Download EVPP 110 Lecture - Populations - Evoluti

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EVPP 110 Lecture
Instructor: Dr. Largen Fall 2002
Populations: Evolution and Natural Selection
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4Evidence of evolution
4Darwin’s theory and the modern synthesis
4Variation and natural selection
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A sea voyage helped Darwin frame his theory of evolution
4 Ideas about evolution originated before Darwin
– Aristotle (mid-350’s BC) noted evidence of natural similarities and relationships among
organisms
• leading him to arrange all the organisms he knew into a “Scale of Nature”
– that extended from the most simple to the most complex
• he visualized living organisms as being imperfect but “moving toward a more
perfect state”
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A sea voyage helped Darwin frame his theory of evolution
4 Ideas about evolution originated before Darwin
– “fossil” was coined in the early 1500s to describe the remains of ancient organisms
– some fossils corresponded to parts of familiar living organisms
– sometimes fossils were found in unexpected contexts
• such as marine invertebrates imbedded in rocks on high mountains
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A sea voyage helped Darwin frame his theory of evolution
4 Ideas about evolution originated before Darwin
– other fossils were strangely unlike any any known form
– Leonardo da Vinci was the first to correctly interpret these finds
• as the remains of animals that that had existed in the past but had become extinct
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A sea voyage helped Darwin frame his theory of evolution
4 Ideas about evolution originated before Darwin
– modern scientific thought emerged in the 1600s
– in the 1700s, more continents were explored, new species were discovered, more
fossils were found
• many scientists began to think that the the world of living organisms must be
guided by natural laws
– similar to the way the physical world was governed by physical laws
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A sea voyage helped Darwin frame his theory of evolution
4 Ideas
about evolution originated before Darwin
– in 1809, French naturalist Jean Baptiste de Lamarck, in Philosophie Zoologique, expressed
most accepted view of evolution the that time
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• that all living organisms were endowed with a vital force that drove them to change
toward greater complexity over time
• that organisms could pass traits acquired during their lifetimes on to their offspring
– ancestral giraffe stretched neck reaching for tree leaves and passed its stretched
neck on to its offspring
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A sea voyage helped Darwin frame his theory of evolution
4 Darwin’s life and experiences led to the development of his theory of evolution
– born in 1809
– father was a physician
– was 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
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A sea voyage helped Darwin frame his theory of evolution
4 Darwin’s life and experiences
– in 1831 (age 22) he embarked on a 5-year round-the-world voyage as naturalist on H.M.S.
Beagle
• that profoundly influenced his thinking
• during the voyage he
– read extensively about geology
– collected 1000s of species of plants & animals, and fossils, including marine snail
fossils in the Andes
– made & documented observations on unique adaptations of organisms in South
America
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A sea voyage helped Darwin frame his theory of evolution
4 By the time Darwin returned to Great Britain at the end of the voyage in 1836
– his reading and experiences had led him to seriously doubt
• that the Earth and living organisms were relatively new (having been specially created
only a few thousand years ago) and unchangeable
– he had come to believe that the Earth was very old and constantly changing
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A sea voyage helped Darwin frame his theory of evolution
4 By the early 1840s, Darwin had composed an essay describing the major features of his theory
– but he knew his theory would cause a social furor so he delayed publishing it
the meantime, in the mid-1850s a British naturalist named Alfred Wallace, who had been
doing field work in Indonesia,
– conceived a theory identical to Darwin’s
4 In 1858, Wallace’s work and excerpts from Darwin’s work were jointly presented to the scientific
community
4 In
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A sea voyage helped Darwin frame his theory of evolution
4 In
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1859, Darwin’s text On the Origin of Species by Means of Natural Selection, was published
he didn’t use “evolution” at first, referring instead to “descent with modification”
he perceived a unity among species
with all organisms related through descent from unknown organisms that lived in the past
as descendants spread into various habitats over millions of years
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• they accumulated adaptations that accommodated them to diverse ways of life
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A sea voyage helped Darwin frame his theory of evolution
4Darwin’s phrase for evolution, descent with modification, captured the idea that an
ancestral species could diversify into many descendant species by the
accumulation of different adaptations to various environments
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The study of fossils provide strong evidence for evolution
4 The fossil record provides some of the strongest evidence of evolution
– an ordered array in which fossils appear within layers, or strata, of sedimentary rack
• each strata can bear a unique set of fossils representing a local sample of the
organisms that lived when the sediment was deposited
• younger strata are on top of older strata
– thus the position of fossils in the strata reveals their relative age
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The study of fossils provide strong evidence for evolution
4 The fossil
record shows that organisms have appeared in a historical sequence
– the oldest known fossils are prokaryotes dating from about 3.5 billion years ago
• prokaryotes are though to have been the ancestors of all life
– fossils in younger layers of rock reveal the evolution of various groups of eukaryotes
• including the successive appearance of the various classes of vertebrates
– fishlike, then amphibians, then reptiles, then mammals and birds
