BIOL 4120 Principles of Ecology Phil Ganter 320 Harned Hall 963-5782
A field of both wind- and insect-pollinated plants

Lecture 2 Evolution and Ecology

Email me

Back to:

Overview - Link to Course Objectives

• Defining evolution and its relationship to ecology
  • Importance
  • Definition
  • Adaptive Evolution (Darwinism)
• How Does Evolution Happen?
  • Heritable Genetic Variation
  • Hardy-Weinberg
    • Mutation
    • Migration
    • Non-random Mating
    • Genetic Drift
    • Adaptive Evolution
• Species and Speciation
  • Species Concepts
  • Speciation
  • Geographic Variation
• Effective Population Size

Evolutionary Ecology

Relationship between ecology and evolution

• Ecology is the study of the abundance and distribution of organisms. 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 factors
  • The evolutionary history of a species may mean that not all areas that could support a population of a particular species actually have a population 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 will 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 through Acquired Characters because 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 variants that were more successful in terms of survivorship and reproduction (two ecological measures) were those that gained greater representation in subsequent generations (Adaptive Evolution)

What is biological evolution?

Heritable change in a population over time

A population is a group of organisms belonging to the same species that live together in the same place at the same time.  A species is made up of one or more populations scattered over a larger area (sometimes over more than one continent).  Most species have many populations.  What happens in one population may or may not affect what happens in other populations (depending on whether or not migration takes place between the populations).

The population is the smallest scale for evolution and evolution is an emergent property of populations.  This scale is often referred to as microevolution  Evolution does not occur on the scale of individual organisms.

Individuals live, give birth, and die, but their evolutionary success can only be seen over a time longer than their lifespans and by comparing their success with that of other members of the population.  Living a year and giving birth to three offspring is relatively successful if the average life span is 10 months with two offspring but relatively unsuccessful if the average is two years with six offspring.  In the first case, the individual is favored by evolution but the same individual is not favored in the second scenario.  Evolution is a property of populations

Evolution also occurs at larger scales (species, new types of species) and over longer periods of time than the lifespan of a population.  Evolution on large scales is often called macroevolution.

We will not much concern ourselves with macroevolution but will concentrate on microevolution.  The exception will be when we consider the ecology of speciation.

 

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 in the context of their habitats.

• 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

How Does Evolution Occur

• Heritable Variation is an essential prerequisite for 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 variants 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
• Sexual Recombination produces different combinations of gene variants (alleles)

How is genetic variation inherited?

• Genetic information is passed from parent to offspring as the sequence of base pairs in DNA molecules
  • Mendel first to propose particulate inheritance with rules for gene expression
  • non-Mendelian inheritance also occurs
    • maternal inheritance
    • horizontal gene transfers

Are all differences among individuals in a population heritable genetic variation?

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

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

How much variation exists and how do we measure it?

• Phenotypic -
  • morphological
  • protein polymorphism
• Genetic -
  • chromosomal
  • allelic
  • RFLP
  • RAPD
  • microsatellite
  • SNP (single nucleotide polymorphism)
  • Sequences of
    • Nuclear DNA
    • mtDNA, cpDNA

• Hardy-Weinberg Predictions and Genetic Variation

  What should we expect to happen over time when variation exists in a population?

  Hardy-Weinberg expectations are predictions of future population variation when that variation is not altered by ecological or statistical processes

  H-W Assumptions - Hardy-Weinberg predicts no change but is only accurate if its 5 assumptions are met.  Below we list the assumptions and discuss what happens when the assumption is violated.

  • No mutation,

    Mutations generate differences between generations

  • No migration,

    If populations differ in their genetic composition (maybe A is 90% of the genes at a locus in one population and only 10% in another population), migration between the populations can change their genetic composition

  • Random Mating,

    Assortative mating (also called Non-Random Mating)

    • Positive Assortative Mating - if like mates with like (due to choice or to small population sized not allowing much choice) then intermediates and heterozygotes are lost - a decrease in genetic variation
    • Negative Assortative Mating - like mating with unlike will increase the proportion of heterozygous intervals and preserve genetic variation

    Inbreeding

    • has the same effect as positive assortative mating - loss of genetic 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

  • Large Populations,

    Genetic Drift

    • loss of genetic variation due to chance events
    • more likely in small populations than in large
      • Neighborhoods can enhance the effect of drift
      • if populations are subdivided into small neighborhoods, then drift will be more important for the entire population
    • Bottleneck - a sudden low point in populations numbers, followed by expansion of the population
    • Bottlenecks can reduce genetic variation in a generation through genetic drift, even though population numbers are generally high
    • If you come along when the population has recovered its large size, you would think that genetic drift was not important in that population, but a recent bottleneck event might have greatly reduced genetic variation in your study population.

