1. Evolution of Populations

2. Individual organisms do not evolve. This is a misconception.
While natural selection acts on individuals, evolution is only apparent in the changes in a population of organisms over time

3. Microevolution
Microevolution: a change in allele frequencies in a population over generations
Microevolution is evolution on the smallest scale
Microevolution can be caused by natural selection, genetic drift, and gene flow
Only natural selection improves the match between organisms and their environments

4. Genetic variation is responsible for making evolution possible
Genetic variation: differences among individuals in the composition of their DNA sequences/genes
Genetic variation often causes phenotypic variation
However, some phenotypic variation is the result of environmental influences, not heredity

5. Sources of Genetic Variation
Formation of new alleles
Altering gene number or position
Rapid reproduction
Sexual reproduction

6. Sources of Genetic Variation
Formation of new alleles
-New alleles arise by mutation
Altering gene number or position
-Chromosomal changes that delete, disrupt, or rearrange many loci at once may be beneficial
-An important source of variation is the duplication of genes due to errors in meiosis
-Ex. Certain species can smell well because their olfactory genes have been duplicated many times

7. Sources of Genetic Variation
3. Rapid reproduction
-Mutations can quickly generate genetic variation in populations of organisms that reproduce rapidly
Ex. Prokaryotes, viruses, etc.
4. Sexual reproduction
-Most of the genetic variation in a population that reproduces sexually comes from the unique combination of alleles that each individual receives from its parents
-Sexual reproduction rearranges existing alleles into fresh combinations each generation

8. The Hardy-Weinberg Equation
Used to test whether evolution is occurring in a population
Population: a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring
Gene pool: all the copies of every type of allele at every locus in all members of the population
Used to characterize a population’s genetic makeup
Every allele has a frequency (proportion) in the population
If an allele is fixed in the gene pool, that means only one allele exists for a particular locus in a population, and all individuals are homozygous for that allele
When studying a locus with two alleles, p is used to represent the frequency of one allele and q is used to represent the frequency of the other

9. The Hardy-Weinberg Equation
One way to assess whether evolution is occurring at a particular locus in a population is to determine what the genetic makeup of the population would look like if evolution were not occurring and compare the two
Hardy-Weinberg principle: describes the gene pool of a population that is not evolving
The frequencies of alleles and genotypes in a population will remain constant from generation to generation, provided that natural selection is not acting upon that population
A gene pool that is not evolving is said to be in Hardy-Weinberg equilibrium

10. Conditions for Hardy-Weinberg Equilibrium
No mutations
Random mating
-Allows for random mixing of gametes
No natural selection
Extremely large population size
-Allele frequencies fluctuate more in smaller populations
No gene flow.
-If one of these requirements is not met, evolution usually occurs.
-It is common for a population to be in H-W equilibrium for a specific gene, not necessarily all of its genes at once.

11. Natural selection, genetic drift, and gene flow can alter allele frequencies in a population
Any time a population deviates from the 5 conditions required for H- W equilibrium, evolution can occur.
Three mechanisms alter allele frequencies directly and cause evolutionary change: natural selection, genetic drift, and gene flow

12. Natural Selection
Natural selection occurs when individuals have different levels of success in survival and reproduction
This allows a selection of alleles to be passed on to the next generation in proportions that differ from those in the present generation

13. Genetic Drift
Genetic drift: chance events that can cause allele frequencies to fluctuate unpredictably from one generation to the next
Affects smaller populations more than bigger ones

14. Genetic Drift
Founder effect: a type of genetic drift that occurs when a few individuals become isolated from a larger population and they establish a new population whose gene pool differs from the original population

15. Genetic Drift
Bottleneck effect: a type of genetic drift that occurs when a severe drop in population size creates a new population with a different gene pool than the original

16. Effects of genetic drift
Genetic drift is significant in small populations
Genetic drift can cause allele frequencies to change at random
Genetic drift can lead to a loss of genetic variation within populations
Genetic drift can cause harmful alleles to become fixed

17. Gene Flow
Gene flow: the transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes
Gene flow tends to reduce the genetic differences between populations because alleles are being transferred between two populations
If it is extensive enough, gene flow can result in two populations combining into a single population with a common gene pool

18. Natural selection is the only mechanisms that consistently causes adaptive evolution
Adaptive evolution: evolutionary changes that are adaptive to the given environment
Natural selection doesn’t create perfectly fit individuals over time; it creates individuals that have greater relative fitness
Relative fitness: the survival of an individual relative to that of other individuals
Three types of natural selection: directional selection, disruptive selection, and stabilizing selection

19. Directional selection: when conditions favor individuals at one extreme of a phenotype

20. Disruptive selection: when conditions favors individuals at both extreme of a phenotype

21. Stabilizing selection: when conditions favor the intermediate phenotype over the extremes

22. The key role of natural selection is adaptive evolution
Natural selection causes the match between a species and its environment to improve
However, an organism’s physical environment is constantly changing, so adaptive evolution is a continuous, dynamic process
Natural selection is the only evolutionary mechanisms that consistently leads to adaptive evolution; genetic drift and gene flow are not consistent in doing this

23. Sexual Selection
Sexual selection: a form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates
Sexual selection can result in sexual dimorphism
Sexual dimorphism: a difference in secondary sexual characteristics between males and females of the same species
Types of sexual selection:
Intrasexual selection: selection within the same sex where individuals compete for mates (usually among males)
Intersexual selection: individuals of one sex (usually females) are choosy in selecting their mates from the other sex

24. The Preservation of Genetic Variation
Diploidy and balancing selection preserve genetic variation when directional and stabilizing selection reduce it
Diploidy: When two alleles are contained for one trait
In diploidy, a considerable amount of genetic variation is hidden from selection the form of recessive alleles
Balancing selection: when natural selection maintains two or more forms in a population. Two types:
Heterozygote advantage: when individuals who are heterozygous at a particular locus have greater fitness than either type of homozygote
Frequency-dependent selection: when the fitness of a phenotype depends on how common it is in the population

25. Why natural selection cannot make perfect organisms
Selection can only act on existing variations
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection, and the environment interact