Unit 4: Evolution

Definitions: Evolution

the relative change in the characteristics of populations that occurs over successive generations

Adaptation a particular structure, physiology or behaviour

that helps an organism survive and reproduce in a particular environment

Variation differences among traits occur among members

of the same species. Therefore no two individuals are exactly alike these variations are passed on to the next

generation

Peppered Moth P. 664-665

Industrial melanism when air pollution levels are high, the trees are

dark. This “favours” the survival of dark-winged moths

when air pollution levels are low, the trees are light. This “favours” the survival of light-winged moths

Survival of the “fittest” wing colour supports the camouflage of the moth

and allows it to survive to reproduce.

Natural selection Pp 347-349

process in which characteristics of a population of organisms change because individuals with certain inheritable traits survive specific local environmental conditions

there must be diversity within a species for this to occur

the environment exerts a selective pressure on a population, selecting individuals with certain characteristics and eliminating others

Artificial selection a breeder selects desired characteristics

in an organism

Eg: Dog breeders

Charles Lyell P. 655 developed theory of uniformitarianism said that all geological processes

operated at the same rates in the past as they do today

important because it indicated the world was much older than 6000 years, and that slow processes happening over long periods of time could result in substantial changes

Thomas Malthus P. 656 plant and animal populations grew

faster than their food supply

eventually a population is reduced by starvation, disease or war

Alfred Wallace P 657 wrote Darwin with essentially

identical theory of evolution

forced Darwin into publishing theory of evolution

Jean Baptiste Lamarck P. 651 presented first theory that discussed the

possibility of evolution believed that organisms have an imaginary force

or desire to change themselves for the better believed in the idea of inheritance of acquired

characteristics

Although this theory has been rejected Lamarck’s main contribution was to show that evolution is adaptive, and that the diversity of life is the result of adaptation

Georges Cuvier P. 650 developed the science of paleontology realized the Earth’s history was

recorded in the fossil record

recognized that extinction was a fairly common occurrence

strongly opposed to the theory of evolution

Darwin when Darwin observed living armadillos and the fossils

of ancient armadillo-like creatures in the same location, He began to wonder why one had survived and the other had not. He would later conclude that one form had evolved from the other

while exploring the Galapagos Islands, he noted slight variations among similar species of organisms from island to island 14 species of finch that were similar to a species of finch

found on the mainland The notable difference in finches lay in the shape of their

beaks. different beak shapes were adaptations for eating a

certain kind of food characteristic of the various geographic location

Darwin assumed that these different species had evolved from a single ancestral mainland species

started to formulate ideas about evolution worked out the process of natural selection in

1838

Published 21 years later (pushed by Wallace)

Theory of Natural Selection 1. Overproduction the number of offspring produced by a species is greater than the

number that can survive, reproduce and live to maturity 2. Struggle of existence (competition)

because of overproduction, organisms of the same species, as well as those of different species, must compete for limited resources such as food, water and a place to live

3. Variation differences among traits occur among members of the same

species. Therefore no two individuals are exactly alike these variations are passed on to the next generation

4. Survival of the fittest (natural selection) those individuals in a species with traits that give them an

advantage (i.e., are well-adapted to their environment) are better able to compete, survive and reproduce. All others die without leaving offspring since nature selects the organisms which survive, the process is called natural selection

5. Origin of new species (speciation) over numerous generations, new species arise by the accumulation

of inherited variations when a type is produced that is significantly different from the original, it becomes a new species

Mendel - work built upon by later scientists to reveal: P 675 there is MUCH genetic variation within

populations variations can arise through mutations

and are inheritable evolution, therefore, depends on both

random genetic mutations (which provide variation) and

mechanisms such as natural selection

Modern Evidence of Evolution Fossil Record P 659

The fossil record shows us: the earliest organisms were small and

simple in structure over millions of years organisms became

larger and more complex the number of different kinds of organisms

has increased over time many species of organisms have

disappeared and have been replaced by new and different species

The fossil record provides evidence of constantly changing life forms.

Biogeography P 663-664 study of the geographical distribution of

species (continental drift)

isolation is a key factor in the evolution of species

1) Geographic isolation occurs when a single breeding population is divided by a

geographic boundary e.g.,Canis lupus beothicus

barriers include: mountain ranges bodies of water barriers created by humans

gene flow between the isolated group and the main population ceases

different adaptations of populations in the separate environments, different gene frequencies within the separate populations and different mutations within the population

may all allow the population to become so different that interbreeding is impossible

2) Reproductive isolation may occur because of geographic isolation

occurs when organism in a population can no longer mate and produce offspring, even following the removal of the geographic barriers

factors that contribute to reproductive isolation include:

differences in mating habits courtship patterns seasonal differences in mating (very few species can

mate and reproduce at any time) inability of the sperm to fertilize eggs

Comparative Anatomy (homologous, analogous and vestigial structures) P 664-665

organisms with similar structures evolved from a common ancestor becomes increasingly obvious.

