Natural Selection affects populations in a variety of ways Stabilizing selection Directional selection Disruptive selection Sexual selection

Natural Selection affects populations in a variety of ways

Stabilizing selection

Directional selection

Disruptive selection

Sexual selection

Originalpopulation

Evolvedpopulation

Pressure ofnatural selection

Stabilizing selection Directional selection Disruptive selection

Natural Selection acts on phenotypes present

Original population

Originalpopulation

Fre

qu

ency

of

ind

ivid

ual

sEvolvedpopulation

Phenotypes (fur color)

Stabilizing selection Directional selection Disruptive selection

Sexual Selection

• Differential mating of males

• Competition among males for dominance and mating privileges with the group.

• Female choice

Sources of Variation

Mutation

Sexual Reproduction

Diplody- 2 copies of each chromosome allows for recessive alleles that maintain variation in a gene pool.

Polyplody- even more variation

Organisms typically show individual variation.

However, in The Origin of Species, Darwin could not explain

– the cause of variation among individuals or

– how variations were passed from parents to offspring.

We now know that mutations are

– changes in the nucleotide sequence of DNA and

– the ultimate source of new alleles and new traits.

Mutation and sexual reproduction produce the genetic variation that makes

evolution possible

© 2012 Pearson Education, Inc.

A gene pool is the total collection of genes in a population at any one time.

Microevolution is a change in the relative frequencies of alleles in a gene pool over time. A change within a population.

Population genetics studies how populations change genetically over time

Evolution occurs within populations


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

To understand how microevolution works:

– Consider an imaginary population of iguanas with individuals that differ in foot webbing.

– Let’s assume that foot webbing is controlled by a single gene and the allele for non-webbed feet (W) is completely dominant to the allele for webbed feet (w).


The shuffling of alleles that accompanies sexual reproduction does not alter the genetic makeup of the population.

– No matter how many times alleles are segregated into different gametes, and united in different combinations by fertilization, the frequency of each allele in the gene pool will remain constant unless other factors are operating.

– This equilibrium is the Hardy-Weinberg principle, named for the two scientists who derived it independently in 1908.


To test the Hardy-Weinberg principle, let’s look at two generations:

Figure 13.10B shows the frequencies of alleles in the gene pool of the original population.

From these genotype frequencies,

we can calculate the frequency

of each allele in the population.


If a population is in Hardy-Weinberg equilibrium, allele and genotype frequencies will remain constant, generation after generation.

The Hardy-Weinberg principle tells us that something other than the reshuffling processes of sexual reproduction is required to change allele frequencies in a population.


For a population to be in Hardy-Weinberg equilibrium, it must satisfy five main conditions. There must be

1. a very large population,

2. no gene flow between populations,

3. no mutations,

4. random mating, and

5. no natural selection.

Rarely are all five conditions met!!

CONNECTION: The Hardy-Weinberg equation is useful in public health science

Public health scientists use the Hardy-Weinberg equation to estimate how many people carry alleles for certain inherited diseases.

One out of 10,000 babies born in the United States has phenylketonuria (PKU), an inherited inability to break down the amino acid phenylalanine.

The health problems associated with PKU can be prevented by strict adherence to a diet that limits the intake of phenylalanine.

CONNECTION: The Hardy-Weinberg equation is useful in public health science

PKU is a recessive allele.

The frequency of the recessive allele for PKU in the population, q, equals the square root of 0.0001, or 0.01.

– The frequency of the dominant allele would equal 1 – q, or 0.99.

– The frequency of carriers = 2pq = 2 0.99 0.01 = 0.0198 = 1.98% of the U.S. population.

– Thus, the equation tells us that about 2% (actually 1.98%) of the U.S. population are carriers of the PKU allele.


The Hardy-Weinberg principle states that

– within a sexually reproducing, diploid population,

– allele and genotype frequencies will remain in equilibrium,

– unless outside forces act to change those frequencies.

The Hardy- Weinburg equation can measure microevolution.


Stable Gene PoolThe frequency of an allele will not change IF:

1. There is no mutation.

2. The population is large.3. The population is isolated. (no immigration or emigration)

4. Mating is random.5. All individuals survive and produce the same number of offspring. (no natural selection)

This is known as Hardy-Weinberg Equilibrium.

Hardy-Weinberg Equation

• Measures changes in allele frequency

• Uses a formula to track changes from equilibrium.

p2 (AA) + 2pq(Aa)+ q2 (aa) = 1.0 (100%, whole

population)

What factors will change allele frequencies?

1. Mutation.

2. Small population

3. Emigration and immigration

4. Mate selection

5. Natural Selection

Alleles are changing = Species are evolving!

• Genetic Drift

• Genetic Bottleneck

• Founder Effect

Small populations have BIG genetic problems…..

…reduced genetic variation, allele frequency changes

Originalpopulation

Bottleneckingevent

Survivingpopulation

Natural selection, genetic drift, and gene flow can cause

microevolution

The origin of species is the source of biological diversity

Microevolution is the change in the gene pool of a population from one generation to the next.

Macroevolution is the rising of new species from existing species.

Speciation is the process by which one species splits into two or more species.

– Every time speciation occurs, the diversity of life increases.

– The many millions of species on Earth have all arisen from an ancestral life form that lived around 3.5 billion years ago.


MICROEVOLUTION v. MACROEVOLUTION

The origin of species is the source of biological diversity

Over the course of 3.5 billion years,

– an ancestral species first gave rise to two or more different species,

– which then branched to new lineages,

– which branched again,

– until we arrive at the millions of species that live, or once lived, on Earth.

