Organic Evolution Exam 3

Altruism

Behavior by an individual that increases the fitness of another individual while decreasing the fitness of the actor.

Problem of Altruism

Any heritable trait that decreases the fitness of an individual should eventually lead to its own extinction.

Possible Explanations for Altruism

Kin selection, reciprocity, and group selection.

Kin Selection

Decreases in the actor’s reproduction may still lead to more alleles being passed on if others who share those alleles have more offspring.

Reciprocity

Donating resources to other individuals may lead to increases in personal fitness later if the recipient pays even more back to the actor.

Group Selection

Sacrifices for the group can lead to benefits to the actor, if the actor benefits more by success of the group than by personal success.

Hamilton’s Rule

In 1964, WD Hamilton formalized an idea by JBS Haldane with a simple rule that would predict when altruistic behavior would be favored.

Hamilton’s Rule Formula

rB > C

r = relatedness to actor

B = benefit to recipient

C = cost to actor

Why does Hamilton’s rule help solve the problem of altruism?

It suggests that altruistic acts toward a relative might benefit the donor’s genes more than keeping the resources for itself.

Gene-Centered View

Altruistic behavior should be favored when the behavior results in higher numbers of the altruistic alleles of the actor being produced in the next generation.

Relatedness r

The probability that an allele carried by the actor will be shared by the recipient. It can be estimated via Mendelian rules.

Benefit (B)

The average fitness benefit to the recipient of the altruism, measured in offspring equivalents, as a result of the altruistic behavior of the actor.

Cost (C)

The fitness cost to the actor, measured as the expected decrease in offspring equivalents, that the actor will experience as a result of the altruistic behavior.

Eusocial Species

Often have sterile castes.

Haploid Males

In many eusocial bee species, only the females are diploid. This genetic structure changes the nature of relatedness among kin.

Highly Related Sisters

In haplodiploid bee species, sisters share on average 75% of their genes.

Strong Sisterhood

A female worker shares more genes with her sister (75%) than with her own offspring (50%).

Ladies Before Babies

From the perspective of the individual bee’s alleles, their host chooses to raise sisters instead of offspring increases their selection coefficients by 50%.

Reciprocal Altruism

I’ll increase your fitness, if you increase mine. In 1972, Robert Trivers postulated that altruistic behavior could be favored if the behavior was reciprocated in the future.

Evolutionarily Stable Strategy (ESS)

A strategy (or set of strategies) that is impermeable when adopted by a population in adaptation to a specific environment. An EES cannot be displaced by an alternative strategy which may be novel or initially rare.

Elements of Natural Selection

Variation, inheritance, and differential reproductive success.

Selfish Genetic Elements have 3 Mechanisms to Achieve Drive

Interference, overreplication, and gonotaxis.

Interference

Disruption of the transmission of the alternative allele.

Overreplication

Getting themselves replicated more often than the other genes in the genome.

Gonotaxis

Moving preferentially toward the germline, rather than the somatic cells.

Selfish DNA in the Human Genome

40-66% seems to be derived.

LINEs (long interspersed nuclear elements)

Make up 1/5 of the genome. Code for their own reverse transcription and integration proteins, leading to their being copied into other parts of the genome.

SINEs (e.g. ALU)

Shorter elements have common sequences with LINEs and as a result are copied by the LINE machinery.

Selfish Origins of Introns Hypothesis

The original symbiont unleashed a wave of selfish DNA that sometimes landed in the middle of coding sequences.

Barbara McClintock

Discovered transposable elements and suggested that the genome reorganizes itself in order to generate variation in times of stress.

Meiotic Driving Alleles

Can bias allele transmission in their favor so that they are found in more than half of the functional meiotic products generated by a heterozygote.

Killer Meiotic Drive

Occurs when gametes with the killer allele destroy the gametes without it.

Poison-Antidotes

Many meiotic drives have two alleles:

1. Produce poisons to all gametes

2. Produces antidotes that only protect the poison producing cells

Genetic Police Hypothesis

Perhaps crossing over didn’t evolve to increase genetic combinations but was instead a policing mechanism against selfish genes.

Genetic police?

Meiotic drives sometimes act via killer alleles that partner with antidote alleles. Crossing over can separate poison and antidote genes.

Sex-Specific Expression

In mammals, males and females put epigenetic marks on genes, producing sex-specific expression patterns in these genes within the offspring.

Mitochondria and Chloroplasts

Contain DNA and replicate themselves. Can contain DNA that is different from each other and the nuclear genomes of their cells. Inheriting mitochondria only from the mother likely represents a way to limit conflict within cells.

Genetic Differences

Can build up between cells in the same body, leading to conflicts like cancer.

Contagious Cancers

Cell lineages that are transmitted among individuals.

Canine Transmissible Venereal Tumor Disease (CTVT)

Grows on dog genitals and is transmitted during copulation. Seems to have originated 11,000 years ago and shows adaptations to avoid dog immune systems.

