illustrate genetic drift, consider a simple model where colored marbles represent alleles within a population. Even if all marbles (alleles) are equally "fit" (no natural selection), random selection of a limited number of marbles to represent the next generation can significantly change the proportions of colors.
For example, if a population has 50\% red alleles and 50\% blue alleles, but only a small number of individuals reproduce, simply by random chance, more red alleles might be passed on than blue alleles. Over several generations, this random "sampling error" can cause the frequency of red alleles to increase and blue alleles to decrease.
In very small populations, these random shifts can be dramatic and rapid. It's possible for an allele to become fixed (reach 100\% frequency) or to be eliminated (reach 0\% frequency) from the population entirely due to genetic drift, even if it was not particularly advantageous or disadvantageous. This highlights how randomness plays a crucial role in shaping the genetic diversity of populations, especially when population sizes are limited.
Non-Random Mating
Non-random mating occurs when individuals do not choose mates purely by chance. Instead, their mate choice is influenced by preferences, proximity, or other factors, biasing the combination of alleles that come together to form genotypes.
Importantly, non-random mating primarily alters genotype frequencies without directly changing the overall allele frequencies in the population (at least in the short term). However, by changing the distribution of genotypes, it can indirectly affect the efficiency of natural selection or genetic drift on specific alleles over time.
Examples of non-random mating include:
Inbreeding: This refers to mating between closely related individuals (e.g., siblings, cousins). Inbreeding significantly increases the proportion of homozygous genotypes (AA or aa) in a population while decreasing heterozygosity (Aa). This can be detrimental, leading to the expression of harmful recessive alleles if they become homozygous.
Selfing (Self-fertilization): A specific form of inbreeding common in plants, where an individual fertilizes itself. This even more rapidly increases homozygosity.
Assortative Mating: This involves individuals mating based on phenotypic similarities or dissimilarities for a particular trait:
Positive Assortative Mating: Individuals with similar phenotypes tend to mate more frequently (e.g., tall individuals mate with tall individuals). This increases homozygosity for the genes controlling the chosen trait.
Negative Assortative Mating (Disassortative Mating): Individuals with dissimilar phenotypes mate more frequently (e.g., tall individuals mate with short individuals). This tends to increase heterozygosity for the genes controlling the chosen trait.
Natural Selection
Natural selection is a fundamental mechanism of evolution where there are changes in allele frequency due to differential reproductive success among genotypes. This means that individuals with certain heritable traits (encoded by their genotypes) are better suited to their environment, leading to greater survival and more offspring, thereby passing on their advantageous alleles at a higher rate.
There are three main modes of selection, describing how phenotypes in a population are affected:
Directional Selection: This mode favors one extreme phenotype over others in the population. For example, if an environment becomes colder, individuals with thicker fur (an extreme phenotype) might have higher survival and reproduction, causing the average fur thickness of the population to increase over generations. This shifts the allele frequencies in a consistent direction.
Disruptive (Diversifying) Selection: This mode favors individuals at both extreme ends of the phenotypic range, while individuals with intermediate phenotypes are selected against. An example might be a population of birds with two distinct food sources, one requiring a very small beak and another requiring a very large beak, but neither suiting a medium-sized beak. This can lead to increased genetic variation and potentially the formation of two distinct phenotypic groups.
Stabilizing Selection: This mode favors intermediate phenotypes and acts against individuals with extreme variations. For instance, human birth weight often experiences stabilizing selection: babies who are too small or too large have higher mortality rates, favoring an intermediate birth weight. This reduces phenotypic variation and tends to maintain the status quo for a particular trait.
Sexual Selection
Sexual selection is a specific type of natural selection that focuses exclusively on an individual's success in obtaining a mate. It results in changes in allele frequencies due to differential mating success, meaning individuals with certain traits are more successful at finding partners and reproducing.
This form of selection often leads to the development of exaggerated traits that may seem counterproductive to survival but increase mating opportunities. A classic outcome of sexual selection is secondary sexual trait dimorphism, which refers to clear physical differences between males and females of a species that are not directly involved in reproduction or primary survival tasks (e.g., peacock's tail, lion's mane, brighter plumage in male birds).
Sexual selection can operate in two main ways:
Intrasexual selection: Competition among individuals of the same sex (usually males) for access to mates (e.g., male deer fighting for territory).
Intersexual selection: Individuals of one sex (usually females) choose mates based on certain desirable traits displayed by the other sex (e.g., a female bird choosing a male with the brightest feathers).
