Microevolution A change in allele frequency
Q: How do scientists know when this occurs? • A: They compare it to a non-changing population • = Ideal population (like a “perfect” car… it only exists in a showroom)
5 Characteristics of an “Ideal Population” 1. Population is very large • Large depends on the population, but thousands or hundreds are assumed 2. Mating is random • No phenotype has a greater chance of mating
3. No mutation of alleles 4. No immigration or emigration • No movement into or out of population 5. No selection occurs • No survival advantage goes to one phenotype over another
If these 5 conditions are met… • There is no change in allele/genotypic frequencies… • and no evolution • When these rules are broken, microevolution is said to be occurring.
For Hardy-Weinberg Equilibrium: • p stands for the frequency of the dominant allele Ex. “A” • q stands for the frequency of the recessive allele. Ex. “a” • In H-W, or 100% of the alleles
p2 = frequency of homozygous dominant genotype Ex. “AA” • q2 = frequency of homozygous recessive genotype Ex. “aa” • 2pq = frequency of heterozygous genotype Ex. “Aa”
Example 1: • Approximately 9% of Americans of African descent suffer from sickle cell anemia, which is inherited as a recessive trait. • What is the frequency of the sickle cell allele? • Approximately what percentage of this sub-population carries the sickle cell allele?
Example 2: • Cystic fibrosis is known to occur as a recessive trait in human populations. In a genetic study, the frequency of the recessive allele for a population was found to be 2.0%. • What percentage of the population would be expected to exhibit Cystic Fibrosis? • What percentage of the population would be normal, but carry a CF allele?
Causes of Evolution • 1. Genetic Drift • Significant in small populations • Can cause allele frequencies to change at random • Can lead to a loss of genetic variability • Can cause harmful alleles to become fixed • Founder effect • Small founding population does not reflect source gene pool
Genetic Drift -Bottleneck effect
Causes of Evolution • 2. Gene flow • Immigration/ emigration- movement of alleles into/ out of populations • May increase or decrease variation in population
Causes of Evolution 3. Natural Selection Only Natural Selection is likely to adapt a population to its environment Do you remember the basic tenets of natural selection?
A closer look at natural selection Fitness? Not necessarily • Evolutionary fitnessrefers to beneficial adaptations that allow an organism to survive and reproduce • Not necessarily the biggest, fastest, or strongest, rather, the best suited to a particular place at a particular time.
Directional Selection • One extreme of the variation spectrum is favored • Results in a shift in the make-up of the population
Disruptive Selection • Both extremes of the variation spectrum is favored • Can result in speciation
Stabilizing Selection • Both extremes are selected against • Reduces variation- maintains status quo
Causes of Evolution Sexual Selection • Intrasexual Selection (within the same sex) • Usually male-to-male competition • Most encounters don’t come down to actual fighting (that could be harmful to both.)
Causes of evolution • Sexual Selection • Intersexual Selection (between sexes) • a.k.a. mate choice – usually females choosing males with the most ornate displays. • There can be a trade-off- Attract a mate? More conspicuous to predators?
Practice Quiz Questions
Which of the following pairs of structures is least likely to represent homology? A) The wings of a bird and those of an insect B) The brain of a cat and that of a dog C) The mitochondria of a plant and those of an animal D) The hemoglobin of a baboon and that of a gorilla E) The wings of a bat and the arms of a human
Which of the following provides evidence that modern species have evolved from prior species? A) molecular biology B) comparative anatomy C) biogeography D) comparative embryology E) All of the choices are correct.
Which of the following is NOT a basic tenet of natural selection? A) Variation exists within a species B) There are limited resources for which organisms much compete C) More offspring are born that can survive D) Some genes are dominant, some are recessive
The fossil record shows all of following except that A) the earliest fossils of life are about 3.5 billion years old. B) younger layers are on top of older layers. C) within the vertebrates, fish were the first to evolve. D) some fossils represent an evolutionary series of changes that provide strong documentation of evolution. E) the first life forms were eukaryotes.