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A mass of evidence validates the evolutionary view of life
4 Evidence that reinforces the fossil evidence for evolution
– biogeography
– comparative anatomy
– comparative embryology
– molecular biology
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A mass of evidence validates the evolutionary view of life
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biogeography = the geographical distribution of species
– is what first suggested to Darwin that organisms evolve from common ancestors
• although the environment of the Galapagos islands resembled that of tropical islands
from distant parts of the world, Darwin noted
– that the animals more closely resembled the species of mainland South America than
the species on environmentally similar but distant islands
A mass of evidence validates the evolutionary view of life
4 Comparative anatomy
= the comparison of body structures in different species
– anatomical similarities among many species give sign of common descent
• the same same skeletal elements make up forelimbs of humans, cats, whales & bats
• since the forelimbs of these animals function differently, we would expect their designs
would be different, unless
– they all descended from a common ancestor with the same basic limb structure
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A mass of evidence validates the evolutionary view of life
4 Comparative anatomy
– homologous structures are features that have different functions but are structurally
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similar because of common ancestry
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A mass of evidence validates the evolutionary view of life
4 Comparative embryology
= the study of structures that appear during the development of
different organisms
– closely related organisms often have similar stages in their embryonic development
• one sign that vertebrates evolved from a common ancestor is that all of them have an
embryonic stage in which structures called gill pouches appear on the sides of throat
– at that stage, the embryos of fishes, frogs, snakes, birds, apes look more alike than
different
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A mass of evidence validates the evolutionary view of life
4 Molecular biology = the study of the molecular basis of genes and gene expression
– the universality of the genetic code is strong evidence that all life is related
– related individuals have greater similarity in their DNA than do unrelated individuals of the
same species
– and two species judged to be closely related have a greater proportion of their DNA in
common than more distantly related species
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Darwin proposed natural selection as the mechanisms of evolution
4 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 the environment can support
• individuals of a population vary in their traits
• organisms’ variations can be inherited by their offspring
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Darwin proposed natural selection as the mechanisms of evolution
4 all species tend to produce excessive numbers of offspring (overproduction )
– the 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
– the rest are starved, eaten, frozen, diseased, unmated, unable to reproduce for
some other reason
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Darwin proposed natural selection as the mechanisms of evolution
4 Individuals of a population vary extensively in their characteristics
– those individuals whose characteristics make them best suited (adapted) to their
environment are most likely to survive
• and therefore, most likely to reproduce
– and therefore, tend to leave more offspring than less “fit” (adapted) individuals
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Darwin proposed natural selection as the mechanisms of evolution
4 Many of the varying traits of individuals in a population can be passed from one
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generation to the next (heritable variations)
– individuals whose traits make them best suited to an environment are more likely to
survive and reproduce and
• the traits that made them well adapted to their environment are likely to be
inherited by their offspring
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Darwin proposed natural selection as the mechanisms of evolution
4 Darwin proposed that the basic mechanism of evolution was
natural selection
– the essence of which is differential, or unequal, success in reproduction
• individuals that are well adapted to their environment are more likely to survive and
reproduce than are individuals who are poorly adapted to their environment
– the traits of the well adapted individuals will be more heavily represented in the next
generation than will the traits of the poorly adapted individuals
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Darwin proposed natural selection as the mechanisms of evolution
4 Darwin proposed that the basic mechanism of evolution was natural selection
– those individuals that are well adapted to their environment can be said to be the most fit for that
environment, or the “fittest”
• hence the 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
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Darwin proposed natural selection as the mechanisms of evolution
4 Darwin proposed that the basic mechanism of evolution was natural selection
– the unequal ability of individuals to survive and reproduce leads to a gradual change
in the characteristics of a population of organisms, with the favored characteristics
accumulating and the unfavored characteristics disappearing over the generations
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Darwin proposed natural selection as the mechanisms of evolution
4 Darwin found evidence for his ideas on natural selection in the results of artificial
selection
– which is the selective breeding of domesticated plants and animals
– by selecting individuals with the desired traits as breeding stock, humans were
playing the role of the environment and bringing about differential reproduction
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Darwin proposed natural selection as the mechanisms of evolution
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selection examples
– in 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
– in animals
• hundreds of varieties of the domestic dog, a single species called Canis familiaris, are the
result of 1000s of years of artificial selection
