Evolution

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What is evolution?•Evolution is a process that results in heritable changes in a population spread over many generations.

•“Evolution can be precisely defined as any change in the frequency of alleles within a gene pool from one generation to the next."

- Helena Curtis and N. Sue Barnes, Biology, 5th ed. 1989 Worth Publishers,

p.974

How does Evolution Occur?• Jean Baptiste Lamarck was one of the

first to propose that organisms change through time (1744-1829)

– “Inheritance of acquired traits”– He proposed that a change in the

environment produced a need to change in the animals

– If an animal uses one part of its body frequently, that part will become stronger

– If an animal doesn’t use a part, it will become smaller and disappear.

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Charles Darwin• Influences:

– geologist Charles Lyell• Wrote Principles of Geology

– geologic forces produced changes on Earth in the past and those same forces will continue to produce changes in the future.

– Economist Thomas Malthus• Wrote in Essay on the Principle of

Population – “population growth would always overpower

food supply growth, creating perpetual states of hunger, disease, and struggle”

If evolution was a car, the theory of natural selection would be the engine

• These ideas about evolution were discussed long before there was any scientific research done to support them.

• Before Darwin, the evolutionary concept was never able to gain any real steam because it lacked a mechanism. – Scientists wanted to believe that species

evolved from one form to another, but had no plausible process to make it happen.

– The theory of natural selection provides that reasonable method of evolution (or the engine behind the machine). animation

Darwin’s Theory of Natural Selection

1. Overpopulation- All species have the potential to

overpopulate the earth, but usually remain stable over time.

2. Competition- a large # of the population must die at an early age.

3. Variation- Due to variation, different individuals among populations have different traits

Darwin’s Idea Cont. 4. Survival of the Fittest-

members of a population whose trait makes them better adapted to their environment are more likely to survive to reproductive age and produce more offspring.

5. Reproduction- members of a population whose trait makes them better adapted are more likely to reproduce.

6. Speciation- given time, the process of evolution by natural selection can account for the formation of new species and thus for the diversity of life on earth.

Cosmos

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How long does evolution take?

Vocabulary to understand:• Population

– a group of individuals living in the same geographical area and sharing a common gene pool

• Gene Pool – the sum of all genetic information carried by the members of

a population • Mutation

– any heritable change in DNA (gene-level and the chromosome-level)

• Gene Flow– The transfer of alleles between populations through

interbreeding• Natural Selection

– Differential survival and reproduction of organisms with a result of increase in frequency of best adapted traits.

• Nonrandom Mating– artificial selection

This stud bull is the product of generations of selective breeding that has resulted in a double muscled freak that can hardly walk

Populations and Gene Pools

What’s the difference between microevolution

and macroevolution?

• Microevolution is change within species which can occur over dozens or hundreds of generations.

• Macroevolution usually involves much longer periods of time and includes the origin of new species.

What is a gene pool?

The collection of alleles available among

reproductive members of a

population

Evolution occurs on the population level

• What’s a population?– a group of individuals of the same

species in a given area whose members can interbreed.

• Because the individuals of a population can interbreed, they share a common group of genes (gene pool). – Each gene pool contains all the alleles

for all the traits of all the population. – Ex. Kermode Bears

• For evolution to occur in real populations, some of the gene frequencies must change with time.

Recall: The Hardy-Weinberg LawThis law states an equilibrium of allele frequencies in a gene pool (using a formula p2 + 2pq + q2) remains in effect in each succeeding generation of a sexually reproducing population if five conditions are met: 1. Large population-

The population must be large to minimize random sampling errors.

2. Random mating-There is no mating preference. For example an AA male does not prefer an aa female.

3. No mutation-The alleles must not change.

4. No migration –No exchange of genes between the population.

5. No natural selection- Natural selection must not favor any particular individual.

• In nature, the conditions of the Hardy-Weinberg law are rarely met

• Allele frequencies in the gene pool of a population do change from one generation to the next– thus evolution happens.

• The H-W equation provides a baseline to judge whether evolution has occurred. Remember that the effect of natural selection on gene

frequencies can be quantified.

So, if a change in allele frequencies occurs over time, you can assume evolution is happening.

A hypothetical “gene pool”

• Find the Frequencies of A and a and the genotypic frequencies of AA, Aa and aa.

Solution: f(A) = 12/30 = 0.4 = 40% f(a) = 18/30 = 0.6 = 60%

Then, p + q = 0.4 + 0.6 = 1 and p2 + 2pq + q2 = AA + Aa + aa

= .16 + .48 + .36 = 1

As long as the conditions of Hardy-Weinberg are met, the population can increase in size and the gene frequencies of A and a will remain the

same.

Thus, the gene pool does not change and evolution is NOT happening.

• Now, suppose more 'swimmers' dive in as shown in What will the gene and genotypic frequencies be?

Solution: f(A) = 12/34 = .35 = 35 % f(a) = 22/34 = .65 = 65%

f(AA) = .12, f(Aa) = .23 and f (aa) = .42

• The results show that H-W Equilibrium was not maintained. – The migration of swimmers (genes) into the pool

(population) resulted in a change in the population's gene frequencies.

– If the migration were to stop and the other agents of evolution (i.e., mutation, natural selection and non-random mating) did not occur, then the population would maintain the new gene frequencies generation after generation.

– It is important to note that a fifth factor affecting gene frequencies is population size.

• The larger a population is, the number of changes that occur by chance alone becomes insignificant.

• In the previous analogy, a small population was used to simplify the explanation.

