THINK ABOUT IT
Darwin developed his theory of evolution without knowing how heritable traits are passed from one generation to the next or where heritable variation came from.
How is evolution defined in genetic terms?
In genetic terms, evolution is any change in the relative frequency (%) of alleles in the gene pool of a population over time. (Microevolution)
- Researchers discovered that heritable traits are controlled by genes and changes in genes and chromosomes generate variation.
Lesson Overview
Genes and Variation
An organism’s genotype is the particular combination of alleles it carries.
An individual’s genotype, together with environmental conditions, produces its phenotype; all physical, physiological, and behavioral characteristics of an organism.
Natural selection acts directly on phenotype, not genotype.
Some individuals have phenotypes that are better suited to their environment than others. These individuals produce more offspring and pass on more copies of their genes to the next generation. (Natural Selection)
A population is a group of individuals of the same species that mate and produce offspring.
A gene pool consists of all the genes, including all the different alleles for each gene that are present within all the individuals in a population.
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Lesson Overview
Genes and Variation
.40 B + .60 b = 1
Sample Population
48% heterozygous black
36% homozygous brown
16% homozygous black
Frequency of Alleles
allele for brown fur
allele for black fur
P + q = 1
Evolution is any change in the relative frequency of alleles in the gene pool of a population over time.
What are the sources of genetic variation?
Three sources of genetic variation are mutation, genetic recombination during sexual reproduction, and lateral gene transfer.
What determines the number of phenotypes for a given trait?
The number of phenotypes produced for a trait depends on how many genes control the trait.
Vs.
What does nature see?
Lesson Overview
Genes and Variation
How Natural Selection Works
How does natural selection affect single-gene and polygenic traits?
Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies.
Ex. A mutation in a gene that determines body color in lizards can affect their lifespan. So if the normal color for lizards is brown, a mutation may produce red (visible to predators) and black (absorb more energy) forms.
Lesson Overview
Evolution as Genetic Change in Populations
Natural selection on polygenic traits can affect the distributions of phenotypes in three ways: directional selection, stabilizing selection, or disruptive selection.
1) Directional selection occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. The range of phenotypes shifts because some individuals are more successful at surviving and reproducing than others.
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For example, if only large seeds were available, birds with larger beaks would have an easier time feeding and would be more successful in surviving and passing on genes.
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Lesson Overview
Evolution as Genetic Change in Populations
For example, very small and very large babies are less likely to survive than average-sized individuals. The fitness of these smaller or larger babies is therefore lower than that of more average-sized individuals.
2) Stabilizing selection occurs when individuals near the center of the curve have higher fitness than individuals at either end. This situation keeps the center of the curve at its current position, but it narrows the overall graph.
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Lesson Overview
Evolution as Genetic Change in Populations
For example, in an area where medium-sized seeds are less common, birds with unusually small or large beaks would have higher fitness. Therefore, the population might split into two groups—one with smaller beaks and one with larger beaks.
3) Disruptive selection occurs when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle. Disruptive selection acts against individuals of an intermediate type and can create two distinct phenotypes.
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Lesson Overview
Evolution as Genetic Change in Populations
3 types of Natural Selection
Genetic Drift
What is genetic drift?
In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by chance. Over time, a series of chance occurrences can cause an allele to become more or less common in a population (random change in allele frequency).
Lesson Overview
Evolution as Genetic Change in Populations
1) Genetic Bottlenecks
The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population.
For example, a disaster may kill many individuals in a population, and the surviving population’s gene pool may contain different gene frequencies from the original gene pool.
Lesson Overview
Evolution as Genetic Change in Populations
2) The Founder Effect
The founder effect occurs when allele frequencies change as a result of the migration of a small subgroup of a population.
For example, two groups from a large, diverse population could produce new populations that differ from the original group.
Lesson Overview
Evolution as Genetic Change in Populations
Evolution Versus Genetic Equilibrium
What conditions are required to maintain genetic equilibrium (allele frequencies in the population remain the same)?
According to the Hardy-Weinberg principle, five conditions are required to maintain genetic equilibrium: (1) The population must be very large; (2) there can be no mutations; (3) there must be random mating; (4) there can be no movement into or out of the population, and �(5) no natural selection. (No Evolution)
Non-random mating in nature is called sexual selection
Lesson Overview
Evolution as Genetic Change in Populations
Meiosis and fertilization do not change the relative frequency of alleles in a population.
80%R, 20%r to 80%R, 20%r
P2 +2Pq + q2 = 1
Lesson Overview
Evolution as Genetic Change in Populations
Forces of Evolution
- Bottleneck effect
- Founder effect