– many species of canines resulted from 1000s to millions of years of natural selection
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Darwin proposed natural selection as the mechanisms of evolution
4 Darwin reasoned that 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 the gradual
accumulation of hertitable changes that would result in the evolution of new species
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• as in the five species of canines thought to have evolved from a single ancestral
canine
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Natural selection is a prominent force in nature
4 Many examples of natural selection in action have been documented
– for example, the peppered moth
• exists in two forms; light colored with splotches of darker pigment (where it gets its name)
and a uniformly dark variety
• they feed at night, rest during the day, on trees & rocks encrusted with lichens
– the light variety is well-camouflaged against the lichens while the dark variety is
conspicuous, therefore not protected from predators
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Natural selection is a prominent force in nature
4 the peppered moth example, in Great Britain
– prior to the Industrial Revolution (in 19th century), the dark variety of the moth was
rare
• probably in part because it was not camouflaged against the lichens and therefore
became prey for birds before they could reproduce and pass onto the next
generation their genes for dark coloration
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Natural selection is a prominent force in nature
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peppered moth example, in Great Britain
– in the late 1800s, industrial pollution from the Industrial Revolution killed large numbers of
lichens, exposing the darker tree bark or rock
• the dark variety of the moth became increasing more abundant since it now was
camouflaged against the dark surface and the lighter variety was not
• by the early 1900s, in some British industrial centers, the populations of the peppered
moth consisted almost entirely of the dark variety
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Populations are the units of evolution
4 A population
is a group of individuals of the same species living in the same place at the same
time
– it is the smallest unit that can evolve
• in the moth example, it was the population, not the individual moths, that evolved
– evolution can be measured as a change in the prevalence of certain heritable traits in a
population over a succession of generations
• Darwin understood that it is populations that evolve but he did not understand the genetic
basis of population change
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Populations are the units of evolution
4 Darwin could not explain
– the cause of variation among the individuals making up a population
– the perpetuation of parents’ traits in their offspring
4 Because of knowledge that came after Darwin, today we understand that
– mutations in genes may produce new traits
– heritable traits are carried by genes on chromosomes
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Populations are the units of evolution
4 A theory of evolution that includes genetics was developed in the early 1940s and is called the
modern synthesis
– focuses on populations as the units of evolution
– includes most of Darwin’s ideas
– melds population genetics with the theory of natural selection
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– requires an understanding of the relationship between populations and species
• sexual species (biological species)= a group of populations whose individuals have the
potential to interbreed & produce fertile offspring
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Microevolution is change in a population’s gene pool over time
4 Studying evolution at the population level focuses on the gene pool
– is the total collection of genes in a population at any one time
– is the reservoir from which members of the next generation of that population will
derive their genes
– can be studying by observing changes in the relative frequencies of alleles over time
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Microevolution is change in a population’s gene pool over time
4 For most gene loci, there are 2 or more alleles
– the population at a given time can be described by the relative frequencies of a particular
set of alleles
– over time, the relative frequencies of a particular alleles in the population can change as
a result of natural selection
• as in the moth example
• such a change in the gene pool is called microevolution
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The gene pool of an idealized, non-evolving population remains constant over the
generations
4 The frequency of each allele in the gene pool will remain constant unless acted on by
other agents
– a population to which this applied is said to be in Hardy-Weinberg equilibrium
• text uses example of imaginary iguanas to illustrate this
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Five conditions are required for Hardy-Weinberg equilibrium
4 Hardy-Weinberg equilibrium suggests that something other than sexual reproduction is
required to alter a gene pool by changing the allele frequencies from one generation to
the next
4 One way to determine what factors can change a gene pool is to to identify the
conditions necessary to maintain genetic equilibrium
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Five conditions are required for Hardy-Weinberg equilibrium
4 Hardy-Weinberg equilibrium
can be maintained in nature only is the following 5 conditions are
met
– the population is very large
– the population is isolated
• there is no migration of individuals (and therefore their alleles) into or out of the population
– gene mutations do not alter the gene pool
– mating is random
– all individuals are equal in reproductive success
• natural selection does not occur
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Five conditions are required for Hardy-Weinberg equilibrium
4 The five conditions necessary for Hardy-Weinberg equilibrium rarely occur in nature
– equilibrium breaks down
– allele frequencies in natural populations change constantly
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There are five potential causes of microevolution
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4 Five agents, derived from the five conditions necessary for Hardy-Weinberg equilibrium,
can cause changes in gene pools (microevolution)
– genetic drift
– gene flow
– mutation
– nonrandom mating
– natural selection