Why does it affect small populations more?

• In a small population, allele frequencies are likely to be atypical just by chance.– If you were to toss a coin 1000 times and

get heads 750 times you would be very surprised.

– If you tossed a coin 4 times and got 3 heads, you would not be so surprised!

• Remember, chance has a greater influence upon gene frequencies in a small population

How much variation does a large population of sexually reproducing organisms have in their gene pool?

•Typically about 0.5% of the DNA bases in an organism are different

• In fruit flies there are 165 million base pairs,

• so ~1 million nucleotides sites differ.

• In humans with 3 billion base pairs,

• this means there are about 15 million different/variable nucleotides.

Mutation Rate • Gene mutations result in new alleles,

and are the source of variation within populations.

• Due to DNA replication and DNA repair mechanisms, mutation rates of individual genes are low• but because we have so many

genes and so many individuals there are new mutations all the time.

                                                       

Mutations and Peppered MothsIn 1850, f(L)= .95 and f(l) = .05

•light colored moths on light colored trunks were camouflaged

In 1900, f(L) = .05 and f(l) = .95

50 generations later:

•light lichen was killed by pollution

•So the dark tree trunk was exposed

• Therefore, light colored moths became easy prey for birds

Positive, Negative, or Neutral:

• Mutations are random and unpredictable• Some are lethal and kill individuals

before they are born.• Some are harmful, but masked by a

dominant allele – each of us carry 7-8 lethal recessive genes

• Some are neutral and have little to no affect on an organisms long term survival.

• Some are beneficial and help the organism survive to reproductive age and reproduce

Gene Flow

• the exchange of alleles– Alleles move through populations when

organisms interbreed or migrate– Gene flow can increase variation within a

population by introducing new alleles

Genetic Drift

• random changes in the gene pool– Genetic drift causes gene pools of two isolated

populations to become dissimilar – some alleles get lost and other get fixed  – This causes a reduction in the frequency of the

heterozygotes over time. – The smaller the population size the faster the

effect is seen

When is genetic drift likely to occur in nature?

• Most populations are large enough so genetic drift does not occur

• Some populations crash in numbers due to natural disasters or over-harvesting by humans(“bottleneck effect”)

• Another example is when a new habitat (e.g. an island) is colonized by just a few dispersing individuals (“founder effect”)

• The suddenly small population may, purely by chance, contain a different frequency of different genotypes that the original large population.

Founder Effect• Genetic drift can also occur when a

small number of individuals from a large population emigrate to a new area.

•The small number of emigrants are likely to have an genetic structure that differs, purely by chance, from the main population

•In the 1700’s 200 German Amish immigrated to Pennsylvania to start a community.

•These people carried a high concentration of a mutation which causes a form of polydactylism.

•Individuals in this group tended to marry within so there's a greater likelihood that the recessive genes of the founders will come together in the cells that produce offspring.

An Example of Founder’s Effect:

InbreedingBreeding between close relatives.

This causes the gradual increase in homozygosity (or loss of heterozygotes.

Example:

In most species, related individuals share about 80% of the same genes.

With cheetahs, this figure rises to approximately 99%.

The genetic inbreeding in cheetahs has led to low survivorship (a large number of animals dying), poor sperm quality, and greater susceptibility to disease.

The Bottleneck Effect• From an original large population, only few

survive to repopulate the habitat.• The few survivors are likely to have a

genetic structure (a set of genotype frequencies) that is unrepresentative of the ancestral large population

Example of a genetic bottleneck

Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s.

Hunting reduced their population 20 individuals at the end of the 19th century.

Their population has since rebounded to over 30,000—but their genes still carry the marks of this bottleneck:

they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted

Comparing the 2 Effects:

Founder Effect is sampling error as a result of movement of a few individuals away from the main population

Bottleneck Effect is sampling error as a result of only a few individuals surviving a population crash.

What’s inbreeding depression?

The fertility and survival are reduced compared with populations that are not inbred. Caused by an increase in homogous individuals.

Outbreeding enhancement/hybrid vigor is when they manage a species and bring back heterogeneity.

In the adder case, introducing snakes from other populations

Ex.

a population of 40 adders experienced inbreeding depression when they were isolated them from other adder populations.

As a result, a higher proportions of stillborn and deformed offspring were born in the isolated population than in the larger populations

Artificial Selection• Much of what

Darwin learned about Natural Selection, came from his observations from selectively bred crops and domestic animals.

• This showed that continued selection was powerful enough to bring about large-scale changes within a species.

A change in plant or animal population by selective breeding

Fitness• The suitability of an

organism to a given environment

• This is often measured by the number of offspring that an individual has

• However, the offspring must survive to contribute to the following generation

•Consider two zebras:• Both live in the savannah and must escape predation by lions

while also finding food & water. As babies, they were protected by the herd

• Zebra 1- had a mutation with deformed its rear leg• Zebra 2- is fast and strong

• Soon after they both became juveniles, a lion pride attacked the heard. – Zebra one, not being able to

run fast, was caught by the lion pride and eaten.

– Zebra 2 escaped this, and many more attacks. Lived to have many offspring who were also fast and strong runners.

– So Zebra 2 was more fit than Zebra 1, by surviving.

Adaptations that affect Fitness• Camouflage-

– color is not necessarily relevant– Pattern and shades matter– Pattern breaks up an organism’s

outline• Protective Coloration-

– Color of animal blends in with environment

– However, this can allow the organism’s shadow to reveal the outline