2. 5
C H A P T E R
EVOLUTION AND GENETICS
5-2

3. EVOLUTION AND GENETICS
Biochemical, or Molecular, Genetics
Population Genetics and Mechanisms of Genetic Evolution
The Modern Synthesis 3

4. EVOLUTION AND GENETICS
What is evolution, and how does it occur?
How does heredity work, and how is it studied?
What forces contribute to genetic evolution?

5. EVOLUTION
Humans have uniquely varied ways—cultural and biological—of adapting to environmental stresses
Many scholars became interested in biological diversity and our position within the classification of plants and animals during the 18th century

6. EVOLUTION
Creationism: biological similarities and differences originated at the Creation
Linnaeus (1707–1778) developed the first comprehensive and still influential classification, or taxonomy, of plants and animals
Fossil discoveries during the
18th and 19th centuries raised
doubts about creationism

7. EVOLUTION
Catastrophism: modified version of creationism that accounts for the fossil record by positing divinely authored worldwide disasters that wiped out creatures represented in the fossil record
Both theories assert that similarities and differences from creation have not changed

8. THEORY AND FACT
Evolution: transformation of species; descent with modification
Alternative to creationism and catastrophism
Darwin best known of evolutionists

9. THEORY AND FACT Darwin influenced by:
Theory of evolution – belief that species arise from others through a long and gradual process of transformation; all life forms are related and the number of species has increased over time
Grandfather, Erasmus Darwin, who proclaimed a common ancestry of all animals
Lyell’s principle of uniformitarianism: the present is the key to the past;
- past geological events can best
be explained by observing ongoing
events of the present and generalizing backward through time
Darwin applied uniformitarianism to living things; contributed a theory of evolution through natural selection – how evolution occurred

10. THEORY AND FACT
Darwin proposed natural selection to explain the origin of the species, biological diversity, and similarities among related life forms
Reached the conclusion along with Alfred Wallace
Natural selection: the process by which nature selects the forms most suited to survive and reproduce in a given environment
Variety within that population
Competition for strategic resources
Giraffes – long versus short necks
Argues that organisms that have a better fit within their environment, will reproduce more frequently than those less fit
Reproduction is the key
Natural selection continues today
Peppered moth

11. Theory and Fact

12. GENETICS
Genetic science helps explain causes/origin of biological variation
Mendelian genetics: ways in which chromosomes transmit genes across generations
Biochemical genetics: examines structure, function, and changes in DNA
Population genetics: investigates natural selection and the causes of genetic variation, stability, and change

13. MENDEL’S EXPERIMENTS
Austrian monk Gregor Mendel began series of experiments that revealed basic principle of genetics in 1856
Studied inheritance of seven contrasting traits in pea plants
Concluded that heredity is determined by discrete particles or units (genes) that may disappear in one generation and reappear in the next

14. MENDEL’S EXPERIMENTS Observed two traits: dominant and recessive
Dominant forms appear in each generation/recessive forms are masked when paired with dominant form of same trait
Concluded that a dominant form could mask another form in hybrid individuals, without destroying the recessive trait
Basic genetic units Mendel described were factors (now called genes or alleles) located on chromosomes

15. Figure 5.1: Mendel’s Second Set of Experiments with Pea Plants15

16. MENDEL’S EXPERIMENTS
Chromosome: a paired length of DNA, composed of multiple genes
Gene: a place (locus) on a chromosome that determines a particular trait
Allele: a variant to a particular gene

17. Figure 5.2: Simplified Representation of a Normal Chromosome Pair17

18. MENDEL’S EXPERIMENTS
Heterozygous: dissimilar alleles of a gene in an offspring
Homozygous: two identical alleles of a gene in an offspring
Genotype: organism’s hereditary makeup
Phenotype: evident biological traits
Dominance produces a distinction between genotype and phenotype

19. Figure 5.3: Punnett Squares of a Homozygous Cross and a Heterozygous Cross19

20. Figure 5.4: Determinants of Phenotypes (Blood Groups) in the ABO System20

21. INDEPENDENT ASSORTMENT AND RECOMBINATION
Independent assortment: traits are inherited independently of one another
Recombination: traits may appear in new combinations with other traits; new types in an offspring on which natural selection can operate; two main ways produces variety

