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Natural Selection & Adaptation

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Presentation on theme: "Natural Selection & Adaptation"— Presentation transcript:

1 Natural Selection & Adaptation
Unit 9 7C

2 Natural Selection Gradual process by which biological traits become either more or less common in a population


4 Nature basically “selects” certain individuals to be successful and reproduce and over time the population changes due to this selection.

5 What has happened to the mice population below?
The population has changed from 50% brown & 50% white to 80% white & 20% brown. WHY?

6 Adaptations Cause Fitness
Adaptation – any inherited characteristic that increases an organism’s chance of survival. (Remember the source of any new trait is a mutation.) Adaptations make an organism better suited to the environment. So in the struggle for survival, organisms with adaptations suited to the environment will be more likely to survive and reproduce.

7 Fit offspring then survive and reproduce and even more offspring are “born” that have the adaptation. Over generations, the DNA in the whole population changes, so that all the population has the adaptation that makes the population fit for its environment.

8 Fitness The ability of an organism to survive and reproduce

9 If fitness is the ability to survive & reproduce
Which lion is more fit?

10 This lion will hunt for more food, have the energy to reproduce, and pass on his genes to his offspring.

11 Traits that help an organism survive
Survive a predator Survive disease Compete for food Compete for territory

12 Traits that help individuals reproduce
Attracting a mate Compete for nesting sites Successfully raising young

13 “Survival of the Fittest”
Faster, stronger, good hunter, better camouflaged Best organism that survives and reproduces

14 Mesquite have the longest tap root of any desert species

15 Adaptation Any characteristic that increases fitness



18 Types of Selection

19 Types of Selection Disruptive Selection
Describes changes in population genetics in which extreme values for a trait are favored over medium values. The variance of the trait increases and the population is divided into 2 distinct groups.



22 Stabilizing Selection
Types of Selection Stabilizing Selection Genetic decreases as the population stabilizes on a particular trait or value. It varies medium traits. “Average Joe wins” Opposite of disruptive selection.



25 Directional Selection
Types of Selection Directional Selection A single phenotype is favored, causing the allele frequency to continuously shift in 1 direction. Occurs under environmental changes when populations migrate to new areas with different environmental pressures.



28 Natural Selection Summary Points
Inherited variation – there are genetic variations (differences) between individuals of a population The more genetic diversity, the more variations of alleles that are suited for the environment. The more offspring will survive and reproduce. The population will then continue.

29 Natural Selection Summary Points
Overproduction of offspring – there are more offspring are produced than can survive Struggle for existence – there is competition for survival among the members of the population

30 Witness to Evolution Peppered Moth 2 types: dark vs. light

31 Peppered moth Why did the population change?
early 1800s = pre-industrial England low pollution lichen on trees = light colored bark late 1800s = industrial factories = soot coated trees killed lichen = dark colored bark mid 1900s = pollution controls clean air laws return of lichen = light colored bark

32 Peppered moth: Evolution in action
Year % dark % light clean air, light-colored bark pollution, dark-colored bark Clean Air Act, light-colored bark industrial melanism

33 Natural Selection & Fitness
Unit 9 7D

34 Genetic or Inherited Variation
variation in alleles of genes that occurs both within and among populations



37 What determines these different traits or variations?
Mutations in our DNA crossing over & independent assortment in meiosis.

38 Homozygous black, heterozygous black, or homozygous brown
Let’s remember… Genotype is the individuals set of alleles for a trait… BB, Bb, or bb Phenotype is the appearance or expression of the trait… Homozygous black, heterozygous black, or homozygous brown


40 64% of the total mouse population has black fur
The black allele (B) occurs 40% of the time while the brown allele (b) occurs 60% of the time. The brown allele occurs more frequently than the black. What change in the habitat could change the frequency of alleles?

41 Elements of Natural Selection
Inherited Variation Producing More offspring than can survive Limited (finite) supply of Environmental Resources

42 Inherited Variation In an environment that favors black fur, mice with brown fur will be eaten, and the frequency of the allele for brown fur (b) will decrease. Black mice will have a better chance of surviving and passing on their alleles to their offspring.

43 The frequency for the while allele in the mouse population below will decrease.

44 Producing More offspring than can survive
Most populations produce more offspring than can survive. Many offspring are lost due to predation, starvation, etc… The offspring that do survive to adulthood will have phenotypes that give them an advantage within that environment over those that didn’t survive & reproduce.

45 Individuals compete for food, water, space, & shelter.
Limited (finite) Supply of Resources Individuals compete for food, water, space, & shelter. When resources become scarce, such as during a drought, competition increases. Populations decline, and the individual with advantageous traits will survive and reproduce.


47 The ground finches with larger beaks can eat a wider variety of seeds & nuts.
If a drought occurs, the finches with smaller beaks will run out of food & the population of finches with larger beaks will increase.

48 Natural Selection & Diversity
Unit 9 7E

49 Speciation the evolutionary process by which new biological species arise

50 Remember… Every species has different DNA.
Species can breed and produce fertile offspring.

51 Body has become red with spots
How has this population changed over time? Body has become red with spots

52 What has caused this change?
Mutations with redder body & blacker spots have been beneficial & allowed these insects to survive and reproduce. The mutations in the nucleotide sequences have now changed the DNA and a new species has evolved.

53 How do new species evolve?

54 Adaptation Due to natural selection, individuals with adaptations that give them an advantage over other individuals in the same environment are more likely to produce and pass the adaptations to their offspring. If the adaptation (mutation) continues to be beneficial, over time, the trait is passed on to generations, and a larger portion of the population with have that adaptation.

55 Diversity in a Species Let’s remember, individuals within a species are genetically different.

56 If the snow & ice melt, will this polar bear be able to grow thin brown fur?

57 But if a few mutated polar bears are born with thin, brown fur…

58 Diversity among Species
The process of natural selection and other mechanisms of evolution have lead to the millions of species of living organisms living on Earth today and millions of different species that lived in the past.


60 How does speciation occur?
Originally, there was 1 species of tortoise on 1 of the Islands.

61 How does speciation occur?
An event occurred, like a storm, that stranded some tortoises on another island. These individuals started a new population in a different environment. Over time, mutations occurred, & the genetic variation increased so the 2 populations couldn’t mate & produce fertile offspring and were now 2 separate species.

62 How does speciation occur?
Now, each island has at least 1 different species of tortoise.

63 Reproductive Isolation
occurs when two populations cannot interbreed and a new species is formed (speciation occurs).

64 A single species has evolved through natural selection into new and different species



67 Things that CAN (but don’t always) cause reproductive isolation:
1. Habitat isolation 2. Behavioral isolation 3. Geographic isolation 4. Temporal isolation

68 Habitat Isolation 2 populations breed in different areas of a habitat
Ex. snakes that breed in the water vs. the land

69 Habitat Isolation (a) Water-dwelling Thamnophis Terrestrial Thamnophis
Figure 24.4 Reproductive barriers Water-dwelling Thamnophis Terrestrial Thamnophis

70 Behavioral Isolation 2 populations can breed, but behavior causes them not to. Ex. Eastern & Western meadowlarks using 2 different songs to attract mates


72 Courtship ritual of different populations of
Behavioral Isolation Courtship ritual of different populations of Blue-Footed Boobies

73 Geographic Isolation 2 populations are separated by a geographic barrier (river, mountain, etc.). Ex. Colorado River separated squirrels

74 Geographic Isolation

75 Geographic Isolation

76 Geographic Isolation

77 Temporal Isolation 2 populations breed at different times.
Ex. orchids that pollinates once a year

78 Wood & Leopard Frog


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