Eg: flipper of a seal, the leg of a pig, the wing of a bat, and the human arm all have the same basic structure and the same pattern of early growth.

1) Vestigial organs Def: small, incomplete organs with no

apparent function provide evidence of ancestry

e.g., snakes once had legs

2) Analogous structures similar functions but different

anatomically (insect/bird wings)

good indicators that these organisms didn’t come from common ancestors

Comparative Embryology P 665

Embryology is the study of organisms in the early stages of development. During the late 1800s, scientists noted a striking similarity between the embryos of different species (see page 683, Nelson).

At a later date, biologists suggested that the similarity of the embryos was due to their evolution from a common ancestor. This doesn’t mean that birds necessarily evolved from reptiles, or mammals from birds, but rather that the young forms of these organisms resemble the young of related species.

In a broad sense, there is a theory that every organism repeats its evolutionary development in its own embryology.

Scientists believe that many of the structures in an embryo are similar to those found in common ancestors

Heredity P 666 Mendel’s laws explain many variations

Since the laws of inheritance and the science of genetics are more clearly understood than in Darwin’s time, the variations in organisms required for natural selection to occur can be explained

Molecular Biology P 666-667

evolutionary relationships among species are reflected in their DNA

the closeness of species can be determined by comparing DNA patterns

http://www.youtube.com/watch?v=IFACrIx5SZ0&safety_mode=true&persist_safety_mode=1&safe=active

DNA similarity reveals a common ancestor also

shows that all life forms on earth are related, to some extent, to the earliest organisms

How to date a fossil P 662 the oldest layers are the ones laid down

first and, therefore, are found at the bottom of the site

the younger layers, added later, are on top since fossils form along with a given layer of sedimentary rock, the relative ages of the fossils can also be determined

The oldest will be on the bottom; the youngest will be on the top It takes about 1000 years to form 30

cm of sedimentary rock

Absolute dating provides a much more accurate method of determining a fossil’s age via radioactive dating techniques. A radioactive isotope has an unstable nuclear

structure, and will break down, releasing particles and energy.

The breakdown often results in a more stable element.

Radioactive dating involves measurements of the decay of radioactive isotopes such as: potassium-40, which becomes argon-40 uranium-238, which changes to lead-206 carbon-14, which becomes nitrogen-14

For example, if a rock contains thorium 232 and lead 208 in equal amounts, then one half of the original thorium 232 has decayed;

one half-life has passed and the rock must be 14 billion years old

Nucleic Acid evidence for evolution Pp 666-667

Cytochrome C protein found in mitochondria amino acid sequence is so similar among organisms that it can

be used to indicate relatedness e.g., chimps and rhesus monkeys differ by one amino acid;

chimps and horses by 11

the longer the period of time since an organism evolved from a simple ancestor, the greater the number of differences in the nucleotide sequences for the cytochrome c gene

http://www.youtube.com/watch?v=W-pc_M2qI74&feature=related&safety_mode=true&persist_safety_mode=1

Hardy-Weinberg law Pp 681-686 Gene pool - the entire genetic content of a

population

If all other factors remain constant, the gene pool will have the same composition generation after generation.

This stability is called genetic equilibrium.

Only if that equilibrium is upset can the population evolve.

The principle can be expressed mathematically by the formulae:

p2 + 2pq + q2 = 1 and p + q = 1

where

p = frequency of dominant allele

q = frequency of recessive allele

If the values for p and q are known, this equation can be used to calculate the frequency of all three genotypes, PP, Pq, and qq.

If the frequencies of the three genotypes are known, the frequencies of the alleles can be calculated.

The conditions under which no change in the gene pool will occur are:

1)large populations. This condition is necessary to ensure that changes in gene frequencies are not the result of chance alone

2) random mating. 3) no mutations 4) no migration. No new genes enter or

leave the population 5) equal viability, fertility and mating

ability of all genotypes (i.e., no selection advantage)

Example: Consider a simple situation - one gene with two

alleles, A and a. The genotypes that might be found in a large

population will be AA, Aa and aa.

In mathematical terms, the frequencies with which the alleles will occur must add up to one (and so must the frequencies of the genotypes)

if the dominant allele, A, is found in 70% of the population (i.e., has a frequency of 0.7),

the recessive allele will have a frequency of 1 - 0.7 = 0.3, or 30%.

