BIOL 2390 - Topic 5

Natural selection

The differential survival and reproduction of individuals based on their interactions with their environment, such that some live and some don’t

Variation

Variation exists within a population, which is essential, as some traits will offer an advantage that allows individuals to survive and/or have better reproductive success

Natural selection drives ________

Evolution

Evolution

The change in gene frequencies, driven by differences in fitness (i.e. survival and reproduction), such that the favourable traits will increase in frequency over time, while the unfavourable ones decrease

Natural selection vs. evolution unit

Natural selection acts on individuals, but evolution acts on populations

Natural selection on camouflaged groupers

Groupers with darker coloured bodies are selected for, such that they are more likely to pass on their genes and make up a majority of the population in the next generation, while those with light coloured bodies are selected against

Adaptations

A heritable trait, either behavioural, morphological, or physiological that is shaped by natural selection over time

Effect of adaptations

1.) It enhances overall growth, survival, and reproduction

2.) It can also affect interactions within and between species, such that it can form positive relationships between them

Adaptations in different conditions

1.) Adaptations are optimized for specific environmental conditions, meaning they will not work when put into other conditions

2.) In fact, they are likely to act as hindrances instead

Phenotypes vs. genotype

Genotypes are an organism’s genetic makeup, which determines the observable traits, i.e. the phenotype, that they possess

Simple dominance

Phenotypic variation is limited, usually only dominant and recessive, making it easy to interpret

Are all traits easily observable?

No, such as body biochemistry

Complex phenotypes

More complex phenotypes can be observed in codominant and continuous distributions

Codominant phenotypes

When the heterozygous genotype results in a blended product

Are codominant and continuous phenotypes bad?

No, they generally just increase phenotypic variation

Continuous phenotypic traits

Refers to when more than one set of genes are responsible for a particular trait, such as height, skin colour, blood pressure, etc

Advantages of continuous traits

It leads to greater genetic and phenotypic diversity

Influence on continuous traits

1.) They are not only determined by genetics, but also by environmental factors

2.) For example, your genes can dictate your optimal height, but not receiving the proper nutrients growing up can prevent you from reaching your full potential height

Phenotypic plasticity

1.) Refers to organisms that can change their phenotype based on environmental conditions

2.) They have a set genotype, but the flexibility is encoded in their genome

Degree of plasticity

It varies among species, some have a lot, some have very little

Phenotypic plasticity example

Salmon, who can change their morphological appearance when they go from the ocean back into the freshwater lake they were born in

Advantage of plasticity

It increases fitness, as it allows organisms to survive and utilize multiple different environmental conditions

Organization of species

1.) Species rarely consist of a single continuous interbreeding population

2.) Instead, they are organized into a metapopulation, which is divided into groups of subpopulations consisting of interbreeding individuals

Linkage of subpopulations

They are linked by the movement of individuals between one subpopulation to another (aka migration/gene flow)

Genetic variation in subpopulations

It can occur within and among subpopulations

Evolution within each subpopulation

Each subpopulation will likely have different selection pressures, therefore each subpopulation will have different evolutionary trajectories

Evolution requires…

Variation among individuals in some heritable trait, resulting in differential success of said trait

Natural selection acting on Daphne finch birds during a drought

1.) Beak size was the target of selection, with the selective agent being the food size and abundance

2.) When the drought occured, smaller seeds were unavailable, leaving behind the larger seeds, which is why the birds selected towards larger beaks rather than small ones

Types of natural selection

1.) Directional selection

2.) Stabilizing selection

3.) Disruptive selection

Directional selection

When selection (the mean value of a trait) prefers one extreme over the other

Directional selection example

Long lizard tails scare predators, therefore selection shifts towards long tails over short tails

Stabilizing selection

When selection favours the mean, at the expense of the extremes

Stabilizing selection example

Baby birth weights: too big and both the mother and offspring dies, too small and the offspring dies, therefore selection favours the middle

Disruptive selection

It is when selection favours both extremes, but not the mean, resulting in a bimodal distribution of the characteristic

When does disruptive selection occur

It typically occurs when a population is subjected to multiple selection pressures at the same time

Disruptive selection can lead to…

It can lead to the formation of a new species if the selection pressures continue to exist for a long time

Disruptive selection example

1.) Short squirrel tails help keep predators from catching you on the ground

2.) Long tails provide good balance in the trees

3.) Medium tails don’t help at all

Mutations

1.) A random event that causes a change in a gene or a chromosome

2.) It is the main source of genetic variation

Cause of mutations

They can be caused by mutagens, such as UV radiation, or due to errors that occur during replication

Inheritance of mutations

It can only be inherited when the mutation occurs in reproductive cells

Environment and mutations

1.) The environment determines whether a mutation is neutral, beneficial, or harmful

2.) Neutral and beneficial mutations stay

3.) Harmful mutations disappear

Genetic drift

Random fluctuations in allele frequencies, usually when genes are lost in a population, due to the random nature of sexual reproduction

Example of genetic drift

Some individuals are not able to find a mate, despite their high reproductive rate, meaning that they cannot pass on their favourable genes, resulting in a simplified population

Occurrence of genetic drift

1.) It is mainly seen in small populations

2.) It is commonly observed in new colonizations or after population crashes

Genetic drift act like…

It acts like a sampling error, due to each generation only representing a small subset of the gene pool

Negative consequences of genetic drift

It reduces genetic diversity, resulting in increased extinction risk

Why does genetic drift increase extinction risk?