    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

  • No Selection

    Natural selection is the outcome of fitness differences between individuals

    • Natural selection requires that there is heritable genetic variation in a population
    • if some of those genetic variants are more fit (better able to survive and reproduce) than others, the fit genetic variants will leave more offspring that also have their "fit" genotypes
    • as time goes on, more of the population are descended from the more fit individuals
      • An example - 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
      • Prevalence of resistance to herbicides, insecticides, rat poisons, and antibiotics are also examples of natural selection

    Natural Selection can enhance, reduce, or maintain variability

    • Natural selection can, under the right conditions, 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's (a large land snail) shell banding varies with the background and can hide the snail from bird predation
      • populations are made up of different forms, each form with an environment in which it is the fittest
      • natural selection favors more than one phenotype within a single population here
    • Natural selection can have different effects on a population, which we have divided into three "modes of selection."
      • 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 produces Adaptations

    • Adaptations are those characteristics of organisms that allow one organisms to be more fit than another
    • Populations adapt to environments as natural selection increases the proportion of individuals that have the most fit adaptation
    • All three modes of selection (disruptive, directional, and stabilizing) will produce adaptation (in the case of disruptive, more than one adaptation).

Species

We mentioned above that species occur as local populations

Species are important because they represent natural grouping of individuals

• Most taxonomic groups (genus, family, up to phylum and kingdom) are groups with boundaries set by biologists and are considered "artificial."
• Species and population are groups with boundaries that are set by nature and are only discovered by biologists, so these groups are seen as "natural."

Species concepts or is there one definition of "species" in biology?

Biological SC

• useful when sexual reproduction occurs
• biological 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
• Reproductive Isolating Mechanisms are ways in which organisms avoid mating with members of other, similar species
  • most involve failure to form a zygote, but avoiding mating or by some inability for egg and sperm to recognize one another but some mechanisms involve the failure of the zygote to develop into a mature adult organism capable of reproduction (maturity in biology means sexual maturity)

Phylogenetic SC

• 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
• Hybridization is a means of sympatric speciation (although the hybrid's parent species ranges may only abut one another, rather than overlap)
  • Best known from plants
  • Genomics is demonstrating that hybridization may be important in general.
    • Ancient hybridization events have been found at the base of many new lineages, so this process may be important in their origin

Geographic Variation

We have already described the local nature of species, that species are collections of local populations sometimes linked by migration.  This structure means that populations far from other populations or separated from them by a geographical barrier (mountain chain, large river or lake, etc.) might become more and more different as time passes.  These differences result in Geographic Variation

• these differences can be so great that members of one population do not recognize members from a distant population as the same species, although members of intermediate populations see both extremes as the same species.
• Geographic variation means that species ecological needs and impacts may be different in different places.  This can complicate ecological studies and make it difficult to understand local interactions based on information gathered in distant populations

Effective Population Size

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) is a corrected population size. Methods of calculating the loss of genetic variation over time due to drift or inbreeding depend on a population that has a 1:1 sex ratio and which does not vary through time (there are other assumptions, but we will not deal with them here) so when you have a population that has a skewed sex ratio or varies in size through time, you must correct the actual population size for these departures from assumptions. Calculation of N_e reduces the actual population size to a size that will undergo drift or inbreeding loss at the same rate as the actual population but the N_e size conforms to the assumptions of the models for calculating drift.
  • 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 N_e (=85) is lower for the second population than the first (Ne = 100), the rate of loss of genetic variation from drift or inbreeding is greater in the second population than in the first, even though there are actually more individuals in it! If the sex ratio was 1:1 in both populations, the second would be more affected by drift
• Formula for correcting population size when the number and sex of the breeding population are known:

• N_e is the effective population size, 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 fluctuates during that time (t = number of generations, N_i = size of the population at generation i).

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

Terms

Acquired Characters, Adaptive Evolution, Biological Evolution, Population, Microevolution , Macroevolution, Natural Selection, Heritable Genetic Variation, Point mutations, Frameshift Mutations, Chromosomal Rearrangements, Phenotypic Variation, Environmental Variation, Genetic Variation, Interaction Variation. Random Mating. Positive & Negative Assortative mating, Inbreeding, heterosis, Genetic Drift, Bottleneck, Founder Effect, Natural selection, Polymorphism, Balanced Polymorphism, Disruptive (Diversifying) Selection, Stabilizing Selection, Directional Selection, Adaptation, Species, Species concepts, Biological SC, Reproductive Isolating Mechanisms, Phylogenetic SC, Shared Derived Characters, Hybridization, Allopatric speciation, Sympatric speciation, Geographic Variation, Effective Population Size

Last updated January 20, 2007