4. Mutation and sexual recombination produce the
variation that makes evolution possible
• Two processes, mutation and sexual
recombination, produce the variation in gene
pools that contributes to differences among
individuals
5. Mutation
• Mutations are changes in the nucleotide
sequence of DNA
• Mutations cause new genes and alleles to arise
6. Sexual Recombination
• Sexual recombination is far more important
than mutation in producing the genetic
differences that make adaptation possible
• This is particularly true with plants and animals,
but clearly isn’t as important in microbes, who
don’t have sex and can obtain genes through
lateral gene transfer (transformation)
• In addition, bacteria have the highest mutation
rates, so it contributes more to genetic
differences in them
17. LE 23-4
Generation
3 25% CR
CR
Generation
4
50% CR
CW 25% CW
CW
50% CW
gametes
50% CR
come together at random
25% CR
CR
50% CR
CW 25% CW
CW
Alleles segregate, and subsequent
generations also have three types
of flowers in the same proportions
gametes
Generation
2
Generation
1
CR
CR
CW
CW
genotypegenotype
Plants mate
All CR
CW
(all pink flowers)
50% CR 50% CW
gametes gametes
come together at random
X
21. Natural Selection and Microevolution:
Darwin’s Finches
• Peter and Rosemary Grant have been
doing research on finches in the
Galapagos Islands since 1973
• They captured and tagged all of the birds
on Daphne Major, an island where
conditions swing between wet and dry
years
• They measured the length, width, and
depth of the birds’ beaks
23. Darwin’s Finches
• When it’s wet, grasses grow and produce a lot of
small seeds
• When it’s dry, the grasses don’t grow and the
available seeds from other plants are larger
• Smaller beaked birds are unable to crack larger
seeds and perish
• The Grants found that average beak size
increased during dry seasons and decreased
during wet seasons
• Thus, microevolution occurs as the environment
selects for beak size
46. Natural Selection and Microevolution:
Sickle-Cell Anemia
• A mutation in the gene for hemoglobin results
in sickle-cell anemia if an individual has 2
copies of the gene (one from each parent)
• Red blood cells form a sickle shape and are
ineffective at transporting oxygen
• They tend to clump together in joints, cause
pain, and may break (hence, the anemia)
• Death before reaching reproductive age is
quite likely
47. Sickle-Cell Anemia
• If sickle-cell disease often causes death, shouldn’t the
frequency of the allele in the human population
decrease?
• It should be selected against, but in areas of the world
with high malaria rates, the frequency of the allele
increases
• Tony Allison showed that the sickle-cell allele in
heterozygotes provides protection against malaria,
known as the heterozygote advantage
• The parasite that causes malaria invades red blood
cells (RBCs) but doesn’t do well in sickle-shaped cells,
and about half of a heterozygote’s RBCs will sickle
48. Sickle-Cell Anemia
• Thus, nature has selected for sickle-cell
anemia because it protects people from
the parasite that causes malaria
• A heterozygous baby born in an area with
a high incidence of malaria has a 26%
better chance of surviving their birth and
childhood than a baby that is homozygous
for normal hemoglobin
49. Figure 23.17
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
Key
Frequencies of the
sickle-cell allele
0–2.5%
2.5–5.0%
5.0–7.5%
7.5–10.0%
10.0–12.5%
>12.5%