1. 16-2 Evolution as Genetic Change
17-2 Evolution as Genetic Change in Populations
16-2 Evolution as Genetic Change
Photo credit: ©MURRAY, PATTI/Animals Animals Enterprises
Copyright Pearson Prentice Hall

2. 16-2 Evolution as Genetic Change
In a genetic view of evolution, evolutionary fitness is marked by an organism's success in passing on its genes. An evolutionary adaptation is any genetically controlled trait that increases an individual’s ability to pass along its alleles.
Natural selection will affect which individuals are able to survive and reproduce and which are not.
If an individual dies without reproducing, it does not contribute its alleles to the population’s gene pool.
If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency.
Copyright Pearson Prentice Hall

3. 16-2 Evolution as Genetic Change
For example, insect populations often contain a few individuals that are resistant to a particular pesticide. Those insects pass on the alleles that code for resistance to their offspring and soon the pesticide-resistant offspring dominate the population. Natural selection AND genetics help us to understand how pesticide resistance develops.
Remember that evolution is any change over time in the relative frequencies of alleles in a population and that populations, not individual organisms, can evolve over time.
Copyright Pearson Prentice Hall

4. Natural Selection on Single-Gene Traits
How does natural selection affect single- gene traits?
Natural selection on single-gene traits can lead to changes in allele frequencies and, thus, to changes in phenotype frequencies.
Copyright Pearson Prentice Hall

5. Natural Selection on Single-Gene Traits
Imagine a population of lizards.
The population is normally brown, but has mutations that produce red and black forms.
Red lizards are more visible to predators, so they will be less likely to survive and reproduce. Therefore, the allele for red color will become rare.
Copyright Pearson Prentice Hall

6. Natural Selection on Single-Gene Traits
Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution. Organisms of one color, for example, may produce fewer offspring than organisms of other colors.
Copyright Pearson Prentice Hall

7. Natural Selection on Single-Gene Traits
Black lizards may warm up faster on cold days. This may give them energy to avoid predators. In turn, they may produce more offspring.
The allele for black color will increase in relative frequency.
Copyright Pearson Prentice Hall

8. Natural Selection on Polygenic Traits
How does natural selection affect polygenic traits?
Natural selection can affect the distributions of phenotypes in any of three ways:
directional selection
stabilizing selection
disruptive selection
Copyright Pearson Prentice Hall

9. Copyright Pearson Prentice Hall
Natural Selection on Polygenic Traits
The effects of natural selection are much more complex when traits are controlled by more than one gene.
Recall that the action of multiple alleles produces a graph called a bell curve.
Fitness from one end of the curve to the other end may vary a great deal. This is where natural selection becomes important.
Copyright Pearson Prentice Hall

10. Natural Selection on Polygenic Traits
Directional Selection 
When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place.
The range of phenotypes shifts as some individuals survive and reproduce while others do not.
Copyright Pearson Prentice Hall

11. Natural Selection on Polygenic Traits
If, for example, only large seeds were available, birds with larger beaks would have a higher fitness. These birds would pass on their genes and, over time, average beak size would increase.
Directional selection occurs when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. In this example, a population of seed-eating birds experiences directional selection when a food shortage causes the supply of small seeds to run low. The dotted line shows the original distribution of beak sizes. The solid line shows how the distribution of beak sizes would change as a result of selection.
Copyright Pearson Prentice Hall

12. Natural Selection on Polygenic Traits
Stabilizing Selection 
When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place.
This keeps the center of the curve at its current position, but it narrows the overall graph.
Copyright Pearson Prentice Hall

13. Natural Selection on Polygenic Traits
Human babies born at an average mass are more likely to survive than babies born either much smaller or much larger than average.
In this example of stabilizing selection, human babies born at an average mass are more likely to survive than babies born either much smaller or much larger than average.
Copyright Pearson Prentice Hall

14. Natural Selection on Polygenic Traits
Disruptive Selection 
When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place.
If the pressure of natural selection is strong enough and long enough, the curve will split, creating two distinct phenotypes.
Copyright Pearson Prentice Hall

