MicroevolutionMicroevolution

Chapter 16Chapter 16

Under Stalin the teaching of genetics was forbidden. One scientist was imprisoned and died for teaching it. After Stalin’s death the Premier, Khrushev had to buy wheat from the U.S. He even came to Morris, IL to see how closely corn rows could be planted.

Darwin did not know where variation came from. He and most other scientists believed inheritance was blended. Ex. Zebra and horse produce a zebroid but it is sterile.

Blending inheritance and natural selection are incompatible. Embryologists noted that tissue that forms ovaries or testes separates from the human body early in development – differentiation takes place in the 4th week.

Weisman concluded that nothing in development or the environment of the body influences the germ cells or (DNA). Genetic material, he thought was passed from generation to generation.

In 1883 biologists observed that sexual reproduction provides new combinations – but it wasn’t known where new traits came from.

In 1900 3 biologists in different countries rediscovered Mendel’s work, de Vries, Correns, and Tschermak rediscovered 3:1 ratios in monohybrid crosses.

In the 1930’s a population genetics arose. It described in mathematical terms processes by which organisms generate new traits and pass them on – called microevolution - changes in the frequencies of genes in a population.

Ex. In flowers if the number of red flowers exceeds the number of white eventually the population will contain all red. Genetic variation begins with mutation that changes 1 or a few nucleotides and alters a single protein.

Chromosome mutations change the number of chromosomes or arrangement of genes. If harmless the mutation is passed on. But inheritance in a diverse population can have many alleles. If these are 2 or more alleles the gene is polymorphic such as eye color.

Most traits are also polygenic (eye color) 2 genes with 2 alleles can result in at least 5 phenotypes. AABB = dark brown. Grey, green hazel, and blue are caused by different combinations. Environmental influences (internal or external) can also change phenotype slightly.

Other traits influenced by polygenes are height, weight, bone structure, hormone levels, personality, intelligence, and skin color. Genetic variation is different from population to population.

What determines genetic variability in a Population?

1) How fast the mutations accumulate in DNA. 2) How fast mutations spread through a population. 3) How fast selection eliminates mutations from a population. Each new baby carries one or two new mutations.

Mutations arise constantly. The rate at which a mutation spreads through a population determines the evolutionary rate of a population. In humans about 7% of an individuals genes are heterozygous. In most natural populations heterozygosity is between 4-15%.

Natural selection acts on phenotype. Selection eliminates organisms less able to survive but environments can also quickly change.

Genetic Equilibrium-

In 1908 Hardy and Weinberg considered the idea that sexual reproduction eliminates variation. Sexual reproduction does not eliminate the frequency of alleles in a population. This idea have become known as the “Hardy-Weinberg principle”.

All the genes in a population are the gene pool. The frequency of a given allele within the gene pool is the number of times the allele is present divided by the number of times it could be present if every person had the allele in both chromosomes.

Consider a gene with 2 alleles; A and a. AA and Aa are blue, aa is brown. The fraction of the population containing gene A is p.

The fraction of the population containing gene a = q.

If p = .6, q=.4 There are only 2 alleles for the gene so p + q = 1.0. p and q are allele frequencies. If mussels release sperm into sea water 60% carry an A allele. 40% carry an a allele.

The frequency that a AA homozygote = .6 x .6 = .36. The frequency that an aa homozygote = .4x .4 = .16. The frequency of an Aa = .6 x.4 = .24 and the frequency of aA = .4 x .6 = .24. The probability of p + 2 pq + q = 1.0 (.36 + .24 + .24 + .16 = 1.0.

Despite sexual reproduction allele frequencies do not change. If there are 3 alleles A, a, and a’, p + q + r =1. A = p, a = q, and a” = r.

A= .5, a =.3, a’ =.2

A = .5 x .5 = .25

A = .3 x.3 = .09

A’ = .2 x .2 = .04

Aa = ,5 x .3 = .15 (x2) = .30

Aa’ = .5 x .2 = .10 (x2) = .20

aa’ = .3 x .2 = .06 (x2) = .12

.25 + .09 + .04 + .30 + .20 + .12 = 1.0

The stability of genotype frequencies is called Hardy-Weinberg Equilibrium. It exists when populations meet all of the following 5 requirements:

1. Random mating

2. Large population size.

3. No mutations

4. No gene flow (interbreeding with another population)

5. No selection

In reality this does not exist in nature. Gene frequencies change so evolution does occur. The Hardy-Weinberg Principle showed sexual reproduction does not dilute variation – supporting Darwinism.

Factors that change allele frequencies are nonrandom mating, small population size, mutation, interbreeding between populations (gene flow), and natural selection. Random mutations can generate every possible change in an amino acid sequence.

Mutations have little immediate effect on gene frequencies. One or 2 mutations out of 3 billion nucleotides isn’t going to change overall allele frequencies in an entire population of thousands of people.

