BIO 412 Principles of Ecology Phil Ganter 320 Harned Hall 963-5782
A field of both wind- and insect-pollinated plants

Chapt. 2 Evolution

Fall, 2001

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Evolutionary Ecology

Relationship between ecology and evolution

• Ecology is the study of the abundance and distribution of organisms. Some factors affecting abundance and distribution are also important in evolution
  • Differences among the members of a population mean that not all are affected in the same way by ecological pressures
  • The evolutionary history of a species may mean that not all areas that could support a population acutally have a popultation present
  • Organisms from different species that interact with one another may, over time, respond to the presence of the other by changing themselves through the process of evolution. Thus, in the chapters on herbivory and predation, we wil look for the importance of these ecological processes by assessing how plants have changed to reduce the effect of herbivores and how prey have changed to escape their predators.
• Theories of biological evolution have included ecology from the start
  • Lamarck opened the discussion of the relationship with his ideas on adaptation as it was to be more successful in their environment that induced organisms to change
  • Darwin/Russell pointed out how we could relate random variation to adaptation through natural selection so that those vairants that were more successful in terms of survivorship and reproduction (two ecological measures) were those that gained greater representation in subsequent generations

What is evolution?

change in a population over time

What is adaptive evolution?

change in a population over time caused by natural selection. Adaptive evolution alters a population of organisms such that the changes allow the organisms to increase survivorship and/or reproductive success

Darwin/Wallace contended that adaptation happens because:

• individuals vary
• variation is heritable
• variation in characteristics is linked to variation in relative success (fitness) of an individual
  • fitness defined as the ability to survive and reproduce in a particular environment

Variation is key to evolution - without it no change is possible and if it is not linked to fitness no adaptive evolution is possible

• ECOLOGY and EVOLUTION are linked by variation -- it is the ecological situation that determines the fitness of a variant
• If there were no heritable (genetic) variation among the members of a species, no change would be possible
• Variation in the ecological setting (in both physical factors and other organisms) means that the course of adaptive evolution is not always in the same direction

How does genetic variation arise?

• Plant and animal breeders have known for millennia that new variant arise spontaneously
• Now we know that mutations are the source of new variants
• mutations include
  • Point mutations - can change primary sequence of proteins
  • Frameshift mutations (due to insertions or deletions) - can cause genes to code for useless proteins
  • Chromosomal rearrangements - inversions, crossing over errors, gene conversion, aneuploidy, polyploidy

How is genetic variation inherited?

• Mendel first to propose particulate inheritance with rules for gene expression
• non-Mendelian inheritance also occurs
  • maternal inheritance
  • horizontal gene transfers

What should we expect to happen when variation exists

Hardy-Weinberg expectations are predictions when variation is not altered by ecological processes

H-W Assumptions - notice that each assumption involves some ecological process not occurring

• No mutation,
• No migration,
• Random Mating,
• Large Populations,
• No Selection

What kinds of Variation are there?

Environmental (V_E) + Genetic (V_G) = Phenotypic (V_p)

Genes may have different effects when in different environments

• many genes are expressed differently when temperature differs
• expression of many genes depends on genetic environment - what alleles are present at other loci - dominance is a good example of this effect
• Therefore, we must added a term for gene-by-environment interactions (V_G+E)

Phenotypic (V_p) = Environmental (V_E) + Genetic (V_G) + Interaction (V_G+E)

How much exists and how do we measure it?

• Phenotypic -
  • morphological
  • protein polymorphism
• Genetic -
  • allelic
  • RFLP
  • RAPD
  • microsatellite
  • Sequences of
    • Nuclear DNA
    • mtDNA, cpDNA
• Heterozygosity is a measure of variation at a locus (which might have several alleles) in a population
  • defined as the probability that any two genes (at a particular locus) chosen at random from a population are the same allele
  • heterozygosity occurs at both the level of an individual and at the level of a population. Each are different, although related, and should not be confused. Populations can be heterozygous even if no individuals are:
    • consider a population with two subpopulations of equal size
    • subpopulation A has all allele X₁
    • subpopulation B has all allele X₂
    • No heterozygous individuals occur
    • Population is heterozygous as there is only a 50% chance that two alleles drawn from total population will be the same allele

Partitioning of genetic variation can help us understand the ecology of the species

We can see above that there are two levels of heterozygosity one can measure in the example above:

