1. The Evolution of Populations
Chapter 23:
The Evolution of Populations

2. The Smallest Unit of Evolution
Microevolution – the smallest unit of evolutionary change in populations. The changes in allele frequencies in a population over generations.
There are three main mechanisms that can cause allele frequency change:
Natural selection
Gene flow
Genetic drift

3. Genetic Variation
Darwin provided evidence on how life changed over time. He knew that natural selection was the primary mechanism for change in a species population.
Darwin could not explain how organisms pass heritable traits to their offspring.
Gregor Mendel – proposed that organisms transmit discrete heritable units (genes) to their offspring.
Worked with pea plants to show inheritance of genes.
Genetic differences are produced by:
Mutation
Sexual reproduction

4. Variation Within a Population
Characters that vary within a population may be discrete or quantitative.
Discrete characters – can be classified on an either-or basis, and are determined by a single gene locus with different alleles that produce distinct phenotypes.
Quantitative characters – vary along a continuum within a population. Usually results from the influence of two or more genes on a single phenotypic character.

5. Discrete and Quantitative Characters

6. Variation within a Population
Average heterozygosity – the average percent of loci that are heterozygous.
Average heterozygosity is usually estimated by surveying the protein products of genes using gel electrophoresis.

7. Variation Between Populations
Besides observing variation in a population, species also exhibit geographic variation.
Geographic variation- the differences in the genetic composition of separate populations.
When isolated from one another, populations can evolve independently from the other population resulting in genetic variations from chance events (drift) rather than natural selection.

8. Geographic Variation

9. Variation Between Populations
Cline – a graded change in a character along a geographic axis.
Some clines are produced by a gradation in an environmental variable, as illustrated by the impact of temperature on the frequency of cold-adaptive allele in mummichog fish.
Clines usually result from natural selection.

10. A Cline Determined by Temperature

11. Mutation Mutations are the ultimate source of new alleles.
Mutation – a change in the nucleotide sequence of an organism’s DNA.
It cannot be predicted which segments of DNA will be altered.
In multicellular organisms, only mutations in cell lines that produce gametes can be passed to offspring.
Most mutations occur in somatic cells and are lost when the organism dies.
Point mutations – a change of as little as one base in a gene. Can have significant impact on phenotypes.
Sickle-cell disease

12. Mutations that alter Gene Number or Sequence
Chromosomal changes that delete, disrupt, or rearrange many loci at once are almost certain to be harmful.
When these large-scale mutations leave genes intact, their effects on organisms may be neutral.
Sometimes these mutations can be beneficial.
A translocation of part of one chromosome to a different chromosome could link DNA segments in a way that results in a positive effect.

13. Mutation Rates
Mutation rates are really low in plants and animals and even lower in prokaryotes.
One every 100,000 genes per generation.
However, prokaryotes have short generation spans, so mutations can quickly generate genetic variation in a population.

14. Sexual Reproduction
Sexual reproduction produces the most genetic variation in a population that reproduces sexually.
Sexual reproduction shuffles the existing alleles and deals them at random to determine individual genotypes.
Shuffling is due to three mechanisms:
Crossing over
Independent assortment of chromosomes
Fertilization

15. Hardy-Weinberg and Evolving Populations
The individuals in a population must differ genetically for evolution to occur.
Population – a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring.
Gene pool – a populations genetic makeup that consists of all the alleles for all the loci in all individuals of the population.
If there are two or more alleles for a particular locus in a population, individuals may be either homozygous or heterozygous.

16. Hardy-Weinberg Principle
The gene pool of a population that is not evolving can be described by the Hardy-Weinberg principle.
This principle state that the frequencies of alleles and genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work.
This type of gene pool is in Hardy-Weinberg equilibrium.