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There are five potential causes of microevolution
4 Genetic drift is a change in the gene pool of a small population due to chance
– if a population is small, a chance event can have a disproportionately large effect
• altering the gene pool in the next generation
– in the iguana example, assume a small population (3 WW, 2 Ww and 5 ww)
» an earthquake kills 3 iguana
» the 3 dead iguanas were all WW
» the frequency of the W allele in the next generation would be reduced
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There are five potential causes of microevolution
4 Genetic
drift, subtypes
– the bottleneck effect results from an event that drastically reduces population size
• such as occurs when natural disaster kills large numbers of individuals unselectively
– producing a small surviving population that is not likely to have the same genetic
makeup as the original population
» after the event, certain alleles will be present at higher frequencies while other
alleles will be present at lower frequencies
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There are five potential causes of microevolution
4 Genetic
drift, subtypes
– the founder effect results from random change in a gene pool that occurs in a small colony
• the colonization of a new location by a single pregnant individual or a small individuals
– the gene pool of subsequent generations will be derived from just these few
individuals
• this effect is thought to have been important in the evolution of many species in the
Galapagos Islands
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There are five potential causes of microevolution
4 Gene flow = gain or loss of alleles from a population by the movement of individuals or gametes
– occurs when fertile individuals move into or out of a population or gametes are transferred from
one population to another
– tends to reduce the genetic differences between the populations
– reproductive isolation reduces gene flow and increases the genetic differences between
populations
– migration and wars tend to increase gene flow among populations
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There are five potential causes of microevolution
4 Mutation is a random change in an organism’s DNA that creates a new allele
– mutations of a given gene is a rare event, typically occurring about once per gene locus
per 10 5 to 106 gametes
– in a large population, mutation alone does not have much effect in a single generation
– over the long term, mutation is vital to evolution because it is the ultimate source of the
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genetic variation that serves as the raw material for evolution
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There are five potential causes of microevolution
4 Nonrandom mating is the selection of a mate other than by chance
– for Hardy-Weinberg equilibrium to be maintained, every male in the population must
have an equal chance of mating with every female in the population (random mating)
• this rarely occurs and nonrandom mating is the norm in most populations
– for example, in humans, short males tend to marry short females
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There are five potential causes of microevolution
4 Natural selection is the fifth agent of microevolution
– the differential success in reproduction
– is most likely to result in adaptive changes in a gene pool
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Not all genetic variation may be subject to natural selection
4 Some genetic variation in populations seems to have a trivial impact on reproductive success
– therefore may not be subject to natural selection
4 the neutral variation hypothesis proposes that species typically have numerous 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
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The perpetuation of genes defines evolutionary fitness
4 Evolutionary fitness is the contribution an individual makes to the gene pool of the next
generation relative to the contribution made by other individuals
– the fittest individuals in an evolutionary context are those that pass on the greatest
number of genes to the next generation
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Natural selection acts on whole organisms and affects genotypes as a result
4 Individuals with a high degree of fitness are those whose phenotypic traits enable them
to reproduce and contribute genes to more offspring than other individuals
4 Favored genotypes are are those whose positive phenotypic effects outweigh any
harmful effects they may have on the reproductive success of the organism
4 By culling less fit individuals, natural selection also culls unfavored genotypes
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There are three main modes of natural selection
4 There are three main ways in which natural selection can alter the phenotypic variations
in an idealized population
– stabilizing selection
– directional selection
– diversifying selection
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There are three main modes of natural selection
4 Stabilizing selection
– favors intermediate variants
– typically occurs in relatively stable environments where conditions tend to reduce
phenotypic variation
– probably prevails most of the time in most populations
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There are three main modes of natural selection
4 Directional selection
– shifts the overall makeup of the population by acting against individuals at one of the
phenotypic extremes
– most common
• during periods of environmental change
• when members of a species migrate to some new habitat with different
environmental conditions
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There are three main modes of natural selection
4 Diversifying selection
– typically occurs when environmental conditions are varied in a way that favors
individuals at both extremes of a phenotypic range rather than the intermediate
individuals
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Directional selection has produced resistant populations of pests and parasites
4 Natural selection has produced resistant populations of pests and parasites
– when a pesticide, antibiotic, drug is first used, its fairly effective killing all but a few
individuals in target population
• the survivors live and reproduce because, by chance, they have genes that protect
them (provide resistance)
– the survivors pass these protective traits on t\o their offspring
» eventually, most of the population consists of resistant individuals
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