Populations: Evolution and Natural Selection

EVPP 110 Lecture
Instructor: Dr. Largen Fall 2003

Historical background
Evidence of evolution
Darwin’s theory
Natural selection
Microevolution

Historical Background

Ideas about evolution
originated before Darwin

mid-350s BC
1500s
1600s
1700s
1800s

mid-350s BC
Aristotle

noted evidence of natural similarities and relationships among organisms
lead him to arrange all organisms he knew into a "Scale of Nature"
extended from most simple to most complex
visualized living organisms as being
imperfect but "moving toward a more perfect state"

1500s
fossils

term coined in early 1500s
to describe remains of ancient organisms
of familiar living organisms
in unexpected contexts
marine invertebrate fossils imbedded in rocks on high mountains
some unlike any known form
Leonardo da Vinci
first interpret these finds

as remains of animals that had existed in past but had become extinct

1600s
emergence of modern scientific

1700s
exploration of continents
discovery of new species
emergence of idea

natural world of living organisms must be guided by natural laws
as physical world was governed by physical laws

1800s
1809

Jean Baptiste de Lamarck,French naturalist, published Philosophie Zoologique

Charles Darwin born

Jean Baptiste de Lamarck,French naturalist

published Philosophie Zoologique, in 1809
expressed most accepted view of evolution of that time
all living organisms were endowed with vital force that drove them to change toward greater complexity over time
organisms could pass traits acquired during their lifetimes on to their offspring

example, ancestral giraffe

Darwin’s life and experiences led to development of his theory of evolution
born in 1809
son physician
sent to University of Edinburgh to study medicine at age 15

found himself unsuited for medicine

transferred to Cambridge University to study theology and received his degree
1831 (at age 22)

embarked on 5-year round-the-world voyage
as naturalist on H.M.S. Beagle
profoundly influenced his thinking
during voyage
read extensively about geology
collected 1000s of specimens

plants, animals, fossils, including marine snail fossils in Andes

observed unique adaptations of organisms

1836

returned to England at end of voyage
his reading and experiences had led him to
seriously doubt current thinking of the time

Earth and living organisms were relatively new and unchangeable

had come to believe that Earth was very old and constantly changing

early 1840s
Darwin had composed an essay describing major features of his theory

delayed publishing it because
he knew it would cause a social furor

mid-1850s
British naturalist Alfred Wallace, who had been doing field work in Indonesia,

conceived a theory identical to Darwin’s

in 1858
Wallace’s work and excerpts from Darwin’s work were jointly presented to scientific community
in 1859
Darwin’s text On the Origin of Species by Means of Natural Selection, was published

didn’t use term "evolution" at first
referred instead to "descent with modification"
perceived a unity among species

all organisms related through descent from unknown organisms that lived in past

On the Origin of Species by Means of Natural Selection

maintained that
as descendants spread into various habitats over millions of years

they accumulated adaptations that accommodated them to diverse ways of life

Darwin’s phrase for evolution "descent with modification" captured the idea that
an ancestral species could diversify into many descendant species

by accumulation of different adaptations to various environments

Evidence of evolution
fossil record
biogeography
comparative anatomy
comparative embryology
molecular biology

fossil record
provides some of strongest evidence of evolution
an ordered array in which fossils appear within layers, or strata, of sedimentary rock

each strata can bear a unique set of fossils representing a local sample of organisms that lived when the sediment was deposited
younger strata are on top of older strata
position of fossils in strata reveals their relative age

shows that organisms appeared in a historical sequence
oldest known fossils

prokaryotes dating from ~ 3.5 BYA

younger layers of rock reveal evolution of various groups of eukaryotes

including successive appearance of various classes of vertebrates
fishlike, then amphibians, then reptiles, then mammals and birds

biogeography

geographical distribution of species
first suggested to Darwin that organisms evolve from common ancestors

environment of Galapagos islands resembled that of tropical islands from distant parts of world
animals of Galapagos more closely resembled species of mainland South America

Comparative anatomy
comparison of body structures in different species
anatomical similarities among many species give sign of common descent

same skeletal elements make up forelimbs of humans, cats, whales & bats
since forelimbs of these animals function differently
would expect their designs would be different, unless

they all descended from a common ancestor with same basic limb structure

homologous structures

features that have different functions but are structurally similar because of common ancestry

Comparative embryology

study of structures that appear during development of different organisms
closely related organisms often have similar stages in their embryonic development

one sign that vertebrates evolved from a common ancestor
all of them have an embryonic stage in which structures called gill pouches appear on sides of throat

at that stage, embryos of fishes, frogs, snakes, birds, apes look more alike than different

