1. 15-3: Darwin Presents His Case

2. Natural Selection and Artificial Selection
variation exists in all species in nature and domesticated plants and animals
natural variation: differences among individuals of a species
Darwin had noticed farmers and breeders routinely selected individual plants and animals with the most desirable qualities for reproduction –artificial selection
artificial selection
Selecting which individuals that get to reproduce
nature provided the variation and humans selected the most useful variations

3. Evolution by Natural Selection
the struggle for existence
Darwin believed that high birth rates and limited resources would force organisms to compete
Those organisms with variations enabling them to better compete for resources would survive to reproduce offspring with the same beneficial variations
survival of the fittest
fitness: the ability of an individual to survive and reproduce in its environment
adaptation: an inherited characteristic that increases an organism’s chance of survival
an organism’s fitness is the result of adaptation

4. Evolution by Natural Selection
survival of the fittest: those individuals with characteristics well suited to the environment will survive to produce many offspring while those less suited will not
also referred to as natural selection
over time natural selection results in changes in inherited characteristics of a population, thereby increasing its fitness for its environment
descent with modification
each living species has descended, with changes, from other species over time
common descent: all species, living and extinct, were derived from common ancestors
a single “tree of life” for all living things on Earth

5. Summarizing Natural Selection
There are variations within population of species
A population produces more offspring than nature can support
These variations struggle for resources
The variations best suited survive and get to reproduce, thus passing on their genes

6. Evidence of Evolution the fossil record
fossils in different layers of rock are evidence that living things have undergone gradual change over time
it is also evidence that the Earth is hundreds of millions, rather than thousands, of years old
geographic distribution of living species
animals on different continents showed similar modifications, even though they descended from different ancestors
Darwin reasoned the similar modifications were the result of being exposed to similar ecological conditions and similar pressures of natural selection

7. Beaver Beaver Muskrat Beaver and Muskrat Coypu Muskrat Capybara
Coypu and Capybara
NORTH AMERICA
Muskrat
Capybara
SOUTH AMERICA
Coypu

8. Evidence of Evolution homologous body structures
additional evidence for evolution is demonstrated by the structural similarities between arms, wings, legs, and flippers
constructed from same basic bones even though they have totally different functions
homologous structures: structures that have different functions but develop from same embryonic tissues
vestigial organs: traces of organs that have no function and are homologous to organs in other animals
vestigial organs survive because they have no affect on an organism’s survival

9. Turtle
Alligator
Bird
Mammals
Typical primitive fish

10. Evidence of Evolution Molecular Biology
similarities in early development or (Comparative Embryology)
Studying the stages of development
“ontogeny recapitulates phylogeny”
the embryonic development of an organism recreates its evolutionary development
the same groups of embryonic cells develop in the same order and in similar patterns to produce tissues and organs of all vertebrates
Molecular Biology
Studying DNA, genes and proteins
All life has DNA

11. Evolution of Populations
Chapter 16
Evolution of Populations

12. 16-1
Genes and Variation

13. Darwin’s Ideas Revisited
Darwin’s disadvantage was he did not understand the mechanism for heredity
he did not know the source for variation
he did not how inheritable traits were passed from one generation to the next
it was fifty years later that biologists combined Mendel’s work with Darwin’s work

14. Gene Pool
Gene Pools
gene pool: the genetic information of all members of a population
contains two or more alleles for each inheritable trait
relative frequency: the number of times an allele occurs in a gene pool
expressed in percents
Measuring the gene pool frequencies tells if evolution if occurring

15. Single-Gene and Polygenic Traits
the number of phenotypes produced for a given trait depends on how many genes control the trait
single-gene traits are controlled by one gene and have two distinct phenotypes
most traits are polygenic traits controlled by two or more genes

16. The dominant phenotype may be less frequent since allele frequencies may not match Mendelian ratios
100 80 60 40 20
Frequency of Phenotype
(%)
Widow’s peak
No widow’s peak
Phenotype

17. Frequency of Phenotype
Various phenotypes of polygenic traits can be grouped into a bell-shaped curve
Frequency of Phenotype
Phenotype (height)

18. Sources of Genetic Variation
mutations and genetic shuffling are two main sources of genetic variation
mutations
a change in the genetic sequence of DNA occurring through replication mistakes, radiation, or mutagens
can be a few bases or large segments of chromosomes
most mutations have a negative affect on organism’s survival

19. gene shuffling (sexual Reproduction)
most variation comes from genetic shuffling
occurs during the production of gametes because of independent assortment and crossing over
it may change the various combinations of alleles but does not affect the relative frequency of alleles