The Evolution of Populations

Chapter 23

“Nothing in Biology makes sense except in the light of evolution” T. Dobzhansky

Population Genetics

   What was missing from Darwin’s explanation of natural selection? 

A way to explain how chance variations can show up in a population, while also accounting for precise transmission from parent to offspring…

Population Genetics:  Emphasizes the

variation within populations the importance of quantitative characters

and

recognizes those characteristics that vary along a continuum

Modern synthesis:   ties in ideas from paleontology, taxonomy, biogeography, and population genetics Scientists contributing: Dobzhansky, Wright, Mayr, Simpson, Stebbins (page 446)

Quantitative Characters and Discrete Characters



Quantitative Characters:

 traits that vary along a continuum in a population (like plant height) 

Discrete Characters:

 traits that can be classified on an either-or basis (like flower color)

Genes & Variation

   While developing his theory of evolution, Darwin worked under a serious disadvantage – he did not know how heredity worked 1.

2.

Without understanding heredity, Darwin was unable to explain 2 important factors: The source of variation central to his theory How hereditable traits were passed from one generation to the next Today, genetics, molecular biology, and evolutionary theory work together to explain how inheritable variation appears and how natural selection operates on that variation (i.e. how evolution takes place)

Gene Pools

   A

gene pool

is the combined genetic information of all the members of a particular population  Recall that a

population

is a collection of individuals of the same species in a given area which share a common group of genes A gene pool typically contains 2 or more alleles forms of certain genes) (or  Example: mouse populations may have 2 or more alleles for fur color – the gene pool for the trait fur color is the combination of all the alleles in the population The

relative frequency

of an allele is the number of times that allele occurs in a gene pool compared to the number of times other alleles occur

Relative Frequency of Alleles

Sources of Genetic Variation



The two main sources of genetic variation are

mutations

and the genetic shuffling that results from

sexual reproduction

  A mutation is any change in a sequence of DNA  Mutations that affect an organisms phenotype can lead to an increase in fitness for that organism Most inheritable differences are the result of gene shuffling that occurs during sexual reproduction  Example: the 23 pairs of chromosomes found in humans can produce 8.4 million different combinations of genes

Mutations



Mutation is the ultimate source of new alleles within a population



Mutation is also a source of evolution

A A A A A A A A T = 0 A a A A A a a A a A T = 1

Sexual Recombination

  Most of the genetic variation in a population results from the unique combination of alleles that each individual receives.

Three mechanisms contribute to the shuffling of alleles during sexual reproduction:  Crossing over  Independent assortment of alleles  Fertilization

Population Genetics & Hardy Weinberg

 Before we consider the mechanisms that cause a population to evolve, it will be helpful to examine, for comparison, the gene pool of a NONEVOLVING population.

  Such a gene pool is described by the

Hardy-Weinberg theorum

.

The theorum states that the frequencies of alleles and genotypes in a population’s gene pool REMAIN CONSTANT over generations unless acted upon by mechanisms other than Mendelian segregation and recombination of alleles.

 Put another way –

the shuffling of alleles due to meiosis and random fertilization has no effect on the overall gene pool of a population

.

Population Genetics

 

Hardy-Weinberg Principle –

states that allele frequencies tend to remain constant in populations unless something happens OTHER THAN Mendelian segregation and sexual recombination.

 This situation in which allele frequencies remain constant is called

genetic equilibrium

 If allele frequencies do not change, the population will not evolve Hardy-Weinberg is a mathematical model that describes the changes in allele frequencies in a population  Allows us to predict allele and genotype frequencies in subsequent generations (testable)

Hardy-Weinberg Principle

   1.

2.

3.

4.

5.

Model assumptions (conditions required to maintain genetic equilibrium from generation to generation): random mating population Large population size – n > 100 No emigration or immigration (no movement into or out of the population) No mutations No natural selection (all genotypes have an equal chance of survival and reproduction)

If all 5 conditions are met, there should be NO EVOLUTION – no selection, no gene flow, no genetic drift, no mutation Describes a NON-EVOLVING POPULATION

Hardy-Weinburg Principle

 The Hardy-Weinburg Principle is neat because it can serve as a null hypothesis for evolution  It can show that evolution IS OCCURING within a population

Hardy-Weinburg Principle

    

Let p= frequency of allele A Let q= frequency of allele a Let p 2 = frequency of genotype AA Let 2pq= frequency of genotype Aa Let q 2 = frequency of genotype aa

 Law says, given assumptions, that within 1 generation of random mating, the genotype frequencies are found to be in the binomial distribution p 2 +2pq+q 2 =1 (genotype frequencies) and p+q=1 (allele frequencies)

Hardy-Weinberg Example

 The allele for the ability to roll one’s tongue is dominant (R) over the allele for the lack of this ability (r).  In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous?