Molecular biology

study of molecular basis of genes and gene expression
universality of genetic code is strong evidence that all life is related
related individuals have greater similarity in their DNA than do unrelated individuals of same species
two closely related species have a greater proportion of their DNA in common than more distantly related species

Darwin’s Theory

In The Origin of Species

Darwin focused on how organisms become adapted to their environments
his theory arose from several key observations

all species tend to produce more offspring than environment can support
individuals of a population vary in their traits
organisms’ variations can be inherited by their offspring

all species tend to produce excessive numbers of offspring (overproduction)
production of more individuals than an environment can support
leads to a struggle for existence

natural resources are limited
only a percentage of offspring in each generation survive and reproduce
rest are starved, eaten, frozen, diseased, unmated, unable to reproduce for some other reason

Individuals of a population vary extensively in their characteristics

individuals whose characteristics make them best suited (adapted) to their environment are most likely to survive

most likely to reproduce
leave more offspring than less "fit" (adapted) individuals

Many of varying traits of individuals in a population can be passed from one generation to the next (heritable variations)
individuals whose traits make them best suited to an environment are more likely to survive and reproduce and

traits that made them well adapted to their environment are likely to be inherited by their offspring

Natural selection

natural selection
proposed by Darwin as basic mechanism of evolution
essence of which is differential, or unequal, success in reproduction

not all individuals have equal success in reproduction

higher reproductive success

occurs in individuals that are well adapted to their environment
these individuals will reproduce and pass on their traits

their traits will become more heavily represented in the next generation than will the traits of poorly adapted individuals

lower reproductive success

occurs in individuals that are poorly adapted to their environment
these individuals will reproduce less

their traits will become more less and less common in subsequent generations

individuals that are well adapted to their environment can be said to be most fit for that environment, or the "fittest"

hence phrase "survival of the fittest"

natural selection leads to, in subsequent generations,

favored traits (well adapted) will be represented more and more
unfavored traits (poorly adapted) will be represented less and less

unequal ability of individuals to survive and reproduce leads to

gradual change in characteristics of a population of organisms
over generations

favored characteristics accumulate
unfavored characteristics disappear

artificial selection
provided Darwin with evidence for his ideas on natural selection
definition

selective breeding of domesticated plants & animals
by selecting individuals with desired traits as breeding stock, humans were playing role of environment and bringing about differential reproduction

plants

broccoli, cauliflower, cabbages, brussel sprouts, kale and kohlrabi are all varieties of a single species of wild mustard that were produced by artificial selection

animals

hundreds of varieties of domestic dog, a single species called Canis familiaris, are result of 1000s of years of artificial selection
many species of canines resulted from 1000s to millions of years of natural selection

Darwin reasoned
if artificial selection could bring about so much change in a relatively short period of time

then natural selection over vast spans of time would result in gradual accumulation of hertitable changes that would result in evolution of new species
as in five species of canines thought to have evolved from a single ancestral canine

natural selection in action
many examples have been documented
peppered moth

exists in two forms
light colored with splotches of darker pigment (where it gets its name)
uniformly dark variety
feed at night, rest during the day, on trees & rocks encrusted with lichens
light variety is well-camouflaged against lichens, protected from predators
dark variety is conspicuous, therefore not protected from predators
Great Britain, prior to Industrial Revolution
dark variety of moth was rare

not camouflaged against lichens
became prey for birds before they could reproduce and pass onto next generation their genes for dark coloration

late 1800s, pollution from Industrial Revolution killed large numbers of lichens, exposing darker tree bark or rock
dark variety of moth became increasingly more abundant

now was camouflaged against dark surface and lighter variety was not

by early 1900s, in some industrial areas, populations consisted almost entirely of dark variety

Population

group of individuals of same species living in same place at same time

is smallest unit that can evolve

in moth example, it was population, not individual moths, that evolved

population is smallest unit that can evolve

evolution can be measured as

a change in prevalence of certain heritable traits in a population over a succession of generations

Darwin

understood
it is populations that evolve
did not understand
genetic basis of population change

Darwin could not explain
cause of variation among individuals making up a population
perpetuation of parents’ traits in their offspring
Due to knowledge that came after Darwin, it is now understood that
mutations in genes may produce new traits
heritable traits are carried by genes on chromosomes

modern synthesis
current version of theory of evolution that includes genetics
was developed in early 1940s
focuses on populations as units of evolution
includes most of Darwin’s ideas
melds population genetics with theory of natural selection
requires an understanding of relationship between populations and species

sexual species (biological species)
group of populations whose individuals have potential to interbreed & produce fertile offspring

Microevolution

Studying evolution at population level
focuses on

gene pool

total collection of genes in a population at any one time
reservoir from which members of next generation will derive their genes
can be studied by observing changes in relative frequencies of alleles over time