• Total genetic variation
• average sub-population genetic variation

Sub-population genetic variation is only one component of total genetic variation (=GV)

• Total GV = Within Sub-population GV + Between Sub-population GV
• If most of Total Genetic Variation occurs within populations then each population varies almost as much as the overall species
  • each population may be exchanging organisms with other populations, so variation is spread among them all
• If most of Total Genetic Variation occurs between populations then each population is internally homogeneous and the species is fragmented into distinct subpopulations, each of which may be adapting to local circumstances independently of one another

Predictable replacement of one (ALLELE, MORPH) over distance is a cline

Clines form in more than one way

• when the effect of environment changes gradually with distance, then genetic composition may also come to reflect the underlying environmental cline
• when two populations with distinct characteristics both occupy the area between their population centers, migration outward from each will result in a cline between those centers

What reduces the amount of variation in a population? Notice that each one of these is a violation of Hardy-Weinberg

Assortative mating (also called Non-Random Mating)

• if like mates with like (due to choice or to small population sized not allowing much choice) then intermediates and heterozygotes are lost)
• like mating with like is called Positive Assortative Mating

Inbreeding

• has the same effect as positive assortative mating - loss of heterozygosity
• can lead to low levels of within-population variation, even if there is appreciable between-population variation
• can (not must, but can) lead to lower viability of inbred individuals or to lower fecundity
• heterosis - condition where the heterozygous individuals show greater fitness (viability, fecundity) than do individuals homozygous for either of the alleles
• more likely in small populations
• often there are physical or behavioral barriers to inbreeding
• Book documents instances where inbreeding has reduced fitness

Genetic Drift

• loss of heterozygosity due to chance events
• more likely in small populations than in large
• operates on a principle called Mueller's Ratchet :
  • chance can cause allele frequencies (proportions of each allele) to fluctuate
  • occasionally, these fluctuations will reach zero and an allele is lost from the population (this is more likely for alleles already at low frequency)
  • once an allele is lost from a population, it is gone forever (this is not strictly true - migration and mutation can restore alleles to a population)

Neighborhoods can enhance the effect of drift

• if populations are subdivided into small neighborhoods, then drift will be more important for the entire population
• Population structure can also contribute to the loss of genetic variation if not all individuals actually breed
  • this can make the breeding population smaller than the actual population and decreases population size
  • can be due to senility of some individuals, or some may be juveniles, or there may be a social system which restricts breeding to one or a few individuals of each sex
  • variation in sex ratio can also increase loss of variation

Effective Population Size

• a way to correct the total population size for the failure of some individuals to breed of for skewed sex ratios and to compare different populations by reducing the actual population size to a population number in which all individuals are breeding and the sex ratio is 1 to 1.
• Effective Population Size (= N_e)
• Suppose I count two populations, one with 100 individuals and the other with 125 individuals. It is determined that the effective population size of the first is 100 individuals, but there is a large excess of males over females in the second population and the effective population size is calculated to be only 85 in the second population. Because Ne (=85) is lower for the second population than the first (Ne = 100), the rate of loss of genetic variation is greater in the second population than in the first, even though there are more individuals!
• Formula for correcting population size when the number and sex of the breeding population are known:

• N _m and N _f are the number of breeding males and females
• Notice that when all individuals breed and the sex ratio is 1-to-1, then N _e is the same as the population size

Population size fluctuations through time can also lead to a loss in heterozygosity

• Bottleneck - a low point in populations numbers
• Bottlenecks can reduce genetic variation in a generation through genetic drift, even though population numbers are generally high
• Formula for calculating N _e over generations when the population size flucutates during that time

Founder Effect

• if new populations are formed by the migration of just a very few individuals, the population can be said to have gone through a bottleneck at its founding
• founder effect can mean that new populations are different from parent populations through chance alone

Click Here for more on effective pop. size and some problems

Natural Selection

outcome of fitness differences between self-replicating entities

• those more fit (better able to survive and reproduce) will leave more offspring with their enhanced abilities
• as time goes on, more of the population are descended from the more fit individuals

Natural selection can favor a single form at a time

• Peppered Moth melanic forms favored when trees are darkened, light form when trees are lighter
  • selective factor is mortality due to bird predation
  • melanic gene has other effects, but none are strong enough to explain the population changes seen in England
  • in the US, melanic form has declined even though trees are not becoming lichen covered, so NS by bird predation may not work for all cases of Industrial Melanism
• Rise of resistance to herbicides, insecticides, rat poisons, and antibiotics are also examples of natural selection

