H2 Biology Core 4 - Natural Selection and Speciation

Step 1 - Variation is the raw material for evolution

Evolution acts on populations, not individuals.

Step 1 - Variation is the raw material for evolution

For natural selection to occur, there must be variation within a population.

Step 1 - Variation is the raw material for evolution

This variation arises from three main sources: gene mutations that create new alleles, meiosis which produces unique combinations through crossing over and independent assortment, and random fertilisation which brings together different gamete combinations.

Step 1 - Variation is the raw material for evolution

Some of this variation is heritable, meaning it can be passed on to offspring and therefore subjected to selection.

Step 2 - The mechanism of natural selection

Natural selection is a four step process.

Step 2 - The mechanism of natural selection

First, individuals in a population show variation in their phenotypes due to genetic differences.

Step 2 - The mechanism of natural selection

Second, there is a struggle for survival because resources are limited and more offspring are produced than can survive.

Step 2 - The mechanism of natural selection

Third, individuals with phenotypes better suited to the environment have a selective advantage and are more likely to survive and reproduce.

Step 2 - The mechanism of natural selection

Fourth, these individuals pass on their advantageous alleles to the next generation, so the frequency of those alleles increases over time.

Step 3 - Evidence for evolution

Several lines of evidence support evolution by natural selection.

Step 3 - Evidence for evolution

The fossil record shows a progression from simple to complex organisms and reveals transitional forms, such as Archaeopteryx linking dinosaurs to birds.

Step 3 - Evidence for evolution

Comparative anatomy reveals homologous structures, like the pentadactyl limb found in mammals, birds, and amphibians, indicating a shared ancestor.

Step 3 - Evidence for evolution

Molecular evidence, such as comparing D N A or amino acid sequences, shows that closely related species have more similar sequences than distantly related ones.

Step 4 - Species concepts and reproductive isolation

A biological species is defined as a group of organisms that can interbreed to produce fertile offspring and are reproductively isolated from other groups.

Step 4 - Species concepts and reproductive isolation

Reproductive isolation prevents gene flow between populations and is essential for speciation.

Step 4 - Species concepts and reproductive isolation

Pre-zygotic barriers include habitat isolation, temporal isolation where species breed at different times, and behavioural isolation where courtship signals differ.

Step 4 - Species concepts and reproductive isolation

Post-zygotic barriers include hybrid inviability, where the hybrid embryo fails to develop, and hybrid sterility, where the offspring is viable but infertile, like a mule.

Step 5 - Allopatric versus sympatric speciation

Allopatric speciation begins when a physical barrier, such as a mountain range, river, or ocean, splits a population into two geographically isolated groups.

Step 5 - Allopatric versus sympatric speciation

Each group experiences different selection pressures and undergoes independent genetic changes through mutation, natural selection, and genetic drift.

Step 5 - Allopatric versus sympatric speciation

Over many generations, the two populations diverge genetically to the point where they can no longer interbreed, even if the barrier is removed.

Step 5 - Allopatric versus sympatric speciation

Sympatric speciation occurs without geographic separation.

Step 5 - Allopatric versus sympatric speciation

It can happen through polyploidy in plants, where a chromosome doubling event instantly creates a reproductively isolated population, or through ecological specialisation where subgroups exploit different niches within the same habitat.