How Hardy-Weinberg Works

       The equation: p 2 + 2pq + q 2 = 1 Therefore, p + q = 1 500 organisms, 25% are rr; thus q 2 = .25

so 125 organisms are rr

If q 2 = .25, then q=.5

Thus, p + .5 = 1, leaves p = .5

So, p 2 = .25,

so 125 organisms are RR

2pq leaves the heterozygotes, so 2(.5)(.5) = .5 or 50%,

so 250 organisms are Rr

Causes of Microevolution

 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population and cause MOST evolutionary changes.



Microevolution:



generation to generation change in a population’s allele frequencies

 Three main causes:

1.

2.

natural selection genetic drift 3.

gene flow

Natural Selection on Polygenic Traits

  

Natural selection

: differential survival and reproduction among members of a population Natural selection is NOT random – it leads to

adaptive evolution

– evolution that results in a better match between organisms and their environment.

Can affect the distribution of genotypes in any of three ways:

1.

2.

3.

Stabilizing selection Directional selection Disruptive selection

Genetic Drift

 Natural selection is not the only force that can lead to evolution:  In addition to natural selection,

genetic drift

is a way by which allele frequencies can change  In the real world, population sizes fluctuate   Because populations fluctuate in size, sometimes there can be changes in allele frequencies due to random chance These changes are called random

genetic drift

Genetic Drift

 In small populations, individuals that carry a particular allele may leave more descendants than other individuals, just by change  Over time, a series of chance occurrences of this type can cause an allele to become common in a population

The Power of Genetic Drift

 Genetic drift is a powerful force when a population size is very small  Can and does lead to allele fixation 

Allele fixation

means that a population changes (evolves) from many alleles represented to only 1 allele represented  Depends on starting frequency (which allele becomes fixed)

Consequences of Genetic Drift

 Consequences of genetic drift:  Can and does lead to fixation of alleles  Effect of chance is different from population to population  Small populations are effected by genetic drift more often than larger ones  Given enough time, even in large populations genetic drift can have an effect  Genetic drift reduces variability in populations by reducing heterozygosity 

REAL WORLD EXAMPLES OF GENETIC DRIFT: 1.

2.

The Bottleneck Effect The Founder Effect

Real World Examples of Genetic Drift



The

Bottleneck Effect

   Occurs when only a few individuals survive a random event, resulting in a shift in allele frequencies within the population Small population sizes facilitate inbreeding and genetic drift, both of which decrease genetic variation

Reduces genetically variability because at least some alleles are likely to be lost from gene pool

Figure 23.5 The bottleneck effect: an analogy

Real World Examples of Genetic Drift



The Founder Effect

 Occurs when individuals from a source population move to a new area and start a new population  This new population is often started by relatively few individuals that do not represent the population well in terms of all alleles being represented

The Founder Effect

http://bcs.whfreeman.com/thelifewire/content/chp24/2402002.html

 What determines which variants survive the event or get to the new location?

 Random chance  Genetic drift has the larges effect on small populations (10-100 individuals)

The Founder Effect

Sample of Original Population Founding Population A Descendants Founding Population B

The Founder Effect

Sample of Original Population Founding Population A Descendants Founding Population B

The Founder Effect

Sample of Original Population Founding Population A Descendants Founding Population B

Gene Flow

    Gene flow can also change allele frequencies

Gene flow is the physical flow of alleles into or out of a population.

 

Immigration Emigration

– alleles coming in (added) – alleles moving out (lost) Gene flow counteracts differences that arise through mutation, natural selection, and genetic drift.

Gene flow helps keep separated populations genetically similar – reduces differences between populations

The Power of Natural Selection

 Natural selection is the ONLY mechanism that consistently causes

adaptive evolution

.

  Evolution by natural selection is a blend of chance and “sorting” – chance is the creation of new genetic variations and sorting as natural selection favors some alleles over others.

 Because of this sorting effect leads to adaptive evolution.

– ONLY natural selection consistently increases the frequencies of alleles that provide reproductive advantage and thus

Relative fitness

is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals.

 Relative fitness conferred by a particular allele depends on the entire genetic and environmental context in which it is expressed.

Three Modes of Selection

 The effect of selection on a varying characteristic can be:    Stabilizing Directional Disruptive (Diversifying)  This effect on allele frequency depends on which phenotypes in a varying population are being favored.

Stabilizing Selection

 selection is against phenotype with arrows  selection is against both extreme phenotypes  intermediate survives and reproduces at a higher rate than others  phenotypic extremes are eliminated, variance has decreased  population has stabilized around mean  But…remember that mutation and gene flow can increase variance by counteracting selection

Stabilizing Selection

 When individuals near the center of the curve have higher fitness than individuals at either end of the curve  Intermediate forms of a trait are favored and alleles that specify extreme forms are eliminated from a population 

Counteracts the effects of mutation, gene flow, and genetic drift – preserves the most common phenotypes.