Mutation
A mutation is defined as a permanent change in the nucleotide sequence of DNA (or RNA in some viruses). Mutations are the ultimate source of all new genetic variation upon which other evolutionary forces, like natural selection, can act.
While most mutations are neutral or harmful, a small proportion can be beneficial, providing raw material for adaptation.
There are various types of mutations:
Point mutation: This is the simplest type, involving a change in a single nucleotide base within a DNA sequence (e.g., A is replaced by T). This can alter the codon, potentially leading to a different amino acid in the resulting protein.
Insertion or Deletion mutation (Indel): These mutations involve the addition (insertion) or removal (deletion) of one or more nucleotide bases from a DNA sequence. Indels can cause a frameshift mutation if they are not a multiple of three bases, drastically changing the protein produced.
Gene duplication: This occurs when a segment of DNA, which may include an entire gene, is copied, resulting in an extra copy of that gene. These duplicated genes can then accumulate new mutations independently and potentially evolve new functions, contributing significantly to evolutionary novelty.
Chromosomal changes: These are larger-scale mutations involving entire segments or rearrangements of chromosomes:
Insertion: A segment of a chromosome is added elsewhere.
Deletion: A segment of a chromosome is lost.
Inversion: A segment of a chromosome is reversed end-to-end.
Translocation: A segment of one chromosome breaks off and attaches to another, non-homologous chromosome.
Duplication: A segment of a chromosome is copied, similar to gene duplication but on a larger scale.
Whole Genome Duplication (Polyploidy): This is a dramatic event where an organism ends up with more than two complete sets of chromosomes (e.g., triploid (3n), tetraploid (4n)). This is common in plants and can lead to immediate speciation.
DNA and Mutation – Conceptual Example
To understand how a mutation impacts an organism, consider a simplified DNA sequence example.
Imagine a segment of the coding strand of DNA with the sequence:
... ATG GAA TGC ...In this sequence,
ATGcodes for the amino acid Methionine,GAAcodes for Glutamic Acid, andTGCcodes for Cysteine.
Now, let's say a point mutation occurs where the second 'A' in the
GAAcodon is replaced by a 'T'. The DNA sequence would change to... ATG GTA TGC ...When this mutated DNA is transcribed into mRNA, the codon
GAAwould becomeGUA.GUAnow codes for the amino acid Valine, instead of Glutamic Acid.
This single base change has resulted in an altered amino acid sequence in the resulting protein (Glutamic Acid replaced by Valine).
Such a change in an amino acid, especially in a critical region of the protein, can potentially affect its structure and function. For example, it might alter the protein's ability to bind to other molecules, its enzymatic activity, or its overall stability, leading to downstream cellular consequences and potentially a different phenotype for the organism.
Sickle-Cell Allele – A Point Mutation Case
The sickle-cell allele is a well-known example of a point mutation with profound consequences.
Mutation Type: It originates from a single point mutation within the gene that codes for the beta-globin chain of hemoglobin, a protein essential for oxygen transport in red blood cells.
Mechanism: Specifically, a change occurs in the DNA template strand where a single nucleotide (Adenine, A) is substituted by another (Thymine, T). This DNA change is transcribed into messenger RNA (mRNA) as a change from GAG to GUG. This codon change in the mRNA then leads to a different amino acid being incorporated into the hemoglobin (Hb) protein.
Molecular Consequence: At the sixth position of the beta-globin chain, the amino acid Glutamic acid (Glu) is replaced by Valine (Val). This specific Glu ( \rightarrow ) Val substitution at position 6 is the classic sickle-cell mutation.
Cellular Consequence: The replacement of a hydrophilic (water-loving) Glutamic acid with a hydrophobic (water-fearing) Valine is critical. Under conditions of low-oxygen (hypoxia), this altered hemoglobin (HbS) tends to aggregate and form long, rigid, abnormal hemoglobin fibers. These fibers distort the normally round red blood cells into a characteristic sickle shape.
Physiological Note: These sickled cells are less efficient at carrying oxygen, can block small blood vessels, and are prematurely destroyed, leading to anemia and other health problems. However, individuals who are heterozygous for the sickle-cell trait (carrying one normal allele and one sickle-cell allele) often exhibit increased resistance to malaria. This provides a balanced selective advantage in regions where malaria is prevalent, demonstrating how a seemingly harmful mutation can persist in a population due to a beneficial effect in a specific environment.