Genetic drift can be demonstrated by flipping a coin. If you flip it 100 x you may get close to 50 H : 50 T. But if you flip it 4x there is a much higher chance of not getting close to 50/50. Chance fluctuations that affect small sample sizes are called “sampling errors”.

In small populations, sampling errors will cause allele frequencies to change randomly from generation to generation. The effects of chance in small populations lead to genetic drift, change in gene frequency due to random events (not selection).

Population Bottlenecks occur when a population is reduced by hunting or other factors. The few remaining individuals have fewer alleles to reproduce so they are inbred and lack genetic diversity. A single epidemic could wipe them all out. African cheetahs have the least heterozygosity of any animal species , .07%

In bottleneck populations the gene pool is less than what the environment and natural selection chooses. The smaller the population the less time it takes for genetic drift to change allele frequencies.

If a new mutant appears in a gene pool it can be eliminated by other things than natural selection (volcanos). Even in large populations genetic drift will cause alleles to go to 0% or 100% slowly or quickly but leads to a homozygous population.

Evolution would cease. New mutations restock genetic variation- the key to evolution. The Hardy-Weinberg Principle assumes random mating. However in nature genotype is chosen as reflected by phenotype.

Mate selection occurs as “sexual selection”. Mates are chosen on basis of similarities to an individual’s own genotype. Humans prefer those of similar skin color. When individuals choose mates from among their own relatives children are more likely to carry 2 identical alleles.

Ex. Ellis – van Creveld syndrome is more frequent among Amish because of years of inbreeding. Recessive genes have negative effects that are expressed in children homozygous of that allele. (inbreds are less healthy). Breeding between populations changes allele frequencies.

Hardy –Weinberg equilibrium requires that a population not interbreed with other populations. Animals immigrate to nearby populations and take new alleles.

The resulting gene flow causes changes in allele frequencies and is nearly universal. 2 effects of gene flow :

1.From the outside it causes genetic variation.

2.Makes adjacent populations more alike.

Natural selection can decrease the frequency of some genes and increases the frequency of others. Ex. Tay-Sachs is fatal before age 4 if it is homozygous therefore it survives in low numbers in heterozygotes. Lethal alleles persist indefinitely in low percentages.

Selection increases the frequency of traits. Insects have developed resistance to poisons and reproduced. Ex. DDT was used in insecticide to kill flies in Lake Tahoe. Some flies died but the few resistant ones reproduced and became as large a nuisance as the original strain.

DDT is toxic to other animals and fish there. Because fish consume many fly larvae they had large concentrations in their livers. When humans consumed the fish they became affected by the carcinogenic effects of DDT.

When penecillin was developed it was called a “magic bullet”. Now days most bacteria are resistant to it. Some bacteria are resistant to all known antibiotics.

The peppered moth in Great Britain existed in 2 forms, light and dark. They are preyed on by birds. 100 years ago they were mostly light but after the industrial revolution soot turned tree trunks dark. 98% of the population were dark due to light forms being more visible to birds.

Kettlewell proved this was true. He marked moths with paint, recaptured them. Where air was cleaner the majority of the population was light. In sooty areas more dark forms were recaptured. After laws were passed reducing soot in the air lighter forms were common.

Types of Natural Selection

Directional- shifts frequency of traits in a direction. Ex: Industrial melanism in moths

Stabilizing selection- acts against extremes in phenotypes so the average is selected for. EX: Horseshoe crabs have survived hundreds of millions of years unchanged.

Disruptive selection- encourages development of new species. Ex. Antibiotic resistant bacteria.

Coevolution – selection from the environment causes separate species to mutually benefit one another. EX: hummingbirds are the sole pollinators of certain flowers in St. Lucia. Male birds defend the flowers. Females must feed from a different species of flowers. The male coevolved with one flower and females with another flower.

Sexual selection - in birds males are brightly colored and females drab. Male plumage attracts the females. Females are camouflaged to guard nests.

In mammals males are larger than females and more aggressive. Males fight for territories. The difference in appearance of males and females is called sexual dimorphism. Females choose males with characteristics she chooses. Courtship varies with species.

Maintaining genetic heterogeneity in a population is not surprising if all the genes are useful. Ex: orange ladybugs are favored in the fall and red in the spring. This creates a balance of different alleles in a population called “balanced polymorphism”.

Populations may contain high frequencies of alleles lethal to an organism. In these cases polymorphism results from the selective superiority of the heterozygote. EX: sickle cell anemia. In the U.S. sickle cell homozygotes die young.

In Africa homozygotes die in their teens and 20’s. The BS allele persists in populations because it has strong BABS selective value. Heterozygotes are not sick and resistant to the parasite that causes malaria in BABA people.

In Africa the BS allele is 20 % , BA = 80%. Using the Hardy-Weinberg Principle the frequency of the heterozygote is 2(pq), 2 x .2 x .8 = 32%. The frequency of the homozygote is .2 x .2 = 4%.

Homozygotes die without reproducing. In the U.S. the BS

allele has no selective value and is lower. Change in allele frequency is microevolution.