22. BIOCHEMICAL OR MOLECULAR GENETICS
Mutation: changes in the DNA molecules of which genes and chromosomes are built; produces variety (source of new forms on which natural selection may operate)
BIOCHEMICAL OR MOLECULAR GENETICS

23. CROSSING OVER Crossing over:
Chromosomes temporarily intertwine in the course of reduplication and exchange lengths of their DNA
the process wherein homologous chromosomes exchange segments by breakage and recombination
Can occur with any chromosome pair
An important source of variety on which natural selection operates

24. Figure 5.6: Crossing Over 24

25. MUTATION
Mutations: the most important source of variety upon which natural selection acts
Chromosomal rearrangement: pieces of a chromosome break off and reattach someplace else on that chromosome
A mismatch of chromosomes resulting from arrangement can lead to congenital disorders, cancer, and possibly to speciation
Chromosomes may also fuse:
When ancestors of humans split off from those of chimpanzees around six milion years ago, two ancestral chromosomes fused together in the humans line
Humans have 23 chromosome pair versus 24 for chimps

26. MUTATION Approximately three mutations will occur in every sex cell
Most mutations are neutral
Evolution depends on mutations
Mutations are major source of genetically transmitted variety
Mutations may be neutral, harmful, or acquire an adaptive advantage through changing selective forces

27. NATURAL SELECTION Genotype: the genetic makeup of an organism
Phenotype: organism’s biological traits (outward physical appearance as well as internal organs, tissues, and cells and physiological processes and systems)
Natural selection acts only on phenotypes
Human biology has considerable plasticity
The environment works on a genotype to build a phenotype
Diet and altitude affect how a person grows

28. DIRECTIONAL SELECTION
After several generations of selection, gene frequencies change
Adaptive traits (favored by natural selection) will be selected from generation to generation
Directional selection (long-term selection of same traits) continues as long as environmental sources stay the same
Maladaptive genes removed from gene pool
If environment changes, new selective forces start working, favoring different phenotypes
Selection operates only on traits that are present in a population
Favorable mutation may occur but doesn’t usually happen just because one is needed or desirable; many species are extinct because they couldn’t adapt to environmental shifts
Humans adapt rapidly to environmental variation by modifying biological responses and learned behavior

29. SEXUAL SELECTION Selection also operates through competition for mates
Sexual selection: based on differential success in mating; a selection of traits that enhances mating success

30. GENE FLOW
Gene flow: exchange of genetic material between populations of the same species
Direct or indirect interbreeding
Allele that isn’t advantageous in one environment might reach an environment in which it has selective advantage
Alleles spread through gene flow even when selection not operating on the allele
Species: group of related organisms whose members can interbreed to produce offspring that live and reproduce
Gene flow tends to prevent speciation: the formation of new species

31. Figure 5.8: Gene Flow between Local Populations31

32. THE MODERN SYNTHESIS Currently accepted view of evolution:
Modern Synthesis: combination of Darwin’s theory of evolution by natural selection and Mendel’s genetic discoveries
Speciation occurs when related populations become reproductively isolated from one another
Microevolution: small-scale changes in allele frequencies over just a few generations, but without speciation
Macroevolution: large-scale changes in allele frequencies in a population over a longer time period, which result in speciation

33. PUNCTUATED EQUILIBRIUM
Punctuated equilibrium: long periods of stasis (stability) may be interrupted by evolutionary leaps (revealed in fossil record)
Occurs through:
Extinction of one species followed by invasion of closely related species
Replacement of one species by a more fit related group in particular environment
Period of sudden environmental change that permits survival of radically altered species with significant mutations or a combination of genetic changes
Sudden environmental change offers possibility for the pace of evolutions to speed up
Species can survive radical environmental shifts, but extinction is more common
Extinction of dinosaurs was accompanied by rapid spread of mammals and birds

34. Darwin Cartoons