Bar-Headed Goose Hemoglobin Adaptation (Example from Molecular Studies)
The Bar-Headed Goose Hemoglobin Adaptation is a compelling molecular example of evolution through mutation and natural selection. These geese are renowned for their ability to fly over the Himalayas, an extremely high-altitude, low-oxygen environment (hypoxia).
Reference Example: Molecular studies have analyzed the structural model of bar-headed goose hemoglobin (Hb), particularly in its deoxy (oxygen-unbound) state, revealing specific amino acid substitutions compared to related species that inhabit lower altitudes.
Key Substitution: A crucial change is the replacement of the ancestral Alanine with Valine at the [\alpha63] position (Val ( \alpha63 )) in the alpha-globin chain.
Effect: This substitution is significant because Valine has a larger side-chain volume than Alanine. This increased volume can cause minor steric clashes (physical hindrances) with nearby amino acid residues, specifically Glycine at ( \alpha25 ) and Glycine at ( \alpha59 ). These subtle changes alter the overall structure and dynamics of the hemoglobin molecule.
Context: These precise molecular changes are directly linked to the enhanced oxygen binding affinity of bar-headed goose hemoglobin, allowing it to efficiently capture and transport oxygen even at very low atmospheric pressures. This beautifully illustrates how specific mutations (like a single amino acid substitution) can lead to physiological adaptations that enable survival and reproduction in challenging environments, driven by natural selection.
Structural Detail: Researchers use precise measurements, such as distances between non-hydrogen atoms (measured in Ångströms, Å), to quantify these steric effects and understand the subtle structural changes that confer these critical functional properties.
Mutations That Alter Gene Number or Arrangement
Beyond single-nucleotide changes or small indels, mutations can also alter gene number or arrangement on a larger scale, impacting significant portions of the genome. These are often referred to as chromosomal mutations.
Chromosomal mutations can involve:
Deleting large segments of DNA, leading to loss of genes.
Disrupting existing genes or regulatory sequences by breakage and rearrangement.
Rearranging DNA segments within or between chromosomes (e.g., inversions, translocations).
One particularly important type is the duplication of large chromosomal segments. When an entire gene or even a set of genes is duplicated, the organism gains an extra copy.
The existence of these duplicated genes is crucial for evolution. One copy can maintain its original function, while the redundant second copy is free to accumulate new mutations without immediately harming the organism. Over long evolutionary timescales, these mutations can lead the duplicated gene to take on new functions, serving as a raw material for evolutionary novelty.
An even more dramatic change is Whole Genome Duplication (Polyploidy). This occurs when an organism ends up with more than two complete sets of chromosomes. For example, a diploid organism (2n) might become tetraploid (4n) if its entire genome is duplicated. Polyploidy is common in plants and can lead to rapid speciation, allowing new species to arise with distinct genetic characteristics.
Mutation Summary
In summary, mutations are fundamental to evolution, representing heritable changes in the nucleotide sequence of DNA.
Their evolutionary significance lies in their ability to create new gene variants, also known as alleles. For instance, a mutation in a gene might lead to a different version of the protein it encodes, thus creating a new allele for that trait. In more complex scenarios like gene duplication, mutations can also give rise to entirely new genes over evolutionary time.
Crucially, from an evolutionary standpoint, only mutations that occur in cells that produce gametes (known as germline mutations)—sperm or egg cells—can be transmitted to the next generation and thereby contribute to population-level changes and evolution. Mutations occurring in somatic (body) cells may affect the individual but are not passed on to offspring.
Sex & Recombination
While mutation creates new alleles, sexual reproduction plays a critical role in generating vast genetic diversity by shuffling existing alleles into new and varied combinations. This reshuffling doesn't create new alleles but arranges them in novel ways.
The key mechanisms by which sexual reproduction achieves this recombination are:
Random assortment of chromosomes during meiosis: During gamete formation, homologous chromosomes (one from each parent) align independently and are separated into daughter cells randomly. This means that each gamete receives a unique mix of maternal and paternal chromosomes, vastly increasing the number of possible chromosome combinations.
Crossing over during meiosis: Sister chromatids of homologous chromosomes exchange segments of genetic material. This physical exchange shuffles alleles on homologous chromosomes, creating new combinations of alleles on a single chromosome that were not present on either parental chromosome.
This constant reshuffling generated by recombination and the independent assortment of chromosomes, often means that a population's adaptability stemming from existing variation may be even more immediately critical for producing genetic diversity that enables adaptation than the infrequent occurrence of completely new mutations. It allows beneficial combinations of alleles to arise quickly, even if the individual alleles themselves are old.
