1. EVOLUTION
Crash Course – Natural Selection
Crash Course – Speciation

2. Evolution Change in a population over time
Change in genetic make-up (allele frequencies)
Results in new species
Species are defined by their genetic characteristics. Each species has a typical number of chromosomes and a particular set of genes. Within a species, however organisms differ. Because individuals have different alelles (versions of a gene), they have different traits. As species evolve over time, some traits (and the alleles that cause them) become more common and others disappear entirely.

3. Population
A group of organisms of the same species that share a geographical area and breed with each other

4. Adaptation
Any trait or behavior that makes an organism likely to survive in its environment

5. Can you spot the adaptation?

6. Mechanisms of Evolution
“Descent with Modification”

7. Mechanisms of Evolution
Natural Selection = “survival of the fittest”
organisms most suited for survival will reproduce and pass genes for the beneficial trait
Alleles for a trait that give an advantage increase
Ex: giraffe neck length

8. Mechanisms of Evolution
Genetic Drift = random changes in allele frequencies due to chance
not based on adaptations or fitness
Ex: landslide

9. Mechanisms of Evolution
Migration = movement of individuals into and out of a population
Individuals bring their alleles with them
More likely to impact smaller populations

10. Mechanisms of Evolution
Bottleneck Effect = when an entire population is reduced to just a few individuals
Leads to decreased genetic variation
Population susceptible to extinction
EX: overhunting of cheetahs

11. Mechanisms of Evolution
Founder Effect = formation of a new population when a few individuals leave a larger population
New gene pool can be very different from the original
EX: island species

12. Speciation through Isolation
Formation of new, genetically distinct species from populations of existing species
Isolating mechanisms prevent mating and breeding between two species
Geographical isolation = physical barriers
Behavioral isolation = differences in courtship rituals
Temporal isolation = differences in mating times

13. Evidence for Evolution
“Descent with Modification”

14. Evidence for Evolution
Fossils = remains or traces of organisms that once existed on Earth
Support the ideas that life on Earth has changed and that new species arise from existing species
EX: fish existed before four-limbed creatures

15. Evidence for Evolution
Anatomy and Development
Homologous structures = similarities in structure due to evolution from common ancestor
EX: limbs
Analogous structures = function similarly because they are adaptations to similar conditions
EX: wings
Vestigial structures = structures that have lost their original function
EX: leg bones in whales

16. Homologous Structures
Forelimbs of mammals

17. Analogous Structures
Wings built very differently

18. Vestigial Structures
Leg bones in whales
Tail bone in humans

19. Evidence for Evolution
Genes and Molecules (genetic codes)
Genetic code is nearly identical in all living things
The more similar two gene sequences are the more closely they are related
Can be used to create a lineage

20. Patterns in Evolution
“Descent with Modification”

21. Convergent evolution
Evolution toward similar characteristics in unrelated species due to similar environments
Analogous structures

22. Divergent Evolution
When closely related species evolve in different directions
Homologous structures

23. Coevolution
Evolution of one species drives the evolution of another

24. Effect of Natural Selection on Trait Distribution
“Descent with Modification”

25. Normal Distribution
Most individuals fall in the middle with few outliers
Ex: human height
Evolution by natural selection can change the distribution of traits ib a population so the “shape” of the graph can change or the direction can shift

26. Stabilizing selection
The “middle” trait is selected for; more individuals fall in the middle and even fewer fall at either end

27. Directional Selection
one of the extremes will be selected for and become the new normal; direction of distribution shifts

28. Disruptive selection
“middle” trait selected against so that both extremes become more common

29. Keystone Prep
EVOLUTION

30. Keystone Prep
Which of the following is a result of cheetahs having gone through a population bottleneck?
The cheetah species is more likely to become extinct
Mutations occur more frequently in the cheetah genome
Individual cheetahs have different alleles for many genes
The cheetah population cannot increase past a certain size

31. Keystone Prep
Which is one way that natural selection differs from genetic drift?
Natural selection causes the frequencies of alleles in a population to change
Natural selection affects smaller populations more often than larger populations
Natural selection depends on some traits helping individuals in their environment

32. Keystone Prep
A particular gene has two alleles, G and g. Each allele has a frequency of 50% in a population. Which is most likely to preserve these allele frequencies?
The population doubles in size over several generations
A number of individuals with one allele migrate from the population
One allele results in a phenotype that better enables organisms to reproduce

33. Keystone Prep
Recently, bears have been found in the Arctic Circle that are hybrids between grizzly bears and polar bears. Which type of isolating mechanism usually keeps these species from producing offspring?
Mating and breeding in different habitats
Production of a nonviable embryo or fetus
Inability of sperm to reach the egg during mating
Failure of he fertilized egg to implant in the uterus

34. Keystone Prep
Under which of the following conditions would a dominant allele that coded for a fatal disease most likely remain in a population?
The allele is acted on by natural selection
The gene is expressed only in members of one sex
The gene is expressed in individuals later in life
The allele mutates to a nonfatal form during an individual’s lifetime

35. Keystone Prep
Imagine that DNA replication and cell division were completely error-free and that cell DNA were completely protected from substances that cause mutation. How would natural selection be affected?
It would become more efficient since it removes mutations from the population
It would become ineffective since it depend on variations within populations

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