Adaptive Radiation-one species give rise to other species

Why? … To better fit their particular environment,Often accompanied by reproductive isolation,

Often in response to competition

There are several ways to define a species

The morphological species concept

The ecological species concept

The phylogenetic species concept

There are several ways to define a species

The word species is from the Latin for “kind” or “appearance.”

Although the basic idea of species as distinct life-forms seems intuitive, devising a more formal definition is not easy and raises questions.

In many cases, the differences between two species are obvious. In other cases, the differences between two species are not so obvious.

Reproductive barriers keep species separate

Reproductive barriers

– serve to isolate the gene pools of species and

– prevent interbreeding.

Depending on whether they function before or after zygotes form, reproductive barriers are categorized as

– prezygotic or

– postzygotic.


Five types of prezygotic barriers prevent mating or fertilization between species.

1. Habitat isolation,

2. Temporal isolation

3. Behavioral isolation

4. Mechanical isolation.

5. Gametic isolation




Habitat isolation(lack of opportunities to encounter each other)

The garter snake Thamnophisatratus lives mainly in water.

The garter snake Thamnophis sirtalis lives on land.


Temporal isolation(breeding at different times or seasons)

The eastern spotted skunk(Spilogale putorius) breeds inlate winter. The western spotted skunk

(Spilogale gracilis) breeds inthe fall.


Behavioral Isolation

Blue-Footed Boobies Courtship Ritualhttps://www.youtube.com/watch?v=z922by9_6Fw

https://www.youtube.com/watch?v=wCzZj21Gs4U

The Dance of the Male Bowerbird


Mechanical Isolation

• Mechanical isolation in plants- two sage species, the black sage and white sage. Even though they grow in the same area, they are pollinated by different insects.

• Mechanical isolation in animals- incompatible reproductive parts/genitalia


Gametic Isolation

Three types of postzygotic barriers operate after hybrid zygotes have formed.

1. In reduced hybrid viability, most hybrid offspring do not survive.

2. In reduced hybrid fertility, hybrid offspring are vigorous but sterile.

3. In hybrid breakdown,

– the first-generation hybrids are viable and fertile but

– the offspring of the hybrids are feeble or sterile.




Habitat isolation(different habitats)

Temporal isolation(breeding at different times)

Behavioral isolation(different courtship rituals)

Mechanical isolation(incompatible reproductive parts)

Gametic isolation(incompatible gametes)

Reduced hybrid vitality(short-lived hybrids)

Reduced hybrid fertility(sterile hybrids)

Hybrid breakdown(fertile hybrids with

sterile offspring)

PREZYGOTIC BARRIERS

POSTZYGOTIC BARRIERS

Zygote

Gametes Prezygotic barriers Postzygotic barriersViable,fertile

offspring• Habitat isolation• Temporal isolation• Behavioral isolation• Mechanical isolation• Gametic isolation

• Reduced hybrid viability• Reduced hybrid fertility• Hybrid breakdown

Gene flow is interrupted when a population is divided into geographically isolated subpopulations.

Allopatric speciation takes place due to geographical isolation

Sympatric speciation takes place without geographic isolation

Sympatric speciation occurs when a new species arises within the same geographic area as its parent species.

Gene flow between populations may be reduced by

– polyploidy,

– habitat differentiation

– sexual selection.

Patterns of Evolution

Divergent Evolution: two or more species that become different over time. (HOW?)

Convergent Evolution: unrelated species with similar traits

Parallel Evolution:

– Marsupital and placental mammals

Coevolution:

– Predator and prey

– Plants and pollinators

The Darwin Orchid- green tube that drops down to the ground in the picture is where the moth would stick its proboscis to get to the nectar.

Speciation can occur rapidly or slowly

There are two models for the tempo of speciation.

1. The punctuated equilibria model draws on the fossil record, where species

– change most as they arise from an ancestral species and then

– experience relatively little change for the rest of their existence.

2. Other species appear to have evolved more gradually.


Also known as phyletic gradualism

The more recently two species have branched from a common ancestor, the more similar their DNA sequences should be.

The longer two species have been on separate evolutionary paths, the more their DNA should have diverged.

Phylogenetic trees and cladograms illustrate evolutionary relationships.

An organism’s evolutionary history is documented in its genome


1. Explain how the organisms Darwin saw while on his voyage on the Beagle influenced his thinking.

2. Explain how the work of Thomas Malthus and Charles Lyell influenced Darwin.

3. Explain why individuals cannot evolve and why evolution does not lead to perfectly adapted organisms.

4. Explain how the fossil record, comparative morphology (anatomy), comparative embryology and comparative biochemistry(molecular biology) support evolution.

At the end of this unit-You should now be able to

5. Explain what an evolutionary trees illustrates.

6. Define the gene pool, population genetics, & microevolution, macroevolution.

7. Explain why mutation is required for the emergence of new traits in a population.

8. Describe the five conditions required for the Hardy-Weinberg equilibrium (no change/evolution in a species).

9. Explain the purpose of the Hardy-Weinberg principles.

You should now be able to

10.Define genetic drift, the bottleneck effect and the founder effect.

11.Describe adaptive radiation.

12.Describe 5 types of prezygotic barriers & 3 types of postzygotic barriers to species interbreeding.

13.Compare the gradual model and the punctuated equilibrium model of evolution.

14.Compare and contrast phylogenetic trees & cladograms.

You should now be able to

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Natural Selection affects populations in a variety of ways Stabilizing selection Directional selection Disruptive selection Sexual selection