bio lecture 8: Population structure, genes + phenotypes

How populations can be differentiated from one another

1. Genetically different populations

   • Gene flow

   • Genetic drift

   • Geographical variation

2. Phenotypically different populations

   • Locally adapted traits

   • Phenotypic plasticity

   • Transplant & genome studies

migration vs gene flow: can one cause the other?

Migration: movement of individuals from one population to another

Gene flow: the movement of alleles from one population to another

• Migration doesn't always lead to gene flow and vice versa (like the org might die before it reproduces)

interpret this

Population A vs Population B. they live in slightly different environments

The effects of selection and drift on one side vs gene flow on the other side:

• B/c the environments are different, natural selection is going to lead the populations to becoming more different.

• Genetic drift is also a DIVERGENT FORCE like natural selection

• Have random subsampling of diversity in each of the populations, the subsampling is going to be different and end up with genetic differences within the pop

How do populations become more divergent from one another? (gene flow vs. genetic drift)

How to measure gene flow:

1. Gene flow experiment: manipulative experiment in nature, create 2 populations and study the gene flow between them (pretty rare though)

2. NEUTRAL genetic markers (genes): to look for signatures of allele sharing between populations

   • IMPORTANT to use neutral genes and not genes under selection b/c hard to identify what is due to natural selection and what Is due to gene flow

   • Ex: can make 2 pops full of 2 diff homozygotes (FF and SS) and over time see if any heterozygotes are made (FS)

stochastic vs deterministic

Random (stochastic): unpredictable evolutionary forces

• Mutation, recombination, genetic drift

Non-random (deterministic): predictable evolutionary force

• Natural selection

Genetic drift (stochastic)

• Stochastic changes in allele frequency due to random variation in fecundity and mortality

  1. By Bottleneck effect: sudden reduction in the number of alleles (followed by rebound)

  2. By founder effect: colonization that a few individuals that start a new population somewhere else

• Affects small pops more

interpret this graph. what does it represent

Allele frequency is 50% initially. As u randomly put gametes into the next gen, the allele frequency is going to go up and down stochastically. As time goes on the frequencies fluctuate and more variation in the outcome as time passes.

 

In the pop with just 9 individuals, the stochastic sub-sampling is going to lead to a much bigger frequency change.

1. The fluctuations are more dramatic in allele frequencies over time in small pops

2. In small pops, more likely BY CHANCE to have some alleles go to fixation (one will go to 100%, the other to 0%) so more rapid loss of genetic diversity

3. Less consistency across replicate populations

interpret this

• Every color represents a diff allele

• Starting with a very LARGE diverse pop and continues to grow bigger after a few gens

• 2 smaller pops go colonize islands and just by chance there is less diversity

• This middle pop has increased its pop size and gotten thru the bottleneck

• In the smaller one, all genetic diversity will be lost because of persistent bottleneck

interpret this. what does it say abt genetic drift and humans

• Humans originated in Africa and hunters started colonizing new areas

• Caused major founder effect events, reduces genetic diversity

• On the graph you can see the gradual loss of genetic diversity as you get further and further away from Africa

Low gene flow vs high gene flow

Low gene flow:

• Happens when a very large barrier is separating the 2 pops (like a mountain)

• With genetic drift acting, it's gonna cause the genes to fix over a long time and some due to natural selection

• The 2 pops will diverge and get unique polymorphisms

 

High gene flow:

• Happens when pops are separated but can still have gene flow (like hills)

• Orgs can bring their genetic material from one side to another

• Shared polymorphisms (homogenize), makes the pops more similar

 

 

• Phenotypic differences can be due to:

1. Local adaptations

2. Genetic drift

3. Phenotypic plasticity

When studying phenotypic patterns, scientists have to determine whether the variation is due to local evolution or plasticity

2 ways:

1. Reciprocal transplant study: diff genotypes from diff environments all planted the same environments they were originally from and from the environments that the others were from

2. Genomic analysis

• Look at geographic patterns of genetic variation in traits that we think are under selection

Ex: the evolution of skin color in human

interpret this (phenotypic plasticity & local adaptation)

First column: the environment is the same for the 3 plants so the differences are due to genetics

First row: tells us that there is a local adaptation that make them unfit at the highest elevation (a trade off)

Second row: they do best in their own environment (local adaptation)

Third row: they're clones and the phenotypic differences are due to plasticity

 

• Can conclude that the differences between pops can be due to BOTH plasticity and genetics

 

what is phenotypic plasticity

Phenotypic plasticity: an organism's ability to change their phenotype in response to their changing environment. NOT an adaptation because plastic traits are not heritable

• The ABILITY to plastically change in response to the environments is genetic

• The specific phenotype you get is a product of the environment interacting with developmental processes

• Common in plants, corals (sedentary orgs)

plastic or not?

1. If the trait doesn’t change because of the environment, then there's no plasticity (left)

2. Trait values change a bit (plastic)

3. Trait values don’t always have to be parallel (right) to be plastic

what are the different types of plasticity (3)

1. Developmental plasticity

   • The environment shapes how it's growing during early life (non-reversible)

2. Acclimation

• Changes the functions/behavior of org in env, but short term. (reversible). allows an org to adjust back and forth depending on the env.

3. Seasonal

• changes in appearance based on the seasons (cyclical). ex: coat color change in snowshoe hare and fox, shedding antlers

Adaptive vs Maladaptive plasticity (and examples?)

not all plasticity is beneficial. it is only ADAPTIVE if it actually improves an org's chances of survival n reproduction. plasticity can have side effects that prevent an org from being perfectly suited to its env

ex of adaptive plasticity:

• seed beetles can change the size of their eggs depending on the plant.

  • acacia (high-quality seeds): beetles lay MANY SMALL eggs

  • paloverde (bad quality seeds): beetles lay LESS LARGE eggs

    • this maximizes the survival of offspring

 

ex of maladaptive plasticity:

• freshwater snails grow thick, round shells when they detect fish to prevent being eaten

• however these shells limit body growth and ability to have more kids

• they react to ALL fish (even ones that don't eat snails) so they waste energy growing these shells when they don't need to

Why aren't all traits phenotypically plastic?

 not enough genetic variation

- inherent costs and limitations

- even if variation is enough, a selective pressure can occur against the plastic trait

- genes associated with plasticity might be linked to genes with low fitness

- or if an org doesn't properly acclimate properly to the env, can create maladapted phenotype

- it takes time to acclimate so in this period they are less fit

 

phenotypic variation is essential for evolution by natural selection

if orgs look/behave the same way, there is nothing for selection to act upon

pre-zygotic barriers?

1. prevents mating/fertilization

   • Temporal/Habitat isolation: differences in breeding season

   • Behavioral isolation: differences in mating displays to prevent individuals from mating

   • Mechanical/Gametic isolation: fail to fertilize eggs

post-zygotic barriers?

1. prevents viable fertile offspring

   • Hybrid inviability: mating between 2 diff species where the embryo or the baby dies

   • Hybrid sterility: the baby hybrid survives but it's sterile (like mules - come from horse x donkey)

what is Intermediate Stages of Speciation

Incomplete Isolation: Speciation is a gradual process, so intermediate stages exist where populations are partially isolated

Molecular Clock Principle

• Big population → lots of individuals → more new mutations

• Small population → fewer mutations, BUT genetic drift has a stronger effect

• These effects approximately balance for neutral mutations

• Therefore, the rate of neutral DNA divergence is largely determined by the mutation rate