Section 16-2

Stabilizing Selection

Directional Selection

 selection against 1 extreme in favor of the other extreme  after time we see a shift in the direction of the population toward 1 of the 2 homozygous extremes  Variation is reduced here and alleles can be lost from the population

Directional Selection

Directional Selection

Disruptive/Diversifying Selection

 selection is against phenotype with arrows  selection is against intermediate phenotype in favor of BOTH extremes  number of intermediates after a few generations is low, but variation is maintained here  in the real world, this can lead to speciation  if this occurs long enough and there is barrier to gene flow, speciation can occur

Disruptive or Diversifying Selection

 

When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle.

Forms at both ends of the range of variation are favored and intermediate forms are selected against –

selection creates two, distinct phenotypes

 Ex. Bird beak size – no middle sized seeds, only large seeds and small seeds; thus, small and large beaks are favored

Disruptive Selection

Selection Graphs

Figure 23.12 Modes of selection http://bcs.whfreeman.com/thelifewire/content/chp23/2302001.html

Key Role of Natural Selection in Adaptive Evolution

 Natural selection increases the frequencies of alleles that enhance survival and reproduction, thus improving the match between organisms and their environment.

 The physical and biological components of an organism’s environment may change over time.

 As a result, what constitutes a “good match” between an organism and its environment can be a moving target – making adaptive evolution a continuous, dynamic process!

Sexual Selection

   Sexual selection may lead to pronounced secondary differences between the sexes: Sexual selection is a form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates  Maintained by natural selection May lead to pronounced differences between sexes –

sexual dimorphism



a marked difference between the two sexes in secondary sexual characteristics

Figure 23.16x1 Sexual selection and the evolution of male appearance

Types of Sexual Selection

 

Intrasexual Selection

means selction within the same sex – typically males   Individuals of one sex compete directly for mates of the opposite sex Often it is based on rituals and displays that don’t risk injury

Intersexual Selection

is also called “mate choice” – typically females   Female choice is typically based on showiness of the male’s appearance and/or behavior Males will often weight the attraction of predators versus the attraction of mates

The Preservation of Genetic Variation

 Tendency for directional and stabilizing selection to reduce variation is countered by mechanisms that preserve or restore it:    Diploidy Balanced Polymorphism Neutral Variations

Diploidy

 Diploidy refers to organisms carrying genes in pairs:  Recessive traits can be preserved in heterozygotes – this maintains a large pool of genes that may not be useful today, but could be in the future.

Balanced Polymorphism

 Balancing selection maintains two or more forms in a population:  Heterozygous Advantage: sometimes a heterozygote has an advantage to homozygotes and survives  Frequency Dependent Selection: the fitness of a phenotype declines if it becomes too common in a population

Figure 23.0 Shells

Figure 23x2 Polymorphism

Neutral Variation

 Changes in the DNA (typically non-coding) that provide no selective advantage or disadvantage.

 However, these variations MAY influence survival and reproduction in ways that are difficult to measure.

  The variation may also be neutral in ONE environment but beneficial in ANOTHER environment.

The point is that this variation is an enormous reservoir of raw material for natural selection!

Figure 23.7 A nonheritable difference within a population

Genetic Variation is the raw material for natural selection Genetic variation occurs within and between populations many are at molecular level and cannot be seen…not all are heritable – some are environmentally induced (Ex. Map butterflies – figure 23.7)

Measuring Genetic Variation

  Population Geneticists use whole gene measurements and molecular measurements – gene diversity and nucleotide diversity  Ex. Fruit flies – gene diversity using

loci

, nucleotide diversity using

DNA fingerprinting Note: humans have little genetic variation compared to other species – same nucleotide sequence at 999 out of every 1000 nucleotide sites in your DNA

Geographic Variation

 Differences in gene pools between populations or subgroups of populations  Due to fact that at least SOME environmental factors are likely to differ from one place to another; thus, natural selection can contribute to this….

 Ex. In population, one type of geographic variation is a

Cline -

graded change in trait along a geographic axis

Figure 23.8 Clinal variation in a plant

What Can’t Natural Selection Do?



NATURAL SELECTION CANNOT FASHION PERFECT ORGANISMS!!!

   

Limited by historical constraints

Ex. Body structure for erect posture

Adaptations are often compromises

Ex. Seal on land vs. water

Not all evolution is adaptive

Ex. Storm blows ALL orgs to new place, not just best suited

Selection can only edit existing variations

See page 461