Evolution And Natural Selection

Chapter 13

Evolution

• Evolution is a change in the frequency of genetically determined characteristics within a population over time.

Evolution???

Evolution

• Microevolution occurs when there are minor differences in allele frequency between populations of the same species. This results in genetic differences between subspecies.

• Macroevolution occurs when there are major differences that have occurred over long periods of time that have resulted in so much genetic change that new kinds of species are produced.

Microevolution

Macroevolution

History of Evolutionary Thought

• French naturalist Georges-Louis Buffon wondered if animals underwent change (evolved) over time. He observed changes of animals in the fossil record.

• In 1809, Buffon’s student, Jean-Baptiste de LaMarck, proposed that acquired characteristics were transmitted to offspring.

Georges-Louis Buffon

Jean-Baptiste de LaMarck

History of Evolutionary Thought

• Acquired characteristics are traits gained during an organism’s life and not determined genetically.

• Lamarck’s theory has since been proven wrong because we know that acquired characteristics are not inherited; however, it stimulated evolutionary thought.

Theory of Natural Selection

• In 1858, Charles Darwin and Alfred Wallace suggested the theory of natural selection as a mechanism for evolution.

Charles Darwin

Alfred Wallace

Theory of Natural Selection

• The theory of natural selection is the idea that some individuals whose genetic combinations favor life in their surroundings are more likely to survive, reproduce, and pass on their genes to the next generation than are individuals who have unfavorable genetic combinations.

Natural Selection

Assumptions Necessary for the Theory of Natural Selection

• 1. All organisms produce more offspring than can survive.

• 2. No two organisms are exactly alike.• 3. Among organisms, there is a constant

struggle for survival.

Assumptions Necessary for the Theory of Natural Selection

• 4. Individuals that possess favorable characteristics for their environment have a higher rate of survival and produce more offspring.

• 5. Favorable characteristics become more common in the species, and unfavorable characteristics are lost.

Applying Gregor Mendel’s Genetics Concepts To These Assumptions

• 1. An organisms ability to overproduce results in surplus organisms.

• 2. Mutation occurs and new genetically determined traits enter the gene pool. Sexual reproduction allows for genetically unique offspring. Meiosis and fertilization allow for new genetic combinations in every generation. The genetic information is expressed in the phenotype of the organism.

Applying Gregor Mendel’s Genetics Concepts To These Assumptions

• 3. Resources such as food, nutrients, water, mates, and nest materials are in short supply, so some individuals do without. Environmental factors such as disease organisms, predators, and helpful partnerships with other species affect survival. All the environmental factors that affect survival are called selecting agents.

Applying Gregor Mendel’s Genetics Concepts To These Assumptions

• 4. Selecting agents favor the individuals that are most likely to survive and reproduce, thereby passing their genes on to the next generation. An organism is selected against if it has fewer offspring than other individuals with a more favorable combination of alleles. The organism does not have to die to be selected against.

Applying Gregor Mendel’s Genetics Concepts To These Assumptions

• 5. Alleles or allele combinations that produce characteristics favorable to survival become more common in the population and, on the average, the members of the species will be better adapted to the environment.

Gregor Mendel

Evolution

• Evolution results when there are changes in allele frequency in a population.

• Individual organisms cannot evolve, only populations can.

• The mechanisms that bring evolution about operate at the level of the individual.

Natural Selection in Evolution

• Natural selection brings about evolution by selecting which organisms will survive, reproduce, and pass their genes on to the next generation.

• These processes affect the phenotype displayed and not the genes directly.

Factors That Affect How A Species Changes Over Time

• Environmental Factors• Sexual reproduction• The amount of genetic diversity

Environment

• The reproductive success of an individual within a population is determined by how well an individual’s characteristics match the demands of the environment in which it lives.

Fitness

• Fitness is the success of an organism in passing on its genes to the next generation, compared with other members of its population.

Genetic Diversity

• A large gene pool with great genetic diversity is more likely to contain genetic combinations to allow some individuals to adapt to a changing environment.

• Structural, behavioral, biochemical, and metabolic characteristics all determine how well an organism can adapt to its environment.

Common Misconceptions About The Theory Of Evolution

• Evolution happened only in the past and is not occurring today.

• Evolution has a predetermined goal, or “it was meant to be.”

• Changes in the environment cause the mutations that are needed to survive under the new environmental conditions.

Common Misconceptions About The Theory Of Evolution

• Individual organisms evolve.• Many of the current species can be shown to

be derived from other present-day species (e.g. apes gave rise to humans).

• Alleles that are valuable to an organisms survival become dominant.

Common Misunderstandings About Natural Selection

• 1. “Survival of the fittest.” While individual survival is important for reproduction, the more important factor is the number of descendants an organism leaves.

• 2. “Struggle for life”. This struggle does not necessarily refer to open conflict and fighting.

Common Misunderstandings About Natural Selection

• 3. Phenotypic characteristics that are acquired during life, but not genetically determined. Acquired characteristics may be important to an individual’s success; however, they are not genetically determined and therefore cannot be passed on to future generations.

Common Misunderstandings About Natural Selection

• 4. Relationship between the mechanism of natural selection (death, reproductive success, mate choice) and the outcomes of the selection process. While the effects of natural selection appear at the population level, the actual selecting events take place at the level of the individual organism.

