Natural Selection & Adaptation
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Transcript of Natural Selection & Adaptation
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Natural Selection & Adaptation
Unit 97C
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Natural Selection
• Gradual process by which biological traits become either more or less common in a population
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Nature basically “selects” certain individuals to be successful and
reproduce and over time the population changes due to this
selection.
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What has happened to the mice population below?
The population has changed from 50% brown & 50% white to 80% white & 20% brown. WHY?
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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.
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• 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.
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Fitness • The ability of an organism to survive
and reproduce
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Which lion is more fit?
If fitness is the ability to survive
& reproduce
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This lion will hunt for more food, have the energy to reproduce, and pass on his genes to his offspring.
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Traits that help an organism survive
1. Survive a predator2. Survive disease3. Compete for food4. Compete for territory
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Traits that help individuals reproduce
• Attracting a mate• Compete for nesting sites• Successfully raising young
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“Survival of the Fittest”
• Faster, stronger, good hunter, better camouflaged• Best organism that survives and
reproduces
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Adaptation
• Any characteristic that increases fitness
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Types of Selection
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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.
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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.
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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.
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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.
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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
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Witness to Evolution• Peppered Moth– 2 types: dark vs. light
Peppered moth
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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
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Peppered moth: Evolution in actionYear % dark % light1848 5 951895 98 21995 19 81
clean air, light-colored bark
pollution, dark-colored bark
Clean Air Act, light-colored bark
industrial melanism
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Natural Selection & Fitness
Unit 97D
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Genetic or Inherited Variation
• variation in alleles of genes that occurs both within and among populations
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What determines these different traits or variations?
• Mutations in our DNA• crossing over & independent
assortment in meiosis.
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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
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48%
16%
36%Bb heterozygous blackBB homozygous blackbb homozygous brown
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• 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?
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Elements of Natural Selection
1. Inherited Variation2. Producing More offspring than can
survive3. Limited (finite) supply of
Environmental Resources
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1. 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.
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The frequency for the while allele in the mouse population below will decrease.
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2. 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.
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3. 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.
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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.
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Natural Selection & Diversity
Unit 97E
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Speciation
• the evolutionary process by which new biological species arise
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Remember…
• Every species has different DNA.• Species can breed and produce
fertile offspring.
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• How has this population changed over time?
Body has become red with spots
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• 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.
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How do new species evolve?
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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.
Adaptation
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Let’s remember, individuals within a species are genetically different.
Diversity in a Species
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If the snow & ice melt, will this polar bear be able to grow thin brown fur?
NO!
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But if a few mutated polar bears are born with thin, brown fur…
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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.
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How does speciation occur?Originally, there was 1 species of tortoise on 1 of the Islands.
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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.
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How does speciation occur?Now, each island has at least 1 different species of tortoise.
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Reproductive Isolation
• occurs when two populations cannot interbreed and a new species is formed (speciation occurs).
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• A single species has evolved through natural selection into new and different species
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Things that CAN (but don’t always) cause reproductive isolation:
1. Habitat isolation2. Behavioral isolation3. Geographic isolation4. Temporal isolation
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Habitat Isolation
• 2 populations breed in different areas of a habitat • Ex. snakes that breed in the water vs.
the land
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Habitat Isolation(a)
Water-dwelling Thamnophis Terrestrial Thamnophis
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Behavioral Isolation• 2 populations can breed, but
behavior causes them not to. • Ex. Eastern & Western meadowlarks
using 2 different songs to attract mates
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http://bio1100.nicerweb.com/sketches/sounds/Chordata/Aves/Melospiza_melodia.mp3
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Behavioral IsolationCourtship ritual of different populations of
Blue-Footed Boobies
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Geographic Isolation
• 2 populations are separated by a geographic barrier (river, mountain, etc.). • Ex. Colorado River separated
squirrels
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Geographic Isolation
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Geographic Isolation
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Geographic Isolation
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Temporal Isolation
• 2 populations breed at different times.• Ex. orchids that pollinates once a
year
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Wood & Leopard Frog
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