CHAPTER 13: THE EVOLUTION OF POPULATIONS

Population

“…evolution can be precisely defined as any change in the frequency of alleles within a gene pool from one generation to the next." - Helena Curtis and N. Sue Barnes, Biology, 5th ed. 1989 Worth Publishers, p.974

Old Theories of Evolution

Jean Baptiste Lamarck

proposed:“The inheritance of acquired

characteristics”

Lamarck was among the first scientists torecognize that living things have changed over time and that all speciesdescended from other species.He also recognized organisms weresomehow adapted to their environments.

Example:A giraffe acquired its long neck

because its ancestor stretched higher and higher into the trees to reach leaves, and that the animal’s increasingly lengthened neck was passed on to its offspring.

Charles Lyell

Published “Principles of Geology” which publication led Darwin to realize that natural forces gradually change Earth’s surface and that the forces of the past are still operating in modern times.

Charles Darwin

Charles Darwin was born on February 12, 1809 in Shrewsbury, England.

From 1831 to 1836 Darwin served as naturalist aboard the H.M.S. Beagle on a British science expedition around the world.

He observed much variation in related or similar species of plants and animals that were geographically isolated from each other.

These observations were the basis for his ideas.

He wrote the book “On the Origin of Species by Means of Natural Selection” in 1859.

The Galapagos Islands

• Darwin was fascinated in particular by the land tortoises and marine iguanas in the Galápagos.

• Giant tortoises varied in predictable ways from one island to another.

• The shape of a tortoise's shell could be used to identify which island a particular tortoise inhabited.

On His Journey Home

Darwin Observed that characteristics of many plants and animals vary greatly among the islands.

Hypothesis: Separate species may have arisen from an original ancestor.

On The Origin of Species by Means of Natural Selection

Stressed two main points:

1. Species were not created in their present form, but evolved from ancestral species.

2. Proposed a mechanism for evolution:

NATURAL SELECTION.

NATURAL SELECTION

A process in which organisms with certain inherited characteristics are more likely to survive and reproduce than are organisms with other characteristics.

Observation 1: Organisms generally have more offspring than can survive to adulthood.

Inference 1: Individuals whose inherited traits give them higher probability of surviving and reproducing in a given environment tend to leave more offspring than other individuals.

“Survival of the Fittest”

Observation 2: Offspring are not identical. There is variation in their appearance, size, and other characteristics.

Inference 2: This unequal production of offspring will cause favorable traits

to accumulate in a population over generations.

Evolution of pesticide resistance in an insect population

3 Key points about Evolution by Natural Selection

1. Although natural selection occurs through interactions between individual organisms and their environment, INDIVIDUALS DO NOT EVOLVE.

Evolution refers to generation-to

generation changes in populations.

2. Natural selection can amplify or diminish only heritable traits.

3. Evolution is not goal directed; it does not lead to perfectly adapted organisms.

A trait may be favorable or useless or even detrimental in different

circumstances.

DESCENT WITH MODIFICATION

An ancestral species could diversify into many descendant species by the accumulation of adaptations to various descendant environments.

ARTIFICIAL SELECTION

The selective breeding of domesticated plants and animals by man to promote the occurrence of desirable traits.

Animal characteristics are changed by artificial means.

Humans are the “environmental pressure” behind artificial selection.

Artificial Selection vs Natural Selection

In Artificial Selection, humans choose the desirable traits and breed only organisms with those traits.

In Natural Selection, the environment does the choosing: individuals with traits best suited to the environment survive and reproduce most successfully, passing those adaptive traits to offspring.

EVIDENCE OF EVOLUTION

1. FOSSIL RECORD

- Strongest evidence

Skull of Homo erectus

Dinosaur Tracks

2. BIOGEOGRAPHY

Biological distribution of species.

Kangaroos in Australia evolved in isolation after geologic activities separated the island continent from

the landmasses on which placental mammals

diversified.

3. TAXONOMY

Classification of species.

4. COMPARATIVE ANATOMY

Homologous Structures

Structures that are similar because of common ancestry.