Factors That Influence Natural Selection

• Genetic Diversity within a species.• The degree of genetic expression.• The ability of most species to produce excess

offspring.

Mechanisms That Affect Genetic Diversity

• Mutation and Migration• Sexual Reproduction and Genetic

Recombination

Genetic Diversity in Humans

Genetic Diversity in Maize

Spontaneous Mutations

• Spontaneous Mutations are changes in DNA that cannot be tied to a particular factor.

• Mutations may alter specific genes.• Cosmic radiation or naturally occurring

mutagenic chemicals might be the cause of these mutations.

Spontaneous Mutations

• Mutation rates in nature are low (1 in 100,000).

• Most mutations are harmful; however, over millions of individuals with thousand of genes, over thousands of generations, beneficial mutations can occur.

• For mutations to be important, they must be in cells that give rise to the gametes.

Migration

• Migration is another way in which new genetic material can enter a population.

• When individuals migrate from one population to another, they bring alleles that may be rare or absent.

• When they leave, they can remove alleles from the population.

Migration

Migration

Sexual Reproduction

• Sexual reproduction creates new genetic combinations in individuals.

• Sexual reproduction does not generate new genetic information, but it does allow for the mixing of genes into new combinations.

Genetic Recombination

• Genetic recombination is the mixing of genes that occurs when the genes from the male are intermingled with those from the female.

• Genetic recombination can sometimes occur between organisms of a different species (hybridization). This often results in sterile offspring.

Cama Hybrid

Liger Hybrid

Zeedonk Hybrid

The Role of Gene Expression

• Degrees of expression.• Why some genes may avoid natural selection.• Natural selection works on the total

phenotype.

Degrees of Gene Expression

• For genes to be selected for or against, they must be expressed in the phenotype of the individual expressing them.

• Penetrance is a term used to describe how often an allele expresses itself.

• Expressivity is a term used to describe situations in which the allele is penetrant, but not expressed equally in an individuals.

Polydactyly

Why Some Genes May Avoid Natural Selection.

• Some genetic characteristics can be expressed only during specific periods in the life of the organism.

• The organism may die before the characteristic is expressed. Consequently, the characteristic does not contribute to the overall fitness of an organism.

Why Some Genes May Avoid Natural Selection.

• Some genes require environmental triggers before being expressed.

• Recessive alleles must be present in the homozygous condition before they can be expressed.

Natural Selection Works On The Total Phenotype.

• “Good” genes can be passed on in combination with “bad” genes.

The Importance of Excess Reproduction

• Successful organisms reproduce at rates in excess of that necessary just to replace the parents when they die.

• High death rates tend to offset high birth rates and the population size remains relatively stable.

The Processes That Drive Selection

• Differential Survival• Differential Reproductive Rates• Differential Mate Choice – Sexual Selection

Differential Survival

• If a population consists of a large number of individuals that are genetically and phenotypically different, some of them will possess characteristics that make their survival difficult.

• Some individuals will have characteristics that make their survival more likely in a given environment.

Differential Reproductive Rates

• Two organisms may have similar survival rates and very different reproductive rates.

• The organism with the greater reproductive rate will be selected for and more of its genes will be in the subsequent population.

Differential Mate Choice – Sexual Selection

• Sexual selection occurs within animal populations when some individuals are chosen as mates more frequently than others.

Differential Mate Choice – Sexual Selection

• Those that are more frequently chosen have more opportunities to pass on more copies of their genetic information.

• The chosen characteristics could include body size, aggressiveness, or specific conspicuous characteristics attractive to the opposite sex.

Sexual Selection – Elephant Seals Fighting

Sexual Selection

Sexual Selection

Patterns of Selection

• Stabilizing Selection• Directional Selection• Disruptive Selection

Stabilizing Selection

• Stabilizing selection occurs when individuals at the extremes of the range of characteristic are consistently selected against.

Directional Selection

• Directional selection occurs when individuals at one extreme of the range of characteristics are consistently selected for.

• A consistent change in the environment can call for this kind of selection.

• Changes in allele frequency result from this.

Disruptive Selection

• Disruptive selection occurs when both extremes of a range for a characteristic are selected for and the intermediate condition is selected against.

• This kind of selection occurs when there are sharp differences in the nature of the environment where the organisms live.

The Hardy-Weinberg Concept

• English mathematician G.H. Hardy and German physician Wilhelm Weinberg developed and applied a simple mathematical relationship to study gene frequencies.

The Hardy-Weinberg Concept

• Constant gene frequencies over several generations indicated that evolution is not taking place at that time.

• Changing gene frequencies indicate that evolutionary changes were happening.

Necessary Conditions For Gene Frequencies To Remain Constant• 1. Mating must be completely random.• 2. Mutations must not occur.• 3. The migration of individual organisms into

and out of the population must not occur.• 4. The population must be very large.• 5. All genes must have an equal chance of

being passed on to the next generation. (Natural selection is not occurring.)

Why Hardy-Weinberg Conditions Rarely Exist

• Random mating does not occur.• Spontaneous mutations occur.• Immigration and emigration of individual

organisms are common.• Populations are not infinitely large.• Genes are not all equally likely to be passed

on to the next generation.

The Next Phase???