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Agents of Evolutionary Change. The Five Forces of Evolution and How We Measure Them. I. What is Evolution? . First, remember that… Evolution is a two-stage process: Production and redistribution of variation (inherited difference between individuals).

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Agents of Evolutionary Change

The Five Forces of Evolution and How We Measure Them


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I. What is Evolution?

  • First, remember that…

    Evolution is a two-stage process:

    • Production and redistribution of variation (inherited difference between individuals).

    • Natural selection (whereby genetic differences in some individuals lead to their higher reproductive success.


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I. What is Evolution?

  • A change in the frequency of alleles in a population from one generation to the next.

    • An allele frequency is the percentage of all the alleles at a specific locus accounted for by one specific allele.

    • For evolution to occur, a new allele must spread through the population and change in frequency.

    • Still, the frequencies of all alleles ill add to 1.0 (100 %)

    • Evolution is driven by five known factors or agents.


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II. Agents of Evolutionary Change

  • Mutation

  • Gene Flow

  • Nonrandom (Assortative) Mating

  • Genetic Drift

  • Selection


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II A. Mutation

  • Mutation is a molecular alteration in genetic material:

  • For a mutation to have evolutionary significance it must occur in a gamete (sex cell).

  • Such mutations will be carried on one of the individual's chromosomes.

  • During meiosis the chromosome carrying the mutation will assort giving a 50 percent chance of passing the allele to an offspring.


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II A. Mutation, continued

  • Mutation is the only way new variations can be produced.

  • Since mutation occurs so infrequently at any particular locus, it would rarely have an effect on allele frequencies.

  • Most mutations are "hidden" as recessive alleles.

    example: About 1 in in 12,000 babies carry the homozygous form of the allele for PKU (phenylketonuria). Without early detection and treatment, this disorder will lead to mental retardation.


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II B. Gene Flow

  • Gene flow is the exchange of alleles between populations. Also called migration.

  • Long-term patterns of mate selection can cause significant changes in gene frequency.

    e.g. African Americans are largely of West African descent, but there has been considerable influx of alleles from non-African populations.


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II C. Assortative Mating

  • Purposeful selection of mates based on the presence or absence of certain traits in potential mating partners.

  • Also called non-random mating or sexual selection

  • Undesirable traits will diminish over time as the organisms having those traits are less reproductively successful.

  • One example of this in history was choice of antler rack size among Red Deer in the British isles.


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II D. Genetic Drift

  • Random fluctuation in allele frequencies

  • Can change in either direction.

  • Associated with small populations

  • Two variants.

    • Founder Effect occurs when a small migrant band of "founders" colonizes a new area away from the parent group.

    • Bottleneck Effect occurs when circumstances dictate that only a select number of individuals will survive to reproduce.


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Tristan de Cunha: Example of Founder Effect

  • An Island in the South Atlantic was settled in early 1800s by one Scottish family of about 15 people.

  • Population was added to by shipwrecks and births

  • The 1987 population numbered 296 people with 7 family names, and mtDNA studies have shown that they are descended from five female founders.

  • The population has a high frequency of a hereditary eye disorder (retinitis pigmentosa). Four islanders have the disease and nine are carriers at last count.


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Northern Elephant Seal: Example of Bottleneck

  • Hunted down to 20 individuals in 1890’s

  • Population has recovered to over 30,000

  • No genetic diversity at 20 loci.

  • Concern about resistance to pathogens


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II E. Natural Selection

  • Differential reproductive success in a population due to heritable variants among the population

  • Malaria is found in areas where the sickle-cell allele is found and it was discovered that heterozygous carriers of sickle-cell anemia are more resistant to malarial infection than the homozygous "normals".

  • Natural selection did not cause the sickle cell allele to appear; it merely acted to increase its frequency in some populations.


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III. Evolution in Population Genetics

  • Some things to remember:

    • Allele frequencies are the proportions of alleles in a population.

    • Each population has a particular complement of genes, a level of variability, called its gene pool.

    • A change in variability in the gene or allele pool across generations is thus microevolution

    • As a species changes over time, macroevolution (speciation) may occur


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III. Evolution in Population Genetics

  • Physical anthropologists a formula called the Hardy Weinberg Equilibrium to determine whether allele frequencies in an isolated human population are changing.

  • The Hardy Weinberg is good for two things:

    • Estimating allele frequencies for a population in which actual counts are impractical

    • Acting as a model against which to test assumptions about the evolutionary status of a known population.

  • The formula is: p2+2pq+q2=1.0


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III. Evolution in Population Genetics

  • For the Hardy Weinberg to be true…

    NONE OF THE FOLLOWING CAN OCCUR:

    • New variation (mutation).

    • Redistributed variation through gene flow

    • Redistributed variation through genetic drift

    • Select "advantageous" allele combinations that promote reproductive success (natural selection)

    • Assortative mating


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I. Types of Human Variation

  • Humans vary in their:

    • Physiology

    • Body proportions

    • Skin color


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I A. Physiologic Variation

  • Lactose Tolerance

    • Ability to produce enzyme lactase throughout life

    • Found in populations with a pastoral heritage

  • Vasoconstriction and Vasodilation

    • Ability to control restriction of blood vessels

    • Found in populations in extreme environments


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I B. Body Proportions

  • Bergmann’s Rule

    • Climate controls body mass to surface ratio

  • Allens’ Rule

  • Climate controls limb lengths


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I C. Skin Color

Factors affecting skin color

  • Sunlight

    Vitamin D needs, cancer, and folate

  • Nutrition

    Excess dietary pigment can change color

  • Environment

    Short term responses (I.e. tanning)

  • Genes

    What your parents pre-determined about you


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II. Historical Views of Human Variation

  • Biological determinism

    cultural variations are inherited in the same manner as biological variations

  • Eugenics

    Galton’s "race improvement" through forced sterilization & encouraged reproduction

  • Mono and Polygenism

    One or many origins for humanity (A&E?)


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III. The Concept of Race

  • All Modern Humans Belong to the Same polytypic species, Homo sapiens.

  • A polytypic species consists of local populations that differ from one another in the expression of one or more traits.


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III. The Concept of Race, cont.

  • Variation (race) is geographic in nature

  • Variation can be plotted on clines


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IV. The Traditional Race Concept

  • Since the 1600s race has been used to refer to various culturally defined groups.

  • While race is usually used as a biological term, it is also one with enormous social significance.

  • In the 1950's the use of the term "race" was replaced with the term "ethnicity "


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V. Race and Intelligence

  • Both genetic and environmental factors contribute to intelligence.

  • Many psychologists say that IQ scores measure life experience.

  • Innate differences in abilities reflect individual variation within populations, not inherent differences between groups.

  • There is no convincing evidence that populations vary with regard to cognitive abilities.


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VI. Adaptive Significance of Variation

  • Human variation is the result of past and present adaptations to environmental conditions.

  • Physiological response to environmental change is under genetic control and environmental influence and operates at two levels:

    • Long-term (i.e. genetic) evolutionary changes characterize all individuals within a population or species.

    • Short-term physiological response to environmental change is called acclimatization; such physiological change is temporary.


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