© 2014 Pearson Education, Inc.. Ecology and Evolution Outcomes 1.Apply the Hardy-Weinberg principle...


Ecology and Evolution

Outcomes

1. Apply the Hardy-Weinberg principle to test for changes in allele frequency.

2. Apply logical deduction using the conditions of the H-W principle to determine the mechanism of evolutio n.

3. Distinguish between adaptive and non-adaptive mechanisms of evolution.



Why is evolution important to the discipline of ecology?



The oldfield mouse (Peromyscus polionotus) is widely distributed in the southeastern U.S. It is preyed upon by a variety of visually hunting predators such as hawks and owls.

The mouse displays considerable variation in coat color both within and between populations across its range.



Figure 3.5 Coat color variation in mice. Two color variants of Peromyscus polionotus: (A) the darker inland form, and (B) the lighter beach-dwelling form.



Figure 3.6 Variation in coat color and genotypes at the Mc1R locus. Peromyscus polionotus exhibits extensive coat color variation across localities in Florida. Red areas indicate the distribution of beach populations; gray areas denote the distribution of inland populations. Characteristic phenotypes for each population are indicated by the coat coloration sketches, but coat color varies within populations as well. Adapted from Hoekstra et al. (2006).



Most populations of the mouse are dark colored, but populations on beaches and barrier islands have lighter colored coats.

Does coat color affect the survival and ultimately reproduction (i.e. fitness) of oldfield mice?

Two experiments suggest it does.



Kaufman (1974) carried out an experiment in which pairs of mice (one dark-coated, one light coated) along with an owl were placed in large cages located in habitats with different backgrounds (light or dark and with different vegetation densities).



In all cases mice that better matched the background survived better than mice that matched less well.



Kaufman et al. carried out a follow-up experiment in which they made silicone mouse models painted light or dark to mimic either the dark or light background.

They placed the models in different habitats and measured from beak and claw marks how often the models were attacked. They found clear differences in attack rates. Models that matched their background were attacked much less.



Figure 3.10 Predation, coat color, and fitness in the oldfield mouse, using plastic models in the field. Hoekstra and colleagues placed light and dark silcone mouse models in light and dark environments to test predation rates. (A) The experimental sites: a light beach environment and a dark inland environment. (B) Proportion of attacks against light and dark mice in the light environment. (C) Proportion of attacks against light and dark mice in the dark environment. Adapted from Vignieri et al. (2010).


Observations made by Charles Darwin that were the building blocks of his theory on Evolution by Natural Selection

1) Individuals of a population are different from one another. Variation among individuals arises through: ►Mutation ►Sexual Reproduction (genetic recombination) 2) Many of these behavior, morphological or physiological differences,

which we term traits, are inherited from the parents. 3) All plants and animals produce more offspring than are needed to simple

replace the parents. 4) Among the different individuals in a population, some are better suited for

survival than others. 5) Different ancestors leave different numbers of descendants.



Charles Darwin's Theory of Evolution by Natural Selection

Natural Selection is the differential success in reproduction, and its product is adaptation of organisms to their environment.

What this means: Organisms that possess the characteristics (traits) that are most helpful in a given environment are more likely to survive and increase their numbers.



Why check for Hardy-Weinberg?


The Hardy-Weinberg equation can be used to test whether a population is evolving

The Hardy-Weinberg Equilibrium Principle has two fundamental conclusions:

1. The allele frequencies in a population will not change, generation after generation.

2. If the allele frequencies in a population are given by p and q, then the genotype frequencies will be given by p2, 2pq q2.


CRCR

CRCW

CWCW

Activity - The Hardy-Weinberg Equilibrium Principle

Consider a population of 500 flower plants.

320

160

20


Conditions for Hardy-Weinberg Equilibrium

The Hardy-Weinberg theorem describes a hypothetical population that is not evolving.

It is a model which has assumptions.

1. No mutations

2. Random mating

3. No natural selection

4. Extremely large population size

5. No gene flow

If the population is not in H-W equilibrium, then one of the assumptions has been violated.


Non-random mating alters genotype frequency, not allele frequency


Non-random mating alters genotype frequency, not allele frequency

Second assumption of H-W is violated:

2. If the allele frequencies in a population are given by p and q, then the genotype frequencies will be given by p2, 2pq q2.

Allele frequencies of A1 = 0.5 ,A2 = 0.5 at the start and at generation 3.

But! Genotype frequencies have changed.


Natural selection, genetic drift, and gene flow can alter allele frequencies in a population

Zimmer and Emlen 2013


Three major factors alter allele frequencies and bring about most evolutionary change Natural selection

Genetic drift

Gene flow Why not mutation?

Although mutation can change allele frequencies, even a beneficial mutation will not increase in frequency without a selection pressure.

Natural selection, genetic drift, and gene flow can alter allele frequencies in a population


Violation of Hardy-Weinberg - Mutation

Mutation is not a strong force for evolution.

∆32 is a 32 base pair deletion in the CCR5 gene. The gene encodes a cell surface protein called CCR-5.

The 32 base pair deletion confers resistance to it carriers from HIV.

But most carriers do not live in areas with high incidence of HIV.


Violation of Hardy-Weinberg - Mutation

But if an allele mutates from one form (A1) to another form (A2) at a constant rate and is not eliminated, it can change the allele frequency of a population – over a very long period of time.

Bergstrom and Dugatkin 2012


Violation of Hardy-Weinberg - Genetic Drift

The smaller a sample, the more likely it is that chance alone will cause deviation from a predicted result.

Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next.

Genetic drift tends to reduce genetic variation through losses of alleles, especially in small populations.


