Download Basics of Evolutionary Theory

Basics of Evolutionary Theory
ANTH 468 (R. Quinlan)
Terms & Concepts
Adaptation: A heritable trait that increases the probability of survival and/or reproduction in a particular
environment. An adaptive trait for one environment may be maladaptive in another.
Carrying capacity: the maximum number of individuals a particular environment can support.
Coefficient of relationship (r): the proportion of genes (or alleles) that two individuals share by common
descent. Coefficient of relationship is a consequence of meiosis and sexual reproduction. Parents
share 50% of their genes with their offspring (r=.50). Grandparents share 25% of their genes with
their grand-offspring (r=.25). Half sibling share 25% of their genes, on average (r=.25), while full
siblings share 50% of their genes on average (r=.50).
Female choice: a mechanism of sexual selection where females mate with males of their choosing.
Among mammals females are generally the choosier sex because they are physically tied to
reproduction through pregnancy and lactation; males are not.
Fitness: a gene’s genetic representation in future generations. A gene found in 10% of the individuals in
generation “A” and 50% in generation “Z” has higher fitness than a gene found in 10% of
generation “A”, but only 20% in “Z”.
Fixation: under directional selection an advantageous heritable trait will dominate a population; i.e., all
members of the population have the same trait. If there is sufficient interbreeding among the
populations of a species, then the entire species will be monomorphic with respect to the trait (i.e.,
everyone will have it, like we all have five fingers and grasping hands).
Gene: fundamental physical unit of heredity that transmits biological “instructions” from parents to
offspring.
Genotype: underlying genetic make up of an individual organism.
Hamilton’s rule: If Br>C then help your relative; if Br<C, then don’t help. B = fitness benefit to your
relative, r =coefficient of relationship to your relative, and C = fitness cost to you of helping your
relative.
Heritable trait: a trait passed on from parent to offspring through genes.
Heritable variation: variation in heritable traits within a population.
Heritability: proportion of phenotypic variation (see phenotype) in a population due to underlying
genotypic variation (see genotype).
Historical constraints: constraints on evolutionary possibilities. Complex adaptations must evolve
through a series of steps. Each step modifies the previous step.
Kin selection: a process by which behavioral traits evolve that benefit closely related kin. These
behavioral traits involve helping behavior among relatives. The strength of kin selection and the
degree to which related individuals should be willing to help each other depends on a kind of costbenefit analysis. If the benefits of help given to a relative, times the coefficient of relationship
between you and the relative exceed your costs of helping, then you should help your relative: Or
if Br>C then help, else don’t help (B = benefit to your relative, r=coefficient of relationship,
C=costs to you). Costs and benefits are ultimately measured in terms of fitness.
Level of selection: the level at which genes are functionally organized to enhance their ability to replicate
and survive from one generation to the next. Usually, the individual is the strongest level of
selection because at that level genes are forced to cooperate for survival and reproduction of
“their” individual or person. Kin groups may also be a level of strong selection.
Male-male competition: a mechanism of sexual selection where males compete physically (and/or
psychologically) for access to mates, which results in sexual dimorphism.
Monogamy: a mating system where one male and one female have an exclusive mating relationship.
Often involves a long-term bond between the pair.
Natural selection: process of evolution through differential survival and reproduction of individuals with
different heritable traits. Evolution by natural selection occurs through the survival and
reproduction of individuals with heritable traits that are best suited to their particular environment.
Norm of reaction: differences in the phenotypic (see phenotype) expression of a particular genotype
occurring in different environments. For example, a genotype that results in normal body weight
in an environment where food is scarce may contribute to obesity in an environment where food is
normally available. Another genotype may contribute to emaciation under food scarcity and
normal weight with a high calorie diet. The two genotypes have different norms of reaction.
Phenotype: observable characteristics of an individual organism.
Polygyny: when one male has multiple mates.
