AQA A Level Biology Topic 7: Genetics, populations, evolution and ecosystems

Vocabulary-style flashcards covering genetics, inheritance, population equations, evolutionary mechanisms, and ecological principles as required for AQA A Level Biology Topic 7.

Genotype

The genetic constitution of an organism.

Phenotype

The expression of the genetic constitution ($genotype$) and its interaction with the environment.

Alleles

Variations of a particular gene found at the same locus, arising by mutation.

Dominant allele

An allele that is always expressed in the phenotype.

Recessive allele

An allele only expressed when two copies are present ($homozygous recessive$) or not expressed when a dominant allele is present ($heterozygous$).

Codominant alleles

Both alleles are expressed and contribute to the phenotype if inherited together.

Homozygous

When alleles at a specific locus on each homologous chromosome are the same.

Heterozygous

When alleles at a specific locus on each homologous chromosome are different.

Monohybrid cross

The inheritance of one phenotypic characteristic coded for by a single gene.

Dihybrid cross

The inheritance of two phenotypic characteristics coded for by two different genes.

Sex-linked gene

A gene with a locus on a sex chromosome, normally the $X$ chromosome.

Autosomal linkage

When two genes are located on the same autosome ($non-sex chromosome$) and are inherited together.

Epistasis

The interaction of non-linked genes where the product of one gene masks or suppresses the expression of another gene.

Chi-squared ($χ^2$) test

A statistical test used to determine if observed results are significantly different from expected frequencies in categorical data.

Population

A group of organisms of the same species in one area at one time that can interbreed.

Gene pool

All the alleles of all the genes in a population at any one time.

Allele frequency

The proportion of an allele of a gene in a gene pool, expressed as a decimal or percentage.

Hardy-Weinberg principle

A model stating allele frequencies do not change across generations in large populations with no migration, mutation, or selection, and where mating is random.

Hardy-Weinberg equation

$p^2 + 2pq + q^2 = 1$, where $p$ is the frequency of the dominant allele and $q$ is the frequency of the recessive allele ($p + q = 1$).

Evolution

The change in allele frequency over time or many generations in a population via natural selection.

Stabilising selection

Selection where organisms with average/modal variations of a trait have a selective advantage, reducing the range or standard deviation.

Directional selection

Selection where organisms with alleles coding for one extreme variation of a trait have a selective advantage.

Disruptive selection

Selection where organisms with alleles coding for either extreme variation of a trait have a selective advantage, potentially leading to speciation.

Speciation

The formation of new species from existing species when genetic differences result in an inability to interbreed and produce fertile offspring.

Allopatric speciation

Speciation occurring due to geographical isolation between populations.

Sympatric speciation

Speciation occurring without geographical isolation, often due to reproductive isolation like different breeding seasons or courtship behaviors.

Genetic drift

A mechanism of evolution where allele frequencies change over generations due to chance, most impactful in small populations.

Community

All the populations of different species living in the same habitat at the same time.

Ecosystem

A dynamic system comprising a community and the non-living ($abiotic$) components of its environment.

Niche

The specific role of a species within its habitat, governed by its adaptation to both abiotic and biotic conditions.

Carrying capacity

The maximum stable population size of a species that an ecosystem can support.

Interspecific competition

Competition for resources between organisms of different species.

Intraspecific competition

Competition for resources between organisms of the same species.

Mark-release-recapture

A method to estimate the population size of motile organisms using the formula: ($number in sample 1 × number in sample 2$) / $number marked in sample 2$.

Succession

The change in a community over time due to changes in abiotic factors and species interactions.

Pioneer species

The first species to colonise an area, starting the process of succession by changing abiotic conditions.

Climax community

The final stable community at the end of succession where abiotic factors and populations are fairly constant.

Transect line

A tape measure placed across an area with an environmental gradient used for systematic sampling.