The expected frequencies of the 3 possible genotypes can be calculated with a Punetsquare, or with the Hardy-Weinberg equation:

Sperm______________

A (0.7) A (0.3)________

A(0.7) AA (0.49) Aa (0.21)______

a (0.3) Aa (0.21) aa (0.09)______

Eggs

The equation predicts that the frequencies of the 3 genotypes possible in the next generation will be:

p2 + 2pq + q2 = 1

(0.7)2 + 2(0.7 x 0.3) + (0.3) = 1

Genotypes: 49% AA; 42% Aa; 9% aa

Given this distribution of genotypes, it’s possible to predict the frequency of the A and a alleles in the population:

F1 generation 0.49 AA ; 0.42 Aa; 0.09 aa potential gametes A A ; A a; a a

A = 0.49 + 0.21 = 0.7 a = 0.21 + 0.09 = 0.3

Problem: Suppose a recessive genetic disorder occurs in 9% of the population. Whatpercentage of the population is heterozygous, or carriers, of the disorder?

a = 0.09 = q2 ; q = 0.3

AA = ? = p2; 1 - q = p; 1 = 0.3 = 0.7

p2 + 2pq + q2 = 1

(0.7)2 + 2(0.7 x 0.3) + (0.3) = 1

0.49 + 0.42 + 0.9 = 1

The Hardy-Weinberg law: compares natural populations with an ideal

situation; such comparisons are a measure of change

In nature, allele frequencies are not constant and populations do change over time, or evolve it shows that meiosis and sexual reproduction by

themselves do not cause populations to change Merely recombining genes does not change

allele frequencies in a gene pool Other factors must be at work

Mutations P 688 may provide new alleles in a population

and, as a result, may provide the variation required for evolution to occur if a mutation provides a selective

advantage it may result in certain individuals producing a disproportionate number of offspring as a result of natural selection

Genetic Drift P 689 in small populations the frequencies of

particular alleles can be changed by chance alone the smaller the population the less likely

the parent gene pool will be reflected in the next generation

Bottleneck effect p. 690 as a result of chance certain alleles are over

represented and others are under represented in the reduced population

genetic drift then follows and the genetic variation in the surviving population is reduced

eg Northern elephant seal; Hunting reduced

population to as few as 20 individuals. The population today has reduced genetic variation as a result.

Founder effect p. 691 when a small number of individuals

colonize a new area the chances are high that they do not contain all the genes represented in the parent population

eg NL moose: since these founders are in a

new environment, they will experience different selection pressure

Gene Flow p 692 the movement of new alleles into a

gene pool can reduce genetic differences between

populations

Non-random Mating p. 692 1) inbreeding 2) self-fertilization 3) assortative mating (choosing mates

with a similar phenotype) This is the basis for artificial selection

e.g., breeding dogs

Natural Selection p. 693 some individuals in a population will leave more offspring

than others due to selective pressures 1) Stabilizing selection favours an intermediate

phenotype and acts against extreme variants e.g., baby weights are between 3 and 4 kg

2) Directional selection favours the phenotypes at one extreme over another. Common during periods of environmental change e.g., in the

wild budgies are usually green

3) Disruptive (diversifying) selection takes place when extremes of a phenotypic range are favoured relative to intermediate phenotypes. As a result, intermediates will be eliminated from the

population

Sexual Selection p. 695 characteristics used in sexual selection

may not be adaptive in the sense that they help an individual survive.

E.g., peacock tail However, they may increase the chances of being chosen as a mate and therefore of passing genes along to the next generation

Biological barriers to reproduction may contribute to speciationBiological barriers:

1) Pre-zygotic barriers p.709 either impede mating between species or prevent fertilization of the ova if

individuals from different species attempt to mate 2) Behavioural isolationism

bird song, courtship rituals, pheromones ... species-specific signals 3) Habitat isolation

because of differing habitats, species may not encounter each other 4) Temporal isolation p 710

differences in times for mating (season, year, time of day) 5) Mechanical isolation

anatomically so different that mating is impossible 6) Gametic isolation

gametes of different species will not fuse

Post-zygotic barriers P 710 when the sperm of one species successfully fertilizes the

ovum of another and a zygote is produced, these barriers prevent the hybrid from developing into normal, fertile individuals

Hybrid inviability genetic incompatibility of the interbred species may stop

development of the hybrid zygote Hybrid sterility p 711

hybrid is sterile e.g., donkey + horse = mule, which is usually sterile

Hybrid breakdown 1st hybrid generation is viable and fertile. Subsequent

offspring of hybrids are sterile or weak

http://www.youtube.com/watch?v=vJFo3trMuD8

Convergent and divergent evolution P 721 Divergent evolution

is adaptive radiation (homologous structures will be present between species)

Convergent evolution occurs when the environment selects similar adaptations in

unrelated species If the environments are similar, it is logical to assume that

some of the same kinds of traits would be favored in the different populations

(Analagous structures will be present among species)

E.g., Wings - birds, bats, bees fins/streamlined shape - dolphins and sharks eye structure - humans and octopus

The process of coevolution P 722 This process of joint evolution of two or

more species is called coevolution. Flowering plants and insects, predator prey relationships, parasites and their hosts

http://www.youtube.com/watch?v=WeuQfToa254&feature=PlayList&p=6362D791829F413A&playnext_from=PL&index=54&playnext=3