Population: all the individuals of a species inhabiting a particular locality or region in which they find
mates. If two groups of individuals belong to one species and live in the same region, but do not
interbreed (because of geographical or behavioral barriers), then they comprise separate
populations.
Selfish gene theory: a metaphor for the “interests” of an individual gene and for the functional
relationships among genes. Genes cooperate with each other to the extent that cooperation
increases their individual chances of “immortality.” Genes cooperate to create functional
individuals because it helps them to replicate.
Sexual dimorphism: differences in the average phenotype of males and females of a particular species.
Sexual selection: a process by which sex differences evolve usually through male-male competition or
female choice.
Testes determining factor: a segment on the Y-chromosome that causes the zygote to develop as a male.
Unit of selection: the gene, the smallest biological unit that may increase or decrease in frequency over
multiple generations.
Darwin's fundamental idea: Natural Selection
Darwin made a series of observations:
1. All species have high potential fertility.
2. Resources (food, nesting spots etc.) are limited.
3. More individuals are born than resources can support, resulting in competition for survival and
reproduction. (Competition does not necessarily mean fighting.)
4. There is great phenotypic variation among the individuals within a population.
5. A lot of variation is heritable.
6. Survival and reproduction often depends on the heritable traits of individuals.
These observations led Darwin to a conclusion:
Heritable traits that increase the probability that an individual survives and/or reproduces tend to
become more common in a population. Heritable traits that decrease the probability that an individual
survives and/or reproduces tend to become less common in a population. Darwin called this process
of differential survival and/or reproduction “natural selection.” Over many generations, he
concluded, natural selection can lead to gradual change in the population and the origin of new
species.
ANTH 468, R. Quinlan
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Visualizing Natural Selection
Figure: Visualizing Natural Selection. Imagine a population of a bird species that has heritable variation
for size and shape of beaks. One heritable trait is a heavy-duty beak. Another heritable trait is a small
beak. These birds eat seeds. Normally there are a lot of different kinds of seeds in the environment. Birds
with small beaks are particularly efficient at eating very small soft-shell seeds, but large hard seeds are
difficult for them to eat. Birds with heavy-duty beaks are particularly efficient at eating large hard-shell
seeds, but small seeds are difficult for them. In the first generation the population is evenly divided
among small beak and heavy-duty beak individuals. Imagine that a disease kills most of the plants that
produce small soft seeds. Because of the environmental change, small beak birds are only able to find
enough food to support a single chick. Heavy-duty beak birds have no trouble finding food and they’re
able to support two chicks. Over several generations there are many more heavy-duty beak birds than
small beak birds in the population.
ANTH 468, R. Quinlan
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Evolution of Psychological Mechanisms
Psychological mechanisms can evolve in ways very similar to other complex adaptations. Psychological
mechanisms are specialized neuropsychological processes designed by natural selection to solve certain
recurrent problems of survival and reproduction present during a species’ evolutionary history. For
example, wolves (and other wild dogs) have specialized mechanisms designed to sense and track
prey. For wolves, these hunting psychological mechanisms focus their attention on cues (usually smells)
in their environment that indicate the presence and direction of game. Wolves also have a set of
psychological mechanisms that help them regulate social behavior that involves dominance relations and
coordinated hunting. These psychological mechanisms for social behavior are just as important for wolf
survival and reproduction as are mechanisms for sensing prey.
Like wolves, humans have a set of psychological mechanisms for complex social behavior. During the
course of human evolution our ancestors faced environmental challenges that required behavioral
solutions. “Variants” of mental abilities that generated better solutions to environmental challenges had
an advantage over other variants that generated inferior solutions. These variants were subject to natural
selection if they were heritable. Heritable variants that conferred selective advantage over a long period
of time become “fixed” in a population (see fixation above). Today we recognize that humans have
psychological mechanisms for many “domains” of thought and behavior. Some domains include the
following:
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Language
Mating behavior
Parenting behavior
Reciprocal altruism & social exchange
Risk assessment
Nepotism
ANTH 468, R. Quinlan
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