Modern View of Evolution:Genetic Change

Genes and Variation

Evolution and Genetics

• Variations are inherited from one generation to the next leading to natural selection

• Differences that help organisms survive and reproduce become more common and differences that are not beneficial become less common

• These differences in traits are caased by differences in the genetic information.

• Selection changes the genes!!!

Darwin and Mendel?

• Darwin did not know about how things are inherited yet (Mendel).

• We can use our knowledge from Mendel’s study of heredity and combine it with Darwin’s study of evolution to explain how inheritable variation appears and how natural selection effects that variation.

Genes and Variation• Biologists studying evolution focus on a population =

collection of individuals of the same species in a given area

• When discussing populations, the idea of a gene pool is important. Gene pool= the combined genetic information of all members of a population– Contain two or more alleles (forms of a gene) for each

inheritable trait• Relative frequency (%) = the number of times an allele

occurs in a gene pool compared to the number of times other alleles occur

Sample Population

48% heterozygous

black

36% homozygous

brown

16% homozygous

black

Frequency of Alleles

allele for brown fur

allele for black fur

Figure 16–2 Relative Frequencies of Alleles

Section 16-1

Sources of Genetic Variation• There are 2 main sources of genetic variation:• Mutations = any change in a sequence of DNA

– Remember: mutations result as a mistake during replication or mutagen (chemicals/radiation)

– Some mutations effect phenotypes (physical characteristics), which can effect an organism’s fitness (ability to survive)

– Mutations that occur in sex cells can be inherited • Gene shuffling = different gene combinations created during

gamete production (sexual reproduction only): which leads to different genotypes (genetic makeup), different phenotypes and more variation– Crossing over increases number of different genotypes– Does not change the relative frequency of alleles in a population

– Think of a deck of cards – there are many possible combinations, but frequency remains same

Single-Gene vs. Polygenic Traits• The number of phenotypes

produced for a given trait depends on how many genes control the trait.– If it is a trait controlled by a

single-gene with 2 alleles, there will be only 2 phenotypes.• Phenotypes of a single-gene

is represented by a bar graph– If it is a trait is polygenic with

2 or more alleles, there will be many genotypes and even more phenotypes.• Phenotypes of a polygenic

trait is represented by a normal distribution (bell curve)

•Fr

eque

ncy

of

Phen

otyp

e•

(%)

• Phenotype

Freq

uenc

y of

Ph

enot

ype

Phenotype (height)

Advantage of Variation• Through time, the environment always changes– Past climate is different that present climate

• Ice ages, plate tectonics, etc.

• The advantage of variations is that they make it more likely that individuals within a population will have a trait that will allow them to adapt to a new environment– More variation= more likely that population will

survive– If a particular variation is not present, it can’t aid in

survival

Evolution as Genetic Change

Evolution as Genetic Change

• Natural selection acts on phenotypes, survival and reproduction determine which alleles are inherited, changing relative frequencies of alleles in a population over time.

• Thus evolution is any change in the relative frequencies of alleles in a population’s gene pool and acts on populations, not individuals.

Evolution of Single-Gene Traits

• Natural selection on single-gene traits can lead to changes in allele frequencies and thus to evolution.– One of the two phenotypes may make an

organisms better fit, thus under pressure from natural selection and its relative frequency will increase

Single Allele Selection

• Which phenotype has higher fitness?– Orange

• Which allele’s frequency will decrease?– Green

Evolution of Polygenic Traits• Natural selection can affect the distributions of

phenotypes in any of three ways: – Directional selection– Stabilizing selection– Disruptive selection

Directional Selection

Food becomes scarce.

KeyLow mortality, high fitness

High mortality, low fitness

Graph of Directional Selection• Directional Selection: When the entire bell moves left/right

because there’s a higher fitness and increase in the number of individuals with the trait at one end of the curve.– Traits at one end or the other are selected for

Section 16-2

Directional Selection

Stabilizing Selection

• Stabilizing selection: When the bell becomes more narrow, because there’s a higher fitness and increase in the number of individuals with the trait in the center of the curve• The average trait is selected

for

Section 16-2

Key

Perc

enta

ge o

f Pop

u lati

on

Birth Weight

Selection against both extremes

keep curve narrow and in same place.

Low mortality, high fitness

High mortality, low fitness

Stabilizing Selection

Evolution of Clutch Size

Disruptive Selection

• Disruptive selection: The bell can split into two, because there’s a higher fitness and increase in the number of individuals at both ends of the curve• Traits at both “extremes” are selected for

Disruptive Selection

Largest and smallest seeds become more common.

Num

ber o

f Bird

sin

Pop

ulati

on

Beak Size

Population splits into two subgroups specializing in different seeds.

Beak SizeN

umbe

r of B

irds

in P

opul

ation

KeyLow mortality, high fitness

High mortality, low fitness

Section 16-2

Disruptive Selection

Genetic Drift• Genetic drift = random change in allele frequencies leading to

evolution without selection pressure• In small populations, individuals that carry a particular allele

may leave more descendants than other individual, just by chance. Over time, a series of chance occurrences can cause an allele to become common in a population.

• Genetic drift can happen when a small group of individuals colonize a new habitat carrying different relative frequencies that the larger population.

• 2 special Cases:• Founder effect = allele frequencies change as a result of the migration

of a small subgroup of a population• Bottle Neck= a population experiences a great reduction in the gene

pool, leaving only a small subset of alleles behind. Results in inbreeding.

Genetic Drift- Chance

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift- Founder EffectSection 16-2

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift- Founder EffectSection 16-2

Sample of Original Population

Founding Population A

Founding Population B

Descendants

Genetic Drift- Founder EffectSection 16-2

Genetic Drift Bottleneck

Genetic Equilibrium• Hardy-Weinberg principle states that allele frequencies

in a population will remain constant unless one or more of a set of factors causes the population to change.

• The following conditions that must be met to avoid evolution:– Random mating- no mate preferences, or choice (rare)– Large population- lots of diversity (less chance of genetic

drift)– No movement into or out of the population- individuals

don’t move between populations, carrying new alleles– No mutations- mutations change the DNA– No natural selection- all individuals have an equal chance

of surviving and reproducing

Hardy-Weinberg(p2) + (2pq) + (q2) = 1