1960s Concept of Group Selection

Organisms act virtually always for the good of their species. Only dominant males mate so that populations don’t grow too large and consume all the available resources.

Group Selection

Selection in which traits evolve according to the fitness (survival and reproductive success) of groups. Probably requires group level traits.

Sex Ratio Test

In 1966, GC Williams proposed an empirical test, suggesting that extraordinary sex ratios would be evidence of group-level selections and 1:1 sex ratios would be evidence of the dominance of individual-level selection.

Multilevel Selection Theory

Selection operates at all levels simultaneously with variable relative strengths.

Altruism will be Favored via Group Selection when

1. The force of group selection is stronger than that of individual selection.

2. Altruists become more common in some populations than others

3. The benefit of the recipient is greater than the cost to the actor.

Enforcement Mechanism

Germ cells enforce agreements among somatic cells to cooperate.

Morphological Concept

Delineating species based on physical characteristics. Advantage is that species are sorted into groups that look proper. Drawbacks are continuous variation in many traits, and dimorphisms within species and cryptic species.

Biological Concept

Delineating species based on the ability to interbreed and exchange genes. Advantage is maybe less arbitrary, evolutionarily relevant. Drawbacks are that it cannot apply to extinct or asexual species.

Genetic Concept

Delineating species based on DNA sequence similarity. Advantage is sequences can be easily determined. Drawbacks are DNA similarity is also often continuous.

Phylogenetic Concept

Delineating species based on phylogenetic relationships and distinctive traits. Advantages is that it’s evolutionarily relevant. Drawbacks are the phylogenetic relatedness is often continuous, so it isn’t obvious how close individuals must be considered a single species.

Unified Concept

A species is a distinct evolving lineage, and these lineages can be categorized based on a combination of DNA, ecology, behavior, etc. It sounds nice, but such an approach will certainly require a level of art in addition to science.

Anagenesis

Speciation without splitting. Probably results from additive changes from long bouts of microevolutionary processes. Cut-offs are arbitrary.

Cladogenesis

When an ancestral species splits into two or more descendant species. Change likely still results from micro processes.

Ernst Mayr and Theodosius Dobzhansky

Biologists that recognized that geographic isolation is a key factor in genetic divergence.

Lab Evidence for Allopatry

More than half of laboratory studies on fruit flies show that habitat isolation for a year results in some isolation effects.

Island Biogeography Evidence for Allopatry

Closest living relative species are usually non-overlapping

Gene Flow and Divergence

Lack of gene flow will result in divergence. Increase in gene flow may result in homogenization.

Isolation vs Gene Flow

Selection, mutations, and drift can lead to divergence, but gene flow counteracts this divergence.

Sympatric Speciation

Species diverge while in the same geographic location. Such as behavioral or temporal isolation, must lead to reproductive isolation and then divergence.

Polyploidy (Sympatric Speciation)

A multiplication of chromosome number, usually seen in plants, results in instant reproductive isolation. Is perhaps the most common form.

Disruptive Selection

Two very different soils on the island led to two different fitness peaks.

Temporal Isolation

Eventually flowering time diverges leading to reproductive isolation and further divergence.

Ecological Speciation Evidence from the Lab

In 1973, this study showed that Drosophila subjected to different selection will diverge even when there is a considerable (40%) gene flow.

Speciation due to Sexual Selection

Isolation could in theory be driven by mate choice, if there are two preferences with little overlap. Lack of mating interest in a phenotype could lead to reduced gene flow between the phenotype groups allowing for divergence changes.

Homogenizing Force

Gene flow

Diverging Forces

Natural selection, genetic drift, and sexual selection.

The Great Chain of Being

Was widely accepted up until the time of Darwin. It was a ranking of the closeness to God of the various organisms and objects of Earth. It did not reflect a common ancestry.

A Branching Tree

Jean-Baptiste Lamarck produced in 1809 a tree with branches, seemingly suggesting some form of common ancestry.

Darwin’s Sketch of a Tree

In 1837, Charles Darwin drew his first phylogenetic tree.

Ernst Haeckel

Seems to be the first in 1866 to put groups on universal phylogenetic tree, giving us three major clades: plants, protists, and animals.

Major Homology Evidence Used to Determine Phylogenies

Morphological, developmental, genetic.

Constructing Phylogenetic Trees

1. Select traits upon which to base relationships

2. Establish polarity: need to determine which traits are ancestral and which are derived

3. Choose the most parsimonious solution: try to determine which relationship requires the fewest mutational events

The Genetic Revolution

In 1962, Linus Pauling (1901-1994) and Emile Zuckerkandl (1922-2013) introduced two important ideas: “chemical paleogenetics” and the molecular clock.

Molecular Systematics

Parsimonious trees are built, but instead of morphological data, gene sequences are used. Today, trees are often constructed from complex data sets from sequences of many genes.