Migration (Gene Flow)
Migration, also known as gene flow, is the movement of alleles between different populations. It involves the physical exchange of DNA (in the form of individuals or gametes) when organisms move from one geographically distinct population to another and successfully reproduce.
Gene flow can occur through:
Immigration: Individuals moving into a population from elsewhere, introducing their alleles.
Emigration: Individuals moving out of a population, taking their alleles with them.
The most significant effect of gene flow is that it tends to homogenize allele frequencies between populations. This means that if two populations have different allele frequencies for a particular gene, gene flow will make them more similar over time.
By homogenizing allele frequencies, gene flow often counteracts local adaptation. If natural selection favors a specific allele in one population but not in a neighboring one, gene flow will introduce the disfavored allele, potentially preventing the population from becoming perfectly adapted to its unique local conditions. However, it can also introduce beneficial alleles to populations where they were previously absent.
Key Terms (Expanded Definitions)
Allele: A variant form of a gene. For example, a gene for flower color might have an allele for red flowers and an allele for white flowers.
Genotype: The genetic makeup of an organism, referring to the specific combination of alleles an individual possesses for a particular gene (e.g., AA, Aa, or aa).
Dominant Allele: An allele that expresses its phenotype even when only one copy is present (e.g., 'A' in 'Aa'). It masks the effect of a recessive allele.
Recessive Allele: An allele whose phenotype is only expressed when two copies are present (e.g., 'a' in 'aa'). Its effect is masked by a dominant allele.
Hardy-Weinberg Theorem: A mathematical model that describes a theoretical population whose allele and genotype frequencies remain constant across generations, meaning it is not evolving.
Homozygous Dominant: A genotype consisting of two identical dominant alleles (e.g., AA).
Homozygous Recessive: A genotype consisting of two identical recessive alleles (e.g., aa).
Heterozygous: A genotype consisting of two different alleles for a particular gene (e.g., Aa), one dominant and one recessive.
Equilibrium: In the context of Hardy-Weinberg, a state of balance where allele and genotype frequencies remain constant over generations, indicating no evolutionary change.
Assumptions of Hardy-Weinberg Equilibrium: The five conditions that must be met for a population's allele and genotype frequencies to remain constant: infinitely large population size, random mating, no natural selection, no mutation, and no migration.
Large Population Size: A condition under Hardy-Weinberg equilibrium where the population is so large that random fluctuations (genetic drift) do not significantly alter allele frequencies.
Random Mating (Panmixis): A condition under Hardy-Weinberg equilibrium where individuals mate irrespective of their genotype or phenotype; there is no mate choice based on traits.
No Selection: A condition under Hardy-Weinberg equilibrium where all genotypes have equal survival and reproductive rates, meaning no differential success due to traits.
No Mutation: A condition under Hardy-Weinberg equilibrium where no new alleles are created, nor are existing alleles transformed into others.
No Migration (Gene Flow): A condition under Hardy-Weinberg equilibrium where no alleles enter or leave the population through the movement of individuals.
Genetic Drift: Random changes in allele frequencies from one generation to the next, especially prevalent in small populations, due to chance events.
Bottleneck Effect: A sudden and drastic reduction in population size, often due to an environmental event, that reduces genetic variation and causes a shift in allele frequencies simply by chance.
Founder Effect: A specific type of genetic drift that occurs when a new population is established by a small number of individuals, whose allele frequencies may differ by chance from the larger source population.
Non-Random Mating: Mating patterns where individuals do not mate randomly, often due to mate choice or geographic proximity, leading to changes in genotype frequencies (e.g., inbreeding, assortative mating).
Inbreeding: A type of non-random mating where individuals mate with close relatives, leading to an increase in homozygosity in the population.
Assortative Mating: A type of non-random mating where individuals with similar phenotypes tend to mate more frequently (positive assortative mating), or where individuals with dissimilar phenotypes mate more often (negative assortative mating or disassortative mating).
Migration (Gene Flow): The transfer of alleles or genes from one population to another, usually through the movement of individuals that then reproduce.
Natural Selection: The process by which individuals with certain heritable traits survive and reproduce at higher rates than others due to those traits, leading to an increase in the frequency of advantageous alleles in the population.
Directional Selection: A mode of natural selection that favors one extreme phenotype over others, causing a shift in the population's allele frequencies towards that extreme.