Natural selection can favor polymorphism (two or more alleles or phenotypes in a population)

• can result in a Balanced Polymorphism if each phenotype has an environment in which it is most fit form
  • Cepaea snail banding varies with the background and can hide the snail from bird predation, so that populations are made up of different forms, each with an environment in which it is the fittest

Natural Selection can enhance, reduce, or enhance variability

• Disruptive (Diversifying)
  • when the extremes are fittest and intermediates are less fit
  • Can split a population into two phenotypes with few intermediate forms
• Stabilizing
  • when the fittest individuals are the average, then those with more extreme (larger and smaller) phenotypes are less fit and NS will act to reduce the number of individuals with extreme phenotypes
• Directional
  • when a new, fitter type originates, the population will move from the older type to the newer type over time

Natural Selection is the consequence of variation in fitness, and if there is none (neutral alleles or phenotypes) it will not have an effect

Speciation

Species are important because they represent largest natural grouping of individuals

• genus, family, ... phylum, kingdom are groups with boundaries set by biologists
• species, population are groups with boundaries that may be set by nature and are only discovered by biologists

Species concepts

Biological

• useful when sexual reproduction occurs
• species are clusters of individuals which may exchange genes in forming the next generation
• species are separated by reproductive barriers that isolate their gene pools
• Isolating Mechanisms
  • Prezygotic
    • Habitat - no cross mating because habitats differ
    • Temporal - no cross mating because time of day or season of reproduction differs
    • Ethological (= study of behavior) - no cross mating because the mating behaviors are incompatible
    • Mechanical - no cross mating because there is a failure in the ability of sperm and egg to come into contact
    • Physiological - chemical means of gamete recognition may fail, male gamete may not survive in female environment (common for pollen from wrong species) or egg and sperm may not chemically recognize one another
  • Postzygotic
    • Hybrid inviability or sterility -

Phylogenetic

• based on common ancestry (relatedness)
• probability of common ancestry measured by the number of shared derived characters between individuals
  • shared means that the characters had the same origin but are found in both individuals
  • derived means that their immediate ancestor had them but not the entire group so that these characters set these individuals off from other, similar individuals
• useful when sexual reproduction does not occur
  • many bacteria
  • many fungi
  • some plants and animals

when phylogenetic concept is applied to sexual species, the two concepts are usually in good agreement

often difficult to define species in practice, no matter which concept is used

• bacteria show significant horizontal transfer of genes between species
• many plants and animals have sub-populations that look different and which can interbreed but, in fact, almost never do
• Hybridization
  • phenomenon that occurs between sexually reproducing species
  • occurs when members of two different gene pools interbreed
  • common in some plants but found in all kingdoms

How do species originate

Allopatric speciation

• formation of species through the differences that arise when populations are physically isolated by distance or by a geographic barrier
• differences must be accompanied by formation of isolation mechanism so that the reproductive barrier is in place when new species come into contact
• foundation of isolated populations and the associated founder effect seen as most common mechanism for origin of species

Sympatric speciation

• new species arises within territory of older species
• controversial because reproductive isolation has to develop without geographical isolation
• Apple-maggot flies may be a case of sympatric speciation occurring

Extinction

There has been an overall increase in species richness in the fossil record

• early increase in total followed by long period of no more change
• may be partly due to better preservation of recent fossils
• Origination of taxa is correlated with extinction, suggesting that new species replace older ones
  • Overall number of species can then only increase when the maximum allowed number increases
  • This may be related to the origin of novel adaptations (new way to feed, new resources exploited, etc.)