Violation of Hardy-Weinberg - Genetic Drift

Ten simulations of random genetic drift using 3 different population sizes.

The random loss of one allele in small populations leads to fixation of the other allele.

Large populations do not exhibit dramatic effects of drift because it is unlikely that all of one allele would be lost due to chance.


The Founder Effect

The founder effect occurs when a few individuals become isolated from a larger population.

Allele frequencies in the small founder population can be different from those in the larger parent population due to chance.

Zimmer & Emlen 2013


Genetic Drift- The Bottleneck Effect

The bottleneck effect can result from a drastic reduction in population size due to a sudden environmental change or other indiscriminate reduction in population size.

By chance, the resulting gene pool may no longer be reflective of the original population’s gene pool.

If the population remains small, it may be further affected by genetic drift.


Case Study: Impact of Genetic Drift on the Greater Prairie Chicken

Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in Illinois.

The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched.

Prairie chicken video. http://www.youtube.com/watch?v=s2_wdMmEupQ

http://www.youtube.com/watch?v=s2_wdMmEupQ

http://www.youtube.com/watch?v=s2_wdMmEupQ


Figure 21.11

Pre-bottleneck(Illinois, 1820)

Post-bottleneck(Illinois, 1993)

Rangeof greaterprairiechicken

Illinois 1930–1960s

1993

Greater prairie chicken

Kansas, 1998 (no bottleneck)

Nebraska, 1998 (no bottleneck)

1,000–25,000

50

75,000–200,000

5.2

3.7

Location Populationsize

750,000

Numberof allelesper locus

Percentageof eggshatched

93

50

5.8

5.8

99

96

http://www.youtube.com/watch?v=s2_wdMmEupQ#t=0




Effects of Genetic Drift: A Summary

1. Genetic drift is significant in small populations.

2. Genetic drift can cause allele frequencies to change at random.

3. Genetic drift can lead to a loss of genetic variation within populations.

4. Genetic drift can cause harmful alleles to become fixed.


Violation of Hardy-Weinberg - Gene Flow

Gene flow consists of the movement of alleles among populations.

Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen).

Gene flow tends to reduce genetic variation among populations over time

Freeman & Herron 2007


Gene flow can decrease the fitness of a population Consider, for example, the great tit (Parus major) on the Dutch

island of Vlieland

Violation of Hardy-Weinberg - Gene Flow

Immigration of birds from the mainland introduces alleles that decrease fitness in island populations.

Natural selection reduces the frequency of these alleles in the eastern population where immigration from the mainland is low.

In the central population, high immigration from the mainland overwhelms the effects of selection.

13% first time breeders43% first time breeders


Directional, Disruptive, and Stabilizing Selection

There are three modes of natural selection Directional selection favors individuals at one end

of the phenotypic range

Disruptive selection favors individuals at both extremes of the phenotypic range

Stabilizing selection favors intermediate variants and acts against extreme phenotypes


Figure 21.13

Originalpopulation

Evolvedpopulation

Originalpopulation

Fre

qu

ency

of

ind

ivid

ual

sPhenotypes (fur color)

(a) Directional selection (b) Disruptive selection (c) Stabilizing selection


Directional Selection


Disruptive Selection


Stabilizing Selection


Overview: The Smallest Unit of Evolution

• Question: Do organisms evolve during their lifetimes?

Answer: No• Question: What is the smallest unit of evolution?

Answer: The population

Natural selection acts on individuals, but only populations evolve.


• Evolution by natural selection involves both chance and “sorting”– New genetic variations arise by chance– Beneficial alleles are “sorted” and favored by natural

selection• Only natural selection consistently results in

adaptive evolution, an increase in the frequency of alleles that improve fitness

Natural selection is the only mechanism that consistently causes adaptive evolution


Relative Fitness

The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals.

Reproductive success is generally more subtle and depends on many factors.

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

Selection indirectly favors certain genotypes by acting directly on phenotypes.


There is a temptation to regard the match between organism and environment as an example of evolved perfection.

But, there is nothing in the process of evolution by natural selection that implies perfection.

Natural selection can only favor those that are fittest from among the range of genetic variants available. But, no population contains the full range of genetic variation that might exist, so the range of genetic variants might be limited.

Organisms come to match their environments by being ‘the fittest available’ or ‘the fittest yet’; they are not ‘the best imaginable’.

Natural Selection does not predict perfection


A proximate explanation is the immediate cause of a phenomenon.

It explains how a trait operates or is regulated.

An ultimate explanation provides more general reasons, involving evolutionary arguments and adaptations. Ultimate explanations are remote in time and space.

It explains why a trait exists.

Proximate and Ultimate explanations


Why Natural Selection Cannot Fashion Perfect Organisms

1. Selection can act only on existing variations.

2. Evolution is limited by historical constraints.

3. Adaptations are often compromises.

4. Chance, natural selection, and the environment interact.

Natural selection operates on a “better than” basis.


Why Natural Selection Cannot Fashion Perfect Organisms – Historical Factors

Organisms live where they do for reasons that are often, at least in part, accidents of history.

Moreover, they have lived in environments that were quite different from the present: some of the features that they acquired in other environments now hang like millstones around their necks.

These characters that they have acquired limit and constrain where they can now live and what they do.

These characters can also constrain how these organisms respond to additional changes in their environment.


Why Natural Selection Cannot Fashion Perfect Organisms – Historical Factors


Activity - Hardy-Weinberg

(B. Kettlewell, 1959)

Peppered moth, Biston betularia

© 2014 Pearson Education, Inc.. Ecology and Evolution Outcomes 1.Apply the Hardy-Weinberg principle...

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