1. Evolution by Natural Selection25
Evolution by Natural Selection

2. Please highlight the items that are in blue.

3. Introduction
Evolution by natural selection is one of the best-supported and most important theories in modern biology
Populations of organisms evolve, or change through time.
Charles Darwin published On the Origin of Species by Means of Natural Selection in 1859.
Radically different from the idea of special creation, which held that:
Species are independent (unrelated)
Life on Earth is young (~ 6000 years old)
Species are immutable (incapable of change)

4. The Pattern of Evolution
Darwin described evolution as descent with modification which includes change over time produced modern, modified species from ancestral species.
The pattern component of the theory of evolution by natural selection predicts that:
Species change through time
Species are related by common ancestry 4

5. Evidence for Change through Time
Fossils are traces of organisms that lived in the past.
The many fossils that have been found and described in the scientific literature make up the fossil record.
The Vastness of Geologic Time
Fossils were initially organized according to their relative ages.
Sedimentary rocks form from sand or mud and form in layers.
Younger layers are deposited on top of older layers. 5

6. Vestigial Traits
A vestigial trait in an organism is a reduced or incompletely developed structure that has no (or reduced) function.
Clearly similar to functioning organs or structures in closely related species.
Examples of vestigial traits include:
Nonfunctional hip and leg bones in some snakes and whales
Reduced wings in flightless birds
Eye sockets in eyeless cave-dwelling fish
Brief eggshell formation and nonfunctional “egg tooth” in some marsupial species
Coccyx bone (vestigial tail) and goose bumps in humans 6

7. Current Examples of Change through Time
Hundreds of contemporary populations have been documented undergoing evolutionary changes. For example:
Bacteria have evolved resistance to drugs
Insects have evolved resistance to pesticides
Weedy plants have evolved resistance to herbicides
Bird migration, insect emergence, and blooming of flowering plants have evolved in response to climate change. 7

8. Similar Species in the Same Geographic Area
A phylogenetic tree is a diagram that illustrates the ancestor–descendant relationships among taxa. 8

9. Homology: Descent from a Common Ancestor
Another line of evidence comes from homologies.
Homology is a similarity that exists in species descended from a common ancestor.
Homology can be recognized and studied at three interacting levels:
Genetic
Developmental
Structural 9

10. Genetic Homology
Genetic homology is a similarity in the DNA nucleotide sequences, RNA nucleotide sequences, or amino acid sequences.
For example, the eyeless gene in fruit flies and the Aniridia gene in humans are so similar that their resulting amino acid sequences are 90% identical. 10

11. Developmental Homology
Developmental homology is seen in embryos of different species.
For example:
Tails and gill pouches are found in the embryos of chickens, humans, and cats.
One explanation for these embryonic similarities is that the common vertebrate ancestor of these species had gill pouches and a tail. 11

12. Structural Homology
Structural homology is a similarity in adult morphology.
For example, most vertebrates have a common structural plan in the limb bones. 12

13. Current Examples of Common Ancestry
Speciation is a process that results in one species splitting into two or more descendant species.
Biologists have documented dozens of contemporary populations that are undergoing speciation.
Powerful evidence that extant species are the descendants of extinct species.
Supports the claim that all organisms are related by descent from a common ancestor. 13

14. How Does Natural Selection Work?
While many researchers had proposed the fact of evolution, Darwin’s contribution was describing a process, natural selection, that could explain the pattern of descent with modification. 14

15. Biological Definitions
Biological fitness
The ability of an individual to produce surviving, fertile offspring relative to that ability in other individuals in the population.
Adaptation
A heritable trait that increases an individual’s fitness in a particular environment relative to individuals lacking that trait.
Selection
Differential reproduction as a result of heritable variation. 15

16. Recent Research on Natural Selection
The theory of evolution by natural selection is testable.
Examples include:
Drug resistance in bacteria
Beak size and shape and body size changes in the Galápagos finches 16

17. Which Genes Are under Selection?
Many characteristics, including beak size, are polygenic—many genes each exert a relatively small effect.
For example:
Beak depth is a polygenic trait
Research has shown that the amount of expression of a gene called Bmp4 correlates with beak depth.
Thus, this gene is thought to be under selection in the medium ground finch.
This illustrates a connection between natural selection on phenotype and evolutionary change in genotype. 17

18. Geospiza magnirostris
Figure 22.15
Lower Bmp4 expression
Higher Bmp4 expression
(little yellow) in embryo’s beak
(bright yellow) in embryo’s beak
2 mm
2 mm
Shallow
adult beak
Deep
adult
beak
Figure Changes in Bmp4 Expression Change Beak Depth.
Geospiza fortis
Geospiza magnirostris
© 2017 Pearson Education, Inc.

19. Natural Selection and Adaptation Misconceptions
Although natural selection appears to be a simple process, research has shown that it is often misunderstood. 19

20. Organisms Do Not Act for the Good of the Species
Individuals with self-sacrificing alleles die and do not produce offspring.
Individuals with selfish, cheater alleles survive and produce offspring.
As a result, selfish alleles increase in frequency while self-sacrificing alleles decrease in frequency.
Thus, it is not possible for self-sacrificing alleles to evolve by natural selection. 20

21. Nonadaptive Traits Vestigial traits confer no known benefit.
Organisms possessing them do not have higher fitness than those without.
Silent mutations:
Changes in the DNA sequence that do not result in a change in the amino acid sequence of the protein encoded by the gene.
Extremely common
Do not change the phenotype and thus cannot be acted on by natural selection and are not adaptive. 21

22. Genetic Constraints
Selection is not able to optimize all aspects of a trait due to certain genetic constraints
Genetic correlation occurs when selection favoring alleles for one trait causes a correlated but suboptimal change in an allele for another trait.
Lack of genetic variation can also constrain evolution, because natural selection can work only on existing variation in a population 22

23. Fitness Trade-Offs
A fitness trade-off is a compromise between traits, in terms of how those traits perform in the environment.
Because selection acts on many traits at once, every adaptation is a compromise.
Examples of fitness trade-offs include compromises between.
The size of eggs or seeds that an individual makes and the number of offspring it can produce.
Rapid growth and long life span.
Bright coloration and tendency to attract predators. 23