1.) Genetic drift causes reduced genetic diversity, resulting in a simplified population

2.) Therefore, when they respond to a something, it is all uniform and there is no variation, therefore they are all likely to die

Gene flow

Refers to when individuals switch back and forth between subpopulations and even metapopulations, specifically when they reproduce in different sub/meta-populations

Significance of gene flow

It acts as a strong force that reduces genetic differences between subpopulations, preventing them from diverging too much from each other

What happens when gene flow stops

Since traits are no longer being shared between the subpopulations, it causes them to drift apart and may even eventually cause speciation

Non-random mating

1.) When individuals select mates based on specific phenotypic traits, therefore those that don’t have the trait are selected against

2.) It is aka sexual selection and is known to drive evolution

Genetic differentiation

Refers to species that have a wide range of environmental conditions/distributions and can therefore result in…

1.) Clines

2.) Ecotypes

3.) Geographic isolates

Clines

1.) It describes gradual, measurable changes in a phenotypic trait across a geographic area

2.) It is often correlated with environmental gradients, such that the trait changes as environmental conditions change over a region

Clines environmental gradients examples

1.) Temperature

2.) Moisture

3.) Altitude

Clines example

1.) Deer gets heavier as they enter colder climates

2.) This is because the bigger their bodies, the better they are able to conserve heat

Ecotypes

Refers to species with an abrupt and dramatic difference in characters, usually well adapted towards the local environment they are in

Do ecotypes have a gradual transitional gradient?

No

Ecotypes example

1.) Lions in Savannah grasslands have very thick mane

2.) Lions in shrublands barely have any mane and this is because it would likely get tangled while they are running through the shrubs

Geographic isolates

1.) Refers to populations of species that are separated by extrinsic barriers that prevents movement between the different subpopulations, therefore limiting gene flow

2.) How isolated they are from one another depends on the efficiency of the barrier

Consequences of geographic isolates

Over time, the geographic isolation can lead to more and more genetic divergence, until speciation eventually occurs because they no longer have any shared traits

Gene flow in clines and ecotypes vs. geographic isolates

In clines and ecotypes, gene flow still occurs, but gene flow in geographic isolates is very restricted

Complete isolation in geographic isolates

It is rare?

Geographic isolates example

Salamanders

Sub-species

1.) Term used when very little gene flow exists between populations, based on having 1 or more differences in major characteristics

2.) It does not only apply to geographic isolates, but ecotypes too

Species (old definition)

They are defined by similar characters

Species (modern definition)

Groups of interbreeding populations that produce fertile offspring

Speciation

1.) The formation of a new species from evolutionary processes

2.) Usually occurs when different populations change over time, until reproductive isolation occurs

Adaptive radiation

Speciation where one species gives rise to many new species, who exploit the environment in unique ways

Adaptive radiation example

Salamanders are at this point, but have not yet fully speciated

Reproductive isolation

Isolating mechanisms/reproductive barriers that restrict the flow of genes between populations

Reproductive isolating mechanisms examples

1.) Morphological (different reproductive parts)

2.) Behavioural (different courtship rituals)

3.) Ecological conditions (reproduce in different places)

4.) Genetic incompatibility (different number of chromosomes)

Reproductive isolating mechanisms may be _________ or __________.

Pre-mating (morphological, behavioural, and ecological conditions) or post-mating (genetic incompatibility)

Horse + Donkey =

Infertile mule

What kind of isolating mechanism does horse+donkey have?

Post-mating mechanism caused by genetic incompatibility

How are donkey’s and horses able to reproduce in the first place?

It was only recently that they speciated, therefore they only have the one barrier between them (i.e. the genetic incompatibility)

Types of speciation

1.) Allopatric speciation

2.) Sympatric speciation

Allopatric speciation

1.) It occurs when a geographical barrier separates a population into isolated subpopulations with no gene flow

2.) The different local environments create different selective pressures, causing them to diverge, until reproductive isolation and speciation eventually occur

Example of allopatric speciation

London Underground mosquito, who are in the process of allopatric speciation as they only produce fertile offspring in the lab

Sympatric speciation

Speciation occurs due to the formation of a reproductive barrier within the same geographic location (i.e. there is no geographic barrier)

Sympatric speciation is caused by…

1.) Polyploidy

2.) Disruptive selection

Polyploidy

1.) Refers to the doubling of the number of chromosomes, resulting in genetic incompatibility

2.) It occurs when errors happen during meiotic cell division

Disruptive selection

1.) Selection that favours the extremes, caused by multiple selection forces at the same time, such as sexual selection, habitat and food source specialization, etc

2.) It is difficult to observe in progress

Disruptive selection example 2

Arctic charr

Hybridization

The mating of two closely related species that can lead to hybrids, which may or may not be sterile or fertile

Fertile hybrids

They blur the lines between two species and challenges the species concept definition

Are hybrids new species

No

Number of fertile hybrids

An estimated 6% of animals and up to 70% of insects/plants can form fertile hybrids

Junco

1.) Prefers coniferous forest habitats across North America, giving it a wide distribution

2.) Eats mostly ground-level insects and seeds and nests near the ground, low in trees, or on the ground itself

3.) Its evolutionary history and speciation fascinates scientists