Uploaded July 2023 | Updated September 2026, 2 days ago

---

You can find all my A Level Biology videos fully indexed at freesciencelessons.co.uk/a-level-revision-videos/a-level-biology

In this video, we look at genetic and environmental causes of variation. First we explore what is meant by interspecific and intraspecific variation. We then look at how variation can have genetic causes such as alleles, independent assortment of chromosomes and crossing-over during meiosis and random fertilisation of gametes. Finally, we look at how variation can be caused by the environment.

Image credits

Astronotus ocellatus
commons.wikimedia.org/wiki/File:Astronotus_ocellatus.jpg
Jón Helgi Jónsson (Amything), CC BY-SA 3.0 creativecommons.org/licenses/by-sa/3.0/, via Wikimedia Commons

Fungus
commons.wikimedia.org/wiki/File:Fungi_at_Springhead,_Shottermill_08.jpg
Simon Burchell, CC BY-SA 4.0 creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons

Hyena
commons.wikimedia.org/wiki/File:Spotted_hyena_(Crocuta_crocuta).jpg
Charles J. Sharp, CC BY-SA 4.0 creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons

Peacock
commons.wikimedia.org/wiki/File:P%C3%A5f%C3%A5gel_(peacock).jpg
Magnus Johansson, CC BY-SA 2.0 creativecommons.org/licenses/by-sa/2.0, via Wikimedia Commons

Mamba
commons.wikimedia.org/wiki/File:Mamba_Dendroaspis_angusticeps.jpg
Holger Krisp, CC BY 3.0 creativecommons.org/licenses/by/3.0, via Wikimedia Commons

Snail
commons.wikimedia.org/wiki/File:Snail_(139998665).jpeg
Lauren, CC BY 3.0 creativecommons.org/licenses/by/3.0, via Wikimedia Commons

Black cat
commons.wikimedia.org/wiki/File:Tesla_the_cat_2.jpg
Sage Ross, CC BY-SA 3.0 creativecommons.org/licenses/by-sa/3.0/, via Wikimedia Commons

Orange tabby
commons.wikimedia.org/wiki/File:Orange_tabby_cat_sitting_on_fallen_leaves-Hisashi-01A.jpg
Orange_tabby_cat_sitting_on_fallen_leaves-Hisashi-01.jpg: Hisashi from Japanderivative work: Caspian blue, CC BY-SA 2.0 creativecommons.org/licenses/by-sa/2.0, via Wikimedia Commons

Black and white cat
commons.wikimedia.org/wiki/File:Black_and_white_cat_sitting_on_a_street-Hisashi-01.jpg
Hisashi from Japan, CC BY-SA 2.0 creativecommons.org/licenses/by-sa/2.0, via Wikimedia Commons

Orange, black and white cat
commons.wikimedia.org/wiki/File:Kirby.jpg
Fquestier, CC BY-SA 3.0 creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

Ob mouse
commons.wikimedia.org/wiki/File:Fatmouse.jpg
The original uploader was Bigplankton at English Wikipedia.Later versions were uploaded by Sunholm at en.wikipedia., Public domain, via Wikimedia Commons

Melanoma
commons.wikimedia.org/wiki/File:Melanoma.jpg
See page for author, Public domain, via Wikimedia Commons

Havanese litter
commons.wikimedia.org/wiki/File:Havanese_Litter.png
Cartman0052007 at English Wikipedia, CC BY 3.0 creativecommons.org/licenses/by/3.0, via Wikimedia Commons

Meiosis
commons.wikimedia.org/wiki/File:Meiosis_(261_35).jpg
Doc. RNDr. Josef Reischig, CSc., CC BY-SA 3.0 creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

Fertilisation
commons.wikimedia.org/wiki/File:Sperm-egg.jpg
pdimages.com/web9.htm, Public domain, via Wikimedia Commons

Scar
commons.wikimedia.org/wiki/File:Scar,_two_months_after_total_thyrodectomy.jpg
Starvee44 at English Wikipedia, Public domain, via Wikimedia Commons

Blue hydrangea
commons.wikimedia.org/wiki/File:Hydrangea_macrophylla_02.jpg
I, MJJR, CC BY 2.5 creativecommons.org/licenses/by/2.5, via Wikimedia Commons

Pink hydrangea
commons.wikimedia.org/wiki/File:Hydrangea_macrophylla_01.jpg
I, MJJR, CC BY 2.5 creativecommons.org/licenses/by/2.5, via Wikimedia Commons

Basketball
commons.wikimedia.org/wiki/File:Spain_national_basketball_team_2011_01.jpg
Christopher Johnson, CC BY-SA 2.0 creativecommons.org/licenses/by-sa/2.0, via Wikimedia Commons

Blood
commons.wikimedia.org/wiki/File:Blood_Test_(15575812743).jpg
Alden Chadwick from Leeds, UK, CC BY 2.0 creativecommons.org/licenses/by/2.0, via Wikimedia Commons