Mass Extinctions

• a sudden drop in the number of higher order taxa (usually families and above) in the fossil record
• can be mapped for different groups of organisms
• good agreement between groups on when these events occurred, but not in the severity of the mass extinction
• Fossil record for animals includes evidence for 5 periods of mass extinctions
  • best known for marine molluscs (shells form good fossils) and fish (bones)
  • plants do not show such sharp drops in richness
  • KT boundary extinction is most significant for land vertebrates
  • Permian extinction most significant for all organisms across all the fossil record
  • current extinction rate indicates that we are in the midst of a mass extinction

We can not distinguish the record from random events

• There seems to be no pattern of one group taking over from another
• each lineage seems to go extinct based on an independent probability of extinction
• Problem with the record??
  • older fossils tend to be lost
  • many organisms do not fossilize
  • difficult to define taxa
    • gaps in record exist

Evidence for 500,000,000 to 1,000,000,000 species in all

• 2-30 million extant
• widespread species, generalist species tend to survive longer
• island species more vulnerable (related to island size)
• no evidence that some lineages are less prone to extinction

Book proposes that some factors may affect probability of extinction

• Rarity - rarer species go extinct at a greater rate
• Dispersal ability - more sedentary go extinct at a greater rate
• Specialization - more specialized go extinct at a greater rate
• Population variability (phenotypic) - recall inbreeding - more homozygous go extinct at a greater rate
• Trophic level (animals) - top predators have lower numbers - predators go extinct at a greater rate
• Longevity (individual) this one is shaky - shorter-lived organisms go extinct at a greater rate
• Intrinsic rate of population increase - - slower reproducers go extinct at a greater rate - weeds do better

Causes of extinction

• Meteors/comets??
• Exploitation by humans
• Human alteration of habitat
• Introduction of competitor/predator/pathogenic species

Categories of species threatened by extinction

• Endangered species - species likely to go extinct unless steps taken to preserve it and reverse cause of decline
• Vulnerable - those undergoing numerical decline and likely to become threatened unless steps taken to reverse cause of decline
• Rare - at risk because of low numbers, no evidence of decline
• Indeterminate - those rare species for which there is insufficient evidence to determine if decline is occurring

Taxonomic distribution of threatened species

• knowledge of extinctions related to knowledge of group in general
• By proportion, mammals are most at risk (show many of the risk factors)
• may have caused extinction of one virus (smallpox) but no other pathogen
• we have been unable to cause the extinction of any species we have targeted because they are a pest

Geographic distribution

• evidence that larger countries have more threatened species
• Indonesia and Madagascar are exceptions
• Tropics contain most species and most threatened species
• Islands contain many threatened species
• Countries with dense populations (recently) have many threatened species

Habitat distribution

• Tropical Forests contain most species and most threatened species

Kerr and Currie (1995) Model of loss of birds and mammals uses these factors to predict the proportion of endangered species (out of the total number of bird and mammal species present) within national boundaries:

• Size of population (of humans, not of the mammal species) - more people increase pressure on natural environments
• Per capita Gross National Product - poorer nations are less able to protect natural environments
• Size of animal preserves - obvious factor, more of it means less risk of extinction
• Size of Cropland - more cropland equals greater risk as cropland does not support many species
• Birth Rate of Human Population - this factor interacts with the GNP and population factors to increase risk of endangerment
• Per capita CO₂ discharge from industry - this is an index of industrialization (it isn't CO₂ the discharge itself that increases the risk of endangerment) but more industrialization means greater pollution levels in general

Terms

Acquired character, Point and Frameshift Mutations, Chromosomal mutations, Deletions, Duplications, Inversions, Translocations, Phenotypic, Environmental, and Genetic variation Gene-by-environment interaction, Genetic (allelic, RFLP, RAPD, microsatellite, mtDNA) variation, Hardy-Weinberg, mutation, migration (gene flow), Heterozygosity, Total GV = Within Sub-population GV + Between Sub-population GV, cline, Hardy-Weinberg, Assortative mating (Non-Random Mating), Inbreeding, heterosis, heterozygosity, Mueller's Ratchet, neighborhoods, sex ratio, Effective Population Size, Bottleneck, Founder Effect, natural selection, Balanced Polymorphism, Cepaea, Disruptive (Diversifying), Stabilizing and Directional selection, Speciation, Biological an Phylogenetic Species concepts, Prezygotic and Postzygotic Isolating Mechanisms, (Habitat , Temporal, Ethological, Mechanical, Physiological, Hybrid inviability or sterility), common ancestry, shared derived characters, Hybridization, Allopatric speciation, geographic barrier, Sympatric speciation, Apple-maggot flies, Extinction, Mass Extinctions, Fossil record, KT boundary extinction, Permian extinction, Endangered, Vulnerable, Rare, Indeterminate species

References

Kerr, T. J. and Currie, D. J. 1995. The effects of human activity on global extinction risk. Conservation Biology 9:1528:1538.

 

Last updated August 24, 2001