Natural selection learning objectives

Explain how natural selection causes evolution

It is a mechanism of evolution.

• Heritable traits that allow for greater reproductive success become more common in a population over time.

  • favoring heritable traits that improve an organism's ability to survive and reproduce.

• Or in a better term, allow for better fitness, which is how reproductively successful one is.

• The alleles that code for traits with better fitness tend to increase in frequency, as the good enough individuals survive.

Describe artificial selection

• Intentional breeding by humans

• Particular characteristics

• Common for plants and animals

• Breeding that is developed to produce more or higher yields than traditional traits.

• Modifies genetic information of an organism

• New traits are produced

Summarize the 3 conditions that define natural selection

1- variation for a trait

2- heritability

3- differential reproductive success

• more offspring produced than can survive

• competition for resources

• some are better competitors, survive & reproduce

Example showing the 3 conditions of natural selection

1- running speed in rabbits can vary from one individual to the next

• some are fast, some are medium, and some are slow

2- the trait of running speed is passed on from parents to their offspring

• fast rabbits will give birth to fast rabbits, slow rabbits will give birth to slow rabbits

3- in a population, rabbits with slower running speeds are eaten by the fox and their traits are not passed on to the next generation

• so only fast/medium rabbits are being produced

• running speed will increase over time

What is fitness

how reproductively successful one is

Why does evolution occur only in populations and not in individuals

• Evolution occurs in populations, not individuals, because it is defined as a change in the inheritable characteristics of a population over successive generations.

• An individual's genetic makeup is fixed during its lifetime, so it cannot change through evolution. Instead, evolution is the process of a population's gene pool changing over time due to factors like natural selection, which favors individuals with advantageous traits that get passed to their offspring.

explain how natural selection brings about increased reproductive success of a population in its environment

relates to rabbit example

explain why it is inaccurate to say that natural selection changes individuals

Natural selection operates on populations over generations, not on individual organisms within their lifetime.

While individual organisms may show temporary changes due to environmental factors or development, these changes are not heritable genetic shifts and do not constitute evolution.

Instead, natural selection acts by favoring individuals with advantageous genetic variations, which then become more common in the population over time, leading to evolutionary change

explain why it is incorrect to say that natural selection is goal-directed

It's a blind, mechanistic process that operates on existing variations, not a conscious force with a plan for the future.

It simply favors traits that provide immediate reproductive advantages in a given environment, rather than "aiming" to create specific outcomes or "improve" a species over time in a purposeful way.

Explain why it is inaccurate to say that natural selection leads to perfection

good enough individuals survive

Intersexual selection

Intersexual Selection: 

• Occurs between members of opposite sexes. 

• Involves females choosing males based on certain traits or behaviors. 

• Examples:

  • Females selecting males with bright plumage or elaborate courtship displays. 

  • Females preferring males with resources or good parental care abilities. . 

Intrasexual selection

Intrasexual Selection:

• Occurs between members of the same sex.

• Involves males competing for access to females through physical combat, displays of dominance, or resource competition.

• Examples:

  • Males fighting for territory or mates.

  • Males showing off their strength or size

compare intersexual and intrasexual selection

Key Differences:

• Participants:

  Intersexual selection involves opposite sexes, while intrasexual selection involves the same sex. 

• Focus:

  Intersexual selection focuses on female mate choice, while intrasexual selection focuses on male competition for mates. 

• Outcome:

  Intersexual selection can lead to the evolution of traits that enhance attractiveness to females, while intrasexual selection can lead to the evolution of traits that enhance competitive ability

The fundamental asymmetry of sex

Bateman-Trivers hypothesis

Pattern- traits that attract opposite sex are more elaborate in males

Process- eggs are more energetically costly to create than sperm

The fundamental asymmetry of sex

• If eggs are more energetically costly to create than sperm, then females, but not males, should be choosy about maters since they invest a lot in each egg

• males will compete with each other

• alleles that increase male’s attractiveness or success in male-male competition should increase in population

• sexual selection should act more strongly on males than on females

think peacocks

describe the relationship between sexual dimorphism and sexual selection

Sexual dimorphism, the physical differences between males and females of a species, is often a result of sexual selection, which drives the evolution of exaggerated traits that enhance mating success. 

Explain the effect of genetic drift on genetic variation

Genetic drift causes a reduction in genetic variation within a population by randomly eliminating alleles, especially in small populations, leading to the loss of some gene variants and the fixation of others.

This process, driven by random chance rather than fitness, decreases the gene pool's diversity, making the population less able to adapt to new environmental pressures and potentially leading to genetic differentiation between isolated populations.

explain the effect of genetic drift on fitness

Neutral-to-negative effect on fitness, as it is a random process that can cause beneficial or neutral alleles to be lost, or even fix deleterious mutations, regardless of their impact on survival and reproduction.

explain how sampling error causes genetic drift

Because the individuals that successfully reproduce in the next generation are a random sample of the parent population, and their genetic makeup may not perfectly reflect the allele frequencies of the entire population.

In small populations, chance fluctuations in which alleles get passed on can lead to significant changes in allele frequencies from one generation to the next, resulting in genetic drift.

explain the founder effect

When a group of individuals establish a new population in a new area

• allele frequencies likely differ from source population if new population is small enough

explain the bottleneck effect

A drastic reduction in a population's size, often due to a random event like a natural disaster, which leads to a significant loss of genetic diversity.

The survivors represent only a small, random fraction of the original gene pool, decreasing the population's ability to adapt to environmental changes.

• This effect is a type of genetic drift and can also increase inbreeding leading to reduced reproductive success and higher mortality rates in the recovering population.

How Genetic Drift Works

Random Chance: Genetic drift is a random process where allele frequencies change from one generation to the next due to chance events, such as random sampling of which individuals successfully reproduce.

Allele Loss: When an allele is rare, it has a greater chance of being completely lost from the population due to these random fluctuations.

Allele Fixation: Conversely, an allele can become "fixed," meaning it is the only allele present at a particular gene locus in the population.

Reduced Genetic Diversity: The combined effect of allele loss and fixation is a significant reduction in the overall genetic variation within the population