3- Genetic Variation and Speciation
Genetic Variation in Humans and Other Species
Humans are skilled at observing phenotypic variation, which refers to differences in observable traits among individuals of the same species.
Despite high phenotypic variation, humans exhibit low overall genetic variation in comparison with other species.
Average genetic divergence between randomly selected humans is one DNA base per 1,000 bases (99.9% identical).
In contrast, two fruit flies differ by ten bases per thousand (99% identical).
The Adelie penguin, looking uniform, is two to three times more genetically variable than humans.
Contributions to Phenotype
Recall from prior discussions that phenotype results from two factors:
Genotype: The set of alleles an individual possesses.
Environment: The conditions in which the individual lives.
To isolate genetic variation, researchers sequence DNA across multiple individuals, removing environmental variables from evaluation.
Population Genetics
Definition: Study of patterns and amounts of genetic variation within populations.
A species consists of individuals capable of sharing alleles through reproduction.
The gene pool encompasses all alleles present in a species. For humans, it includes alleles responsible for various traits (skin color, hair type, eye color).
Each individual carries a unique combination of alleles drawn from this gene pool.
Questions addressed in population genetics include:
What factors influence genetic variation in populations and species?
Why do humans have less genetic variability than fruit flies and Adelie penguins?
How do particular variations distribute?
Sources of Genetic Variation
Two main sources of genetic variation:
Mutation: Generates new genetic variation. Mutations can be:
Somatic: Affect non-reproductive cells; not passed to offspring.
Germ-line: Occur in gametes; passed on to future generations. Evolutionarily significant.
Each human typically has about 60 new mutations at birth, most < neutral or harmful. Advantaged mutations may increase reproductive success.
Recombination: The shuffling of alleles during meiotic cell division.
Produces new allele combinations that contribute to genetic diversity.
Mutation Classification
Mutations categorized by their effect on the organism:
Deleterious Mutations: Harmful effects, often eliminated by natural selection.
Neutral Mutations: No significant effects on survival or reproduction.
Advantageous Mutations: Potentially enhance survival or reproduction; may become prevalent in a population over time.
Understanding Genetic Variation through Allele Frequencies
Definition: Allele frequency of an allele x is the number of x's in a population divided by the total number of alleles for that gene.
For example, pea color in Mendel's plants is determined by alleles A (yellow) and a (green).
Frequencies Defined:
If a population has primarily green peas (aa), allele frequency of a = 100%, while A = 0%.
Fication: If a population is fixed for an allele, it means one allele has replaced others.
Genotype Frequency Measurement Methods
Genotype frequency represents the proportion of each genotype in a population. This can be measured by:
Observable traits (limited application).
Gel electrophoresis: Detects genetic variance by separating proteins based on size. Limitations include inability to detect silent mutations.
DNA sequencing: The definitive standard for identifying all genetic variation, measuring polymorphisms and their frequencies.
Evolution and its Measurement
Evolution: A change in allele frequency or genotype over time.
Hardy-Weinberg Equilibrium: Describes populations where allele and genotype frequency remain stable (no evolution).
Conditions include:
No selection (survival/reproductive advantages).
Large population size (avoiding sampling error).
No migration (gene flow between populations).
No mutation processes altering existing genes.
Random mating must occur.
The equilibrium provides a baseline to identify evolutionary mechanisms when frequencies deviate from expectations.
Mechanisms of Evolution
Natural Selection: Alters allele frequencies based on reproductive success, either favoring or eliminating mutations:
Advantageous traits become more common; the population becomes adapted to the environment.
Genetic Drift: A random process altering frequencies, more pronounced in small populations leading to loss of genetic diversity (bottleneck effect).
Migration: The movement and mixing of individuals between populations, influencing allele frequencies and gene flow.
Mutation: The source of new genetic variation, albeit usually rare. An important driver of longer-term change.
Nonrandom Mating: Alters genotype frequencies without impacting allele frequencies; inbreeding can increase homozygous individuals without changing allelic diversity.
Speciation Overview
Speciation mechanisms include:
Allopatric Speciation: Geological or geographical barriers separate populations, leading to genetic divergence.
Includes dispersal (movement away) and vicariance (geographic change).
Sympatric Speciation: Populations that share habitats diverge through adaptive radiation or disruptive selection.
Examples include hybridization leading to reproductive isolation in plants and animals.
Adaptive Radiation: A burst of diversified speciation in evidence under numerous ecological opportunities. Found notably in Galapagos finches.
Co-speciation: Occurs as two related species evolve in response to one another, seen commonly in parasites and hosts.