Diversifying/Disruptive Selection: A mode of natural selection that favors individuals at both extremes of the phenotypic range over intermediate phenotypes. This can lead to increased genetic variation and potentially the formation of two distinct phenotypic groups.
Stabilizing Selection: A mode of natural selection that favors intermediate phenotypes over extreme ones, leading to a reduction in phenotypic variation and maintaining the status quo for a particular trait.
Sexual Selection: A type of natural selection acting on an organism's ability to obtain or successfully copulate with a mate, resulting in differential reproductive success. Often leads to the development of secondary sexual characteristics.
Mutation: A permanent change in the nucleotide sequence of DNA or RNA. It is the ultimate source of new genetic variation.
Point Mutation: A type of mutation that involves a change in a single nucleotide base in the DNA sequence.
Indel (Insertion/Deletion) Mutation: A type of mutation involving the insertion or deletion of one or more nucleotide bases in a DNA sequence.
Duplication: A type of mutation where a segment of DNA (including entire genes) is copied, resulting in two or more copies of that segment on a chromosome.
Polyploidy: A condition where an organism has more than two complete sets of chromosomes, often resulting from whole genome duplication.
Recombination: The process, especially during meiosis (crossing over and random assortment), that shuffles existing alleles into new combinations on chromosomes, increasing genetic diversity.
Additional Notes on the Transmission and Mutational Processes
To reiterate, mutations are fundamental changes in the nucleotide sequence of DNA and are the ultimate source of new genetic variation.
For these changes to have evolutionary significance for a species, they must be heritable. This means that mutations occurring in germ cells (those that produce gametes like sperm and eggs) can be passed down to offspring. In contrast, somatic mutations (mutations in body cells) affect only the individual in which they occur and are not transmitted to future generations.
Beyond new mutations, existing genetic variation is extensively reshuffled by recombination during meiosis and through sexual reproduction. This process generates novel combinations of alleles, leading to significant genetic diversity within a population.
Ultimately, the complex interplay and combination of mutation, recombination, natural selection, genetic drift, and gene flow are the forces that continually shape the genetic structure of populations over time, driving the process of evolution.
References to Foundational Ideas
When considering the foundational ideas of evolutionary biology:
Charles Darwin, in his historical context, developed the theory of evolution by natural selection without a complete understanding of the mechanism of inheritance. He recognized the concept of heredity—that traits are passed from parents to offspring—but the precise nature of how this occurred was unknown to him.
It was Gregor Mendel's work on pea plants, published around the same time but largely unrecognized until much later, that provided the first insights into the discrete nature of inheritance. Modern genetics later integrated Darwin's ideas with Mendel's principles, clarifying that genes are the units of inheritance and identifying mutation and recombination as the primary sources of genetic variation.
Thus, the modern understanding of evolution is built upon these foundations. It is defined as changes in allele frequencies within populations, driven by the five major mechanisms: mutation, recombination, natural selection, genetic drift, and gene flow. The Hardy-Weinberg equilibrium serves as the crucial null model against which observed evolutionary changes are measured and understood, providing a baseline for a non-evolving population.
Quick Reference Formulas
These formulas are essential for understanding and calculating allele and genotype frequencies in populations, especially in the context of Hardy-Weinberg equilibrium:
Allele frequency sum:
p+q=1
This formula states that the sum of the frequencies of all alleles for a given gene in a population must equal 1 (or 100\%). Here, 'p' is the frequency of the dominant allele, and 'q' is the frequency of the recessive allele.Genotype frequencies under H-W equilibrium:
The expected frequencies of the three possible genotypes in a population that is not evolving are:Homozygous dominant (AA) = p^2
Heterozygous (Aa) = 2pq
Homozygous recessive (aa) = q^2
Sum of genotype frequencies:
p^2+2pq+q^2=1
This formula confirms that the sum of the frequencies of all possible genotypes in a population must also equal 1 (or 100\%). This quadratic equation represents the expansion of (p+q)^2 = 1^2 = 1.Relationship to phenotype (example):
Phenotype frequencies are derived from genotype frequencies. For instance, if 'Yellow' is the dominant phenotype (expressed by genotypes AA and Aa) and 'Green' is the recessive phenotype (expressed only by genotype aa), then:Frequency of Yellow phenotype = p^2 + 2pq
Frequency of Green phenotype = q^2
This demonstrates how the underlying allele and genotype frequencies manifest as observable traits in a population.