For most genes, there are 2 or more alleles (varieties)
a population at a given time can be described by relative frequencies of a particular set of alleles
over time, relative frequencies of particular alleles in population can change as result of natural selection
leads to microevolution

change in gene pool
as in moth example

frequency of each allele in gene pool will remain constant unless acted on by other agents
population to which this applies is said to be in Hardy-Weinberg equilibrium

Hardy-Weinberg equilibrium

suggests that something other than sexual reproduction is required to alter a gene pool

by changing allele frequencies from one generation to next

One way to determine what factors can change a gene pool is
identify conditions necessary to maintain genetic equilibrium
following 5 conditions must be met

population is very large
population is isolated
no movement into or out of population
gene mutations do not alter gene pool
mating is random
all individuals are equal in reproductive success
natural selection does not occur

five conditions necessary for Hardy-Weinberg equilibrium
rarely occur in nature

equilibrium breaks down
allele frequencies in natural populations change constantly

Causes of microevolution

basically reverse of 5 necessary conditions for Hardy-Weinberg equilibrium
5 causes of microevolution
genetic drift
gene flow
mutation
nonrandom mating
natural selection

Genetic drift
change in gene pool of a small population due to chance
in small population, chance event can have a disproportionately large effect

altering gene pool in next generation
iguana example, assume a small population (3 WW, 2 Ww and 5 ww)

an earthquake kills 3 iguana
3 dead iguanas were all WW
frequency of W allele in next generation would be reduced

two subtypes

bottleneck effect

founder effect

bottleneck effect

results from event that drastically reduces population size
event kills large numbers of individuals unselectively
produces small surviving population that is not likely to have same genetic makeup as original population

certain alleles will be present at higher frequencies, other alleles will be present at lower frequencies

founder effect

results from random change in a gene pool that occurs in a small colony
colonization of a new location by a single pregnant individual or a small # of individuals
gene pool of subsequent generations will be derived from just these few individuals
thought to have been important in evolution of many species in Galapagos Islands

Gene flow

gain or loss of alleles from a population by movement of individuals or gametes
occurs when

fertile individuals move into or out of a population
gametes are transferred from one population to another

minimizes genetic differences between populations
reduced by reproductive isolation

which increases genetic differences between populations

increased by

migration
wars

Mutation

random change in an organism’s DNA that creates a new allele
rare event for any given gene

occur ~ once per gene locus per 10⁵ to 10⁶ gametes

little effect on large population in a single generation
over time, vital to evolution because

ultimate source of genetic variation
serves as raw material for evolution

Nonrandom mating

selection of a mate other than by chance

random mating (chance) would require
every male (female) in population have an equal chance of mating with every female (male) in population
is rare in nature

nonrandom mating is the norm in most populations

for example, in humans, short males tend to marry short females

Natural selection

fifth agent of microevolution
differential success in reproduction
most likely to result in adaptive changes in a gene pool

Some genetic variation
seems to have a trivial impact on reproductive success

therefore may not be subject to natural selection

neutral variation hypothesis

proposes that species have some alleles that confer no selective advantage or disadvantage

frequencies of these alleles may increase or decrease as a result of chance genetic drift
but natural selection will not affect them
human fingerprints are probably an example of neutral variation

Evolutionary fitness

contribution an individual makes to gene pool of next generation relative to contribution made by other individuals
fittest individuals in an evolutionary context are those that pass on the greatest number of genes to the next generation
Individuals with a high degree of fitness
those whose phenotypic traits enable them to reproduce and contribute genes to more offspring than other individuals
Favored genotypes
those whose positive phenotypic effects outweigh any harmful effects they may have on reproductive success of organism
By culling less fit individuals, natural selection also culls unfavored genotypes
natural selection can alter phenotypic variations in an idealized population
three main ways

stabilizing selection

directional selection

diversifying selection

Stabilizing selection
favors intermediate variants
typically occurs in relatively stable environments

where conditions tend to reduce phenotypic variation

probably prevails most of time in most populations

Directional selection
shifts overall makeup of population by acting against individuals at one of phenotypic extremes
most common

during periods of environmental change
when members of a species migrate to new habitat with different environmental conditions

Diversifying selection
typically occurs when environmental conditions are varied in a way that favors individuals at

both extremes of a phenotypic range
rather than intermediate individuals

Natural selection can produced resistant populations of pests and parasites
new pesticide, antibiotic, drug is fairly effective killing all but a few individuals in target population when first used

few survivors live and reproduce because, by chance, they have genes that protect them (provide resistance)
they pass these protective traits on to their offspring

eventually, most of population consists of resistant individuals

The End