15. Natural Selection on Polygenic Traits
If average-sized seeds become scarce, a bird population will split into two groups: one that eats small seeds and one that eats large seeds.
In this example of disruptive selection, average-sized seeds become less common, and larger and smaller seeds become more common. As a result, the bird population splits into two subgroups specializing in eating different-sized seeds.
Copyright Pearson Prentice Hall

16. Copyright Pearson Prentice Hall
Genetic Drift
Genetic Drift
What is genetic drift?
A random change in allele frequency is called genetic drift.
Copyright Pearson Prentice Hall

17. Copyright Pearson Prentice Hall
Genetic Drift
In small populations, individuals that carry a particular allele may leave more descendants than other individuals do, just by chance.
Over time, a series of chance occurrences of this type can cause an allele to become common in a population.
Copyright Pearson Prentice Hall

18. Copyright Pearson Prentice Hall
Genetic Drift
Genetic Bottlenecks
The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population.
For example, if a natural disaster were to kill many individuals in a population, the surviving popuation’sgene pool may carry alleles in different relative frequencies than did the larger population from which they came.
The new population will be genetically different from the parent population.
Copyright Pearson Prentice Hall

19. Copyright Pearson Prentice Hall
Genetic Drift
The Founder Effect
When allele frequencies change due to migration of a small subgroup of a population it is known as the founder effect.
Copyright Pearson Prentice Hall

20. Copyright Pearson Prentice Hall
Genetic Drift
Two groups from a large, diverse population could produce new populations that differ from the original group.
In small populations, individuals that carry a particular allele may have more descendants than other individuals. Over time, a series of chance occurrences of this type can cause an allele to become more common in a population. This model demonstrates how two small groups from a large, diverse population could produce new populations that differ from the original group.
Copyright Pearson Prentice Hall

21. Evolution Versus Genetic Equilibrium
When allele frequencies remain constant it is called genetic equilibrium. If allele frequencies don’t change, the population will not evolve.
The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change.
Copyright Pearson Prentice Hall

22. Evolution Versus Genetic Equilibrium
Five conditions are required to maintain genetic equilibrium from generation to generation:
the population must be very large,
there can be no mutations,
there must be random mating,
there can be no movement into or out of the population, and
there can be no natural selection.
Copyright Pearson Prentice Hall

23. Evolution Versus Genetic Equilibrium
Large Population
Genetic drift can cause changes in allele frequency of small populations.
Allele frequencies of large populations are less likely to be changed through the process of genetic drift.
Large population size helps maintain genetic equilibrium.
Copyright Pearson Prentice Hall

24. Evolution Versus Genetic Equilibrium
No Mutations
If genes mutate, new alleles may be introduced into the population, and allele frequencies will change.
Copyright Pearson Prentice Hall

25. Evolution Versus Genetic Equilibrium
Random Mating
All members of the population must have an equal opportunity to produce offspring. Individuals must mate with other members of the population at random.
In natural populations, mating is rarely completely random. Many species select mates based on particular heritable traits, a practice called sexual selection.
Copyright Pearson Prentice Hall

26. Copyright Pearson Prentice Hall
Peacocks, for example, choose mates based on their brightly patterned tail feathers. Such non-random mating means that alleles for those traits are under selection pressure.
Copyright Pearson Prentice Hall

27. Evolution Versus Genetic Equilibrium
No Movement Into or Out of the Population 
Because individuals may bring new alleles into a population (in a process called gene flow), there must be no movement of individuals into or out of a population.
The population's gene pool must be kept together and kept separate from the gene pools of other populations.
Copyright Pearson Prentice Hall

28. Evolution Versus Genetic Equilibrium
No Natural Selection
All genotypes in the population must have equal probabilities of survival and reproduction.
No phenotype can have a selective advantage over another.
There can be no natural selection operating on the population.
Copyright Pearson Prentice Hall