17. Hardy-Weinberg Equilibrium

18. Hardy-Weinberg Equation

19. Conditions for Hardy-Weinberg Equilibrium
No mutations
Random mating
No natural selection
Extremely large population size
No gene flow

20. Natural selection
Natural selection can alter allele frequencies in a population.
Natural selection can cause adaptive evolution (evolution that results in a better match between organisms and their environment.

21. Genetic Drift
Genetic drift can alter allele frequencies in a population.
Genetic drift - Chance events that cause allele frequencies to fluctuate unpredictable from one generation to the next, especially in small populations.

22. Genetic Drift

23. The Founder Effect
The founder effect can also cause a fluctuation in the allele frequencies of a population.
Founder Effect – when a few individuals become isolated from a larger population, this smaller group may establish a new population whose gene pool differs from the source population.
The Founder Effect probably accounts for the relatively high frequency of certain inherited disorders among isolated human populations.

24. The Bottleneck Effect
A certain change in the environment, such as a fire or flood, may drastically reduce the size of a population.
A severe drop in population size can cause the bottleneck effect.
The bottleneck effect is named so because the population has passed through a restrictive “bottleneck” in size.
Some alleles may be overrepresented and some will be underrepresented. Some populations may even cease to exist.

25. Bottleneck

26. Effects of Genetic Drift: a summary
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 a population.
Genetic drift can cause harmful alleles to become fixed.

27. Gene Flow
Gene flow can also cause the alleles frequencies in a population to fluctuate.
Gene flow – the transfer of alleles into or out of a population due to the movement of fertile individuals or their gametes.
Because alleles are exchanged among populations, gene flow tends to reduce the genetic differences between populations.

28. Gene flow and human evolution

29. A closer look at natural selection
Relative fitness – the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals.
The terms “survival of the fittest” and “struggle for existence” are sometimes misleading if taken to mean direct competition.

30. Directional, Disruptive, and Stabilizing Selection
Natural selection can alter the frequency distribution of heritable traits in three ways:
Directional Selection
Disruptive Selection
Stabilizing Selection

31. Directional Selection
Directional Selection – occurs when conditions favor individuals exhibiting one extreme of a phenotypic range, thereby shifting the frequency curve for the phenotypic character in one direction or the other.
Directional selection is common when a population’s environment changes or when members of a population migrate to a new habitat.

32. Disruptive Selection
Disruptive selection – occurs when conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes.
Ex. Members of a population of birds whose members display different beak sizes.

33. Stabilizing Selection
Stabilizing Selection – acts against both extreme phenotypes and favors intermediate variants.
This mode of selection reduces variation and tends to maintain the status quo for a particular phenotypic character.

35. The Role of Natural Selection in Adaptive Radiation
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 – marked differences between the two sexes in secondary sexual characteristics which are not directly associated with reproduction or survival.

36. Sexual Dimorphism

37. How does Sexual Selection Work?
Intrasexual Selection – meaning selection within the same sex, individuals of one sex compete directly for mates of the opposite sex.
In many species, intrasexual selection occurs among males.
Intersexual Selection – also called mate choice, individuals of one sex (usually the females) are choosy in selecting their mates from the other sex.
In many cases, the female’s choice depends on the showiness of the male’s appearance or behavior.

38. Intersexual Selection

39. The Preservation of Genetic Variation
Because most eukaryotes are diploid, a considerable amount of genetic variation is hidden from selection in the form of recessive alleles.
Balancing Selection – occurs when natural selection maintains two or more forms in a population.
This type of selection includes heterozygote advantage and frequency-dependent selection.
Heterozygote advantage – exhibited when individuals who are heterozygous at a particular locus have greater fitness than do both kinds of homozygotes.

40. Heterozygous Advantage

41. Preservation of Genetic Variation cont…
Frequency-Dependent Selection – the fitness of a phenotype declines if it becomes too common in the population.
Neutral Variation –It happens when, in humans, many of the nucleotide differences in noncoding sequences appear to confer no selective advantage or disadvantage.

42. Frequency-dependent selection in scale eating fish.