Analagous Structures

- similar structures in species due to convergent evolution rather than to descent from a common ancestor.

5. COMPARATIVE EMBRYOLOGY

Study of structures that appear during embryonic development.

Similarities in Early Development

6. MOLECULAR BIOLOGY

All forms of life use the same genetic language of DNA and RNA and the genetic code, thus, species descended from one or a few common ancestors that use the genetic code.

POPULATION

A localized group of individuals belonging to the same species.

GENE POOL

• The total collection of genes in a population at any one time.

• The gene pool consists of all alleles in all the individuals making up a population.

• If all members of a population are homozygous for a particular allele, then the allele is fixed in the gene pool.

MICROEVOLUTION

change in allele frequencies over generations.

5 Mechanisms of Microevolution

1. Genetic drift:

Change in the gene pool of a small population due to chance.

Two examples:

a. Bottleneck effect

Genetic drift resulting from a disaster that drastically reduces population size.

E.g. Earthquakes, Volcanic

eruptions

b. Founder effect

Genetic drift resulting from the colonization of a new location by a small number of individuals.

Results in random change of the gene pool.

This may lead to reduced variability when a few individuals from a large population colonize an isolated habitat.

2. Gene Flow:

The gain or loss of alleles from a population by the movement of individuals or gametes.

Example: Migration

3. Mutation

A change in the nucleotide sequence of DNA. Hence, the ultimate source of genetic variation.

Mutations can be transmitted in gametes to offspring, and immediately affect the composition of the gene pool.

The original source of variation.

4. Non-random Mating

The selection of mates other than by chance.

The result of nonrandom mating is that some individuals have more opportunity to mate than others and thus produce more offspring (and more copies of their genes) than others.

5. Natural Selection

“Survival of the Fittest”

FITNESS – the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals. (The fittest are the ones able to produce a large number of viable, fertile offspring and pass the genes to the next generation)

Natural Selection can alter variation in a population in three ways

1. Stabilizing Selection

Acts upon extremes and favors the intermediate.

2. Directional Selection

Eliminates one extreme and moves the population toward the other.

Favors variants of one extreme.

3. Disruptive Selection

Eliminates average phenotypes and encourages the extremes. This tends to result in distinct phenotypes in the same population.

Favors variants of opposite extremes.

HARDY-WEINBERG EQUILIBRIUM

The shuffling of genes that occurs during sexual reproduction by itself cannot change the overall genetic makeup of a population

Calculates Genotype and Allele Frequencies

The gene pool of a non-evolving population remains constant over multiple generations; i.e., the allele frequency does not change over generations of time.

 

The Hardy-Weinberg Equation:

 

1.0 = p2 + 2pq + q2

                                             

where p2 = frequency of AA genotype; 2pq = frequency of Aa plus aA genotype; q2 = frequency of aa genotype

For a population to be in Hardy-Weinberg equilibrium, it must satisfy 5 main conditions:

1. Very large population

2. No gene flow between populations

3. No mutations

4. Random mating

5. No natural selection

SEXUAL SELECTION

A form of natural selection in which individuals with certain characteristics are more likely than other individuals to obtain mates.

May lead to phenotypic differences between males and females.

Sexual Dimorphism

Marked differences between the secondary sex characteristics of males and females.

Examples include colour, size, and the presence or absence of parts of the body used in courtship displays or fights, such as ornamental feathers, horns, antlers, or tusks.

Extreme sexual dimorphism (peacock at right, peahen at left)

2 Forms:

1. Intrasexual Selection

Involves characteristics which affect the outcome of competition among members of one sex for access to members of the other sex.

A contest for access to mates

2. Intersexual Selection

This would influence the evolution of secondary sexual characteristics which determine the relative

"attractiveness" of members of one sex to the other sex. 

“Mate choice”, individuals of one sex are choosy in selecting their mates.

A male grey tree frog calling for mates

What is the advantage of females of being choosy?

Females prefer male traits that are correlated with “good genes.”

Natural Selection does not produce 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.

Prepared by: Maelyn B. RoaReference: BIO: Concepts and Connections, Campbell, Reece, Taylor, Simon, Dickey